The Third Chimpanzee: A Critical Retrospective | Amita Basu

Amidst a global renaissance of xenophobia and antiscientific fundamentalism, Jared Diamond’s book is a valuable reminder that man is after all an animal shaped by his ecology

                       third chimpanzee

When The Third Chimpanzee appeared in 1992, the claim that humans should be classified with the common and the pygmy chimpanzee was novel.  TTC is the first of a series of popular science books in which polymath scientist Jared Diamond has synthesised knowledge from fields as diverse as anthropology, comparative genetics, ecology, and history to introduce the public to novel worldviews. TTC has been continuously in print ever since, influencing both research agendas and the popular imagination.  With our collective future imperilled by threats from climate change to antiscientific fundamentalism, this is an apt moment for a retrospective on one of the 20th century’s most influential books on human origins.  27 years on, how well does the science of TTC hold up?

First, a note on readability. Diamond is an eminently accessible writer: one of a small number of practising scientists who unite bodies of knowledge to present provocative but well-reasoned perspectives on questions of life, origins, and meaning.  Diamond examines diverse strands of evidence – from bonobo ethology to the forensics of prehistoric rubbish-middens – to cull evidence for and against his arguments.  He presents his views, but explicitly invites readers to form their own.  This eminently scientific approach invites laypeople into the process of science itself: of systematic discovery, testing and refuting hypotheses, and serendipity.  But Diamond’s greatest gift is narrative.  Whether chronologising the rise and fall of the various hominid species, or describing the Mixed Reproductive Strategy of nesting seabirds, it’s by constructing lucid narratives that Diamond immerses readers in the problems of science.

TTC claims that, for each characteristic we claim as uniquely human, there’re precedents in the animal kingdom.  Our proclivity for monogamous mating systems, with a side of opportunistic cheating?  Documented in numerous species of nesting seabirds.  Our ability to domesticate animals and plants?  47 species of leaf-cutter ants cultivate fungi; other ant species domesticate aphids – dewinging, capturing, sedating, and ‘milking’ them in practices reminescent of modern farming.  Our penchant for mass violence?  Violent single combats occur in elephant seals and other harem-keeping species; chimpanzees, wolves, and ants war tribe against tribe.  In the decades since TTC, evidence has only strengthened for Diamond’s broad claim that notable human behaviours have wide-ranging precedents.

But today, let’s assess the state of the science on three of Diamond’s specific claims about human origins.

Diamond’s titular claim is that ethology and genetics identify us as a chimpanzee: a third member of the genus Pan.  Behaviourally, many human traits could be considered as progressions on the same continuum with the other great apes: chimpanzees, gorillas, and orangutans. Our variety of social structures and mating systems have precedents among primates.  So does neoteny: a prolonged childhood, characterised by play, learning, and intensive parenting.

The exception is language.  A few captive bonobos have been intensively trained to manipulate symbols in a rudimentary form of human sign-language that may even show syntactic sensitivity. But there’s no evidence that, in the wild, bonobos or any other animal use anything like language.

That is key.  It’s true that most key human behaviours have precedents.  But humans have impacted the earth like no other creature. This uniqueness demands explaining. It’s language that likely allows this uniqueness.  Yes: chimps other great apes forge alliances, plot usurpations, ambush enemies, groom friends, use sticks to weasel out termites, and ape human tool-use.  But the fact that we can observe rudiments of many human behaviours in chimps is not a sufficient argument for classing us behaviourally as the third chimpanzee.

What about the genetic basis for the claim that we’re a third chimpanzee?  Less than 2% of our genome differs from that of Pan paniscus (the bonobo) and Pan troglodytes (the common chimp).  Diamond identifies in Pan precedents for key human behaviours: such as our penchant for flexible mating systems (bonobos are promiscuous; common chimps have a wider array of mating systems).  Common chimpanzees are also one of the few nonhuman species known to wage war. Thus, Diamond argues, we should recognise how slim a genetic chance stands between us strolling through the zoo, and chimps looking out through the bars.

How does Diamond’s claim that we’re a chimpanzee hold up to contemporary science?  It’s true that genetically we’re very similar to Pan.  But genetic proximity between extant species is not the basis for Linnean classification.  It’s true that of all extant species, Pan is our closest relative.  But fossil evidence indicates a series of intermediate ancestors between modern humans and the last common ancestor we share with the two chimps 6 million years ago. This series shows progressively less chimp-like features: straighter gait, increased bipedalism, defter tool use, and increasing encephalisation.  It is because the intermediates of the genus Homo have vanished that we correctly identify chimpanzees as our closest relatives.

Second, even though it is a fact that our genome differs from Pan’s by only a tiny fraction (by only 1.2%), we now know that much of the human genome is noncoding: 98% of our 3-billion-letter genome never gets translated into the protein products that a functioning gene encodes.  Do we then count differences in coding sections only, or in the whole genome?

Third, howsoever we enumerate genetic difference between species, the implications of this number are unclear.  First, a single genetic change can have large effects: e.g. the doubling of the size of the cerebral cortex during embryonic development may come from a single point mutation in one gene. Further, much behaviour is polygenic, with each gene making a contribution that is hard to quantify.  For these and other reasons, ‘percent genetic difference’ may be a catchy but not an accurate way to capture kinships among species.Behaviourally and genetically, humans are a great ape.  Our behaviours and genes illuminate our kinship with all other animals.  But TTC’s argument that we should be classified as a chimpanzee does not hold up to science.  In the sum of our behaviours we’re just too different.

But, then, Diamond’s titular claim is motivated by ethical considerations as much as by genetics.  Confronting us with our affinity to nonhuman primates forces us to recognise the arbitrary basis of our speciesism.  We consume bushmeat, infringe on chimps’ habitats, and dissect chimpanzees for science.  Does our close cousinship demand revising these decisions?  To confront us with such questions is Diamond’s aim.

It is a popular but erroneous belief that science and ethics are separate fields of inquiry.  As Sam Harris has argued, research from the behavioural and biological sciences can inform ethics. Given evidence of the web of our kinships with nonhumans, and given that nearly every ‘uniquely’ human behaviour has far-scattered nonhuman precedents, how do we want to interact with chimpanzees and with our numerous more distant cousins?  The fact that we have a moral sense requires us to confront these moral questions.  The science of our kinship with the web of life redoubles this duty.

Diamond’s second claim is that much human uniqueness can be dated from what he calls the ‘Great Leap Forward’ 35,000 years ago.   Diamond argues that while anatomically modern humans have existed for 2 million years, much of our behavioural repertoire is more recent.  This repertoire includes: well-crafted, specialised, hafted tools; cave art and decorative objects; long-distance trade; and elaborate burials.  Prior to the GLF, he argues, our ancestors scrounged the savannahs for a living: much as they’d done for millennia, and as their extant hominid cousins continued to do.  It’s only after the GLF that we see the rapid mastery of key technologies and the cross-generation accumulation of knowledge that can be described as ‘culture.’  Diamond stakes a claim for the GLF as a discontinuous change in human evolution.  And what drove this leap?  Diamond suggests language: minor genetic changes that enhanced either the brain areas involved in language, or the laryngeal apparatus required to produce a range of phonemes sufficient for language.

Archaeologists and cognitive scientists agree that language was indeed a key milestone in human development.  Language allowed: cooperation for big-game hunting (and later agriculture and city-building); knowledge- sharing for the building of cultural repertoires; and, vitally, internal symbolic thought for individuals to plan, remember, and self-regulate.

Language was key in human development.  But was it the sudden appearance of language 35,000 years ago that caused a putative GLF in human evolution?

On this the science is divided.  GLF champions include Richard Dawkins, who suggests that “a case could even be made … that all modern achievements from the Sistine Chapel to special relativity are a series of footnotes to GLF”.  Another champion, the anthropologist Richard Klein, argues that GLF produced “an organism that could radically alter its behaviour without any change in anatomy.”  Once an organism has this potential for culture, information can travel not just vertically but also horizontally: not just as successful genetic variants from parent to offspring, but also as technologies, habits, and other cultural variants between peers. Klein makes the point that, once culture emerged, it took over from the genome as the major driver of human evolution: modern humans are anatomically identical to, but culturally distinct from, our Palaeolithic ancestors.

On the other hand, some recent evidence suggests that both the hardware (throat) and the software (brain) for language may have been present long before a putative GLF, even predating Homo sapiens.    Cognitive scientist and linguist Steven Pinker sides conditionally with GLF: he speculates that while language may have existed for millennia, a special linguistic category like the conditional, emerging relatively suddenly, might have expanded the utility of language and triggered GLF.  Recent evidence that a specific gene, underlying language, emerged in hominids as long ago as 500,000 years ago, lends weight to the idea that language in some rudimentary form may be very old indeed.  Thus TTC’s claim that the sudden emergence of language itself drove a sudden leap is thrown into question by intervening scientific advances.

But some scientists (including a few leading anthropologists and archaeologists) dispute that a GLF occurred at all. They argue that human evolution progressed in a series of small changes, rather than by way of giant leaps: even periods of rapid progress “were multi-event processes, each lasting hundreds of thousands of years.”

The debate about the Great Leap Forward exemplifies a much larger debate in evolutionary biology between two schools of thought: “punctuated equilibrium” vs. gradualism.  Does evolution occur in rapid fits and starts interspersed with long periods of stagnation?  Or is the rate of evolution a low constant?  This has been a vibrant topic of debate from Darwin on.

Did the GLF occur, and was the sudden appearance of language the trigger?  On this the second of TTC’s claims about the origins of human uniqueness, the jury is out.

Finally, we examine Diamond’s claim that all modern humans are descendants of a single hominid species, with no genetic contribution from any of our co-extant hominid cousins.

The question of ancestry is taxonomically fraught.  Our best-known cousins, the Neanderthals, were classified by earlier archaeologists as a human subspecies: Homo sapiens neanderthalensis as distinct from Homo sapiens sapiens.  Later, Neanderthals were reclassified as a different species: Homo neanderthalensis.  Whatever the neanderthals’ Linnean status, Diamond claims that there’s no evidence our ancestors interbred with them. He cites cultural differences: given that neanderthals were probably nonlinguistic, humans would’ve shied away from mating with them.

What does the science in the years since TTC say about our relationship to the neanderthals?  Were they the subhuman and alien savages of popular imagination?  Did their nonhumanness stop our ancestors interbreeding with them?  New genetic evidence is revealing.

Our primary genetic information resides in the nucleus of each cell in our bodies.  This is the standard-issue double-helix 23-chromosome DNA that we inherit from both parents. In 2010, a  sequencing of nuclear DNA by Dr Green and others overturned Diamond’s hypothesis, and challenged the popular view of neanderthals as subhuman savages.  Their data shows that 1-4% of the genes of all modern humans come from neanderthals. DNA-dating techniques suggest that this interbreeding occurred beginning at 80,000 years ago, and ending 45,000 years ago when neanderthals went extinct. The important conclusion is that Sapiens-neanderthal breeding did occur extensively enough for us all to carry a fraction of neanderthal DNA.  A tiny fraction, but significant as a challenge to beliefs about the unbridgeable gaps among hominid species.

Thus Diamond’s third claim, that humans and neanderthals never interbred, has been largely dislodged.  Evidence for human-neanderthal interbreeding, and its implications for the genetic and cultural similarities between hominid species, brings us back to the questions – how do we define our relationships with other species?  What does that mean for the ethics of cross-species interactions?  Like it or not, we’re all indirectly involved in such interactions.  Whether we consume palm oil from plantations encroaching on the habitat of our close cousins the orangutans in Southeast Asia, or whether we benefit from drugs tested on primates – decisions at the interface of science and ethics are inescapable in life choices.

From its second edition on, TTC has been subtitled The Evolution and Future of the Human Animal.  The book has weathered almost three decades of scientific research.  Its central tenets are still compelling.  And its message that human creativity and destructiveness have roots deep in our nonhuman past is particularly resonant today.  With xenophobia and fundamentalism enjoying a global renaissance, and with our limited ecological resources increasingly threatened, we could stand to be reminded that we are after all an animal: shaped by our ecology and inextricably linked to the web of life.  Our cognitive prowess, that has allowed us mastery over so much of life, does not exempt us from sharing the fate of our fellow-creatures, linked to us in concentric rings of cousinship.  27 years on, The Third Chimpanzee still has many lessons from our past as we collectively make decisions for our future.

Amita Basu is a Ph D candidate at the Centre of Behavioural & Cognitive Sciences at Allahabad University. 


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