A Most Interesting Problem

What Darwin's Descent of Man Got Right and Wrong about Human Evolution

Jeremy DeSilva, Jeremy DeSilva

• Publisher: Princeton University Press
• Year: 2021
• ISBN:
  9780691210810
• Topic: Biological Sciences
• Subtopic: Evolution
• Index: Biological Sciences

Leading scholars take stock of Darwin's ideas about human evolution in the light of modern science In 1871, Charles Darwin published The Descent of Man, a companion to Origin of Species in which he attempted to explain human evolution, a topic he called "the highest and most interesting problem for the naturalist." A Most Interesting Problem brings together twelve world-class scholars and science communicators to investigate what Darwin got right—and what he got wrong—about the origin, history, and biological variation of humans.Edited by Jeremy DeSilva and with an introduction by acclaimed Darwin biographer Janet Browne, A Most Interesting Problem draws on the latest discoveries in fields such as genetics, paleontology, bioarchaeology, anthropology, and primatology. This compelling and accessible book tackles the very subjects Darwin explores in Descent, including the evidence for human evolution, our place in the family tree, the origins of civilization, human races, and sex differences. A Most Interesting Problem is a testament to how scientific ideas are tested and how evidence helps to structure our narratives about human origins, showing how some of Darwin's ideas have withstood more than a century of scrutiny while others have not. A Most Interesting Problem features contributions by Janet Browne, Jeremy DeSilva, Holly Dunsworth, Agustín Fuentes, Ann Gibbons, Yohannes Haile-Selassie, Brian Hare, John Hawks, Suzana Herculano-Houzel, Kristina Killgrove, Alice Roberts, and Michael J. Ryan.

1

The Fetus, the Fish Heart, and the Fruit Fly

Alice Roberts

The sole object of this work is to consider, firstly, whether man, like every other species, is descended from some pre-existing form.

CHARLES DARWIN, THE DESCENT OF MAN¹

IN “THE EVIDENCE OF the Descent of Man from Some Lower Form,” the first chapter of The Descent of Man, his magnum opus on human evolution, Charles Darwin drew on evidence from comparative anatomy and embryology, revealing how similarities in the structure of living animals provided important clues to human evolution, through traces of common ancestry with other animals. He was able to make a convincing case without recourse to the fossil record—which for hominins^a was virtually nonexistent at the time—or indeed to genetics. Looking back from where we find ourselves today, with a specious hominid fossil record showing how our kind evolved in Africa over millions of years, and with our knowledge of genetics, it seems perhaps extraordinary that Darwin could have deduced so much from so little. And perhaps we have forgotten just how much evidence for evolution there really is buried in the bodies of living animals and embryos.

How did we get here? What’s the evidence for our having evolved? The proposition that the human species arose through the same natural processes that produced every other species on this planet is testable. We don’t just have to take it for granted: that’s not how science works. We can test out this proposition, and the way we do that is by looking at the evidence.

Evolution, in the biological sense, is a change in organisms over time, generation by generation. It happens because of the variation in inherited characteristics that is present in any population and because some individuals end up having more offspring than others. The first question, then, is: Do humans vary in ways that can be passed on to their offspring? The answer is clear: of course we do. We’re all very familiar with the fact that children tend to bear similarities to their parents, in all sorts of ways. Nineteenth-century scientists, including Darwin, could observe those patterns of inheritance, even if they didn’t know about the mechanism—DNA.

A further clue to the evolvability of humans, Darwin argued, comes from the pattern of variation seen in species across the globe. Just as we expect children to look more like their parents than like any more distant relatives, we would expect certain characteristics to be more frequent in one population compared with another. We would also expect geographic clusters of those characteristics to exist—even with all the movement of people that has happened over the millennia. Other animal species exhibit geographic variation like this, sometimes so marked that distinct varieties or even subspecies may be identified. While Darwin goes further than biologists would today in recognizing different races of humans (see Chapter 7 of this volume), it is clearly true that certain traits in humans show geographic variation.

But the definition of evolution involves a change in frequency of inherited characteristics (and, as we now understand, in particular genetic variants) over time. It is not enough that there is variation today among human populations; we must ask whether the patterns of variation have actually changed. Have humans, like every other life-form, been subject to natural selection? Do we see any evidence of beneficial variations spreading through ancestral populations and of disadvantageous traits being weeded out? The answer is yes, and we can see this evidence of evolution clearly in the fossil record and in our DNA.

In Darwin’s day, the fossil record of human evolution was practically nonexistent. Fast-forwarding to the twenty-first century, we now have bounteous fossil evidence for human evolution (see Chapter 4 of this volume), including around twenty known fossil hominin species, forming a six-million-year-old family tree of two-legged apes that includes our ancestors. And we can see how particular adaptations—from anatomical features that improve the efficiency of bipedalism to the expansion of brain size—emerged and took hold among our ancestors.

In the century and a half since the publication of Descent of Man, there have of course been great leaps forward in our understanding of the nature of inherited characteristics, with the discovery of DNA and ongoing research into the function of genes and the reading of entire genomes. We know that our genetic makeup influences our anatomy, physiology, and behavior—and that some traits are more tightly controlled by our genes than others. We see that we are, at a fundamental level, made of the same stuff as other animals and subject to the same biological processes. The more detail we’re able to discern, the more similarities we see between ourselves and other forms of life. And we can also see evidence for natural selection written into our genomes—in the conserving of important genes, the promoting of the spread of advantageous mutations, and the weeding out of variants that might have compromised fertility in the past, for instance.

But even back in the nineteenth century, before biologists had worked out what the stuff of inheritance was, before all those hominin fossils were discovered, there was plenty of evidence for evolution, and Darwin knew exactly where to find it: in anatomy. Similarities in the structure of adult bodies, and in embryos, hint at links between living animals—links that Darwin knew were explained by common ancestry.

Further clues could appear when things went wrong with development. Occasionally, a developmental anomaly would seem to hark back to an earlier stage of evolution, producing a “throwback,” or atavism. Darwin was fascinated by these anomalies; their existence—even when their genetic basis was unknown—suggested that humans were subject to the same laws of inheritance and development as other animals. As well as anomalies, he was also interested in anatomical variants that seemed to be nonfunctional and to represent echoes of ancient ancestors. He called these “rudiments,” whereas we tend to refer to them as “vestigial features” today. Some of the vestigial features he wrote about have turned out to be more functional than he suspected; others have proved to be novel characters rather than vestiges left over from earlier ancestors.

Although Darwin may have been wrong about a few details, his brief survey of comparative anatomy and embryology, forming the first chapter of Descent, stands up well today. The fact of our evolution is indeed written into our bodies—with some traces of earlier ancestors very obvious while others are more hidden away. What’s astonishing about such clues is that they come from careful study of living organisms—and provide compelling evidence for evolution on their own, without drawing on fossils or genetics, which now provide us with independent, corroborating bodies of evidence.

Anatomical and Physiological Similarities between Humans and Other Animals

Biologists have realized for centuries that humans are fundamentally similar to other mammals, sharing the same basic body plan. We can look at a human skeleton and find corresponding bones—which are given the same names—in a monkey, a bat, or a seal. We find the same when we turn our attention to the muscles, nerves, blood vessels, and internal organs. Even the human brain—which is larger, compared with body size, than that of any other mammal (see Chapter 2 of this volume)—still appears to be a variation on a theme. The human brain is three times the size of that of our closest living relatives, chimpanzees. The human cerebral cortex—the outer layer, consisting of cell-dense gray matter—contains twice as many neurons as that of chimpanzees. And yet the general anatomy of these two very differently sized brains is strikingly similar, especially in the lobes and to a fair extent in the fissures and folds. There are fewer obvious similarities when we look at the even smaller brain of a more distant relative, the macaque. While the human brain is not just a scaled-up version of other primate brains, it is, as Darwin noted, evidently related to these brains. And that of course is the key to it: all these anatomical similarities represent relatedness. We are connected with these other animals through threads of common ancestry. We can take any animal and trace its genealogy back until we find a shared ancestor of both that species and our own.

These types of similarities, which reveal how closely we’re related to other animals, are not just to be found in our bones, our blood, and our brains. Darwin believed that there was more evidence of common ancestry to be found in human susceptibility to infectious diseases that also affect other animals. Zoonoses are diseases that can cross the species boundary, leaping from other animals to humans. They include bacterial, viral, and fungal infections as well as parasitic infestations. Of all the transmissible diseases to which humans are vulnerable, over 60 percent are zoonotic. It seems reasonable to assume that diseases are more likely to spread successfully (for the diseases) from one species to another if those two species are closely related. And some very significant zoonotic infections are known to have jumped from other primates into humans, including HIV, malaria, and monkeypox. The devastating Ebola outbreak that killed more than 10,000 people in West Africa between 2014 and 2018 was initially thought to have come from great apes, but careful investigation revealed that bats were a more likely source. Bats and horses carry Hendra virus, and mice carry the Lassa virus. Humans contracted our own form of bovine spongiform encephalopathy (BSE) from cattle. Flu viruses are notoriously capable of spreading from birds and pigs into human populations. And mammalian carnivores also turn out to be a particularly important reservoir for zoonotic diseases. So, although humans certainly can and do catch infections from other primates, we can be very vulnerable to pathogens from much more distantly related species.

The problem with using zoonoses to study the relatedness of humans and other animals is that this adds another layer of complexity. After all, the defining characteristic of a successful zoonotic disease is that it isn’t fussy—it is a generalist. These diseases aren’t adapted to the immunological landscape of just one host; their success depends on their ability to survive and thrive in a range of quite different animals.

Darwin does point to some aspects of physiology that seem to provide evidence of common ancestry, and here he is on safer ground than with the infectious diseases. His examples are anecdotal: he writes about monkeys becoming intoxicated when plied with alcohol—and even suffering from hangovers like humans. He notes that humans heal by essentially the same process we see in other animals. The depth of knowledge about animal physiology has grown enormously since Darwin’s day; now we can delve into any physiological aspect and find homologous processes taking place in the bodies of humans and other animals. In fact, our understanding of human physiology has been enhanced through studying and comparing physiological systems among animals. The functions of the cardiovascular, respiratory, endocrine, nervous, musculoskeletal, digestive, and reproductive systems appear as variations on a theme throughout the animal kingdom, just as the anatomical structures of bones, blood, and brains do. The variation reflects adaptations to different lifestyles, different ecological niches, but also reflects the evolutionary history of each species.

Darwin noted the “close correspondence in general structure, in the minute structure of the tissues, in chemical composition and in constitution” between humans and closely related animals.² At the time he was writing and observing, in the nineteenth century, many similarities or correspondences were visible to him, but a whole host were hidden and yet to be discovered. The detailed structure of that “chemical composition” was to be elucidated over the course of the twentieth century, and that work still engages biologists today. We understand now how the hemoglobin protein molecule of a human differs—in its amino acid sequence and its structure, as well as its affinity with oxygen—from that of the extinct woolly mammoth. We know now that the molecule that carries information from one generation to the next, which underpins any heritable characteristic, is deoxyribonucleic acid—DNA. We can see how this molecule differs in its sequence and its expression from one animal to the next, humans among them. Darwin said that it wasn’t possible to exaggerate the similarity between the other great apes and humans. He would surely have been delighted to learn that the genetic sequence of humans is remarkably close to that of chimpanzees, with a 96 to 98 percent similarity, depending on how it is measured.

As we can see, there is a wealth of evidence of relatedness, of common ancestry, with other animals when we look closely at anatomy, physiology, and biochemistry. We can look at structure and function at the level of whole organisms and see similarities there. We can focus right down to the level of molecules within the cells of different species and once again see similarities in structure and function. And what is crucial to understand is that these deep veins of similarity don’t merely reflect adaptations to similar lifestyles. Indeed, they often appear to conflict with those demands. Form and function appear to be strongly influenced or constrained by something else as well, and what Darwin saw very clearly is that the “something else” was common ancestry.

And signs of common ancestry appear even more clearly when we turn our gaze to look not at fully formed, adult organisms but at tiny, developing embryos.

Similarities in Embryos

Darwin had written about the striking similarities among vertebrate embryos in his 1859 book, On the Origin of Species. He illustrated his point with an anecdote about the famous Swiss American anatomist Louis Agassiz: “Having forgotten to [label] the embryo of some vertebrate animal, he cannot now tell whether it be that of a mammal, bird, or reptile.”³ Darwin realized that the resemblances between embryos of different species could provide important clues about the animals’ evolutionary relationships—clues that later become obscured by the appearance of specific adaptions in adult animals. In a creationist view of biology, the similarities between embryos (and adults) represented an abstract connection between animals in the mind of a creator. Under the new evolutionary paradigm, those resemblances spoke of real, physical links between ancestors and descendants.

A brand-new human being starts at conception—as a fertilized egg or ovum. At ovulation, the egg, containing half the genetic material needed to make a human, bursts free from the ovary. The egg, about a tenth of a millimeter in size, possesses the largest diameter of any cell in the human body. As it leaves the ovary, it takes a cluster of smaller supporting cells with it, and the whole mass is picked up by the waving, fingerlike fimbriae fringing the open end of the oviduct. At coitus, sperm are deposited in the upper vagina, and to reach the egg, they must traverse the canal of the cervix, travel through the cavity of the uterus, and enter the correct oviduct. Only a few make it that far: o...