From Wikipedia, the free encyclopedia
The origin of the
domestic dog is not clear. The domestic dog is a member of the
genus Canis, which forms part of the
wolf-like canids, and is the most widely abundant terrestrial
carnivore. The closest living relative of the dog is the
extant gray wolf, and there is no evidence of any other canine contributing to its
genetic lineage. The dog and the extant gray wolf are
sister taxa, as modern wolves are not closely related to the wolves that were first
domesticated. The archaeological record and genetic analysis show the remains of the
Bonn–Oberkassel dog buried beside humans 14,200 years ago to be the first undisputed dog, with disputed remains occurring 36,000 years ago. These dates imply that the earliest dogs arose in the time of human
hunter-gatherers and not
agriculturists. The dog was the first species to be domesticated.
Where the
genetic divergence of dog and wolf took place remains controversial, with the most plausible proposals spanning
Western Europe,
[3][15] Central Asia[15][16] and
East Asia.
[15][17]
This has been made more complicated by the most recent proposal that
fits the available evidence, which is that an initial wolf population
split into East and West
Eurasian groups; these, before going
extinct,
were domesticated independently into two distinct dog populations
between 14,000 and 6,400 years ago. The Western Eurasian dog population
was partially and gradually replaced by East Asian dogs introduced by
humans at least 6,400 years ago.
Canid and human evolution
Six million years ago, towards the close of the
Miocene era, the earth's climate was gradually cooling and this led to the glaciations of the
Pliocene and the
Pleistocene – the
Ice Age. In many areas,
forests and
savannahs were replaced with
steppe or
grasslands and only those creatures that could adapt survived.
In southern
North America, small woodland
foxes grew bigger and better adapted to running, and by the late Miocene the first of the genus
Canis had arisen - the ancestors of
coyotes,
wolves and the
domestic dog. In
eastern Africa, a split occurred among the large
primates.
Some remained in the trees, while others came down from the trees,
learned to walk upright, developed larger brains, and in the more open
country learned to avoid predators while becoming predators themselves.
[20] The two species ultimately met in
Eurasia.
"They were individual animals and
people involved, from our perspective, in a biological and cultural
process that involved linking not only their lives but the evolutionary
fate of their heirs in ways, we must assume, they could never have
imagined." — Mark Derr[21]
Human hunter-gathers did not live in fear of nature and knew that they posed a formidable risk to any potential predators. The
Ju'wasi people of
Namibia
share their land with prides of lions. Both species coexist with
respect and without fear or hostility in a relationship that may go back
to the dawn of modern humans. The lion is a larger and far more
dangerous predator than the wolf. Early modern humans entering Eurasia
and first encountering packs of wolves may have been assisted in living
among them because of the traditional beliefs of their African
ancestors. In historical times, mutual respect and cooperation with
canids can be found in the stories and traditions of the indigenous
peoples of Siberia, East Asia, North America, and Australia.
[22]
Dog evolution
Evolution (from the Latin
evolutio: "unrolling")
[23] is defined by the English naturalist, geologist and biologist
Charles Darwin as biological descent with modification.
[24] Evolution eventually splits ancestral populations into new species which cannot, or will not, interbreed.
[25]
The domestic dog is the first species and the only large
carnivore to have been domesticated. Over the past 200 years, dogs have undergone rapid
phenotypic change and were formed into today's modern
dog breeds due to
artificial selection imposed by humans. These breeds can vary in size and weight from a 0.46 kg (1.0 lb) teacup
poodle to a 90 kg (200 lb) giant
mastiff.
The skull, body, and limb proportions vary significantly between
breeds, with dogs displaying more phenotypic diversity than can be found
within the entire order of carnivores. Some breeds demonstrate
outstanding skills in herding, retrieving, scent detection, and
guarding, which demonstrates the functional and behavioral diversity of
dogs. There have been major advances in understanding the genes that
gave rise to the phenotypic traits of dogs. The first dogs were
certainly wolflike, however the phenotypic changes that coincided with
the dog–wolf
genetic divergence are not known.
[26]
The causes and context of dog domestication are not known.
Deriving a time and a place for domestication could help describe the
conditions that gave rise to an association of a large carnivore with
hunter-gatherers. All other domestication events occurred during or
after the development of
agriculture, which was approximately 10,000
years before present
(YBP). The earlier association of dogs with humans may have allowed
dogs to have a profound influence on the course of early human history
and the development of civilization. However, the timing, geographic
locations, and ecological conditions that led to dog domestication are
not agreed.
[26]
Genetic studies indicate that the
gray wolf is the closest living relative of the dog, with no evidence of any other
canine species having contributed. Attempting to reconstruct the dog's lineage through the
phylogenetic analysis of
DNA sequences
from modern dogs and wolves has given conflicting results for several
reasons. Firstly, studies indicate that an extinct Late Pleistocene wolf
is the nearest common ancestor to the dog, with modern wolves not being
the dog's direct ancestor. Secondly, the genetic divergence between the
dog and modern wolves occurred over a short period of time, so that the
time of the divergence is difficult to date (referred to as
incomplete lineage sorting).
This is complicated further by the cross-breeding that has occurred
between dogs and wolves since domestication (referred to as
post-domestication
gene flow). Finally, there has been only tens of thousands of generations of dogs since domestication, so that the number of
mutations between the dog and the wolf are few and this makes the timing of domestication difficult to date.
[26]
Archaeological evidence
Among archeologists, the proposed timing of the development of a
relationship between humans and wolves is debated. There exists two
schools of thought.
[27]
The early domestication theory argues that the relationship commenced
once humans moved into the colder parts of Eurasia around 35,000 YBP,
which is when the proposed
Paleolithic dogs first began to appear.
[11][28][29]
Wolves that were adjusting to live with humans may have developed
shorter, wider skulls and more steeply-rising foreheads that would make
wolf facial expressions easier to interpret.
[27]
The late domestication theory argues that Paleolithic dogs are an
unusual phenotype of wolf and that dogs appeared only when they could be
phenotypically distinguishable from the wolf, which is usually based on
a reduction in size.
This argument maintains that domesticated dogs are more clearly
identified when they are associated with human occupation, and those
interred side-by-side with human remains provide the most conclusive
evidence,
[31] commencing with the 14,200 years old
Bonn-Oberkassel dog.
The debate centres around
Homo sapiens and if they had
entered into cooperation with wolves soon after they moved into Eurasia,
and if so when and where did these wolves change into domesticated
dogs. In arguing that domestication leads to reduction in size, the late
domestication theory ignores that modern horses and pigs are larger
than their wild ancestors. It also ignores that if hunter-gathers
entered into a hunting relationship with wolves then there would be no
need of selection for a reduction in size. A reduction in size would
have occurred much later when humans moved into agricultural villages.
The late domestication theory does not consider the possibility that
humans may have formed a relationship with non-domesticated wolves and
that dogs in the early stages of domestication might be
indistinguishable from wolves. According to indigenous North Americans,
over the past 20,000 years the canids living with them were wolves that
could not be distinguished as dogs.
[27]
The problem in attempting to identify when and where
domestication occurred is the possibility that the process of
domestication occurred in a number of places and at a number of times
throughout prehistory.
[27]
Early dog remains have been found in different parts of the world. This
suggests that dog domestication may have taken place in different
regions independently by hunter-gatherers, in some cases at the same
time
[35] and in other cases at different times,
[36] with different wolf subspecies producing different dog lineages.
[37][36] Therefore, the number of dog domestication events is not known.
[38] A maternal mDNA study shows that dogs fall within 4
clades,
[3] indicating that dogs are derived from 4 separate lineages and that there was no single domestication event.
[27]
A 2018 domestication study looked at the reasons why the
archeological record that is based on the dating of fossil remains often
differed from the genetic record contained within the cells of living
species. The study concluded that our inability to date domestication is
because domestication is a continuum and there is no single point where
we can say that a species was clearly domesticated using these two
techniques. The study proposes that changes in morphology across time
and how humans were interacting with the species in the past needs to be
considered in addition to these two techniques.
[39]
..."wild" and "domesticated" exist
as concepts along a continuum, and the boundary between them is often
blurred — and, at least in the case of wolves, it was never clear to
begin with. — Raymond Pierotti[22]
The table below lists by location and timing in years before present
the very early co-location of hominid and wolf specimens, followed by
proposed Paleolithic dogs and then the first undisputed dog remains.
Years BP |
Location |
Finding
|
400,000 |
Boxgrove near Kent, England |
Wolf bones in close association with hominid bones. These have been found in Lower Paleolithic sites including Boxgrove (400,000 YBP), Zhoukoudian in North China (300,000 YBP), and Grotte du Lazaret
(125,000 YBP) in southern France. "The sites of occupation and hunting
activities of humans and wolves must often have overlapped."[40] We do not know if the co-location was the result of coincidence or a relationship.
|
300,000 |
Zhoukoudian cave system, China |
Small, extinct wolf skulls of Canis variabilis. At the site, the small wolf's remains were in close proximity to Peking man (Homo erectus pekinensis).[41] We do not know if the co-location was the result of coincidence or a relationship.
|
125,000 |
Grotte du Lazaret, near Nice, France |
Wolf skulls appear to have been set at the entrance of each dwelling
in a complex of Paleolithic shelters. The excavators speculated that
wolves were already incorporated into some aspect of human culture by
this early time. A nearby wolf den intruded on the site.[42] In 1997, a study of maternal mDNA indicated that the genetic divergence of dogs from wolves occurred 100,000–135,000 YBP.[7]
The Lazaret excavation lends credence to this mDNA study, in addition
to indicating that a special relationship existed between wolves and
genus Homo other than Homo sapiens, because this date is well before the arrival of Homo sapiens into Europe.[27] In 2018, a study of paternal yDNA indicated that the dog and the modern gray wolf genetically diverged from a common ancestor between 68,000-151,000 YBP.[43]
|
36,000 – 32,000[27] |
Goyet Cave, Samson River Valley, Belgium |
The "Goyet dog" is proposed as being a Paleolithic dog.[11] The dog-like skull was found in a side gallery of the cave, and Palaeolithic artifacts in this system of caves date from the Mousterian, Aurignacian, Gravettian, and Magdalenian, which indicates recurrent occupations of the cave from the Pleniglacial until the Late Glacial.[11] The Goyet dog left no descendants, and its genetic classification is inconclusive because its mitochondrial DNA
(mDNA) does not match any living wolf nor dog. It may represent an
aborted domestication event or phenotypically and genetically distinct
wolves.[3] A genome-wide
study of a 35,000 YBP Pleistocene wolf fossil from northern Siberia
indicates that the dog and the modern grey wolf genetically diverged
from a common ancestor between 27,000-40,000 YBP.[44][27]
|
33,500 |
Razboinichya Cave, Altai Mountains, Central Asia (Russia) |
The "Altai dog" is proposed as being a Paleolithic dog.[11]
The specimens discovered were a dog-like skull, mandibles (both sides)
and teeth. The morphological classification, and an initial mDNA
analysis, found it to be a dog.[45]
A later study of its mDNA was inconclusive, with 2 analyses indicating
dog and another 2 indicating wolf. In 2017, two prominent evolutionary
biologists reviewed the evidence and supported the Altai dog as being a
dog from a lineage that is now extinct and that was derived from a
population of small wolves that are also now extinct.[26]
|
32,000–13,500 |
Europe and Yakutia, northeastern Siberia |
Numerous Paleolithic dog remains found in Europe and one in Yakutia, northeast Siberia.
|
14,200 |
Bonn-Oberkassel, Germany |
The "Bonn-Oberkassel dog". Undisputed dog skeleton buried with a man and woman. All three skeletal remains were found sprayed with red hematite powder.[10] The consensus is that a dog was buried along with two humans.[46] Analysis of mDNA indicates that this dog was a direct ancestor of modern dogs.[47]
|
Genetics, archaeology and morphology
The domestic dog is the most widely abundant large
carnivore.
When and where dogs were first domesticated has taxed geneticists for
the past 20 years and archaeologists for many decades longer.
[9] Identifying the earliest dogs is difficult because the key
morphological
characters that are used by zooarchaeologists to differentiate domestic
dogs from their wild wolf ancestors (size and position of teeth, dental
pathologies, and size and proportion of cranial and
postcranial
elements) were not yet fixed during the initial phases of the
domestication process. The range of natural variation among these
characters that may have existed in ancient wolf populations, and the
time it took for these traits to appear in dogs, are unknown.
[12]
However, recent studies based on
genetics propose five generalizations about dogs.
[49]
- Over the past million years, numerous wolf-like forms existed
but their turnover has been high, and modern wolves are not the lineal
ancestors of dogs.[2][3][8][50] Although research had suggested that dogs and wolves were genetically very close relatives,[51][7][52][53] later phylogenetic analysis strongly supported the hypothesis that the dog forms a monophyletic clade that is sister to Eurasian wolves, and these together form a sister clade to North American wolves.[2][49]
This indicates that an extant wolf population ancestral to dogs has not
been found, and is presumed that the wolves immediately ancestral to
dogs are extinct.[8][9][51] The dog is not a separate species to the wolf.[2][49]
- Studies propose a divergence time of the dog from the wolf ancestor closer to 27,000 YBP,[2][3][8][18][44] with the most recent proposing 36,900-41,500 YBP followed by domestication occurring between 20,000-40,000 YBP.[54]
- The dog was the first domesticated species[9][12][13][14] and appeared more than 15,000 years before present (YBP). The dog was established across Eurasia before the end of the Late Pleistocene era, well before cultivation and the domestication of other animals around 10,000 YBP,[12] indicating that dogs were domesticated by hunter-gatherers and not early agriculturalists.[55] Studies support two population bottlenecks had occurred to the dog lineage, one due to the initial domestication and one due to the formation of dog breeds.[49]
- Dogs show both ancient and modern lineages. The ancient lineages appear most in Asia but least in Europe because the Victorian era development of modern dog breeds used little of the ancient lineages.[6][16][18] All dog populations (breed, village, and feral) show some evidence of genetic admixture between modern and ancient dogs. Some ancient dog populations that once occupied Europe and the New World no longer exist.[3][9][18][56] This implies that some ancient dog populations were entirely replaced and others admixed over a long period of time.[49]
- There was admixture between dog and regional wolf populations except on the Tibetan Plateau and in the New World wolves.[2][8]
This admixture has occurred throughout history and as dogs expanded
across the landscape. There are some dog populations that show recent
admixture with wolves.[2][49]
The extinction of the wolves that were the direct ancestors of dogs,
and the sustained admixture between different dog and wolf populations
over at least the last 10,000 years, has blurred the genetic signatures
and confounded efforts of researchers at pinpointing the origins of
dogs.
[9][12]
An alternate proposal is that during the ecological upheavals of the
Late Pleistocene, all of the remaining members of a dwindling lineage
joined with humans.
[57][34]
Time of divergence
Paleoecology
Mammoth bone dwelling,
Mezhirich site, Ukraine
During the last
Ice Age, the most recent peak is known as the
Last Glacial Maximum when a vast
mammoth steppe stretched from
Spain eastwards across
Eurasia and over the
Bering land bridge into
Alaska and the
Yukon. The continent of Europe was much colder and drier than it is today, with polar desert in the north and the remainder
steppe or
tundra. Forest and woodland were almost non-existent except for isolated pockets in the mountain ranges of southern Europe.
[58] The
Late Pleistocene
was characterized by a series of severe and rapid climate oscillations
with regional temperature changes of up to 16 °C, which has been
correlated with
Pleistocene megafaunal extinctions. There is no evidence of megafaunal extinctions at the height of the
Last Glacial Maximum,
indicating that increasing cold and glaciation were not factors.
Multiple events appear to have caused the rapid replacement of one
species by another one within the same
genus,
or one population by another within the same species, across a broad
area. As some species became extinct, so too did the predators that
depended on them.
[59] The ancestors of modern humans first reached Europe between 43,000-45,000 years BP, with their remains found in
Italy[60] and in
Britain.
[61]
Probable ancestor
During the Last Glacial Maximum there were two types of wolf. The cold north of the
Holarctic was spanned by a large, robust, wolf
ecomorph
that specialised in preying on megafauna. Another more slender form
lived in the warmer south in refuges from the glaciation. When the
planet warmed and the Late Glacial Maximum came to a close, whole
species of megafauna became extinct along with their predators, leaving
the more gracile wolf to dominate the Holarctic. The more gracile wolf
was the ancestor of the modern gray wolf, which is the dog's sister but
not its ancestor as the dog shows a closer genetic relationship to the
now-extinct
megafaunal wolf.
Evolutionary divergence
DNA
evidence indicates that the dog, the modern gray wolf (above) and the
now-extinct Taimyr wolf diverged from a now extinct wolf that once lived
in Europe.
The date estimated for the evolutionary divergence of a domestic
lineage from a wild one does not necessarily indicate the start of the
domestication process but it does provide an upper boundary. The
divergence of the domestic horse from the lineage that led to the modern
Przewalski’s horse
is estimated at 45,000 YBP but the archaeological record indicates
5,500 YBP. The variance could be due to the modern wild population not
being the direct ancestor of the domestic one, or the impact of a split
due to climate, topography, or other environmental changes. The
divergence time does not imply domestication during this specific
period.
[15] Some researchers propose that the "dog was the dog before it was domesticated"
[62] and that the ancestor of
Canis familiaris was a wild
Canis familiaris,
[63] which had diverged from a common ancestor along with the gray wolf.
[64]
Early
mitochondrial DNA analysis indicated that if the dog had descended from the modern gray wolf then the divergence would have occurred 135,000 YBP.
[7] Two later studies using
whole genome sequencing indicated divergence times of 32,000 YBP
[65] or 11,000-16,000 YBP,
[8] with the assumed
mutation rate "the dominant source of uncertainty in dating the origin of dogs."
[8]
In 2015, a study was conducted on a partial rib-bone (designated as Taimyr-1) found near the Bolshaya Balakhnaya River in the
Taymyr Peninsula, Arctic
North Asia, that was
AMS radiocarbon dated to 34,900 YBP. The sample provided the first draft
genome from the
cell nucleus of a Pleistocene
carnivore and the sequence was identified as belonging to
Canis lupus.
The sequence indicated that the Taimyr-1 lineage was separate to modern
wolves and dogs. Using the Taimyr-1 specimen's radiocarbon date in
addition to its genome sequence compared to that of a modern wolf, a
direct estimate of the mutation rate in dogs and wolves could be made to
calculate the time of divergence.
[44]
The study indicated that the Taimyr-1 population, gray wolves and dogs
diverged from a now-extinct common ancestor before the peak of the Last
Glacial Maximum 27,000-40,000 years ago.
[44][27]
Such an early divergence is consistent with several paleontological
reports of dog-like canids dated up to 36,000 YBP, as well as evidence
that domesticated dogs most likely accompanied early colonizers into the
Americas.
[44]
The study proposed that the timing of this separation of the dog and
wolf did not have to coincide with selective breeding by humans.
In 2017, a study compared the
nuclear genome
sequences of Neolithic dog specimens with sequences from over 5,000
dogs and wolves. These Neolithic dog specimens included a dog sample
from the
Early Neolithic site in
Herxheim, Germany dated 7,000 YBP, one from the
Late Neolithic site of Kirschbaum (Cherry Tree) Cave near
Forchheim, Germany dated 4,700 YBP, and one found in the Late Neolithic passage grave at
Newgrange,
Ireland
dated 4,800 YBP. The study calculated a mutation rate for the 7,000 YBP
Neolithic dog and found that it matched the mutation rate of the
Taimyr-1 specimen, and noted that this also matched the mutation rate
for the
Newgrange dog
that had been calculated in an earlier study. Using the 7,000 YBP
specimen and this mutation rate, the dog-wolf divergence time is
estimated to have occurred 36,900-41,500 YBP and this is consistent with
the timing found with the Taimyr-1 specimen in an earlier study.
[54]
In 2018, a study looked at the
y-chromosome
male lineage of canines, including the ancient fossils of the Herxheim,
Kirschbaum, and Newgrange dogs. The study identified six major dog yDNA
haplogroups, of which two of these include the majority of modern dogs.
The Newgrange dog fell into the most commonly occurring of these
haplogroups. The two ancient German dogs fell into a haplogroup commonly
found among dogs from the Middle East and Asia, with the Kirschbaum dog
sharing a common male lineage with the extant
Indian wolf.
The study concluded that at least 2 different male haplogroups existed
in ancient Europe, and that the dog male lineage diverged from its
nearest common ancestor shared with the gray wolf sometime between
68,000-151,000 YBP.
[43]
Place of divergence
Where the
genetic divergence of dog and wolf took place remains controversial, with the most plausible proposals spanning Western Europe,
[3][15] Central Asia,
[15][16] and East Asia.
[15][17]
This has been made more complicated by the most recent proposal that
fits the available evidence, which is that an initial wolf population
split into East and West Eurasian wolves, these were then domesticated
independently before going extinct into two distinct dog populations
between 14,000-6,400 years ago, and then the Western Eurasian dog
population was partially and gradually replaced by East Asian dogs that
were brought by humans at least 6,400 years ago.
[15][18][19]
Using modern DNA
Europe
In 2007, a study of maternal
mDNA sequences from dogs and wolves indicated that their
haplotypes resolved into four
monophyletic clades
(a grouping that includes its ancestor), of which two clades showed a
strong relationship between dogs and the wolves of Europe.
[7][55]
South East Asia:
In 2009, a study of the maternal
mitochondrial genome indicated the origin in south-eastern Asia south of the
Yangtze River as more dog
haplogroups had been found there.
[67] Paternal
Y-chromosome DNA sequences indicated the south-western part of south-eastern Asia that is south of the Yangtze River (comprising
South-East Asia and the Chinese provinces of
Yunnan and
Guangxi) because of the greater diversity of
yDNA haplogroups found in that region.
[68]
A criticism of this proposal is that no wolf remains have been found in
this region and the earliest archaeological evidence of a dog dates to
only 4,700 YBP.
[12]
Middle East: In 2010, a study using
single nucleotide polymorphisms
indicated that dogs originated in the Middle East due to the greater
sharing of haplotypes between dogs and Middle Eastern gray wolves, else
there may have been significant admixture between some regional breeds
and regional wolves.
[6] In 2011, a study found that there had been dog-wolf
hybridization and not an independent domestication.
[8][69]
East Asia: In 2002, a study of maternal
mDNA
indicated that the dog diverged from its ancestor in East Asia because
there were more dog mDNA haplotypes found there than in other parts of
the world,
[70] but this was rebutted because village dogs in Africa also show a similar haplotype diversity.
[71] In 2015, a
whole genome analysis
of modern dog and wolf sequences concluded that based on the genetic
diversity of today's East Asian dogs, the dog had originated in southern
East Asia, followed by a migration of a subset of ancestral dogs 15,000
YBP towards the Middle East, Africa and Europe and reaching Europe
10,000 YBP. Then, one of these lineages migrated back to northern China
and admixed with endemic Asian lineages before migrating to the
Americas.
[17]
A criticism of this proposal is that no dog remains date beyond 12,000
YBP in this region but date to 14,000 YBP in western Europe, however it
is accepted that archaeological studies in the Far East are generally
lagging behind those in Europe.
[12]
Central Asia: In 2015, a study looked at 85,805
genetic markers of
autosomal, maternal
mitochondrial genome and paternal
Y chromosome diversity in 4,676 purebred dogs from 161 breeds and 549 village dogs from 38 countries. Some dog populations in the
Neotropics and the
South Pacific
are almost completely derived from European stock, and other regions
show clear admixture between indigenous and European dogs. The
indigenous dog populations of Vietnam, India, and Egypt show minimal
evidence of European admixture, and exhibit indicators consistent with a
Central Asian
domestication origin, followed by a population expansion in East Asia.
The study could not rule out the possibility that dogs were domesticated
elsewhere and subsequently arrived in and diversified from Central
Asia. Studies of extant dogs cannot exclude the possibility of earlier
domestication events that subsequently died out or were overwhelmed by
more modern populations.
[16]
No agreement using modern DNA: In 2016, a whole-genome
study of wolves and dogs concluded that admixture had confounded the
ability to make inferences about the place of dog domestication. Past
studies based on
single-nucleotide polymorphisms,
[6] genome-wide similarities with Chinese wolves,
[17] and lower
linkage disequilibrium[16] might reflect regional admixture between dogs with wolves and gene flow between dog populations, with
genetically divergent
dog breeds possibly maintaining more wolf ancestry in their genome. The
study proposed that the analysis of ancient DNA might be a better
approach.
[2]
Using ancient DNA
The 14,500-year-old upper-right jaw of a
Pleistocene wolf found in Kesslerloch Cave,
Switzerland, is the sister to 2/3 of modern dogs
[3] (courtesy Hannes Napierala)
In his 1868 book on
Variation under Domestication,
Charles Darwin assessed the arguments for single or multiple origins of dogs, and noted the paleontological research of
de Blainville who proposed that dogs were descended from a single extinct species.
[72] In 1934, an eminent paleontologist indicated that the ancestor of the dog lineage may have been the extinct
Canis variabilis.
[41]
In 1950, a morphological study of Japanese prehistoric dogs compared to
extant wolves concluded: "Therefore the living wolf kinds have nothing
to do with the ancestral forms of the prehistoric dog races."
[73]
In 1999, a study emphasized that while molecular genetic data seem to
support the origin of dogs from wolves, dogs may have descended from a
now
extinct species of canid whose closest living relative was the wolf.
[52] The dog's lineage may have been contributed to from a
ghost population. The advent of rapid and inexpensive
DNA sequencing
technology has made it possible to significantly increase the resolving
power of genetic data taken from both modern and ancient domestic dog
genomes. Attention was now turned to studies based on
ancient DNA from fossil canids.
[12]
Europe: In 2013, a study analysed the complete and partial
mitochondrial genome
sequences of 18 fossil canids dated from 1,000 to 36,000 YBP from the
Old and New Worlds, and compared these with the complete mitochondrial
genome sequences from modern wolves and dogs.
Phylogenetic analysis showed that modern dog
mDNA haplotypes resolve into four
monophyletic clades with strong statistical support, and these have been designated by researchers as
clades A-D.
[3][7][74] Based on the specimens used in this study, clade A included 64% of the
dogs sampled and these were sister to a 14,500 YBP wolf sequence from
the
Kessleroch cave near
Thayngen in the canton of
Schaffhausen,
Switzerland,
with a most recent common ancestor estimated to 32,100 YBP. This group
of dogs matched three fossil pre-Columbian New World dogs dated between
1,000 and 8,500 YBP, which supported the hypothesis that pre-Columbian
dogs in the New World share ancestry with modern dogs and that they
likely arrived with the first humans to the New World. Clade B included
22% of the dog sequences and was related to modern wolves from Sweden
and the Ukraine, with a common recent ancestor estimated to 9,200 YBP.
However, this relationship might represent mitochondrial genome
introgression
from wolves because dogs were domesticated by this time. Clade C
included 12% of the dogs sampled and these were sister to two ancient
dogs from the
Bonn-Oberkassel cave (14,700 YBP) and the Kartstein cave (12,500 YBP) near
Mechernich in Germany, with a common recent ancestor estimated to 16,000–24,000 YBP. Clade D contained sequences from 2
Scandinavian breeds (
Jamthund,
Norwegian Elkhound)
and were sister to another 14,500 YBP wolf sequence also from the
Kesserloch cave, with a common recent ancestor estimated to 18,300 YBP.
Its branch is phylogenetically rooted in the same sequence as the
"Altai dog"
(not a direct ancestor). The data from this study indicated a European
origin for dogs that was estimated at 18,800–32,100 years ago based on
the genetic relationship of 78% of the sampled dogs with ancient canid
specimens found in Europe.
[51][3] The data supports the hypothesis that dog domestication preceded the emergence of agriculture
[7] and was initiated close to the
Last Glacial Maximum when hunter-gatherers preyed on
megafauna.
[3][75]
The study found that three ancient Belgium canids (the 36,000 YBP
"Goyet dog" cataloged as
Canis species, along with Belgium 30,000 YBP and 26,000 years YBP cataloged as
Canis lupus)
formed an ancient clade that was the most divergent group. The study
found that the skulls of the "Goyet dog" and the "Altai dog" had some
dog-like characteristics and proposed that the may have represented an
aborted domestication episode. If so, there may have been originally
more than one ancient domestication event for dogs
[3] as there was for domestic pigs.
[76]
One theory is that domestication occurred during one of the five cold
Heinrich events
that occurred after the arrival of humans in West Europe 37 000, 29
000, 23 000, 16 500 and 12 000 YBP. The theory is that the extreme cold
during one of these events caused humans to either shift their location,
adapt through a breakdown in their culture and change of their beliefs,
or adopt innovative approaches. The adoption of the large wolf/dog was
an adaptation to this hostile environment.
[77]
A criticism of the European proposal is that dogs in East Asia
show more genetic diversity. However, dramatic differences in genetic
diversity can be influenced both by an ancient and recent history of
inbreeding.
[17]
A counter-comment is that the modern European breeds only emerged in
the 19th century, and that throughout history global dog populations
experienced numerous episodes of diversification and homogenization,
with each round further reducing the power of genetic data derived from
modern breeds to help infer their early history.
[12]
Arctic North-East Siberia: In 2015, a study looked at the
mitochondrial control region
sequences of 13 ancient canid remains and one modern wolf from five
sites across Arctic north-east Siberia. The fourteen canids revealed
nine mitochondrial
haplotypes,
three of which were on record and the others not reported before. The
phylogentic tree generated from the sequences showed that four of the
Siberian canids dated 28,000 YBP and one
Canis c.f. variabilis dated 360,000 YBP were highly divergent. The haplotype designated as S805 (28,000 YBP) from the
Yana River
was one mutation away from another haplotype S902 (8,000 YBP) that
represents Clade A of the modern wolf and domestic dog lineages. Closely
related to this haplotype was one that was found in the
recently-extinct
Japanese wolf. Several ancient haplotypes were oriented around S805, including
Canis c.f. variabilis
(360,000 YBP), Belgium (36,000 YBP – the "Goyet dog"), Belgium (30,000
YBP), and Konsteki, Russia (22,000 YBP). Given the position of the S805
haplotype on the phylogenetic tree, it may potentially represent a
direct link from the
progenitor (including
Canis c.f. variabilis)
to the domestic dog and modern wolf lineages. The gray wolf is thought
to be ancestral to the domestic dog, however its relationship to
C. variabilis, and the genetic contribution of
C. variabilis to the dog, is the subject of debate.
[78]
The
Zhokhov Island
(8,700 YBP) and Aachim (1,700 YBP) canid haplotypes fell within the
domestic dog clade, cluster with S805, and also share their haplotypes
with – or are one mutation away from – the
Tibetan wolf (
C. l. chanco) and the recently-extinct Japanese wolf (
C. l. hodophilax).
This may indicate that these canids retained the genetic signature of
admixture with regional wolf populations. Another haplotype designated
as S504 (47,000 YBP) from
Duvanny Yar
appeared on the phylogenetic tree as not being connected to wolves
(both ancient and modern) yet ancestral to dogs, and may represent a
genetic source for regional dogs.
[78]
Newgrange dog – two domestication events
In 2009, a study looked at the two earliest dog skulls that had been
found at Eliseevich 1 in comparison to other much earlier but
morphologically similar fossil skulls that had been found across Europe
and concluded that the earlier specimens were "
Paleolithic dogs",
which were morphologically and genetically distinct from Pleistocene
wolves that lived in Europe at that time. These also include the
36,000-year-old "Goyet dog" and the 33,000-year-old "Altai dog".
[11]
Later studies suggested that it was possible for multiple primitive forms of the dog to have existed, including in Europe.
[17]
European dog populations had undergone extensive turnover during the
last 15,000 years that has erased the genomic signature of early
European dogs,
[16][79] the genetic heritage of the modern breeds has become blurred due to admixture,
[12]
and there was the possibility of past domestication events that had
gone extinct or had been largely replaced by more modern dog
populations.
[16]
In 2016, a study compared the
mitochondrial DNA and
whole-genome sequences of a worldwide panel of modern dogs, the mDNA sequences of 59 ancient European dog specimens dated 14,000-3,000 YBP, and the
nuclear genome sequence of a dog specimen that was found in the
Late Neolithic passage grave at
Newgrange,
Ireland and
radiocarbon dated
at 4,800 YBP. A genetic analysis of the Newgrange dog showed that it
was male, did not possess genetic variants associated with modern coat
length nor color, was not as able to process starch as efficiently as
modern dogs but more efficiently than wolves, and showed ancestry from a
population of wolves that could not be found in other dogs nor wolves
today.
[18] As the taxonomic classification of the "proto-dog"
Paleolithic dogs as being either dogs or wolves remains controversial, they were excluded from the study.
[18]:Sup The
phylogenetic tree generated from mDNA sequences found a deep division between the
Sarloos wolfdog and all other dogs, indicating that breed's recent deriving from the
German Shepherd
and captive gray wolves. The next largest division was between Eastern
Asian dogs and Western Eurasian (Europe and the Middle East) dogs that
had occurred between 14,000-6,400 YBP, with the Newgrange dog clustering
with the Western Eurasian dogs. The northern breed
Greenland dog and the
Siberian husky were poorly supported in the tree, possibly indicating mixed ancestry.
The Newgrange and ancient European dog mDNA sequences could be largely assigned to mDNA haplogroups
C and D
but modern European dog sequences could be largely assigned to mDNA
haplogroups A and B, indicating a turnover of dogs in the past from a
place other than Europe. As this split dates older than the Newgrange
dog this suggests that the replacement was only partial. The analysis
showed that most modern European dogs had undergone a
population bottleneck
which can be an indicator of travel. The archaeological record shows
dog remains dating over 15,000 YBP in Western Eurasia, over 12,500 YBP
in Eastern Eurasia, but none older than 8,000 YBP in Central Asia. The
study proposed that dogs may have been domesticated separately in both
Eastern and Western Eurasia from two genetically distinct and now
extinct wolf populations. East Eurasian dogs then made their way with
migrating people to Western Europe between 14,000-6,400 YBP where they
partially replaced the dogs of Europe.
[18][80] Two domestication events in Western Eurasia and Eastern Eurasia has recently been found for the domestic pig.
[18][76]
The hypothesis is that two genetically different, and possibly
now extinct, wolf populations were domesticated independently in eastern
and western Eurasia to produce paleolithic dogs.
[18]
The eastern Eurasian dogs then dispersed westward alongside humans,
reaching western Europe 6,400–14,000 years ago where they partially
replaced the western paleolithic dogs.
[51][18]
A single domestication is thought to be due to chance, however dual
domestication on different sides of the world is unlikely to have
happened randomly and it suggests that external factors - an
environmental driver - may have forced wolves to work together with
humans for survival. It is possible that wolves took advantage of
resources that humans had, or humans may have been introduced to wolves
in an area in which they didn’t previously live.
[81]
The study used the radiocarbon age of the Newgrange dog to
calibrate the mutation rate for dogs, which was similar to that
calculated for the Late Pleistocene
Taimyr wolf.
Comparing the sequence of the Newgrange dog using this mutation rate
with two modern wolves from Russia gave a divergence time between
20,000-60,000 YBP. However, these two modern wolves may not have been
closely related to the population that gave rise to the dog, which may
have diverged from their ancestor at a later time.
[18]
Neolithic dogs – one domestication event
In 2017, a study compared the nuclear genome sequences of three
Neolithic dog specimens from Germany and Ireland with sequences from
over 5,000 dogs and wolves. The study found that modern European dogs
descended from their Neolithic ancestors with no evidence of a
population turnover found by an earlier study. The study found that a
single dog-wolf divergence occurred 36,900-41,500 YBP, followed by a
divergence between Southeast Asian and Western Eurasian dogs
17,500-23,900 YBP and this indicates a single dog domestication event
occurring between 20,000-40,000 YBP. The study did not support a dual
domestication event found by an earlier study.
[54]
Dog domestication
The domestication of animals is the
scientific theory of the mutual relationship between animals with the humans who have influence on their care and reproduction.
[84] Charles Darwin
recognized the small number of traits that made domestic species
different from their wild ancestors. He was also the first to recognize
the difference between conscious
selective breeding in which humans directly select for desirable traits, and unconscious selection where traits evolve as a by-product of
natural selection or from selection on other traits.
[72][85][86]
There is a genetic difference between domestic and wild
populations. There is also such a difference between the domestication
traits that researchers believe to have been essential at the early
stages of domestication, and the improvement traits that have appeared
since the split between wild and domestic populations.
[9][87][88]
Domestication traits are generally fixed within all domesticates, and
were selected during the initial episode of domestication of that animal
or plant, whereas improvement traits are present only in a proportion
of domesticates, though they may be fixed in individual breeds or
regional populations.
[9][88][89] The dog was the first domesticant and was established across
Eurasia before the end of the
Late Pleistocene era, well before
cultivation and before the domestication of other animals.
[12]
Time of domestication
An
apex predator is a predator that sits on the top
trophic level of the food chain, while a
mesopredator
sits further down the food chain and is dependent on smaller animals.
Towards the end of the Pleistocene era, most of today's apex predators
were mesopredators and this included the wolf. During the ecological
upheaval associated with the close of the Late Pleistocene, one type of
wolf population rose to become today's apex predator and another joined
with humans to become an apex consumer.
[57]
In August 2015, a study undertook an analysis of the complete
mitogenome
sequences of 555 modern and ancient dogs. The sequences showed an
increase in the population size approximately 23,500 YBP, which broadly
coincides with the proposed separation of the ancestors of dogs and
present-day wolves before the
Last Glacial Maximum (refer
first divergence).
A ten-fold increase in the population size occurred after 15,000 YBP,
which may be attributable to domestication events and is consistent with
the demographic dependence of dogs on the human population (refer
archaeological evidence).
[90]
Socialization
Humans and wolves both exist in complex social groups. How humans and
wolves got together remains unknown. One view holds that domestication
as a process that is difficult to define. The term was developed by
anthropologists with a human-centric view in which humans took wild animals (
ungulates)
and bred them to be "domestic", usually in order to provide improved
food or materials for human consumption. That term may not be
appropriate for a large carnivore such as the dog. This alternate view
regards dogs as being either
socialized
and able to live among humans, or unsocialized. There exists today dogs
that live with their human families but are unsocialized and will
threaten strangers defensively and aggressively no different from a wild
wolf. There also exists a number of cases where wild wolves have
approached people in remote places, attempting to initiate play and to
form companionship.
[91] One such notable wolf was
Romeo, a gentle black wolf that formed relationships with the people and dogs of Juneau, Alaska.
[92] This view holds that before there could have been domestication of the wolf, there had to have been its socialization.
[91][93]
Commensal pathway
Montage showing the morphological variation of the dog.
Animal domestication is a
coevolutionary process in which a population responds to selective pressure while adapting to a novel
niche that included another species with evolving behaviors.
[9]
The dog is a classic example of a domestic animal that likely traveled a
commensal
pathway into domestication. The dog was the first domesticant, and was
domesticated and widely established across Eurasia before the end of the
Pleistocene, well before cultivation or the domestication of other animals.
[12]
It may have been inevitable that the first domesticated animal came
from the order of carnivores as these are less afraid when approaching
other species. Within the carnivores, the first domesticated animal
would need to exist without an all-meat diet, possess a running and
hunting ability to provide its own food, and be of a controllable size
to coexist with humans, indicating the family
Canidae, and the right temperament
[94]:p166 with wolves being among the most gregarious and cooperative animals on the planet.
[95][96]
Ancient DNA supports the hypothesis that dog domestication preceded the emergence of agriculture
[3][7] and was initiated close to the
Last Glacial Maximum 27,000 YBP when hunter-gatherers preyed on
megafauna,
and when proto-dogs might have taken advantage of carcasses left on
site by early hunters, assisted in the capture of prey, or provided
defense from large competing predators at kill-sites.
[3]
Wolves were probably attracted to human campfires by the smell of meat
being cooked and discarded refuse in the vicinity, first loosely
attaching themselves and then considering these as part of their home
territory where their warning growls would alert humans to the approach
of outsiders.
[97]
The wolves most likely drawn to human camps were the less-aggressive,
subdominant pack members with lowered flight response, higher stress
thresholds, less wary around humans, and therefore better candidates for
domestication.
[98] The earliest sign of domestication in dogs was the neotonization of skull morphology
[98][99][100]
and the shortening of snout length that results in tooth crowding,
reduction in tooth size, and a reduction in the number of teeth,
[101][98] which has been attributed to the strong selection for reduced aggression.
[98][99]
This process may have begun during the initial commensal stage of dog
domestication, even before humans began to be active partners in the
process.
[9][98]
A maternal
mitochondrial, paternal
Y chromosome, and
microsatellite
assessment of two wolf populations in North America and combined with
satellite telemetry data revealed significant genetic and morphological
differences between one population that migrated with and preyed upon
caribou, and another territorial
ecotype population that remained in a
boreal
coniferous forest. Though these two populations spend a period of the
year in the same place, and though there was evidence of gene flow
between them, the difference in prey–habitat specialization has been
sufficient to maintain genetic and even coloration divergence.
[9][102] A study has identified the remains of a population of extinct
Pleistocene Beringian wolves with unique mitochondrial signatures. The skull shape, tooth wear, and isotopic signatures suggested these were specialist
megafauna hunters and scavengers that became extinct while less specialized wolf ecotypes survived.
[9][103]
Analogous to the modern wolf ecotype that has evolved to track and prey
upon caribou, a Pleistocene wolf population could have begun following
mobile hunter-gatherers, thus slowly acquiring genetic and phenotypic
differences that would have allowed them to more successfully adapt to
the human habitat.
[9][104]
Even today, the wolves on
Ellesmere Island
do not fear humans, which is thought to be due to them seeing humans so
little, and they will approach humans cautiously, curiously and
closely.
Post-domestication gene flow
Some studies have found greater diversity in the genetic markers of dogs from East
[17][67] and Central
[16]
Asia compared to Europe and have concluded that dogs originated from
these regions, despite no archaeological evidence to support the
conclusions.
[12]
One reason for these discrepancies is the sustained admixture between
different dog and wolf populations across the Old and New Worlds over at
least the last 10,000 years, which has blurred the genetic signatures
and confounded efforts of researchers at pinpointing the origins of
dogs.
[12] Another reason is that none of the modern wolf populations are related to the
Pleistocene wolves that were first domesticated.
[8]
In other words, the extinction of the wolves that were the direct
ancestors of dogs has muddied efforts to pinpoint the time and place of
dog domestication.
[9]
Dog-Wolf hybridization
Phylogenetic analysis shows that modern dog
mDNA haplotypes resolve into four
monophyletic clades with strong statistical support, and these have been designated by researchers as clades A-D.
[3][7][74]
Other studies that included a wider sample of specimens have reported
two rare East Asian clades E-F with weaker statistical support.
[67][70][90] In 2009, a study found that
haplogroups
A, B and C included 98% of dogs and are found universally distributed
across Eurasia, indicating that they were the result of a single
domestication event, and that haplogroups D, E, and F were rare and
appeared to be the result of regional hybridization with local wolves
post-domestication. Haplogroups A and B contained subclades that
appeared to be the result of hybridization with wolves
post-domestication, because each haplotype within each of these
subclades was the result of a female wolf/male dog pairing.
[67][90]
Haplogroup A: Includes 64-65% of dogs.
[3][90] Haplotypes of subclades a2–a6 are derived from post-domestication wolf–dog hybridization.
[67][90]
Haplogroup B: Includes 22-23% of dogs.
[3][90] haplotypes of subclade b2 are derived from post-domestication wolf–dog hybridization.
[67][90]
Haplogroup C: Includes 10-12% of dogs.
[3][90]
Haplogroup D: Derived from post-domestication wolf–dog hybridization in subclade d1 (Scandinavia) and d2 (South-West Asia).
[67][90] The northern Scandinavian subclade d1 hybrid haplotypes originated 480-3,000 YBP and are found in all
Sami-related breeds:
Finnish Lapphund,
Swedish Lapphund,
Lapponian Herder,
Jamthund,
Norwegian Elkhound and
Hällefors Elkhound. The maternal wolf sequence that contributed to them has not been matched across Eurasia
[109] and its branch is phylogenetically rooted in the same sequence as the
Altai dog (not a direct ancestor).
[3] The subclade d2 hybrid haplotypes are found in 2.6% of South-West Asian dogs.
[69]
Haplogroup E: Derived from post-domestication wolf–dog hybridization in East Asia,
[67][90] (rare distribution in South-East Asia, Korea and Japan).
[69]
Haplogroup F: Derived from post-domestication wolf–dog hybridization in Japan.
[67][90] A study of 600 dog specimens found only one dog whose sequence indicated hybridization with the extinct
Japanese wolf.
[110]
It is not known whether this hybridization was the result of humans selecting for
phenotypic traits from local wolf populations
[74] or the result of natural
introgression as the dog expanded across Eurasia.
[12]
In 2018, a study found a small amount of dog ancestry in 62% of
Eurasian wolf specimens looked at, that hybridization had occurred
across a wide number of timescales and not just recently, however in
contrast there was almost no admixture detected in the North American
specimens. There was introgression of the male dog into the wolf, but
also one hybrid detected which was the result of a male wolf crossed
with a female dog. Wolves have maintained their phenotype differences
from the dog, which indicates low-frequency hybridization. The
conclusion is that phenotype is no indication of "purity" and the
definition of pure wolves is ambiguous. Free-ranging dogs across Eurasia
show introgression from wolves.
[111] Another study found that the
β-defensin
gene responsible for the black coat of North American wolves was the
result of a single introgression from dogs in the Yukon dated between
1,600-7,200 years ago. The study proposes that early Native American
dogs were the source.
[112]
The
Greenland dog carries 3.5% shared genetic material with the 35,000 years BP Taymyr wolf specimen.
Taimyr wolf admixture
In May 2015, a study compared the ancestry of the Taimyr-1 wolf lineage to that of dogs and gray wolves.
Comparison to the gray wolf lineage indicated that Taimyr-1 was
basal to gray wolves from the Middle East, China, Europe and North
America but shared a substantial amount of history with the present-day
gray wolves after their divergence from the coyote. This implies that
the ancestry of the majority of gray wolf populations today stems from
an ancestral population that lived less than 35,000 years ago but before
the inundation of the
Bering Land Bridge with the subsequent isolation of Eurasian and North American wolves.
[44]:21
A comparison of the ancestry of the Taimyr-1 lineage to the dog
lineage indicated that some modern dog breeds have a closer association
with either the gray wolf or Taimyr-1 due to
admixture. The
Saarloos wolfdog
showed more association with the gray wolf, which is in agreement with
the documented historical crossbreeding with gray wolves in this breed.
Taimyr-1 shared more alleles (i.e. gene expressions) with those breeds
that are associated with high latitudes - the
Siberian husky and
Greenland dog that are also associated with arctic human populations, and to a lesser extent the
Shar Pei and
Finnish spitz.
An admixture graph of the Greenland dog indicates a best-fit of 3.5%
shared material, although an ancestry proportion ranging between 1.4%
and 27.3% is consistent with the data. This indicates admixture between
the Taimyr-1 population and the ancestral dog population of these four
high-latitude breeds. These results can be explained either by a very
early presence of dogs in northern Eurasia or by the genetic legacy of
Taimyr-1 being preserved in northern wolf populations until the arrival
of dogs at high latitudes. This
introgression
could have provided early dogs living in high latitudes with phenotypic
variation beneficial for adaption to a new and challenging environment.
It also indicates that the ancestry of present-day dog breeds descends
from more than one region.
[44]:3–4
An attempt to explore admixture between Taimyr-1 and gray wolves produced unreliable results.
[44]:23
As the Taimyr wolf had contributed to the genetic makeup of the
Arctic breeds, a later study suggested that descendants of the Taimyr
wolf survived until dogs were domesticated in Europe and arrived at high
latitudes where they mixed with local wolves, and these both
contributed to the modern Arctic breeds. Based on the most widely
accepted oldest zooarchaeological dog remains, domestic dogs most likely
arrived at high latitudes within the last 15,000 years. The mutation
rates calibrated from both the Taimyr wolf and the
Newgrange dog
genomes suggest that modern wolf and dog populations diverged from a
common ancestor between 20,000 and 60,000 YBP. This indicates that
either dogs were domesticated much earlier than their first appearance
in the archaeological record, or they arrived in the Arctic early, or
both.
[15]
Positive selection
Reduction in size under domestication - grey wolf and chihuahua skulls.
The wolf's family portrait reveals a diversity of form among breeds of domestic dogs.
Charles Darwin
recognized the small number of traits that made domestic species
different from their wild ancestors. He was also the first to recognize
the difference between conscious
selective breeding in which humans directly select for desirable traits, and unconscious selection where traits evolve as a by-product of
natural selection or from selection on other traits.
[72][86]
Domestic animals have variations in coat color as well as texture,
dwarf and giant varieties, and changes in their reproductive cycle, and
many others have tooth crowding and floppy ears.
Although it is easy to assume that each of these traits was
uniquely selected for by hunter-gatherers and early farmers, beginning
in 1959
Dmitry Belyayev
tested the reactions of silver foxes to a hand placed in their cage and
selected the tamest, least aggressive individuals to breed. His
hypothesis was that, by selecting a behavioral trait, he could also
influence the phenotype of subsequent generations, making them more
domestic in appearance. Over the next 40 years, he succeeded in
producing foxes with traits that were never directly selected for,
including piebald coats floppy ears, upturned tails, shortened snouts,
and shifts in developmental timing.
[99][113][114]
In the 1980s, a researcher used a set of behavioral, cognitive, and
visible phenotypic markers, such as coat colour, to produce domesticated
fallow deer within a few generations.
[113][115] Similar results for tameness and fear have been found for
mink[116] and
Japanese quail.
[117]
In addition to demonstrating that domestic phenotypic traits could
arise through selection for a behavioral trait, and domestic behavioral
traits could arise through the selection for a phenotypic trait, these
experiments provided a mechanism to explain how the animal domestication
process could have begun without deliberate human forethought and
action.
[113]
The genetic difference between domestic and wild populations can
be framed within two considerations. The first distinguishes between
domestication traits that are presumed to have been essential at the
early stages of domestication, and improvement traits that have appeared
since the split between wild and domestic populations.
[9][87][88]
Domestication traits are generally fixed within all domesticates and
were selected during the initial episode of domestication, whereas
improvement traits are present only in a proportion of domesticates,
though they may be fixed in individual breeds or regional populations.
[9][88][89]
A second issue is whether traits associated with the domestication
syndrome resulted from a relaxation of selection as animals exited the
wild environment or from
positive selection
resulting from intentional and unintentional human preference. Some
recent genomic studies on the genetic basis of traits associated with
the domestication syndrome have shed light on both of these issues.
[9]
A study published in 2016 suggested that there have been negative
genetic consequences of the domestication process as well, that
enrichment of disease-related gene variants accompanied positively
selected traits.
[118]
In 2010, a study identified 51 regions of the dog genome that
were associated with phenotypic variation among breeds in 57 traits
studied, which included body, cranial, dental, and long bone shape and
size. There were 3
quantitative trait loci
that explained most of the phenotypic variation. Indicators of recent
selection were shown by many of the 51 genomic regions that were
associated with traits that define a breed, which include body size,
coat characteristics, and ear floppiness.
[119] Geneticists have identified more than 300 genetic loci and 150 genes associated with coat color variability.
[113][120]
Knowing the mutations associated with different colors has allowed the
correlation between the timing of the appearance of variable coat colors
in horses with the timing of their domestication.
[113][121] Other studies have shown how human-induced selection is responsible for the allelic variation in pigs.
[113][122]
Together, these insights suggest that, although natural selection has
kept variation to a minimum before domestication, humans have actively
selected for novel coat colors as soon as they appeared in managed
populations.
[113][123]
In 2015, a study looked at over 100 pig genome sequences to
ascertain their process of domestication. A model that fitted the data
included admixture with a now extinct
ghost population of wild pigs during the
Pleistocene.
The study also found that despite back-crossing with wild pigs, the
genomes of domestic pigs have strong signatures of selection at
genetic loci
that affect behavior and morphology. The study concluded that human
selection for domestic traits likely counteracted the homogenizing
effect of gene flow from wild boars and created
domestication islands in the genome. The same process may also apply to other domesticated animals.
[76][124]
In 2014, a whole genome study of the DNA differences between
wolves and dogs found that dogs did not show a reduced fear response but
did show greater
synaptic plasticity.
Synaptic plasticity is widely believed to be the cellular correlate of
learning and memory, and this change may have altered the learning and
memory abilities of dogs in comparison to wolves.
[125]
Behavior
Unlike other domestic species which were primarily selected for
production-related traits, dogs were initially selected for their
behaviors.
[126][127]
In 2016, a study found that there were only 11 fixed genes that showed
variation between wolves and dogs. These gene variations were unlikely
to have been the result of natural evolution, and indicate selection on
both morphology and behavior during dog domestication. There was
evidence of selection during dog domestication of genes that affect the
adrenaline and
noradrenaline biosynthesis
pathway. These genes are involved in the synthesis, transport and
degradation of a variety of neurotransmitters, particularly the
catecholamines, which include
dopamine and
noradrenaline. Recurrent selection on this pathway and its role in emotional processing and the fight-or-flight response
[127][128]
suggests that the behavioral changes we see in dogs compared to wolves
may be due to changes in this pathway, leading to tameness and an
emotional processing ability.
[127] Dogs generally show reduced fear and aggression compared to wolves.
[127][129]
Some of these genes have been associated with aggression in some dog
breeds, indicating their importance in both the initial domestication
and then later in breed formation.
[127]
Dietary adaption
The difference in overall body size between a
Cane Corso (Italian mastiff) and a
Yorkshire terrier is over 30-fold, yet both are members of the same species.
AMY2B (Alpha-Amylase 2B) is a gene that codes a protein that assists with the first step in the digestion of dietary
starch and
glycogen.
An expansion of this gene in dogs would enable early dogs to exploit a
starch-rich diet as they fed on refuse from agriculture.
[8][127]
In a study in 2014, the data indicated that the wolves and dingo had
just two copies of the gene and the Siberian Husky that is associated
with hunter-gatherers had just three or four copies, whereas the
Saluki that is associated with the
Fertile Crescent
where agriculture originated had 29 copies. The results show that on
average, modern dogs have a high copy number of the gene, whereas wolves
and dingoes do not. The high copy number of AMY2B variants likely
already existed as a standing variation in early domestic dogs, but
expanded more recently with the development of large agriculturally
based civilizations. This suggests that at the beginning of the
domestication process, dogs may have been characterized by a more
carnivorous diet than their modern-day counterparts, a diet held in common with early hunter-gatherers.
[8] A later study indicated that because most dogs had a high copy number of the AMY2B gene but the arctic breeds and the
dingo
did not, that the dog's dietary change may not have been caused by
initial domestication but by the subsequent spread of agriculture to
most - but not all - regions of the planet.
[130]
In 2016, a study of the dog
genome
compared to the wolf genome looked for genes that showed signs of
having undergone positive selection. The study identified genes relating
to brain function and behavior, and to
lipid
metabolism. This ability to process lipids indicates a dietary target
of selection that was important when proto-dogs hunted and fed alongside
hunter-gatherers. The evolution of the dietary metabolism genes may
have helped process the increased lipid content of early dog diets as
they scavenged on the remains of carcasses left by hunter-gatherers.
[131]
Prey capture rates may have increased in comparison to wolves and with
it the amount of lipid consumed by the assisting proto-dogs.
[131][75][132]
A unique dietary selection pressure may have evolved both from the
amount consumed, and the shifting composition of, tissues that were
available to proto-dogs once humans had removed the most desirable parts
of the carcass for themselves.
[131] A study of the mammal biomass during modern human expansion into the northern
Mammoth steppe
found that it had occurred under conditions of unlimited resources, and
that many of the animals were killed with only a small part consumed or
left unused.
[133]
Natural selection
Dogs can infer the name of an object and have been shown to learn the
names of over 1,000 objects. Dogs can follow the human pointing
gesture; even nine-week-old puppies can follow a basic human pointing
gesture without being taught.
New Guinea singing dogs, a half-wild proto-dog endemic to the remote alpine regions of
New Guinea, as well as
dingoes in the remote
Outback
of Australia are also capable of this. These examples demonstrate an
ability to read human gestures that arose early in domestication and did
not require human selection. "Humans did not develop dogs, we only
fine-tuned them down the road."
[134]:92
A dog's cranium is 15% smaller than an equally heavy wolf's, and the
dog is less aggressive and more playful. Other species pairs show
similar differences.
Bonobos, like
chimpanzees,
are a close genetic cousin to humans, but unlike the chimpanzees,
bonobos are not aggressive and do not participate in lethal inter-group
aggression or kill within their own group. The most distinctive features
of a bonobo are its cranium, which is 15% smaller than a chimpanzee's,
and its less aggressive and more playful behavior. In other examples,
the
guinea pig's cranium is 13% smaller than its wild cousin the
cavy,
and domestic fowl show a similar reduction to their wild cousins.
Possession of a smaller cranium for holding a smaller brain is a
telltale sign of domestication. Bonobos appear to have domesticated
themselves.
[134]:104
In the farm fox experiment, humans selectively bred foxes against
aggression, causing domestication syndrome. The foxes were not
selectively bred for smaller craniums and teeth, floppy ears, or skills
at using human gestures, but these traits were demonstrated in the
friendly foxes. Natural selection favors those that are the most
successful at reproducing, not the most aggressive. Selection against
aggression made possible the ability to cooperate and communicate among
foxes, dogs and bonobos. Perhaps it did the same thing for humans.
[134]:114[135]
The more docile animals have been found to have less testosterone than
their more aggressive counterparts, and testosterone controls aggression
and brain size. One researcher has argued that in becoming more social,
we humans have developed a smaller brain than those of humans 20,000
years ago.
[136]
Dog and human convergent evolution
As a result of the domestication process there is also evidence of
convergent evolution having occurred between dogs and humans.
[134] The history of the two is forever intertwined.
[137]
Biological evidence
Montage showing the coat variation of the dog.
In 2007, a study found that dog domestication was accompanied by
selection at three genes with key roles in starch digestion: AMY2B,
MGAMand SGLT1, and was a striking case of parallel evolution when coping
with an increasingly starch-rich diet caused similar adaptive responses
in dogs and humans.
[138][139]
In 2013, a DNA sequencing study indicated that
parallel evolution
in humans and dogs is most apparent in the genes for digestion and
metabolism, neurological processes, and cancer, likely as a result of
shared selection pressures.
[65][140]
In 2014, a study compared the
hemoglobin levels of village dogs and people on the Chinese lowlands with those on the
Tibetan Plateau.
It found the hemoglobin levels higher for both people and dogs in
Tibet, suggesting that Tibetan dogs might share similar adaptive
strategies as the Tibetan people. A population genetic analysis then
showed a significant convergence between humans and dogs in Tibet.
[141] Like the Tibetan people,
Tibetan mastiffs appear to have derived these benefits partially from the
EPAS1 gene, acquired through
introgression.
[142]
In 2015, a study found that when dogs and their owners interact, extended
eye contact (mutual
gaze) increases
oxytocin levels in both the dog and its owner. As oxytocin is known for its role in
maternal bonding, it is considered likely that this effect has supported the coevolution of human-dog bonding.
[143]
One observer has stated, "The dog could have arisen only from animals
predisposed to human society by lack of fear, attentiveness, curiosity,
necessity, and recognition of advantage gained through
collaboration....the humans and wolves involved in the conversion were
sentient,
observant beings constantly making decisions about how they lived and
what they did, based on the perceived ability to obtain at a given time
and place what they needed to survive and thrive. They were social
animals willing, even eager, to join forces with another animal to merge
their sense of group with the others' sense and create an expanded
super-group that was beneficial to both in multiple ways. They were
individual animals and people involved, from our perspective, in a
biological and cultural process that involved linking not only their
lives but the evolutionary fate of their heirs in ways, we must assume,
they could never have imagined. Powerful emotions were in play that many
observers today refer to as
love – boundless, unquestioning love."
[21]
Behavioral evidence
Convergent evolution is when distantly related species independently evolve similar solutions to the same problem. For example, fish,
penguins and
dolphins have each separately evolved
flippers
as a solution to the problem of moving through the water. What has been
found between dogs and humans is something less frequently
demonstrated: psychological convergence. Dogs have independently evolved
to be cognitively more similar to humans than we are to our closest
genetic relatives.
[134]:60 Dogs have evolved specialized skills for reading human social and
communicative behavior. These skills seem more flexible – and possibly
more human-like – than those of other animals more closely related to
humans phylogenetically, such as chimpanzees, bonobos and other
great apes. This raises the possibility that convergent evolution has occurred: both
Canis familiaris and
Homo sapiens
might have evolved some similar (although obviously not identical)
social-communicative skills – in both cases adapted for certain kinds of
social and communicative interactions with human beings.
[135]
The pointing gesture is a human-specific signal, is referential in its nature, and is a foundation building-block of
human communication. Human infants acquire it weeks before the first spoken word.
[144]
In 2009, a study compared the responses to a range of pointing gestures
by dogs and human infants. The study showed little difference in the
performance of 2-year-old children and dogs, while 3-year-old children's
performance was higher. The results also showed that all subjects were
able to generalize from their previous experience to respond to
relatively novel pointing gestures. These findings suggest that dogs
demonstrating a similar level of performance as 2-year-old children can
be explained as a joint outcome of their evolutionary history as well as
their socialization in a human environment.
[145]
Later studies support
coevolution in that dogs can discriminate the emotional expressions of human faces,
[146] and that most people can tell from a bark whether a dog is alone, being approached by a stranger, playing, or being aggressive,
[147] and can tell from a growl how big the dog is.
[148]
Human adoption of some wolf behaviors
“
|
... Isn't it strange that, our being such an
intelligent primate, we didn't domesticate chimpanzees as companions
instead? Why did we choose wolves even though they are strong enough to
maim or kill us? ...
|
”
|
— Wolfgang Schleidt[95]
|
In 2002, a study proposed that immediate human ancestors and wolves
may have domesticated each other through a strategic alliance that would
change both respectively into humans and dogs. The effects of human
psychology, hunting practices,
territoriality and social behavior would have been profound.
[149]
Wolves and dogs mark their territory with scent; however, humans do not
have a keen sense of smell and may have begun to mark their territories
with symbols, which became the first art and may have been generative
of human culture.
[149][150] Wolves hunt large game; however, there is no evidence of pre-
sapiens
hunting large game. Early humans moved from scavenging and small-game
hunting to big-game hunting by living in larger, socially more-complex
groups, learning to hunt in packs, and developing powers of cooperation
and negotiation in complex situations. As these are characteristics of
wolves, dogs and humans, it can be argued that these behaviors were
enhanced once wolves and humans began to cohabit. Communal hunting led
to communal defense. Wolves actively patrol and defend their
scent-marked territory, and perhaps humans had their sense of
territoriality enhanced by living with wolves.
[149]
One of the keys to recent human survival has been the forming of
partnerships. Strong bonds exist between same-sex wolves, dogs and
humans and these bonds are stronger than exist between other same-sex
animal pairs. Today, the most widespread form of inter-species bonding
occurs between humans and dogs. The concept of friendship has ancient
origins but it may have been enhanced through the inter-species
relationship to give a survival advantage.
[149][151]
In 2003, a study compared the behavior and ethics of chimpanzees,
wolves and humans. Cooperation among humans' closest genetic relative
is limited to occasional hunting episodes or the persecution of a
competitor for personal advantage, which had to be tempered if humans
were to become domesticated.
[95][152] The closest approximation to human morality that can be found in nature is that of the gray wolf,
Canis lupus. Wolves are among the most gregarious and cooperative of animals on the planet,
[95][96]
and their ability to cooperate in well-coordinated drives to hunt prey,
carry items too heavy for an individual, provisioning not only their
own young but also the other pack members, babysitting etc. are rivaled
only by that of human societies. Similar forms of cooperation are
observed in two closely related canids, the
African wild dog and the Asian
dhole,
therefore it is reasonable to assume that canid sociality and
cooperation are old traits that in terms of evolution predate human
sociality and cooperation. Today's wolves may even be less social than
their ancestors, as they have lost access to big herds of
ungulates and now tend more toward a lifestyle similar to coyotes, jackals, and even foxes.
[95] Social sharing within families may be a trait that early humans learned from wolves,
[95][153] and with wolves digging dens long before humans constructed huts it is not clear who domesticated whom.
[55][95][152]
Bison surrounded by gray wolf pack
On the
mammoth steppe the wolf's ability to hunt in packs, to share risk fairly among pack members, and to cooperate moved them to the top of the
food chain above lions, hyenas and bears. Some wolves followed the great
reindeer
herds, eliminating the unfit, the weaklings, the sick and the aged, and
therefore improved the herd. These wolves had become the first
pastoralists hundreds of thousands of years before humans also took to
this role.
[55]
The wolves' advantage over their competitors was that they were able to
keep pace with the herds, move fast and enduringly, and make the most
efficient use of their kill by their ability to "wolf down" a large part
of their quarry before other predators had detected the kill. The study
proposed that during the
Last Glacial Maximum, some of our ancestors teamed up with those pastoralist wolves and learned their techniques.
[95][154] Many of our ancestors remained gatherers and scavengers, or specialized
as fish-hunters, hunter-gatherers, and hunter-gardeners. However, some
ancestors adopted the pastoralist wolves' lifestyle as herd followers
and herders of reindeer, horses, and other hoofed animals. They
harvested the best stock for themselves while the wolves kept the herd
strong, and this group of humans was to become the first herders and
this group of wolves was to become the first dogs.
[55][95]
First dogs
Bonn–Oberkassel dog
In 1914, on the eve of the
First World War, two human skeletons were discovered during basalt quarrying at
Oberkassel, Bonn in Germany. With them were found a right mandible of a "wolf" and other animal bones.
[155] After the end of the First World War, in 1919 a full study was made of these remains. The mandible was recorded as "
Canis lupus, the wolf" and some of the other animal bones were assigned to it.
[156]
The remains were then stored and forgotten for fifty years. In the late
1970s there was renewed interest in the Oberkassel remains and the
mandible was re-examined and reclassified as belonging to a domesticated
dog.
[157][158][159] The
mitochondrial DNA sequence of the mandible was matched to
Canis lupus familiaris - dog,
[3] and confirms that the Oberkassel dog is a direct ancestor of today’s dogs.
[47] The bodies were dated to 14,223 YBP.
[10]
This implies that in Western Europe there were morphologically and
genetically "modern" dogs in existence around 14,500 years ago.
[46]
Later studies assigned more of the other animal bones to the dog until most of a skeleton could be assembled.
[46]
The humans were a man aged 40 years and a woman aged 25 years. All
three skeletal remains were found covered with large 20 cm thick basalt
blocks and were sprayed with red hematite powder.
[10] The consensus is that a dog was buried along with two humans.
[46] A tooth belonging to a smaller and older dog was also identified but it had not been sprayed with red powder.
[10] The cause of the death of the two humans is not known.
[46] A
pathology study of the dog remains suggests that it had died young after suffering from
canine distemper between ages 19 and 23 weeks.
[10] The dog could not have survived during this period without intensive human care.
[46][10] During this period the dog was of no utilitarian use to humans,
[10] and suggests the existence of emotional or symbolic ties between these humans and this dog.
[46]
In conclusion, near the end of the Late Pleistocene at least some
humans regarded dogs not just materialistically, but had developed
emotional and caring bonds for their dogs.
[10]
First dogs as a hunting technology
Image on a
Krater (vase) depicting a boar hunt in Ancient Greece
During the late Paleolithic, the increase in human population
density, advances in blade and hunting technology, and climate change
may have altered prey densities and made scavenging crucial to the
survival of some wolf populations. Adaptations to scavenging such as
tameness, small body size, and a decreased age of reproduction would
reduce their hunting efficiency further, eventually leading to obligated
scavenging.
[16][160]
Whether these earliest dogs were simply human-commensal scavengers or
they played some role as companions or hunters that hastened their
spread is unknown.
[16]
Researchers have proposed that in the past a hunting partnership
existed between humans and dogs that was the basis for dog
domestication.
[161][162][163] The transition from the
Late Pleistocene into the early
Holocene
was marked by climatic change from cold and dry to warmer, wetter
conditions and rapid shifts in flora and fauna, with much of the open
habitat of large herbivores being replaced by forests.
[163] In the early
Holocene,
it is proposed that along with changes in arrow-head technology that
hunting dogs were used by hunters to track and retrieve wounded game in
thick forests.
[162][163]
The dog's ability to chase, track, sniff out and hold prey can
significantly increase the success of hunters in forests, where human
senses and location skills are not as sharp as in more open habitats.
Dogs are still used for hunting in forests today.
[163]
In Japan, temperate deciduous forests rapidly spread onto the
main island of Honshu and caused an adaption away from hunting megafauna
(
Naumann’s elephant and Yabe’s giant deer) to hunting the quicker
sika deer and
wild boar
in dense forest. With this came a change in hunting technology,
including a shift to smaller, triangular points for arrows. A study of
the
Jōmon people that lived on the Pacific coast of
Honshu
during the early Holocene shows that they were conducting individual
dog burials and were probably using dogs as tools for hunting sika deer
and wild boar, as hunters in Japan still do today.
[163]
Hunting dogs make major contributions to forager societies and
the ethnographic record shows them being given proper names, treated as
family members, and considered separate to other types of dogs.
[163][164] This special treatment includes separate burials with markers and grave-goods,
[163][165][166] with those that were exceptional hunters or that were killed on the hunt often venerated.
[163][167]
A dog's value as a hunting partner gives them status as a living weapon
and the most skilled elevated to taking on a "personhood", with their
social position in life and in death similar to that of the skilled
hunters.
[163][168]
Intentional dog burials together with ungulate hunting is also
found in other early Holocene deciduous forest forager societies in
Europe
[169] and North America,
[171] indicating that across the
Holarctic temperate zone hunting dogs were a widespread adaptation to forest ungulate hunting.
[163]
First dog breeds
In 2017, a study showed that 9,000 YBP the domestic dog was present at what is now
Zhokhov Island,
arctic north-eastern Siberia, which was connected to the mainland at
that time. The dogs were selectively bred as either sled dogs or as
hunting dogs, which implies that a sled dog standard and a hunting dog
standard existed at that time. The optimal maximum size for a sled dog
is 20–25 kg based on thermo-regulation, and the ancient sled dogs were
between 16–25 kg. The same standard has been found in the remains of
sled dogs from this region 2,000 YBP and in the modern
Siberian husky
breed standard. Other dogs were more massive at 30 kg and appear to be
dogs that had been crossed with wolves and used for polar bear hunting.
At death, the heads of the dogs had been carefully separated from their
bodies by humans, probably for ceremonial reasons.
[64]
The study proposes that after having diverged from the common ancestor along with the grey wolf, the evolution of
Canis familiaris
proceeded in three stages. The first was natural selection based on
feeding behavior within the ecological niche that had been formed
through human activity. The second was artificial selection based on
tamability. The third was directed selection based on forming breeds
that possessed qualities to help with specific tasks within the human
economy. The process commenced 40,000-30,000 YBP with its speed
increasing with each stage until domestication became complete.