Timeline of human evolution

From Wikipedia, the free encyclopedia

   

Haeckel's Paleontological Tree of Vertebrates (c. 1879).
The evolutionary history of species has been described as a "tree", with many branches arising from a single trunk. While Haeckel's tree is somewhat outdated, it illustrates clearly the principles that more complex modern reconstructions can obscure.

The timeline of human evolution outlines the major events in the development of the human species, and the evolution of humans' ancestors. It includes a brief explanation of some animals, species or genera, which are possible ancestors of Homo.
It does not address the origin of life, which is addressed by abiogenesis, but presents one possible line of descendants that led to humans. This timeline is based on studies from paleontology, developmental biology, morphology and from anatomical and genetic data. The study of human evolution is a major component of anthropology.

Homo sapiens taxonomy

The cladistic line of descent (taxonomic rank) of Homo sapiens (modern humans) is as follows:

Taxonomic rank	Name	Common name	Millions of years ago
Domain	Eukaryota	Cells with a nucleus	2,100
Kingdom	Animalia	Animals	590
Phylum	Chordata	Vertebrates and closely related invertebrates	530
Subphylum	Vertebrata	Vertebrates	505
Superclass	Tetrapoda	Tetrapods	395
Unranked	Amniota	Amniotes, tetrapods that are fully terrestrially-adapted	340
Class	Mammalia	Mammals	220
Subclass	Theriiformes	Mammals that birth live young (i.e. non-egg-laying)
Infraclass	Eutheria	Placental mammals (i.e. non-marsupials)	125
Magnorder	Boreoeutheria	Supraprimates, bats, whales, most hoofed mammals, and most carnivorous mammals
Superorder	Euarchontoglires	Supraprimates (primates, rodents, rabbits, tree shrews, and colugos)	100
Grandorder	Euarchonta	Primates, colugos and tree shrews
Mirorder	Primatomorpha	Primates and colugos	79.6
Order	Primates	Primates	75
Suborder	Haplorrhini	"Dry-nosed" (literally, "simple-nosed") primates (apes, monkeys, and tarsiers)	40
Infraorder	Simiiformes	"Higher" primates (or Simians) (apes, old-world monkeys, and new-world monkeys)
Parvorder	Catarrhini	"Downward-nosed" primates (apes and old-world monkeys)	30
Superfamily	Hominoidea	Apes	28
Family	Hominidae	Great apes (Humans, chimpanzees, bonobos, gorillas, and orangutans)	15
Subfamily	Homininae	Humans, chimpanzees, bonobos, and gorillas	8
Tribe	Hominini	Genera Homo and Australopithecus	5.8
Subtribe	Hominina	Contains only the Genus Homo	2.5
Genus	Homo	Humans	2.5
Species	(Archaic) Homo sapiens	Modern humans	0.5
Subspecies	Homo sapiens sapiens	Fully anatomically modern humans	0.2

Timeline

First living beings

Date	Event
4000 Ma (million years ago)	The earliest life appears. Further information: Abiogenesis
3900 Ma	Cells resembling prokaryotes appear. This marks the first appearance of photosynthesis and therefore the first occurrence of large quantities of oxygen on the earth. Further information: Cell (biology) § Evolution
2500 Ma	First organisms to utilize oxygen. By 2400 Ma, in what is referred to as the Great Oxygenation Event, the pre-oxygen anaerobic forms of life were wiped out by the oxygen consumers.
2100 Ma	More complex cells appear: the eukaryotes. Further information: Eukaryote § Origin and evolution
1200 Ma	Sexual reproduction evolves, leading to faster evolution.[1]
900 Ma	Choanoflagellate The choanoflagellates may look similar to the ancestors of the entire animal kingdom, and in particular they may be the direct ancestors of Sponges.[2][3] Proterospongia (members of the Choanoflagellata) are the best living examples of what the ancestor of all animals may have looked like. They live in colonies, and show a primitive level of cellular specialization for different tasks.
600 Ma	It is thought that the earliest multicellular animal was a sponge-like creature. Sponges are among the simplest of animals, with partially differentiated tissues. Sponges (Porifera) are the phylogenetically oldest animal phylum extant today.
580 Ma	Animal movement may have started with cnidarians. Almost all cnidarians possess nerves and muscles. Because they are the simplest animals to possess them, their direct ancestors were very probably the first animals to use nerves and muscles together. Cnidarians are also the first animals with an actual body of definite form and shape. They have radial symmetry. The first eyes evolved at this time.
550 Ma	Flatworm Flatworms are the earliest animals to have a brain, and the simplest animals alive to have bilateral symmetry. They are also the simplest animals with organs that form from three germ layers.
540 Ma	Acorn worms are considered more highly specialised and advanced than other similarly shaped worm-like creatures. They have a circulatory system with a heart that also functions as a kidney. Acorn worms have a gill-like structure used for breathing, a structure similar to that of primitive fish. Acorn worms are thus sometimes said to be a link between vertebrates and invertebrates.[citation needed]

Chordates

Date	Event
530 Ma	Pikaia Pikaia is an iconic ancestor of modern chordates and vertebrates.[4] Other, earlier chordate predecessors include Myllokunmingia fengjiaoa,[5] Haikouella lanceolata,[6] and Haikouichthys ercaicunensis.[7] The lancelet, still living today, retains some characteristics of the primitive chordates. It resembles Pikaia. Conodont Conodonts are a famous type of early (495 Mya and later) chordate fossil; they are the peculiar teeth of an eel-shaped animal characterized by large eyes, fins with fin rays, chevron-shaped muscles and a notochord. The animal is sometimes called a conodont, and sometimes a conodontophore (conodont-bearer) to avoid confusion.
505 Ma	Agnatha The first vertebrates appear: the ostracoderms, jawless fish related to present-day lampreys and hagfishes. Haikouichthys and Myllokunmingia are examples of these jawless fish, or Agnatha. (See also prehistoric fish). They were jawless and their internal skeletons were cartilaginous. They lacked the paired (pectoral and pelvic) fins of more advanced fish. They were precursors to the Osteichthyes (bony fish).[8]
480 Ma	A Placoderm The Placodermi were prehistoric fishes. Placoderms were some of the first jawed fishes (Gnathostomata), their jaws evolving from the first gill arch.[9] A placoderm's head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked. However, the fossil record indicates that they left no descendents after the end of the Devonian and are less closely related to living bony fishes than sharks are.[citation needed]
410 Ma	The first coelacanth appears;[10] this order of animals had been thought to have no extant members until living specimens were discovered in 1938. It is often referred to as a living fossil.

Tetrapods

Date	Event
390 Ma	Panderichthys Some fresh water lobe-finned fish (Sarcopterygii) develop legs and give rise to the Tetrapoda. The first tetrapods evolved in shallow and swampy freshwater habitats. Primitive tetrapods developed from a lobe-finned fish (an "osteolepid Sarcopterygian"), with a two-lobed brain in a flattened skull, a wide mouth and a short snout, whose upward-facing eyes show that it was a bottom-dweller, and which had already developed adaptations of fins with fleshy bases and bones. The "living fossil" coelacanth is a related lobe-finned fish without these shallow-water adaptations. These fishes used their fins as paddles in shallow-water habitats choked with plants and detritus. The universal tetrapod characteristics of front limbs that bend backward at the elbow and hind limbs that bend forward at the knee can plausibly be traced to early tetrapods living in shallow water.[11] Panderichthys is a 90–130 cm (35–50 in) long fish from the Late Devonian period (380 Mya). It has a large tetrapod-like head. Panderichthys exhibits features transitional between lobe-finned fishes and early tetrapods. Trackway impressions made by something that resembles Ichthyostega's limbs were formed 390 Ma in Polish marine tidal sediments. This suggests tetrapod evolution is older than the dated fossils of Panderichthys through to Ichthyostega. Lungfishes retain some characteristics of the early Tetrapoda. One example is the Queensland Lungfish.
375 Ma	Tiktaalik Tiktaalik is a genus of sarcopterygian (lobe-finned) fishes from the late Devonian with many tetrapod-like features. It shows a clear link between Panderichthys and Acanthostega.
365 Ma	Acanthostega Ichthyostega Acanthostega is an extinct amphibian, among the first animals to have recognizable limbs. It is a candidate for being one of the first vertebrates to be capable of coming onto land. It lacked wrists, and was generally poorly adapted for life on land. The limbs could not support the animal's weight. Acanthostega had both lungs and gills, also indicating it was a link between lobe-finned fish and terrestrial vertebrates. Ichthyostega is an early tetrapod. Being one of the first animals with legs, arms, and finger bones, Ichthyostega is seen as a hybrid between a fish and an amphibian. Ichthyostega had legs but its limbs probably weren't used for walking. They may have spent very brief periods out of water and would have used their legs to paw their way through the mud.[12] Amphibia were the first four-legged animals to develop lungs which may have evolved from Hynerpeton 360 Mya. Amphibians living today still retain many characteristics of the early tetrapods.
300 Ma	Hylonomus From amphibians came the first reptiles: Hylonomus is the earliest known reptile. It was 20 cm (8 in) long (including the tail) and probably would have looked rather similar to modern lizards. It had small sharp teeth and probably ate millipedes and early insects. It is a precursor of later Amniotes and mammal-like reptiles. Αlpha keratin first evolves here which is used in claws in modern lizards and birds, and hair in mammals.[13] Evolution of the amniotic egg gives rise to the Amniota, reptiles that can reproduce on land and lay eggs on dry land. They did not need to return to water for reproduction. This adaptation gave them the capability to colonize the uplands for the first time. Reptiles have advanced nervous systems, compared to amphibians. They have twelve pairs of cranial nerves.

Mammals

Date	Event
256 Ma	Phthinosuchus, an early Therapsid Shortly after the appearance of the first reptiles, two branches split off. One branch is the Diapsids, from which come the modern reptiles and birds. The other branch is Synapsida, from which come modern mammals. Both had temporal fenestrae, a pair of holes in their skulls behind the eyes, which were used to increase the space for jaw muscles. Synapsids had one opening on each side, while diapsids had two. The earliest mammal-like reptiles are the pelycosaurs. The pelycosaurs were the first animals to have temporal fenestrae. Pelycosaurs are not therapsids but soon they gave rise to them. The Therapsida were the direct ancestor of mammals. The therapsids have temporal fenestrae larger and more mammal-like than pelycosaurs, their teeth show more serial differentiation, and later forms had evolved a secondary palate. A secondary palate enables the animal to eat and breathe at the same time and is a sign of a more active, perhaps warm-blooded, way of life.[14]
220 Ma	Cynognathus One sub-group of therapsids, the cynodonts, evolved more mammal-like characteristics. The jaws of cynodonts resemble modern mammal jaws. It is very likely that this group of animals contains a species which is the direct ancestor of all modern mammals.[15]
220 Ma	Repenomamus From Eucynodontia (cynodonts) came the first mammals. Most early mammals were small shrew-like animals that fed on insects. Although there is no evidence in the fossil record, it is likely that these animals had a constant body temperature and milk glands for their young. The neocortex region of the brain first evolved in mammals and thus is unique to them. Monotremes are an egg-laying group of mammals represented amongst modern animals by the platypus and spiny anteaters. Recent genome sequencing of the platypus indicates that its sex genes are closer to those of birds than to those of the therian (live birthing) mammals. Comparing this to other mammals, it can be inferred that the first mammals to gain gender differentiation through the existence or lack of SRY gene (found in the y-Chromosome) evolved after the monotreme lineage split off.
160 Ma	Juramaia sinensis Juramaia sinensis[16] is the earliest known eutherian (placental) mammal fossil.
100 Ma	Last common ancestor of mice and humans (base of the clade Euarchontoglires).

Primates

Date	Event
85–65 Ma	Plesiadapis Carpolestes simpsoni A group of small, nocturnal and arboreal, insect-eating mammals called the Euarchonta begins a speciation that will lead to the primate, treeshrew and flying lemur orders. The Primatomorpha is a subdivision of Euarchonta that includes the primates and the stem-primates Plesiadapiformes. One of the early stem-primates is Plesiadapis. Plesiadapis still had claws and the eyes located on each side of the head. Because of this they were faster on the ground than on the top of the trees, but they began to spend long times on lower branches of trees, feeding on fruits and leaves. The Plesiadapiformes very likely contain the species which is the ancestor of all primates.[17] One of the last Plesiadapiformes is Carpolestes simpsoni. It had grasping digits but no forward-facing eyes.
63 Ma	Primates diverge into suborders Strepsirrhini (wet-nosed primates) and Haplorrhini (dry-nosed primates). Strepsirrhini contain most of the prosimians; modern examples include the lemurs and lorises. The haplorrhines include the three living groups: prosimian tarsiers, simian monkeys, and apes. One of the earliest haplorrhines is Teilhardina asiatica, a mouse-sized, diurnal creature with small eyes. The Haplorrhini metabolism lost the ability to make its own Vitamin C. This means that it and all its descendants had to include fruit in its diet, where Vitamin C could be obtained externally.
30 Ma	Aegyptopithecus Haplorrhini splits into infraorders Platyrrhini and Catarrhini. Platyrrhines, New World monkeys, have prehensile tails and males are color blind. They may have migrated to South America on a raft of vegetation across the relatively narrow Atlantic ocean (approx. 700 km). Catarrhines mostly stayed in Africa as the two continents drifted apart. Possible early ancestors of catarrhines include Aegyptopithecus and Saadanius.
25 Ma	Proconsul Catarrhini splits into 2 superfamilies, Old World monkeys (Cercopithecoidea) and apes (Hominoidea). Our trichromatic color vision had its genetic origins in this period. Proconsul was an early genus of catarrhine primates. They had a mixture of Old World monkey and ape characteristics. Proconsul's monkey-like features include thin tooth enamel, a light build with a narrow chest and short forelimbs, and an arboreal quadrupedal lifestyle. Its ape-like features are its lack of a tail, ape-like elbows, and a slightly larger brain relative to body size. Proconsul africanus is a possible ancestor of both great and lesser apes, including humans.

Hominidae

Date	Event
15 Ma	Hominidae (great apes) speciate from the ancestors of the gibbon (lesser apes).
13 Ma	Homininae ancestors speciate from the ancestors of the orangutan.[18] Pierolapithecus catalaunicus is believed to be a common ancestor of humans and the great apes or at least a species that brings us closer to a common ancestor than any previous fossil discovery. It had special adaptations for tree climbing, just as humans and other great apes do: a wide, flat rib cage, a stiff lower spine, flexible wrists, and shoulder blades that lie along its back.
10 Ma	The lineage currently represented by humans and the Pan genus (chimpanzees and bonobos) speciates from the ancestors of the gorillas.
7 Ma	Sahelanthropus tchadensis Hominina speciate from the ancestors of the chimpanzees. Both chimpanzees and humans have a larynx that repositions during the first two years of life to a spot between the pharynx and the lungs, indicating that the common ancestors have this feature, a precondition for vocalized speech in humans. The latest common ancestor lived around the time of Sahelanthropus tchadensis, ca. 7 Ma [4]; S. tchadensis is sometimes claimed to be the last common ancestor of humans and chimpanzees, but there is no way to establish this with any certainty. The earliest known representative from the ancestral human line post-dating the separation with the chimpanzee lines is Orrorin tugenensis (Millennium Man, Kenya; ca. 6 Ma).
4.4 Ma	Ardipithecus is a very early hominin genus (tribe Hominini or subtribe Hominina). Two species are described in the literature: A. ramidus, which lived about 4.4 million years ago[19] during the early Pliocene, and A. kadabba, dated to approximately 5.6 million years ago[20] (late Miocene). A. ramidus had a small brain, measuring between 300 and 350 cm3. This is about the same size as modern bonobo and female common chimpanzee brain, but much smaller than the brain of australopithecines like Lucy (~400 to 550 cm3) and slightly over a fifth the size of the modern Homo sapiens brain. Ardipithecus was arboreal, meaning it lived largely in the forest where it competed with other forest animals for food, including the contemporary ancestor for the chimpanzees. Ardipithecus was probably bipedal as evidenced by its bowl shaped pelvis, the angle of its foramen magnum and its thinner wrist bones, though its feet were still adapted for grasping rather than walking for long distances.
3.6 Ma	Australopithecus afarensis Some Australopithecus afarensis left human-like footprints on volcanic ash in Laetoli, Kenya (Northern Tanzania) which provides strong evidence of full-time bipedalism. Australopithecus afarensis lived between 3.9 and 2.9 million years ago. It is thought that A. afarensis was ancestral to both the genus Australopithecus and the genus Homo. Compared to the modern and extinct great apes, A. afarensis has reduced canines and molars, although they are still relatively larger than in modern humans. A. afarensis also has a relatively small brain size (~380–430 cm³) and a prognathic (i.e. projecting anteriorly) face. Australopithecines have been found in savannah environments and probably increased its diet to include meat from scavenging opportunities. An analysis of Australopithecus africanus lower vertebrae suggests that females had changes to support bipedalism even while pregnant.
3.5 Ma	Kenyanthropus platyops, a possible ancestor of Homo, emerges from the Australopithecus genus.
3 Ma	The bipedal australopithecines (a genus of the Hominina subtribe) evolve in the savannas of Africa being hunted by Dinofelis. Loss of body hair takes place in the period 3-2 Ma, in parallel with the development of full bipedalism.

Homo

Date	Event
2.5 Ma	Homo habilis Appearance of Homo. Homo habilis is thought to be the ancestor of the lankier and more sophisticated Homo ergaster. Lived side by side with Homo erectus until at least 1.44 Ma, making it highly unlikely that Homo erectus directly evolved out of Homo habilis. First stone tools, beginning of the Lower Paleolithic. Further information: Homo rudolfensis
1.8 Ma	A reconstruction of Homo erectus. Homo erectus evolves in Africa. Homo erectus would bear a striking resemblance to modern humans, but had a brain about 74 percent of the size of modern man. Its forehead is less sloping than that of Homo habilis and the teeth are smaller. Other hominid designations such as Homo georgicus, Homo ergaster, Homo pekinensis, Homo heidelbergensis are often put under the umbrella species name of Homo erectus.[21] Starting with Homo georgicus found in what is now the Republic of Georgia dated at 1.8 Ma, the pelvis and backbone grew more human-like and gave H. georgicus the ability to cover very long distances in order to follow herds of other animals. This is the oldest fossil of a hominid found outside of Africa. Control of fire by early humans is achieved 1.5 Ma by Homo ergaster. Homo ergaster reaches a height of around 1.9 metres (6.2 ft). Evolution of dark skin, which is linked to the loss of body hair in human ancestors, is complete by 1.2 Ma. Homo pekinensis first appears in Asia around 700 Ka but according to the theory of a recent African origin of modern humans, they could not be human ancestors, but rather, were just a cousin offshoot species from Homo ergaster. Homo heidelbergensis was a very large hominid that had a more advanced complement of cutting tools and may have hunted big game such as horses.
1.2 Ma	Homo antecessor may be a common ancestor of humans and Neanderthals.[22][23] At present estimate, humans have approximately 20,000–25,000 genes and share 99% of their DNA with the now extinct Neanderthal [24] and 95-99% of their DNA with their closest living evolutionary relative, the chimpanzees.[25][26] The human variant of the FOXP2 gene (linked to the control of speech) has been found to be identical in Neanderthals.[27] It can therefore be deduced that Homo antecessor would also have had the human FOXP2 gene.
600 ka	A reconstruction of Homo heidelbergensis Three 1.5 m (5 ft) tall Homo heidelbergensis left footprints in powdery volcanic ash solidified in Italy. Homo heidelbergensis may be a common ancestor of humans and Neanderthals.[28] It is morphologically very similar to Homo erectus but Homo heidelbergensis had a larger brain-case, about 93% the size of that of Homo sapiens. The holotype of the species was tall, 1.8 m (6 ft) and more muscular than modern humans. Beginning of the Middle Paleolithic.
338 ka	Y-chromosomal Adam lived in Africa approximately 338,000 years ago, according to a recent study.[29] He is the most recent common ancestor from whom all male human Y chromosomes are descended.
200 ka	Homo sapiens sapiens (Pioneer plaque) Omo1, Omo2 (Ethiopia, Omo river) are the earliest fossil evidence for anatomically modern Homo sapiens.[30]
160 ka	Homo sapiens (Homo sapiens idaltu) in Ethiopia, Awash River, Herto village, practice mortuary rituals and butcher hippos. Potential earliest evidence of anatomical and behavioral modernity consistent with the continuity hypothesis including use of red ochre and fishing.[31]
150 ka	Mitochondrial Eve is a woman who lived in East Africa. She is the most recent female ancestor common to all mitochondrial lineages in humans alive today. Note that there is no evidence of any characteristic or genetic drift that significantly differentiated her from the contemporary social group she lived with at the time. Her ancestors were Homo sapiens as were her contemporaries.
90 ka	Appearance of mitochondrial haplogroup L2.
70 ka	Behavioral modernity according to the "great leap forward" theory.[32]
60 ka	Appearance of mitochondrial haplogroups M and N, which participate in the migration out of Africa. Homo sapiens that leave Africa in this wave start interbreeding with the Neanderthals they encounter.[33][34]
50 ka	Migration to South Asia. M168 mutation (carried by all non-African males). Beginning of the Upper Paleolithic. mt-haplogroups U, K.
40 ka	Migration to Australia[35] and Europe (Cro-Magnon).
25 ka	The independent Neanderthal lineage dies out. Y-Haplogroup R2; mt-haplogroups J, X.
12 ka	Beginning of the Mesolithic / Holocene. Y-Haplogroup R1a; mt-haplogroups V, T. Evolution of light skin in Europeans (SLC24A5).[citation needed] Homo floresiensis dies out, leaving Homo sapiens as the only living species of the genus Homo.

Homo sapiens

From Wikipedia, the free encyclopedia

 

Homo sapiens Temporal range: Pleistocene - Recent
Conservation status
Least Concern (IUCN 3.1)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Primates
Family:	Hominidae
Tribe:	Hominini
Genus:	Homo
Species:	H. sapiens
Binomial name
Homo sapiens Linnaeus, 1758
Subspecies
†Homo sapiens idaltu Homo sapiens sapiens †Homo neanderthalensis? †Homo rhodesiensis?

Homo sapiens (Latin: "wise man") is the binomial nomenclature (also known as the scientific name) for the human species. Homo is the human genus, which also includes Neanderthals and many other extinct species of hominid; H. sapiens is the only surviving species of the genus Homo. Modern humans are the subspecies Homo sapiens sapiens, which differentiates them from what has been argued to be their direct ancestor, Homo sapiens idaltu.

Subspecies

Subspecies of H. sapiens include Homo sapiens idaltu and the only extant subspecies Homo sapiens sapiens. Some sources show Homo sapiens neanderthalensis as a subspecies of H. sapiens. Similarly, the discovered specimens of the Homo rhodesiensis species have been classified by some as a subspecies, but this classification is not widely accepted.

Evolution

 

Skulls of
1. Gorilla 2. Australopithecus 3. Homo erectus 4. Neanderthal (La Chapelle aux Saints) 5. Steinheim Skull 6. Homo sapiens

Scientific study of human evolution is concerned, primarily, with the development of the genus Homo, but usually involves studying other hominids and hominines as well, such as Australopithecus. "Modern humans" are defined as the Homo sapiens species, of which the only extant subspecies is known as Homo sapiens sapiens.

Homo sapiens idaltu (roughly translated as "elder wise human"), the other known subspecies, is now extinct.^[1] Homo neanderthalensis, which became extinct 30,000 years ago, has sometimes been classified as a subspecies, "Homo sapiens neanderthalensis"; genetic studies now suggest that the functional DNA of modern humans and Neanderthals diverged 500,000 years ago.^[2]

Similarly, the discovered specimens of the Homo rhodesiensis species have been classified by some as a subspecies, but this classification is not widely accepted.

Anatomically modern humans first appear in the fossil record in Africa about 195,000 years ago, and studies of molecular biology give evidence that the approximate time of divergence from the common ancestor of all modern human populations was 200,000 years ago.^{[3][4][5][6][7]} The broad study of African genetic diversity found the ǂKhomani San people to express the greatest genetic diversity among the 113 distinct populations sampled, making them one of 14 "ancestral population clusters". The research also located the origin of modern human migration in south-western Africa, near the coastal border of Namibia and Angola.^[8][9]

Evolutionary history of Primates

The evolutionary history of primates can be traced back 65 million years (mya).^[10] Primates are one of the oldest of all surviving placental mammal groups. The oldest known primate-like mammal species (those of the genus Plesiadapis) come from North America, but inhabited Eurasia and Africa on a wide scale during the tropical conditions of the Paleocene and Eocene.

Within family Hominidae, orangutans (Ponginae) were the first to diverge, followed by gorillas. Molecular evidence suggests that the last common ancestor between humans and chimpanzees (Pan) diverged 4–8 million years ago, making humans and chimps the closest relations among the great apes.^[11] Current estimates of suggested concurrence between functional human and chimpanzee DNA sequences range between 95% and 99%.^{[12][13][14][15]} The functional portion of human DNA is approximately 98.4% identical to that of chimpanzees when comparing single nucleotide polymorphisms (see human evolutionary genetics).^[11]

Early estimates indicated that the human lineage may have diverged from that of chimpanzees about five million years ago, and from that of gorillas about eight million years ago. However, a hominid skull discovered in Chad in 2001, classified as Sahelanthropus tchadensis, is approximately seven million years old, and may be evidence of an earlier divergence.^[16]

Human evolution is characterized by a number of important changes—morphological, developmental, physiological, and behavioural—which have taken place since the split between the last common ancestor of humans and chimpanzees. The first major morphological change was the evolution of a bipedal locomotor adaptation from an arboreal or semi-arboreal one,^[17] with all its attendant adaptations (a valgus knee, low intermembral index (long legs relative to the arms), reduced upper-body strength).

The human species developed a much larger brain than that of other primates – typically 1,400 cm³ in modern humans, over twice the size of that of a chimpanzee or gorilla. The pattern of human postnatal brain growth differs from that of other apes (heterochrony), and allows for extended periods of social learning and language acquisition in juvenile humans. Physical anthropologists^[who?] argue that the differences between the structure of human brains and those of other apes are even more significant than their differences in size.

Other significant morphological changes included the evolution of a power and precision grip,^[18] a reduced masticatory system, a reduction of the canine tooth, and the descent of the larynx and hyoid bone, making speech possible. An important physiological change in humans was the evolution of hidden oestrus, or concealed ovulation, which may have coincided with the evolution of important behavioural changes, such as pair bonding. Another significant behavioural change was the development of material culture, with human-made objects becoming increasingly common and diversified over time. The relationship between all these changes is the subject of ongoing debate.^[19][20]

The forces of natural selection have continued to operate on human populations, with evidence that certain regions of the genome display directional selection in the past 15,000 years.^[21]

Denisovan

From Wikipedia, the free encyclopedia

 

Denisovans or Denisova hominins /dəˈniːsəvə/ are a Paleolithic-era species of the genus Homo or subspecies of Homo sapiens. In March 2010, scientists announced the discovery of a finger bone fragment of a juvenile female who lived about 41,000 years ago, found in the remote Denisova Cave in the Altai Mountains in Siberia, a cave which has also been inhabited by Neanderthals and modern humans.^[1][2][3] Two teeth and a toe bone belonging to different members of the same population have since been reported.

Analysis of the mitochondrial DNA (mtDNA) of the finger bone showed it to be genetically distinct from the mtDNAs of Neanderthals and modern humans.^[4] Subsequent study of the nuclear genome from this specimen suggests that this group shares a common origin with Neanderthals, that they ranged from Siberia to Southeast Asia, and that they lived among and interbred with the ancestors of some present-day modern humans, with about 3% to 5% of the DNA of Melanesians and Aboriginal Australians deriving from Denisovans.^[5][6][7] Other ethnicities, such as the Malays, Polynesians, the Dravidians of India, Burmans, and Mon-Khmer-speaking peoples may be included in this category as well.^{[citation needed]} A comparison with the genome of a Neanderthal from the same cave revealed significant local interbreeding, with local Neanderthal DNA representing 17% of the Denisovan genome, while evidence was also detected of interbreeding with an as yet unidentified ancient human lineage.^[8] Similar analysis of a toe bone discovered in 2011 is underway,^[9] while analysis of DNA from two teeth found in different layers than the finger bone revealed an unexpected degree of mtDNA divergence among Denisovans.^[8] In 2013, mitochondrial DNA from a 400,000-year-old hominin femur bone from Spain, which had been seen as either Neanderthal or Homo heidelbergensis, was found to be closer to Denisovan mtDNA than to Neanderthal mtDNA.^[10]

Discovery

Tourists in front of the Denisova Cave, where "X woman" was found

The Denisova Cave is located in southwestern Siberia, in the Altai Mountains near the the border with China and Mongolia. It is named after Denis, a Russian hermit who lived there in the 18th century. The cave was originally explored in the 1970s by Russian paleontologist Nikolai Ovodov, who was looking for remains of cave bears.^{[citation needed]} In 2008, Michael Shunkov from the Russian Academy of Sciences and other Russian archaeologists from the Institute of Archaeology and Ethnology of Novosibirsk investigated the cave. They found the finger bone of a juvenile hominin, dubbed the "X woman" (referring to the maternal descent of mitochondrial DNA^[11]) or the Denisova hominin. Artifacts, including a bracelet, excavated in the cave at the same level were carbon dated to around 40,000 BP. Excavations have since revealed human artifacts showing an intermittent presence going back 125,000 years.^[12]

A team of scientists led by Johannes Krause and Svante Pääbo from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, sequenced mtDNA extracted from the fragment. The cool climate of the Denisova Cave preserved the DNA.^[3] The average annual temperature of the cave remains at 0 °C, which has contributed to the preservation of archaic DNA among the remains discovered.^[13] The analysis indicated that modern humans, Neanderthals, and the Denisova hominin last shared a common ancestor around 1 million years ago.^[4]

The mtDNA analysis further suggested this new hominin species was the result of an earlier migration out of Africa, distinct from the later out-of-Africa migrations associated with modern humans, but also distinct from the earlier African exodus of Homo erectus.^[4] Pääbo noted the existence of this distant branch creates a much more complex picture of humankind during the Late Pleistocene.^[11] This work shows that the Denisovans were actually a sister group to the Neanderthals,^[14] branching off from the human lineage 600,000 years ago, and diverging from Neanderthals, probably in the Middle East, 200,000 years later.^[15]

Later in 2010, a second paper from the Svante Pääbo group reported the prior discovery, in 2000, of a third upper molar from a young adult, dating from about the same time (the finger was from level 11 in the cave sequence, the tooth from level 11.1). The tooth differed in several aspects from those of Neanderthals, while having archaic characteristics similar to the teeth of Homo erectus. They performed mitochondrial DNA analysis on the tooth and found it to have a sequence different from but similar to that of the finger bone, indicating a divergence time about 7,500 years before, and suggesting it belonged to a different individual from the same population.^[16]

In 2011, a toe bone was discovered in the cave, in layer 11, and therefore contemporary with the finger bone. Preliminary characterization of the bone's mitochondrial DNA suggests it belonged to a Neanderthal, not a Denisovan.^[17] The cave also contains stone tools and bone artifacts made by modern humans, and Pääbo commented: "The one place where we are sure all three human forms have lived at one time or another is here in Denisova Cave."^[17]

Anatomy

Little is known of the precise anatomical features of the Denisovans since the only physical remains discovered thus far are the finger bone, two teeth from which genetic material has been gathered and a toe bone. The single finger bone is unusually broad and robust, well outside the variation seen in modern people. Surprisingly, it belonged to a female, indicating the Denisovans were extremely robust, perhaps similar in build to the Neanderthals. The tooth that has been characterized shares no derived morphological features with Neanderthal or modern humans.^[16] An initial morphological characterization of the toe bone led to the suggestion that it may have belonged to a Neanderthal-Denisovan hybrid individual, although a critic suggested the morphology was inconclusive. This toe bone is currently undergoing DNA analysis by Pääbo.^[9]

Some older finds may or may not belong to the Denisovan line. These includes the skulls from Dali and Maba, and a number of more fragmentary remains from Asia. Asia is not well mapped with regard to human evolution, and the above finds may represent a group of "Asian Neanderthals".

Mitochondrial DNA analysis

The mtDNA from the finger bone differs from that of modern humans by 385 bases (nucleotides) in the mtDNA strand out of approximately 16,500, whereas the difference between modern humans and Neanderthals is around 202 bases. In contrast, the difference between chimpanzees and modern humans is approximately 1,462 mtDNA base pairs.^[3] This suggested a divergence time around one million years ago. The mtDNA from a tooth bore a high similarity to that of the finger bone, indicating they belonged to the same population.^[16] From a second tooth, an mtDNA sequence was recovered that showed an unexpectedly large number of genetic differences compared to that found in the other tooth and the finger, suggesting a high degree of mtDNA diversity. These two individuals from the same cave showed more diversity than seen among sampled Neanderthals from all of Eurasia, and were as different as modern-day humans from different continents.^[8]

Nuclear genome analysis

In the same second 2010 paper, the authors reported the isolation and sequencing of nuclear DNA from the Denisova finger bone. This specimen showed an unusual degree of DNA preservation and low level of contamination. They were able to achieve near-complete genomic sequencing, allowing a detailed comparison with Neanderthal and modern humans. From this analysis, they concluded, in spite of the apparent divergence of their mitochondrial sequence, the Denisova population along with Neanderthal shared a common branch from the lineage leading to modern African humans. The estimated average time of divergence between Denisovan and Neanderthal sequences is 640,000 years ago, and the time between both of these and the sequences of modern Africans is 804,000 years ago. They suggest the divergence of the Denisova mtDNA results either from the persistence of a lineage purged from the other branches of humanity through genetic drift or else an introgression from an older hominin lineage.^[16] In 2013, the mtDNA sequence from the femur of a 400,000 year old Homo heidelbergensis from the Sima de los Huesos Cave in Spain was found to be most similar to that of Denisova.^[10]

Interbreeding

 

The Evolution and geographic spread of Denisovans as compared with other groups

A detailed comparison of the Denisovan, Neanderthal, and human genomes has revealed evidence for a complex web of interbreeding among the lineages. Through such interbreeding, 17% of the Denisova genome represents DNA from the local Neanderthal population, while evidence was also found of a contribution to the nuclear genome from an ancient hominin lineage yet to be identified,^[8] perhaps the source of the anomalously ancient mtDNA.

Analysis of genomes of modern humans show that they mated with at least two groups of ancient humans: Neanderthals (more similar to those found in the Caucasus than those from the Altai region)^[8] and Denisovans.^[13][16][18] Approximately 4% of the DNA of non-African modern humans is shared with Neanderthals, suggesting interbreeding.^[16] Tests comparing the Denisova hominin genome with those of six modern humans – a ǃKung from South Africa, a Nigerian, a Frenchman, a Papua New Guinean, a Bougainville Islander and a Han Chinese – showed that between 4% and 6% of the genome of Melanesians (represented by the Papua New Guinean and Bougainville Islander) derives from a Denisovan population. This DNA was possibly introduced during the early migration to Melanesia. These findings are in concordance with the results of other comparison tests which show a relative increase in allele sharing between the Denisovan and the Aboriginal Australian genome, compared to other Eurasians and African populations, however it has been observed that Papuans, the population of Papua New Guinea, have more allele sharing than Aboriginal Australians.^[19]
Melanesians may not be the only modern-day descendants of Denisovans. David Reich of Harvard University, in collaboration with Mark Stoneking of the Planck Institute team, found genetic evidence that Denisovan ancestry is shared by Melanesians, Australian Aborigines, and smaller scattered groups of people in Southeast Asia, such as the Mamanwa, a Negrito people in the Philippines. However, not all Negritos were found to possess Denisovan genes; Onge Andaman Islanders and Malaysian Jehai, for example, were found to have no significant Denisovan inheritance. These data place the interbreeding event in mainland Southeast Asia, and suggest that Denisovans once ranged widely over eastern Asia.^[6][20][21] Based on the modern distribution of Denisova DNA, Denisovans may have crossed the Wallace Line, with Wallacea serving as their last refugium.^[22][23] A paper by Kay Prüfer in 2013 said that mainland Asians and Native Americans had around 0.2% Denisovan ancestry.^[24]

The immune system's HLA alleles have drawn particular attention in the attempt to identify genes that may derive from archaic human populations. Although not present in the sequenced Denisova genome, the distribution pattern and divergence of HLA-B*73 from other HLA alleles has led to the suggestion that it introgressed from Denisovans into humans in west Asia. Indeed, half of the HLA alleles of modern Eurasians represent archaic HLA haplotypes, and have been inferred to be of Denisovan or Neanderthal origin.^[25] The apparent over-representation of these alleles suggests a positive selective pressure for their retention in the human population. A higher quality Denisovan genome published in 2012 reveals variants of genes in humans that are associated with dark skin, brown hair and brown eyes - consistent with features found with Melanesians today.^[26]

It has been suggested,^[27] in the absence of genomic evidence (as of 2013), that the Red Deer Cave people of China were the result of interbreeding between Homo sapiens and Denisovans within a few thousands years of the end of the last glacial period.

There is evidence of a minimum 0.5% Neanderthal gene flow into the Denisovans.^[28] The Denisovan genome shared more derived alleles with the Altai Neanderthal genome from Siberia than with the Vindija Neanderthal genome from Croatia and the Mezmaiskaya Neanderthal genome from the Caucasus, suggesting that the gene flow came from a population that was more closely related to the Altai Neanderthal.^[28]

It has also been observed that the Denisovan genome comprises a component derived from an unknown hominin that diverged long before the modern human/Neanderthal/Denisovan separated, suggesting a possible gene flow from said unknown hominin to Denisovans or a population sub-structure.^[28]

Tibetans have a version of the EPAS1 gene which helps them to adapt to the low oxygen levels at high altitude, and according to a study published in Nature in July 2014, this version of the gene is found in the Denisovan genome, suggesting that they acquired the adaptation by interbreeding between their ancestors and the Denisovans.^[29]