The grandmother hypothesis is a hypothesis to explain the existence of menopause in human life history by identifying the adaptive value of extended kin networking. It builds on the previously postulated "mother hypothesis"
which states that as mothers age, the costs of reproducing become
greater, and energy devoted to those activities would be better spent
helping her offspring in their reproductive efforts. It suggests that by redirecting their energy onto those of their
offspring, grandmothers can better ensure the survival of their genes
through younger generations. By providing sustenance and support to
their kin, grandmothers not only ensure that their genetic interests are
met, but they also enhance their social networks which could translate
into better immediate resource acquisition. This effect could extend past kin into larger community networks and benefit wider group fitness.
Background
One explanation to this was presented by G.C. Williams who was the first to posit, in 1957, that menopause might be an adaptation. Williams suggested that at some
point it became more advantageous for women to redirect reproductive
efforts into increased support of existing offspring. Since a female's
dependent offspring would die as soon as she did, he argued, older
mothers should stop producing new offspring and focus on those existing.
In so doing, they would avoid the age-related risks associated with
reproduction and thereby eliminate a potential threat to the continued
survival of current offspring. The evolutionary reasoning behind this is
driven by related theories.
Kin selection provides the framework for an adaptive strategy by
which altruistic behavior is bestowed on closely related individuals
because easily identifiable markers exist to indicate them as likely to
reciprocate. Kin selection is implicit in theories regarding the
successful propagation of genetic material through reproduction, as
helping an individual more likely to share one's genetic material would
better ensure the survival of at least a portion of it. Hamilton's rule
suggests that individuals preferentially help those more related to them
when costs to themselves are minimal. This is modeled mathematically as
. Grandmothers would, therefore, be expected to forgo their own reproduction once the benefits of helping those individuals (b) multiplied by the relatedness to that individual (r) outweighed the costs of the grandmother not reproducing (c).
Evidence of kin selection emerged as correlated with
climate-driven changes, around 1.8–1.7 million years ago, in female
foraging and food sharing practices. These adjustments increased juvenile dependency, forcing mothers to opt for a low-ranked, common food source (tubers) that required adult skill to harvest and process. Such demands constrained female IBIs (Inter Birth Intervals) thus
providing an opportunity for selection to favor the grandmother
hypothesis.
Parental investment, originally put forth by Robert Trivers, is defined as any benefit a parent confers on an offspring at a cost to its ability to invest elsewhere. This theory serves to explain the dynamic sex difference in investment
toward offspring observed in most species. It is evident first in gamete
size, as eggs are larger and far more energetically expensive than
sperm. Females are also much more sure of their genetic relationship
with their offspring, as birth serves as a very reliable marker of
relatedness. This paternity uncertainty that males experience makes them
less likely than females to invest, since it would be costly for males
to provide sustenance to another male's offspring. This translates into
the grandparental generation, as grandmothers should be much more likely
than grandfathers to invest energy into the offspring of their
children, and more so in the offspring of their daughters than sons.
The grandmother effect
Evolutionary theory
dictates that all organisms invest heavily in reproduction in order to
replicate their genes. According to parental investment, human females
will invest heavily in their young because the number of mating
opportunities available to them and how many offspring they are able to
produce in a given amount of time is fixed by the biology of their sex.
This inter birth interval (IBI) is a limiting factor in how many
children a woman can have because of the extended developmental period
that human children experience. Extended childhood, like the extended
post-reproductive lifespan for females, is relatively unique to humans. Because of this correlation, human grandmothers are well-poised to
provide supplemental parental care to their offspring's children. Since
their grandchildren still carry a portion of their genes, it is still in
the grandmother's genetic interest to ensure those children survive to
reproduction.
The mismatch between the rates of degradation of somatic cells versus gametes in human females provides an unsolved paradox. Somatic cells decline more slowly, and humans invest more in somatic longevity relative to other species. Since natural selection has a much stronger influence on younger generations, deleterious mutations during later life become harder to select out of the population.
In female placentals, the number of ovarian oocytes is fixed during embryonic development, possibly as an adaptation to reduce the accumulation of mutations, which then mature or degrade over the life course. At birth there are, typically, one million ova. However, by menopause, only approximately 400 eggs would have actually matured. In humans, the rate of follicular atresia increases at older ages (around 38–40), for reasons that are not known. In chimpanzees, our closest nonhuman, genetic relatives, recent
research indicates a menopausal age of roughly 50, similar to that of
human females, in captive chimpanzees, with similar findings reflected in a study of the Ngogo (Uganda) wild chimpanzee community reported in October 2023.
The report of the latter study questioned the grandmother hypothesis by
observing that "...chimpanzees have very different living arrangements
than humans. Older female chimpanzees typically do not live near their
daughters or provide care for grandchildren, yet females at Ngogo often
live past their childbearing years." Previously, a very similar rate of
oocyte atresia until the age of 40 had been posited in chimps and
humans, at which point humans experienced a far accelerated rate
compared to chimpanzees.
The aging process in humans leaves a dilemma in that females live
past their ability to reproduce. The question poised to evolutionary
researchers then becomes, why do human bodies live on so robustly and
for so long past their reproductive potential, and could there be an
adaptive benefit to abandoning one's own attempts at reproduction to
assist kin?
Alloparenting
The practice of dividing parenting responsibilities among non-parents
affords females a great advantage in that they can dedicate more effort
and energy toward having an increased number of offspring. While this
practice is observed in several species, it has been an especially successful strategy for humans
who rely extensively on social networks. One observational study of the
Aka foragers of Central Africa demonstrated how allomaternal investment
toward an offspring increased specifically during times that the
mother's investment in subsistence and economic activities increased.
If the grandmother effect were true, post-menopausal women should
continue to work after the cessation of fertility and use the proceeds
to preferentially provision their kin. Studies of Hadza
women have provided such evidence. A modern hunter-gatherer group in
Tanzania, the post-menopausal Hadza women often help their grandchildren
by foraging for food staples that younger children are inefficient at
acquiring successfully. Children, therefore, require the assistance of an adult to gain this
crucial sustenance. Often, however, mothers are inhibited by the care of
younger offspring and are less available to help their older children
forage. In this regard, the Hadza grandmothers become vital to the care of
existing grandchildren, and allow reproductive-age women to redirect
energy from existing offspring into younger offspring or other
reproductive efforts.
Some commentators felt that Hawkes et al. understated the role of
Hadza men, who contribute 96% of the mean daily intake of protein - however the authors have addressed this criticism in numerous publications.Others questioned the extent of behavioral similarities between modern humans (such as the Hadza) and our hominid ancestors.
Maternal v. paternal grandmothers
Because grandmothers should be expected to provide preferential
treatment to offspring she is most certain of her relationship to, there
should be differences in the help she provides to each grandchild
according to that relationship. Studies have found that not only does
the maternal or paternal relationship of the grandparent affect whether
or how much help a grandchild receives, but also what kind of help.
Paternal grandmothers often had a detrimental effect on infant
mortality.Also, maternal grandmothers concentrate on offspring survival, whereas paternal grandmothers increase birth rates. These finding are consistent with ideas of parental investment and
paternity uncertainty. Equally, a grandmother could be both a maternal
and paternal grandmother and thus in division of resources, a daughter's
offspring should be favored.
Other studies have focused on the genetic relationship between
grandmothers and grandchildren. Such studies have found that the
effects of maternal / paternal grandmothers on grandsons /
granddaughters may vary based on degree of genetic relatedness, with
paternal grandmothers having positive effects on granddaughters but
detrimental effects on grandsons, and paternity uncertainty may be less important than chromosome inheritance.
Criticisms and alternative hypotheses
Some critics have cast doubt on the hypothesis because while it
addresses how grandparental care could have maintained longer female
post-reproductive lifespans, it does not provide an explanation for how
it would have evolved in the first place. One theory is that the number
of caregivers has a positive relationship on the likelihood of offspring
reaching adulthood, suggesting that grandparents who contribute to the
care of their grandchildren are more likely to have their genes passed
down. Some versions of the grandmother hypothesis asserted that it
helped explain the longevity of human senescence.
However, demographic data has shown that historically rising numbers in
older people among the population correlated with lower numbers of
younger people. This suggests that at some point grandmothers were not helpful toward
the survival of their grandchildren, and does not explain why the first
grandmother would forgo her own reproduction to help her offspring and
grandchildren.
In addition, all variations on the mother, or grandmother effect, fail to explain longevity with continued spermatogenesis in males.
Another problem concerning the grandmother hypothesis is that it requires a history of female philopatry. Though some studies suggest that hunter-gatherer societies are patriarchal, mounting evidence shows that residence is fluid among hunter-gatherers and that married women in at least one patrilineal society visit their
kin during times when kin-based support can be especially beneficial to a
woman's reproductive success. One study does suggest, however, that maternal kin were essential to the fitness of sons as fathers in a patrilocal society.
It also fails to explain the detrimental effects of losing
ovarian follicular activity. While continued post-menopausal synthesis
of estrogen occurs in peripheral tissues through the adrenal pathways, these women undoubtedly face an increased risk of conditions associated with lower levels of estrogen: osteoporosis, osteoarthritis, Alzheimer's disease and coronary artery disease.
However, cross-cultural studies of menopause have found that
menopausal symptoms are quite variable among different populations, and
that some populations of females do not recognize, and may not even
experience, these "symptoms". This high level of variability in menopausal symptoms across
populations brings into question the plausibility of menopause as a sort
of "culling agent" to eliminate non-reproductive females from competition with younger, fertile members of the species. This also faces the task of explaining the paradox between the typical
age for menopause onset and the life expectancy of female humans.
Recent human evolution refers to evolutionary adaptation, sexual and natural selection, and genetic drift within Homo sapiens populations, since their separation and dispersal in the Middle Paleolithic about 50,000 years ago. Contrary to popular belief, not only are humans still evolving, their evolution since the dawn of agriculture is faster than ever before. It has been proposed that human culture acts as a selective force in human evolution and has accelerated it; however, this is disputed. With a sufficiently large data set and modern research methods, scientists can study the changes in the frequency of an allele occurring in a tiny subset of the population over a single lifetime, the shortest meaningful time scale in evolution. Comparing a given gene with that of other species enables geneticists
to determine whether it is rapidly evolving in humans alone. For
example, while human DNA is on average 98% identical to chimpanzee DNA, the so-called Human Accelerated Region 1 (HAR1), involved in the development of the brain, is only 85% similar.
Following the peopling of Africa some 130,000 years ago, and the recent Out-of-Africa expansion some 70,000 to 50,000 years ago, some sub-populations of Homo sapiens have been geographically isolated for tens of thousands of years prior to the early modern Age of Discovery. Combined with archaic admixture, this has resulted in relatively significant genetic variation. Selection pressures were especially severe for populations affected by the Last Glacial Maximum (LGM) in Eurasia, and for sedentary farming populations since the Neolithic, or New Stone Age.
Single nucleotide polymorphisms
(SNP, pronounced 'snip'), or mutations of a single genetic code
"letter" in an allele that spread across a population, in functional
parts of the genome can potentially modify virtually any conceivable
trait, from height and eye color to susceptibility to diabetes and
schizophrenia. While approximately 2% of the human genome codes for
proteins and a slightly larger fraction is involved in gene regulation,
the remainder has no known function. If the environment remains stable,
the beneficial mutations will spread throughout the local population
over many generations until it becomes a dominant trait. An extremely
beneficial allele could become ubiquitous in a population in as little
as a few centuries whereas those that are less advantageous typically
take millennia.
Simplified phylogeny of Homo sapiens for the last two million years
Genetic evidence suggests that a species dubbed Homo heidelbergensis is the last common ancestor of Neanderthals, Denisovans, and Homo sapiens.
This common ancestor lived between 600,000 and 750,000 years ago,
likely in either Europe or Africa. Members of this species migrated
throughout Europe, the Middle East, and Africa and became the
Neanderthals in Western Asia and Europe while another group moved
further east and evolved into the Denisovans, named after the Denisova Cave
in Russia where the first known fossils of them were discovered. In
Africa, members of this group eventually became anatomically modern
humans. Migrations and geographical isolation notwithstanding, the three
descendant groups of Homo heidelbergensis later met and interbred.
Map
of western Eurasia showing areas and estimated dates of possible
Neandertal–modern human hybridization (in red) based on fossil samples
from indicated sites
Archaeological research suggests that as prehistoric humans swept
across Europe 45,000 years ago, Neanderthals went extinct. Even so,
there is evidence of interbreeding between the two groups as humans
expanded their presence in the continent. While prehistoric humans
carried 3–6% Neanderthal DNA, modern humans have only about 2%. This
seems to suggest selection against Neanderthal-derived traits. For example, the neighborhood of the gene FOXP2, affecting speech and language, shows no signs of Neanderthal inheritance whatsoever.
Introgression of genetic variants acquired by Neanderthal admixture has different distributions in Europeans and East Asians, pointing to differences in selective pressures. Though East Asians inherit more Neanderthal DNA than Europeans; East Asians, South Asians, Australo-Melanesians, Native Americans, and
Europeans all share Neanderthal DNA, so hybridization likely occurred
between Neanderthals and their common ancestors coming out of Africa. Their differences also suggest separate hybridization events for the ancestors of East Asians and other Eurasians.
Following the genome sequencing of three Vindija Neanderthals, a
draft sequence of the Neanderthal genome was published and revealed that
Neanderthals shared more alleles with Eurasian populations—such as
French, Han Chinese, and Papua New Guinean—than with sub-Saharan African
populations, such as Yoruba and San. According to the authors of the
study, the observed excess of genetic similarity is best explained by
recent gene flow from Neanderthals to modern humans after the migration out of Africa. But gene flow did not go one way. The fact that some of the ancestors
of modern humans in Europe migrated back into Africa means that modern
Africans also carry some genetic materials from Neanderthals. In
particular, Africans share 7.2% Neanderthal DNA with Europeans but only
2% with East Asians.
Some climatic adaptations, such as high-altitude adaptation in humans, are thought to have been acquired by archaic admixture. An ethnic group known as the Sherpas from Nepal is believed to have inherited an allele called EPAS1, which allows them to breathe easily at high altitudes, from the Denisovans. A 2014 study reported that Neanderthal-derived variants found in East
Asian populations showed clustering in functional groups related to immune and haematopoietic pathways, while European populations showed clustering in functional groups related to the lipid catabolic process. A 2017 study found correlation of Neanderthal admixture in modern European populations with traits such as skin tone, hair color, height, sleeping patterns, mood and smoking addiction. A 2020 study of Africans unveiled Neanderthal haplotypes, or alleles
that tend to be inherited together, linked to immunity and ultraviolet
sensitivity.
The gene microcephalin (MCPH1), involved in the development of the brain, likely originated from a Homo
lineage separate from that of anatomically modern humans, but was
introduced to them around 37,000 years ago, and has become much more
common ever since, reaching around 70% of the human population at
present. Neanderthals were suggested as one possible origin of this
gene. But later studies did not find this gene in the Neanderthal genome nor has it been found to be associated with cognitive ability in modern people.
The promotion of beneficial traits acquired from admixture is known as adaptive introgression.
A study concluded only 1.5–7% of "regions" of the modern human
genome to be specific to modern humans. These regions have neither been
altered by archaic hominin DNA due to admixture (only a small portion of
archaic DNA is inherited per individual but a large portion is
inherited across populations overall) nor are shared with Neanderthals
or Denisovans in any of the genomes of the used datasets. They also
found two bursts of changes specific to modern human genomes which
involve genes related to brain development and function.
Upper Paleolithic, or the Late Stone Age (50,000 to 12,000 years ago)
Cave paintings (such as this one from France) represent a benchmark in the evolutionary history of human cognition.
Victorian naturalist Charles Darwin was the first to propose the out-of-Africa hypothesis for the peopling of the world, but the story of prehistoric human migration is now understood to be
much more complex thanks to twenty-first-century advances in genomic
sequencing. There were multiple waves of dispersal of anatomically modern humans out of Africa, with the most recent one dating back to 70,000 to 50,000 years ago. Earlier waves of human migrants might have gone extinct or returned to Africa. Moreover, a combination of gene flow from Eurasia back into Africa and
higher rates of genetic drift among East Asians compared to Europeans
led these human populations to diverge from one another at different
times.
Around 65,000 to 50,000 years ago, a variety of new technologies, such
as projectile weapons, fish hooks, porcelain, and sewing needles, made
their appearance. Bird-bone flutes were invented 30,000 to 35,000 years ago, indicating the arrival of music. Artistic creativity also flowered, as can be seen with Venus figurines and cave paintings. Cave paintings of not just actual animals but also imaginary creatures that could reliably be attributed to Homo sapiens
have been found in different parts of the world. Radioactive dating
suggests that the oldest of the ones that have been found, as of 2019,
are 44,000 years old. For researchers, these artworks and inventions represent a milestone in the evolution of human intelligence, the roots of story-telling, paving the way for spirituality and religion.Experts believe this sudden "great leap forward"—as anthropologist Jared Diamond
calls it—was due to climate change. Around 60,000 years ago, during the
middle of an ice age, it was extremely cold in the far north, but ice
sheets sucked up much of the moisture in Africa, making the continent
even drier and droughts much more common. The result was a genetic
bottleneck, pushing Homo sapiens to the brink of extinction, and a
mass exodus from Africa. Nevertheless, it remains uncertain (as of
2003) whether or not this was due to some favorable genetic mutations,
for example in the FOXP2 gene, linked to language and speech. A combination of archaeological and genetic evidence suggests that
humans migrated along Southern Asia and down to Australia 50,000 years
ago, to the Middle East and then to Europe 35,000 years ago, and finally
to the Americas via the Siberian Arctic 15,000 years ago.
Epicanthic folds
are thought to be a particular trait in archaic humans from Eastern and
Southeast Asia, and may have originated already within early humans in
Africa.
DNA analyses conducted since 2007 revealed the acceleration of
evolution with regards to defenses against disease, skin color, nose
shapes, hair color and type, and body shape since about 40,000 years
ago, continuing a trend of active selection since humans emigrated from
Africa 100,000 years ago. Humans living in colder climates tend to be
more heavily built compared to those in warmer climates because having a
smaller surface area compared to volume makes it easier to retain heat. People from warmer climates tend to have thicker lips, which have large
surface areas, enabling them to keep cool. With regards to nose shapes,
humans residing in hot and dry places tend to have narrow and
protruding noses in order to reduce loss of moisture. Humans living in
hot and humid places tend to have flat and broad noses that moisturize
inhaled air and retain moisture from exhaled air. Humans dwelling in cold and dry places tend to have small, narrow, and
long noses in order to warm and moisturize inhaled air. As for hair
types, humans from regions with colder climates tend to have straight
hair so that the head and neck are kept warm. Straight hair also allows
cool moisture to quickly fall off the head. On the other hand, tight and
curly hair increases the exposed areas of the scalp, easing the
evaporation of sweat and allowing heat to be radiated away while keeping
itself off the neck and shoulders. Epicanthic eye folds
are believed to be an adaptation protecting the eye from overexposure
to ultraviolet radiation, and is presumed to be a particular trait in
archaic humans from eastern and southeast Asia.
A cold-adaptive explanation for the epicanthic fold is today seen as
outdated by some, as epicanthic folds appear in some African
populations. Dr. Frank Poirier, a physical anthropologist at Ohio State
University, concluded that the epicanthic fold in fact may be an
adaptation for tropical regions, and was already part of the natural diversity found among early modern humans.
Various theories have been proposed to explain the short stature of pygmies and negritos. Some studies suggest that it could be related to adaptation to low ultraviolet light levels in tropical rainforests.
Physiological or phenotypical changes have been traced to Upper Paleolithic mutations, such as the East Asian variant of the EDAR
gene, dated to about 35,000 years ago in Southern or Central China.
Traits affected by the mutation are sweat glands, teeth, hair thickness
and breast tissue. While Africans and Europeans carry the ancestral version of the gene,
most East Asians have the mutated version. By testing the gene on mice,
Yana G. Kamberov and Pardis C. Sabeti
and their colleagues at the Broad Institute found that the mutated
version brings thicker hair shafts, more sweat glands, and less breast
tissue. East Asian women are known for having comparatively small
breasts and East Asians in general tend to have thick hair. The research
team calculated that this gene originated in Southern China, which was warm and humid, meaning having more sweat glands would be advantageous to the hunter-gatherers who lived there. A subsequent study from 2021, based on ancient DNA samples, has suggested that the derived variant became dominant among "Ancient Northern East Asians" shortly after the Last Glacial Maximum
in Northeast Asia, around 19,000 years ago. Ancient remains from
Northern East Asia, such as the Tianyuan Man (40,000 years old) and the
AR33K (33,000 years old) specimen lacked the derived EDAR allele, while
ancient East Asian remains after the LGM carry the derived EDAR allele. The frequency of 370A is most highly elevated in North Asian and East Asian populations.
The most recent Ice Age peaked in intensity between 19,000 and
25,000 years ago and ended about 12,000 years ago. As the glaciers that
once covered Scandinavia all the way down to Northern France retreated,
humans began returning to Northern Europe from the Southwest, modern-day
Spain. But about 14,000 years ago, humans from Southeastern Europe,
especially Greece and Turkey, began migrating to the rest of the
continent, displacing the first group of humans. Analysis of genomic
data revealed that all Europeans since 37,000 years ago have descended
from a single founding population that survived the Ice Age, with
specimens found in various parts of the continent, such as Belgium.
Although this human population was displaced 33,000 years ago, a
genetically related group began spreading across Europe 19,000 years
ago. Recent divergence of Eurasian lineages was sped up significantly during the Last Glacial Maximum (LGM), the Mesolithic and the Neolithic, due to increased selection pressures and founder effects associated with migration. Alleles predictive of light skin have been found in Neanderthals, but the alleles for light skin in Europeans and East Asians, KITLG and ASIP, are (as of 2012) thought to have not been acquired by archaic admixture but recent mutations since the LGM. Hair, eye, and skin pigmentation phenotypes associated with humans of
European descent emerged during the LGM, from about 19,000 years ago. The associated TYRP1SLC24A5 and SLC45A2 alleles emerge around 19,000 years ago, still during the LGM, most likely in the Caucasus. Within the last 20,000 years or so, lighter skin has evolved in East
Asia, Europe, North America and Southern Africa. In general, people
living in higher latitudes tend to have lighter skin. The HERC2 variation for blue eyes first appears around 14,000 years ago in Italy and the Caucasus.
Inuit adaptation to high-fat diet and cold climate has been traced to a mutation dated the Last Glacial Maximum (20,000 years ago). Humans living in Northern Asia and the Arctic have evolved the ability
to develop thick layers of fat on their faces to keep warm. Moreover,
the Inuit tend to have flat and broad faces, an adaptation that reduces
the likelihood of frostbites.
Australian Aboriginals living in the Central Desert,
where the temperature can drop below freezing at night, have evolved
the ability to reduce their core temperatures without shivering.
Teosinte (left) was cultivated and evolved into modern corn (right).
Populations that cultivated carbohydrate-rich food crops such as rice evolved to produce the enzyme amylase in their saliva.
Impacts of agriculture
The advent of agriculture has played a key role in the evolutionary
history of humanity. Early farming communities benefited from new and
comparatively stable sources of food, but were also exposed to new and
initially devastating diseases such as tuberculosis, measles, and smallpox.
Eventually, genetic resistance to such diseases evolved and humans
living today are descendants of those who survived the agricultural
revolution and reproduced. The pioneers of agriculture faced tooth cavities, protein deficiency and general malnutrition, resulting in shorter statures. Diseases are one of the strongest forces of evolution acting on Homo sapiens.
As this species migrated throughout Africa and began colonizing new
lands outside the continent around 100,000 years ago, they came into
contact with and helped spread a variety of pathogens with deadly
consequences. In addition, the dawn of agriculture led to the rise of
major disease outbreaks. Malaria is the oldest known of human
contagions, traced to West Africa around 100,000 years ago, before
humans began migrating out of the continent. Malarial infections surged
around 10,000 years ago, raising the selective pressures upon the
affected populations, leading to the evolution of resistance.
Around 11,000 years ago, as agriculture was replacing hunting and
gathering in the Middle East, people invented ways to reduce the
concentrations of lactose in milk by fermenting
it to make yogurt and cheese. People lost the ability to digest lactose
as they matured and as such lost the ability to consume milk. Thousands
of years later, a genetic mutation enabled people living in Europe at
the time to continue producing lactase, an enzyme that digests lactose,
throughout their lives, allowing them to drink milk after weaning and
survive bad harvests.
Today, lactase persistence can be found in 90% or more of the
populations in Northwestern and Northern Central Europe, and in pockets
of Western and Southeastern Africa, Saudi Arabia, and South Asia. It is
not as common in Southern Europe (40%) because Neolithic farmers had
already settled there before the mutation existed. It is rarer in inland
Southeast Asia and Southern Africa. While all Europeans with lactase
persistence share a common ancestor for this ability, pockets of lactase
persistence outside Europe are likely due to separate mutations. The
European mutation, called the LP allele, is traced to modern-day
Hungary, 7,500 years ago. In the twenty-first century, about 35% of the
human population is capable of digesting lactose after the age of seven
or eight. Before this mutation, dairy farming was already widespread in Europe.
A Finnish research team reported that the European mutation that
allows for lactase persistence is not found among the milk-drinking and
dairy-farming Africans, however. Sarah Tishkoff
and her students confirmed this by analyzing DNA samples from Tanzania,
Kenya, and Sudan, where lactase persistence evolved independently. The
uniformity of the mutations surrounding the lactase gene suggests that
lactase persistence spread rapidly throughout this part of Africa.
According to Tishkoff's data, this mutation first appeared between 3,000
and 7,000 years ago. This mutation provides some protection against
drought and enables people to drink milk without diarrhea, which causes
dehydration.
Lactase persistence is a rare ability among mammals. Because it involves a single gene, it is a simple example of convergent
evolution in humans. Other examples of convergent evolution, such as
the light skin of Europeans and East Asians or the various means of
resistance to malaria, are much more complicated.
Skin color
Humans evolved light skin after migrating from Africa to Europe and East Asia.
The light skin pigmentation characteristic of modern Europeans is
estimated to have spread across Europe in a "selective sweep" during the
Mesolithic (5,000 years ago). Signals for selection in favor of light skin among Europeans was one of
the most pronounced, comparable to those for resistance to malaria or
lactose tolerance. However, Dan Ju and Ian Mathieson caution in a study addressing 40,000
years of modern human history, "we can assess the extent to which they
carried the same light pigmentation alleles that are present today," but
explain that c. 40,000 BPEarly Upper Paleolithic
hunter-gatherers "may have carried different alleles that we cannot now
detect", and as a result "we cannot confidently make statements about
the skin pigmentation of ancient populations."
Eumelanin, which is responsible for pigmentation in human skin, protects against ultraviolet radiation while also limiting vitamin D synthesis. Variations in skin color, due to the levels of melanin, are caused by
at least 25 different genes, and variations evolved independently of
each other to meet different environmental needs. Over the millennia, human skin colors have evolved to be well-suited to
their local environments. Having too much melanin can lead to vitamin D
deficiency and bone deformities while having too little makes the
person more vulnerable to skin cancer. Indeed, Europeans have evolved lighter skin in order to combat vitamin D
deficiency in regions with low levels of sunlight. Today, they and
their descendants in places with intense sunlight such as Australia are
highly vulnerable to sunburn and skin cancer. On the other hand, Inuit have a diet rich in vitamin D and consequently have not needed lighter skin.
Eye color
Blue eyes are an adaptation for living in regions where the amounts
of light are limited because they allow more light to come in than brown
eyes. They also seem to have undergone both sexual and frequency-dependent selection. A research program by geneticist Hans Eiberg
and his team at the University of Copenhagen from the 1990s to 2000s
investigating the origins of blue eyes revealed that a mutation in the
gene OCA2
is responsible for this trait. According to them, all humans initially
had brown eyes and the OCA2 mutation took place between 6,000 and 10,000
years ago. It dilutes the production of melanin, responsible for the
pigmentation of human hair, eye, and skin color. The mutation does not
completely switch off melanin production, however, as that would leave
the individual with a condition known as albinism. Variations in eye
color from brown to green can be explained via the variation in the
amounts of melanin produced in the iris. While brown-eyed individuals
share a large area in their DNA controlling melanin production,
blue-eyed individuals have only a small region. By examining
mitochondrial DNA of people from multiple countries, Eiberg and his team
concluded blue-eyed individuals all share a common ancestor.
In 2018, an international team of researchers from Israel and the
United States announced their genetic analysis of 6,500-year-old
excavated human remains in Israel's Upper Galilee region revealed a
number of traits not found in the humans who had previously inhabited
the area, including blue eyes. They concluded that the region
experienced a significant demographic shift 6,000 years ago due to
migration from Anatolia and the Zagros mountains (in modern-day Turkey
and Iran) and that this change contributed to the development of the Chalcolithic culture in the region.
Bronze Age to Medieval Era
Sickle cell anemia is an adaptation against malaria.
Resistance to malaria is a well-known example of recent human
evolution. This disease attacks humans early in life. Thus humans who
are resistant enjoy a higher chance of surviving and reproducing. While
humans have evolved multiple defenses against malaria, sickle cell anemia—a
condition in which red blood cells are deformed into sickle shapes,
thereby restricting blood flow—is perhaps the best known. Sickle cell
anemia makes it more difficult for the malarial parasite to infect red
blood cells. This mechanism of defense against malaria emerged
independently in Africa and in Pakistan and India. Within 4,000 years it
has spread to 10–15% of the populations of these places. Another mutation that enabled humans to resist malaria that is strongly
favored by natural selection and has spread rapidly in Africa is the
inability to synthesize the enzyme glucose-6-phosphate dehydrogenase, or
G6PD.
A combination of poor sanitation and high population densities
proved ideal for the spread of contagious diseases which was deadly for
the residents of ancient cities. Evolutionary thinking would suggest
that people living in places with long-standing urbanization dating back
millennia would have evolved resistance to certain diseases, such as tuberculosis and leprosy.
Using DNA analysis and archeological findings, scientists from the
University College London and the Royal Holloway studied samples from 17
sites in Europe, Asia, and Africa. They learned that, indeed, long-term
exposure to pathogens has led to resistance spreading across urban
populations. Urbanization is therefore a selective force that has
influenced human evolution. The allele in question is named SLC11A1
1729+55del4. Scientists found that among the residents of places that
have been settled for thousands of years, such as Susa in Iran, this
allele is ubiquitous whereas in places with just a few centuries of
urbanization, such as Yakutsk in Siberia, only 70–80% of the population
have it.
Evolution to resist infection of pathogens also increased inflammatory disease risk in post-Neolithic Europeans over the last 10,000 years. A study of ancient DNA
estimated nature, strength, and time of onset of selections due to
pathogens and also found that "the bulk of genetic adaptation occurred
after the start of the Bronze Age, <4,500 years ago".
Adaptations have also been found in modern populations living in extreme climatic conditions such as the Arctic, as well as immunological adaptations such as resistance against prion caused brain disease in populations practicing mortuary cannibalism, or the consumption of human corpses. Inuit have the ability to thrive on the lipid-rich diets consisting of
Arctic mammals. Human populations living in regions of high altitudes,
such as the Tibetan Plateau, Ethiopia, and the Andes benefit from a
mutation that enhances the concentration of oxygen in their blood. This is achieved by having more capillaries, increasing their capacity for carrying oxygen. This mutation is believed to be around 3,000 years old.
The Sama-Bajau have evolved to become durable free divers.
A recent adaptation has been proposed for the Austronesian Sama-Bajau, also known as the Sea Gypsies or Sea Nomads, developed under selection pressures associated with subsisting on free-diving over the past thousand years or so.As maritime hunter-gatherers, the ability to dive for long periods of times plays a crucial role in their survival. Due to the mammalian dive reflex,
the spleen contracts when the mammal dives and releases oxygen-carrying
red blood cells. Over time, individuals with larger spleens were more
likely to survive the lengthy free-dives, and thus reproduce. By
contrast, communities centered around farming show no signs of evolving
to have larger spleens. Because the Sama-Bajau show no interest in
abandoning this lifestyle, there is no reason to believe further
adaptation will not occur.
Advances in the biology of genomes have enabled geneticists to
investigate the course of human evolution within centuries. Jonathan
Pritchard and a postdoctoral fellow, Yair Field, counted the singletons,
or changes of single DNA bases, which are likely to be recent because
they are rare and have not spread throughout the population. Since
alleles bring neighboring DNA regions with them as they move around the
genome, the number of singletons can be used to roughly estimate how
quickly the allele has changed its frequency. This approach can unveil
evolution within the last 2,000 years or a hundred human generations.
Armed with this technique and data from the UK10K project, Pritchard and
his team found that alleles for lactase persistence, blond hair, and
blue eyes have spread rapidly among Britons within the last two
millennia or so. Britain's cloudy skies may have played a role in that
the genes for light hair could also cause light skin, reducing the
chances of vitamin D deficiency. Sexual selection could also favor blond
hair. The technique also enabled them to track the selection of
polygenic traits—those affected by a multitude of genes, rather than
just one—such as height, infant head circumferences, and female hip
sizes (crucial for giving birth). They found that natural selection has been favoring increased height
and larger head and female hip sizes among Britons. Moreover, lactase
persistence showed signs of active selection during the same period.
However, evidence for the selection of polygenic traits is weaker than
those affected only by one gene.
A 2012 paper studied the DNA sequence of around 6,500 Americans
of European and African descent and confirmed earlier work indicating
that the majority of changes to a single letter in the sequence (single
nucleotide variants) were accumulated within the last 5,000-10,000
years. Almost three quarters arose in the last 5,000 years or so. About
14% of the variants are potentially harmful, and among those, 86% were
5,000 years old or younger. The researchers also found that European
Americans had accumulated a much larger number of mutations than African
Americans. This is likely a consequence of their ancestors' migration
out of Africa, which resulted in a genetic bottleneck; there were few
mates available. Despite the subsequent exponential growth in
population, natural selection has not had enough time to eradicate the
harmful mutations. While humans today carry far more mutations than
their ancestors did 5,000 years ago, they are not necessarily more
vulnerable to illnesses because these might be caused by multiple
mutations. It does, however, confirm earlier research suggesting that
common diseases are not caused by common gene variants. In any case, the fact that the human gene pool has accumulated so many
mutations over such a short period of time—in evolutionary terms—and
that the human population has exploded in that time mean that humanity
is more evolvable than ever before. Natural selection might eventually
catch up with the variations in the gene pool, as theoretical models
suggest that evolutionary pressures increase as a function of population
size.
Early Modern Period to present
A study published in 2021 states that the populations of the Cape Verde islands off the coast of West Africa have speedily evolved resistance to malaria
within roughly the last 20 generations, since the start of human
habitation there. As expected, the residents of the Island of Santiago,
where malaria is most prevalent, show the highest prevalence of
resistance. This is one of the most rapid cases of change to the human
genome measured.
Geneticist Steve Jones told the BBC that during the sixteenth
century, only a third of English babies survived until the age of 21,
compared to 99% in the twenty-first century. Medical advances,
especially those made in the twentieth century, made this change
possible. Yet while people from the developed world today are living
longer and healthier lives, many are choosing to have just a few or no
children at all, meaning evolutionary forces continue to act on the
human gene pool, just in a different way.
Natural selection
affects only 8% of the human genome, meaning mutations in the remaining
parts of the genome can change their frequency by pure chance through genetic drift, a selectively neutral
form of evolution. If natural selective pressures are reduced, then
more mutations survive, which could increase their frequency and the
rate of evolution. For humans, a large source of heritable mutations is sperm;
a man accumulates more and more mutations in his sperm as he ages.
Hence, men delaying reproduction can affect human evolution.
A 2012 study led by Augustin Kong suggests that the number of de novo
(new) mutations increases by about two per year of delayed reproduction
by the father and that the total number of paternal mutations doubles
every 16.5 years.
For a long time, medicine has reduced the fatality of genetic
defects and contagious diseases, allowing more and more humans to
survive and reproduce, but it has also enabled maladaptive traits that
would otherwise be culled to accumulate in the gene pool. This is not a
problem as long as access to modern healthcare is maintained. But
natural selective pressures will mount considerably if that is taken
away. Nevertheless, dependence on medicine rather than genetic adaptations
will likely be the driving force behind humanity's fight against
diseases for the foreseeable future. Moreover, while the introduction of
antibiotics initially reduced the mortality rates due to infectious diseases by significant amounts, overuse has led to the rise of antibiotic-resistant strains of bacteria, making many illnesses major causes of death once again.
Many humans today have jaws that are too small to accommodate their wisdom teeth.
Human jaws and teeth have been shrinking in proportion with the
decrease in body size in the last 30,000 years as a result of new diets
and technology. There are many individuals today who do not have enough
space in their mouths for their third molars
(or wisdom teeth) due to reduced jaw sizes. In the twentieth century,
the trend toward smaller teeth appeared to have been slightly reversed
due to the introduction of fluoride, which thickens dental enamel, thereby enlarging the teeth.
Recent research suggests that menopause
is evolving to occur later. Other reported trends appear to include
lengthening of the human reproductive period and reduction in
cholesterol levels, blood glucose and blood pressure in some
populations.
Population geneticist Emmanuel Milot and his team studied recent
human evolution in an isolated Canadian island using 140 years of church
records. They found that selection favored younger age at first birth
among women. In particular, the average age at first birth of women from Coudres Island (Île aux Coudres),
80 km (50 mi) northeast of Québec City, decreased by four years between
1800 and 1930. Women who started having children sooner generally ended
up with more children in total who survive until adulthood. In other
words, for these French-Canadian women, reproductive success was
associated with lower age at first childbirth. Maternal age at first
birth is a highly heritable trait.
Human evolution continues during the modern era, including among
industrialized nations. Things like access to contraception and the
freedom from predators do not stop natural selection. Among developed countries, where life expectancy is high and infant
mortality rates are low, selective pressures are the strongest on traits
that influence the number of children a human has. It is speculated
that alleles influencing sexual behavior would be subject to strong
selection, though the details of how genes can affect said behavior
remain unclear.
Historically, as a by-product of the ability to walk upright,
humans evolved to have narrower hips and birth canals and to have larger
heads. Compared to other close relatives such as chimpanzees,
childbirth is a highly challenging and potentially fatal experience for
humans. Thus began an evolutionary tug-of-war (see Obstetrical dilemma).
For babies, having larger heads proved beneficial as long as their
mothers' hips were wide enough. If not, both mother and child typically
died. This is an example of balancing selection,
or the removal of extreme traits. In this case, heads that were too
large or hips that were too small were selected against. This
evolutionary tug-of-war attained an equilibrium, making these traits
remain more or less constant over time while allowing for genetic
variation to flourish, thus paving the way for rapid evolution should
selective forces shift their direction.
All this changed in the twentieth century as Caesarean sections (also known as C-sections) became safer and more common in some parts of the world. Larger head sizes continue to be favored while selective pressures
against smaller hip sizes have diminished. Projecting forward, this
means that human heads would continue to grow while hip sizes would not.
As a result of increasing fetopelvic disproportion, C-sections would
become more and more common in a positive feedback loop, though not
necessarily to the extent that natural childbirth would become obsolete.
Paleoanthropologist Briana Pobiner of the Smithsonian Institution
noted that cultural factors could play a role in the widely different
rates of C-sections across the developed and developing worlds. Daghni
Rajasingam of the Royal College of Obstetricians observed that the
increasing rates of diabetes and obesity among women of reproductive age
also boost the demand for C-sections. Biologist Philipp Mitteroecker from the University of Vienna and his
team estimated that about six percent of all births worldwide were
obstructed and required medical intervention. In the United Kingdom, one
quarter of all births involved the C-section while in the United
States, the number was one in three. Mitteroecker and colleagues
discovered that the rate of C-sections has gone up 10% to 20% since the
mid-twentieth century. They argued that because the availability of safe
Cesarean sections significantly reduced maternal and infant mortality
rates in the developed world, they have induced an evolutionary change.
However, "It's not easy to foresee what this will mean for the future of
humans and birth," Mitteroecker told The Independent. This is
because the increase in baby sizes is limited by the mother's metabolic
capacity and modern medicine, which makes it more likely that neonates
who are born prematurely or are underweight to survive.
Westerners are evolving to have lower blood pressures because their modern diets contain high amounts of salt (NaCl), which raises blood pressure.
Researchers participating in the Framingham Heart Study,
which began in 1948 and was intended to investigate the cause of heart
disease among women and their descendants in Framingham, Massachusetts,
found evidence for selective pressures against high blood pressure
due to the modern Western diet, which contains high amounts of salt,
known for raising blood pressure. They also found evidence for selection
against hypercholesterolemia, or high levels of cholesterol in the blood. Evolutionary geneticist Stephen Stearns and his colleagues reported
signs that women were gradually becoming shorter and heavier. Stearns
argued that human culture and changes humans have made on their natural
environments are driving human evolution rather than putting the process
to a halt. The data indicates that the women were not eating more; rather, the ones who were heavier tended to have more children. Stearns and his team also discovered that the subjects of the study
tended to reach menopause later; they estimated that if the environment
remains the same, the average age at menopause will increase by about a
year in 200 years, or about ten generations. All these traits have
medium to high heritability. Given the starting date of the study, the spread of these adaptations can be observed in just a few generations.
By analyzing genomic data of 60,000 individuals of Caucasian descent from Kaiser Permanente in Northern California, and of 150,000 people in the UK Biobank, evolutionary geneticist Joseph Pickrell and evolutionary biologist Molly Przeworski
were able to identify signs of biological evolution among living human
generations. For the purposes of studying evolution, one lifetime is the
shortest possible time scale. An allele associated with difficulty
withdrawing from tobacco smoking dropped in frequency among the British
but not among the Northern Californians. This suggests that heavy
smokers—who were common in Britain during the 1950s but not in Northern
California—were selected against. A set of alleles linked to later
menarche was more common among women who lived for longer. An allele
called ApoE4, linked to Alzheimer's disease, fell in frequency as carriers tended to not live for very long. In fact, these were the only traits that reduced life expectancy
Pickrell and Przeworski found, which suggests that other harmful traits
probably have already been eradicated. Only among older people are the
effects of Alzheimer's disease and smoking visible. Moreover, smoking is
a relatively recent trend. It is not entirely clear why such traits
bring evolutionary disadvantages, however, since older people have
already had children. Scientists proposed that either they also bring
about harmful effects in youth or that they reduce an individual's inclusive fitness,
or the tendency of organisms that share the same genes to help each
other. Thus, mutations that make it difficult for grandparents to help
raise their grandchildren are unlikely to propagate throughout the
population. Pickrell and Przeworski also investigated 42 traits determined by
multiple alleles rather than just one, such as the timing of puberty.
They found that later puberty and older age of first birth were
correlated with higher life expectancy.
Larger sample sizes allow for the study of rarer mutations. Pickrell and Przeworski told The Atlantic
that a sample of half a million individuals would enable them to study
mutations that occur among only 2% of the population, which would
provide finer details of recent human evolution. While studies of short time scales such as these are vulnerable to
random statistical fluctuations, they can improve understanding of the
factors that affect survival and reproduction among contemporary human
populations.
Evolutionary geneticist Jaleal Sanjak and his team analyzed
genetic and medical information from more than 200,000 women over the
age of 45 and 150,000 men over the age of 50—people who have passed
their reproductive years—from the UK Biobank and identified 13 traits
among women and ten among men that were linked to having children at a
younger age, having a higher body-mass index, fewer years of education, and lower levels of fluid intelligence,
or the capacity for logical reasoning and problem solving. Sanjak
noted, however, that it was not known whether having children actually
made women heavier or being heavier made it easier to reproduce. Because
taller men and shorter women tended to have more children and because
the genes associated with height affect men and women equally, the
average height of the population will likely remain the same. Among
women who had children later, those with higher levels of education had
more children.
Evolutionary biologist Hakhamanesh Mostafavi led a 2017 study
that analyzed data of 215,000 individuals from just a few generations in
the United Kingdom and the United States and found a number of genetic
changes that affect longevity. The ApoE allele linked to Alzheimer's disease was rare among women aged 70 and over while the frequency of the CHRNA3
gene associated with smoking addiction among men fell among middle-aged
men and up. Because this is not itself evidence of evolution, since
natural selection only cares about successful reproduction not
longevity, scientists have proposed a number of explanations. Men who
live longer tend to have more children. Men and women who survive until
old age can help take care of both their children and grandchildren, in
benefits their descendants down the generations. This explanation is
known as the grandmother hypothesis.
It is also possible that Alzheimer's disease and smoking addiction are
also harmful earlier in life, but the effects are more subtle and larger
sample sizes are required in order to study them. Mostafavi and his
team also found that mutations causing health problems such as asthma,
having a high body-mass index and high cholesterol levels were more
common among those with shorter lifespans while mutations leading to
delayed puberty and reproduction were more common among long living
individuals. According to geneticist Jonathan Pritchard, while the link
between fertility and longevity was identified in previous studies,
those did not entirely rule out the effects of educational and financial
status—people who rank high in both tend to have children later in
life; this seems to suggest the existence of an evolutionary trade-off
between longevity and fertility.
In South Africa, where large numbers of people are infected with
HIV, some have genes that help them combat this virus, making it more
likely that they would survive and pass this trait onto their children. If the virus persists, humans living in this part of the world could
become resistant to it in as little as hundreds of years. However,
because HIV evolves more quickly than humans, it will more likely be
dealt with technologically rather than genetically.
The Amish have a mutation that extends their life expectancy and reduces their susceptibility to diabetes.
A 2017 study by researchers from Northwestern University unveiled a mutation among the Old Order Amish
living in Berne, Indiana, that suppressed their chances of having
diabetes and extends their life expectancy by about ten years on
average. That mutation occurred in the gene called Serpine1, which codes
for the production of the protein PAI-1
(plasminogen activator inhibitor), which regulates blood clotting and
plays a role in the aging process. About 24% of the people sampled
carried this mutation and had a life expectancy of 85, higher than the
community average of 75. Researchers also found the telomeres—non-functional
ends of human chromosomes—of those with the mutation to be longer than
those without. Because telomeres shorten as the person ages, those with
longer telomeres tend to live longer. At present, the Amish live in 22
U.S. states plus the Canadian province of Ontario. They live simple
lifestyles that date back centuries and generally insulate themselves
from modern North American society. They are mostly indifferent towards
modern medicine, but scientists do have a healthy relationship with the
Amish community in Berne. Their detailed genealogical records make them
ideal subjects for research.
In 2020, Teghan Lucas, Maciej Henneberg, Jaliya Kumaratilake gave
evidence that a growing share of the human population retained the median artery
in their forearms. This structure forms during fetal development but
dissolves once two other arteries, the radial and ulnar arteries,
develop. The median artery allows for more blood flow and could be used
as a replacement in certain surgeries. Their statistical analysis
suggested that the retention of the median artery was under extremely
strong selection within the last 250 years or so. People have been
studying this structure and its prevalence since the eighteenth century.
Multidisciplinary research suggests that ongoing evolution could
help explain the rise of certain medical conditions such as autism and autoimmune disorders.
Autism and schizophrenia may be due to genes inherited from the mother
and the father which are over-expressed and which fight a tug-of-war in
the child's body. Allergies, asthma,
and autoimmune disorders appear linked to higher standards of
sanitation, which prevent the immune systems of modern humans from being
exposed to various parasites and pathogens the way their ancestors'
were, making them hypersensitive and more likely to overreact. The human
body is not built from a professionally engineered blueprint but a
system shaped over long periods of time by evolution with all kinds of
trade-offs and imperfections. Understanding the evolution of the human
body can help medical doctors better understand and treat various
disorders. Research in evolutionary medicine
suggests that diseases are prevalent because natural selection favors
reproduction over health and longevity. In addition, biological
evolution is slower than cultural evolution and humans evolve more
slowly than pathogens.
Whereas in the ancestral past, humans lived in geographically isolated communities where inbreeding was rather common, modern transportation technologies have made it much easier for people
to travel great distances and facilitated further genetic mixing, giving
rise to additional variations in the human gene pool. It also enables the spread of diseases worldwide, which can have an effect on human evolution. Furthermore, climate change may trigger the mass migration of not just humans but also diseases affecting humans. Besides the selection and flow of genes and alleles, another mechanism of biological evolution is epigenetics,
or changes not to the DNA sequence itself, but rather the way it is
expressed. Scientists already know that chronic illnesses and stress are
epigenetic mechanisms.
A scientist observes a captive tufted capuchin (monkey), who has turned her face away from the researcher.
Primatology is the scientific study of primates. Unlike branches of zoology focused on specific animal groups (such as ornithology,
the study of birds), primatology – and the primate order — includes
both human and nonhuman animals. Thus, the field entails significant
overlap with anthropology, the study of humans, and related sciences.
Primatology encompasses a broad swath of scientists from
different fields of study, each with distinct perspectives. For example,
behavioral ecologists may focus on ways primate species act in different environments or circumstances. Sociobiologists are concerned with genetic inheritance and primates' physical and behavioral traits. Anthropologists tend to focus on humans' evolutionary history; they look to primates for greater insights into how Homo Sapiens have evolved. Comparative psychologists study differences between human and nonhuman primate minds.
Some primatologists work in the field to study animals in their
natural environments; others work in academia in labs conducting
experiments. Many do a mix of both. In the 21st century, primatologists
have often blended approaches, incorporating both experimentation and
observational data to varying degrees.
Many primatologists work outside of academia. In places where
nonhuman primates are indigenous — Asia, Africa, and South America —
they often work in government to balance human-wildlife coexistence and promote conservation. Primatologists also work in animal sanctuaries, NGOs, biomedical research facilities, museums and zoos.
21st century "primatologies"
Primatology was established as a discipline in the 1950s in
America/Europe and in Japan. (See History below.) International programs
— in South America, Africa, and other parts of Asia — began taking off
in the 1970s.
Given the wide variety of disciplines involving primates, some
specialists speak of primatology not as a single discipline but of
multidisciplinary "primatologies." Primatology in the U.S. largely
originated with anthropology and its strong bent toward understanding
humans and defining human uniqueness. In contrast, "establishing the
human-animal divide is generally of little importance to non-Western
primatologies."
Researchers from Brazil, India, Vietnam, Africa and areas with
indigenous primates have adopted many Western practices while focusing
on objectives and approaches that reflect local challenges and cultural
traditions. Human populations in these countries have different
relationships and experiences with wild primates than do those in the
West. The human-primate "interface" (the scientific term for
human-nonhuman interactions) is thus a key point of research. Population
dynamics, with repeated conservation surveys, form a significant part
of research activities for Indian primatologists, for example. Primate
rescue centers are key research hubs in Vietnam. Ecology, demography,
human-wildlife conflict, and conservation of interconnected species and
ecosystems are all possible focal points.
Ethnoprimatology
is a 21st-century subdiscipline focused on the social, cultural, and
ecological contexts of human-primate interactions. (These interactions
have also been viewed as human-wildlife conflict and human-wildlife coexistence.) As habitat loss continues to worsen internationally, primatologists Agustin Fuentes
and Kimberley J. Hockings state that understanding which primates are
best able to adapt and interface with human populations, and how they
are able to do so, is a new frontier for primatology.
Charles Darwin's books On the Origin of Species (1859) and The Descent of Man
(1871) drew widespread attention to humans' closest relatives. His
theory of evolution ignited public fascination with the relationship
between humans and monkeys, even a "gorilla craze." Zoologists Ramona and Desmond Morris
later credited Darwin for setting off two major trends. One: By
connecting humans with other animals, Darwin prompted researchers to
consider the behaviour of living animals, especially monkeys and apes,
as worthy of detailed scientific study. Two: Researchers inspired by
Darwin became prone to highly anthropomorphic interpretations of animal
behavior. Once animals were seen as related to humanity, they were
viewed as potentially highly rational creatures with exalted moral
codes.
A 1910 syndicated news story made R.L. Garner's interpretations of chimp behavior almost comical.
Richard Garner,
arguably among the first dedicated primate field researchers,
personified this tendency. Garner was an innovator in some ways: he
built a cage in the African forest to study gorillas in their natural
habitat. He recorded primate vocalizations and tested the animal's
responses when played back. But his writings included anthropomorphized claims about monkey and ape "speech," stories that provided fodder for outlandish newspaper headlines and illustrations.
While scientists from the late-19th and early-20th century were
deeply interested in researching evolution, they were wary of being seen
as peddling Garner-style primate folklore." In the early 1900s, many Western researchers discounted observational
studies as unprofessional and uncontrolled. They viewed lab experiments
as the scientific ideal but faced serious complications in building out
spaces suitable for primates. Primates are not indigenous to Europe or
North America and importing them was expensive.
In
the early 20th century, scientists struggled to keep captive primates
alive. Yerkes's Chim died a year after Yerkes purchased him. Here, Chim
copies humans, gently paging through a book.
More significantly, those hoping to study primates struggled to keep animals alive. The experience of American scientist Robert Yerkes is illustrative. Yerkes spent $2,000 in 1923 — most of his life savings at that point — to buy his first two ape
study subjects, Chim and Panzee. Within 5 months, Panzee was dead, and
by 12 months, Chim was too. From 1837 to 1965, the average primate in zoos survived about 18 months. Given that apes take a decade or more to reach adulthood, the poor care
practices for captive animals meant that studies were bound to be
short-term and largely restricted to juveniles.
Yerkes improved his animal care methods after traveling to Cuba to visit wealthy animal-keeper Rosalía Abreu, the first person to successfully breed chimpanzees in captivity. He documented Abreu's practices in Almost Human (1925), in which he identified several factors to improve captive primate care:
socially house animals in large, clean spaces with a choice of shade or
sunlight; fresh air; sunlight; a varied, appropriate diet and, where
possible, space for exercise.
Other early pioneers of primate research include:
Clarence Ray Carpenter,
an American student of Yerkes, was one of the first researchers to
scientifically record the behavior of wild primates in the 1930s; He
established rigorous methodologies for field scientists to follow.
Wolfgang Kohler, a German psychologist who conducted seminal experiments on ape cognition, described in his classic The Mentality of Apes (1917).
Élie Metchnikoff,
a Russian immunologist, in 1903 used chimpanzees and orangutans as the
first reliable animal models for studying the progression and treatment
of human disease, in this case, syphilus.
Racism in primate research
Early primate research used science to give cover to racist ideology, such as that popularized by Crookshank in his book The Mongol in Our Midst.
Primate research before the 1950s had roots in eugenics and scientific racism,
reflecting and amplifying racist tropes in Western popular culture.
Robert Yerkes, often considered the founder of American primatology,
promoted primate research in 1925 by arguing that it was the most
practical way to "wisely and effectively regulate or control individual,
social, and racial existence."
It was practical, he argued, because one could conduct experiments on
apes relatively efficiently (compared to humans) without "risk of social
censure or legal infringement."
Yerkes was a key American promoter of eugenics, an ideology
intended to improve the genetic quality of the human race. Eugenics
became hotly criticized and, in the US, started to wane in the 1920s. In
effect, Yerkes worked to build a new discipline (primatology) on the
remains of an old one (eugenics).
Yerkes was far from alone in this effort. Konrad Lorenz,
an Austrian zoologist whose work heavily influenced the development of
European primatology, was also an advocate of eugenics. In the early
1940s, Lorenz defended Nazi efforts to prevent interbreeding of
different human "races." Richard Garner, the attention-seeking professor of "monkey talk"
mentioned above, used his platforms to promote white supremacy in the
late 1890s and early 1900s. F.G. Crookshank, a Fellow of Britain's Royal College of Physicians,
published a book in 1924 claiming that white people descended from
chimpanzees, Black people from gorillas, and "yellow" (Asian) people
from orangutans. Crookshank, in line with other racial pseudoscientists, argued that
racial "mixing" was dangerous and destructive to the white race.
Significant change to anthropology — and, thus, primate research — came after WWII and the Nazi Holocaust.
In the wake of Nazi atrocities perpetrated by beliefs about racial
superiority, sciences studying humankind shifted dramatically away from
differences between races. Instead, scientists began stressing the unity
of the human species. The "new physical anthropology" promoted by Sherwood Washburn, a pioneer in baboon studies, had an antiracist ethos.
But while explicit racism in mainstream science waned after WWII, primatology's racist roots have continued to impact the field. Donna Haraway drew attention to the legacy of racism and sexism in primatology in her critical history of the field, Primate Visions (1989). In 2023, the American Journal of Biological Anthropology
published an editorial by Thomas C. Wilson, a Black primatologist,
outlining ways that the field's racist legacy (in which Black members
constituted only .9% of survey respondents) negatively impacts
contemporary research.
Establishing primatology in Japan
Japan is home to an indigenous species of macaques, which made it easier to develop field research there.
Primatology emerged as its own distinctive field in the 1950s. That decade saw the rise of primatology simultaneously — and largely
independently — in both Japan and in the West (North America and
Europe). Over time, the traditions blended, but Japanese scientific
practice initially differed from that of the Darwin-centered,
objectivity-focused researchers overseas. The relationship between humans and other living beings was a
deep-seated part of Japanese cultural and intellectual traditions, while
the quest for objectivity was not. Japanese scientists assumed monkeys were thinking animals because
nonthinking doesn't make sense from an evolutionary perspective. "The
problem of mind" in animals was not a problem for the Japanese in the
way it was for Western scholars. This opened up Japanese studies to criticism of anthropomorphism and
bias, even in cases where their ideas later proved correct.
Unlike Europe and the US, Japan was home to an indigenous monkey species, Japanese macaques,
making it relatively easy to observe subjects in the wild. In the
1950s, the tropical areas where most primates live were very difficult
(and expensive) for outsiders to access. So while primatology in the West focused on animals in captivity (in
zoos and labs), Japanese scientists focused on field research.
Kinji Imanishi and Junichiro Itani, founders of Japanese primatology, studied primate social groups, seeking insights into the origins of human society. They pioneered the following distinct techniques:
Provisioning: Researchers provided food for the monkeys as a short-cut to habituate them, making them easier to observe. This practice was later discouraged out of concerns that it warps natural behaviors.
Individual identification: Learning to identify every monkey in a
troop as an individual was seen as key to understanding the group's
dynamics. Japanese researchers also identified primate "personalities."
Long-term studies over many years and multiple generations were considered necessary to understand group dynamics and society.
The first scientific journal focused on primate research, Primates,
was published in Japan in 1957, with English translations. More
Japanese studies were translated into English in the 1960s, where they
eventually found an audience in the West. Many of their findings —
regarding dominance hierarchies, matrilineal residence, the existence of
a breeding season — provided foundational understandings of primate
socialization internationally.
Perhaps the most widely popularized reports of Japanese origin were those regarding macaque proto-culture. In 1954,Satsue Mito,
a field assistant, noticed that one of the female monkeys washed her
sweet potatoes before eating them — and that other monkeys in the group
were copying the habit. This led researchers to explore how learned
behaviors spread in populations, eventually igniting debates around
monkey and ape "culture," a subject popularized in the U.S. by Frans de Waal in The Ape and the Sushi Master.
Establishing primatology in Europe and North America
In the period after WWII, primate researchers in the West drew more
heavily upon captive animals than did their colleagues in Japan. So, in
the 1950s, Western primate research could be roughly lumped into two
categories: lab research and field studies. These approaches served
wildly different purposes, goals, and methods.
In Harry Harlow's experiments isolating baby monkeys from their mothers, an infant clings to a soft surrogate.
Innovations in captive animal care and breeding enabled new lines of
inquiry in the 1950s. Biomedical researchers saw monkeys and apes as
ideal animal models
for understanding human disease, and now had the resources to advance
experimental testing. Perhaps the most notable medical breakthroughs
were the use of Rhesus macaques in making the Salk polio vaccine and chimpanzees in developing the hepatitis B vaccine.
Silver Spring monkey, in a restraint chair in 1981 inside the laboratory at the Institute of Behavioral Research, Maryland, 1981
Monkeys and apes were seen as models not only for human anatomy but
psychology, and the 1950s and 1960s saw a proliferation of psychological
studies using primates. Of these, arguably Harry Harlow achieved the most fame and notoriety, as the initiator of "wire" monkey mother surrogate studies.[38]
His research was widely covered in mainstream media, ultimately leading
to broad shifts toward more nurturing methods of prenatal, pediatric,
and psychological care in humans. Harlow's habit of describing his work in gruesome detail came at a cost, however: it ended up inspiring an organized animal rights movement. This period was captured in a series of articles by Deborah Blum, later compiled into The Monkey Wars. (Blum's research on this controversy received the Pulitzer Prize in 1992.)
Scientific efforts to teach great apes human sign language
proved to be more popular with the larger public. Ape sign language
studies had their heyday in the early 1970s before a critical study in Science (1979) was seen as debunking the research. As a result, government and foundations cut funding for ape language research. The sign language studies ultimately illustrated a problem with
experimental psychology more generally: working with traumatized animals
in human-centered, unnatural conditions led to skewed results, even
accusations of "pseudoscience." Rather than trying to teach apes human language, 21st century explorations of primate communication focused on observing species in their natural habitats.
In the 21st century, many countries have banned or eliminated the use of great apes as biomedical subjects in response to public opposition and efforts such as Project R&R and Great Ape Project,
as well as pragmatic concerns. However, researchers continue to use
monkeys as experimental subjects, a practice that remains controversial.
Field research
Post-WWII
prosperity and improvements in international travel opened up new
opportunities for Westerners to study primates in natural environments
in the 1950s. R.L. Carpenter resumed research on rhesus macaques that he had translocated to Puerto Rico. Studies on baboons in Africa proliferated. (Unlike other monkeys, baboons live on savannah — not in trees — and thus were seen as better models of human origins.) Studies on "lower," arboreal primates — lemurs and langurs
— had a slower start but represented an important development in
studying animals for their own sake, not just as "little furry people."
Paleontologist Louis Leakey helped organize long-term studies of chimpanzees and gorillas in Africa with Jane Goodall and Dian Fossey in the 1960s. This work coincided with the rise of color television and a new vogue for nature documentaries. The airing of National Geographic's Miss Goodall and the Wild Chimpanzees
in 1965 thus introduced mass audiences to primate fieldwork. The
magazine continued to popularize a more naturalistic view of great apes
with Fossey's work, as well as that of Biruté Galdikas, who studied orangutans in Indonesia in the 1970s.
Field researchers in the 1980s and 1990s were increasingly forced
to confront the destruction of natural habitats, which had been
increasing over the century but had reached a fever pitch. Goodall's shift from scientific field work to international
conservation and education, reflected a shift in focus that many
researchers adopted.
Women in primatology
Primatology has long been subject to debates regarding researchers'
pre-existing opinions and biases. In particular, the use of
primatological studies to assert gender roles, and to both promote and
subvert feminism has been a point of contention.
The evolution of primatology
Early research on baboon society, starting with Solly Zuckerman's influential The Social Lives of Monkeys and Apes
(1932), emphasized male-male aggression and competition for females.
Females were described as dedicated mothers to small infants and
sexually available to males in order of the males' dominance rank. Female-female competition and choice was ignored. In the 1960s, as more women entered the field, primatologists started looking more closely at female behavior. Studies by Thelma Rowell, Shirley Strum, and Barbara Smuts
found that females are active participants, and even leaders, within
their groups. For instance, Rowell found that female baboons determine
the route for daily foraging. Shirley Strum
found that male investment in special relationships with females had a
greater payoff —in terms of producing offspring — than their rank in a
dominance hierarchy. A field researcher in Madagascar, Alison Jolly, found that females dominated lemur social groups.
In 1970, Jeanne Altmann
drew attention to representative sampling methods in which all
individuals, not just the dominant and the powerful, were observed for
equal periods of time. Prior to Altmann's review, primatologists used
"opportunistic sampling," which only recorded what caught their
attention, thus preferencing the more physically active and aggressive
males.
Sarah Hrdy,
a self-identified feminist, was among the first to apply
sociobiological theory to primates in her studies of infanticide in
langurs.
These female scientists — as well as National Geographic's Jane Goodall and Dian Fossey — forced a reanalysis of how aggression, reproductive access, and dominance affect primate societies.
In the 1970s, media and popular culture portrayed the field of
primatology as a science dominated by women. However, the numbers told a
more complicated story. A 2011 study found that primatology — like
nearly all animal-related studies — drew far more female than male
students. Yet most professors of primatology remained male.
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