The Accidental Species: Misunderstandings of Human Evolution

by Henry Gee

  What this tells us is that our record of hominin fossils is important not by virtue of the fossils that have been found, because these are few, but by the oceans of ignorance that they punctuate. In almost all cases, newly found hominin fossils open up new vistas, new possibilities, that scientists had not imagined before the fossils were found. This tells us that the hominin record is not only sparse, but so sparse that even the general course of events in human evolution cannot clearly be discerned—much less a coherent narrative.

  What else might lurk in the vast gaps between the tiny islets of knowledge represented by the few fossils that have been discovered?

  The case of Homo floresiensis is particularly instructive. This discovery revealed the presence on a remote island of a peculiar hominin that had evolved in isolation for at least 100,000 years, and possibly more than a million, and whose anatomy spoke of an evolutionary divergence from the hominin line before the emergence of Homo erectus, or even the genus Homo itself.

  The implications of Homo floresiensis for understanding the scale of our ignorance are immense. This single discovery showed that hominins might have migrated from Africa perhaps a million years earlier than anyone had thought, which means a million years of entirely unknown hominin evolution in Eurasia as yet completely undocumented by fossils, and of which everyone had been completely ignorant.

  It showed that the usual scenario of human evolution, concerning the emergence of Homo erectus and its migration out of Africa around 1.9 million years ago, is based very much on our idea of human evolution as a narrative of progression, with scant regard paid to the poverty of the evidence required to support such a narrative.

  Most of all, the discovery should prompt questions such as these: How likely do you think it is that researchers, excavating in just one cave on just one island in the vast archipelago that is Indonesia, just happened upon the one and only species of peculiar, endemic, primitive hominin that ever existed in Eurasia? And, given that Homo floresiensis lived until almost historical times, how likely do you think researchers just happened to have stumbled across the only species of archaic hominin to have survived to so late a date?

  More likely, I think, is the alternative view, that the world was full of hominins of all kinds, some of them persisting until relatively recently, in geological terms. Given that fossilization is exceptional, especially for hominins, it would be no surprise if almost none of these species left any trace in the fossil record. The discovery of Homo floresiensis is proof enough, in the breach. If Homo floresiensis existed, then so must many others, in many other places.

  This is not to say that the discovery of Homo floresiensis has not caused some scientists to take a new look at specimens that never quite seemed to fit into the conventional narrative. A puzzling skeleton from Nigeria, for example, was generally dismissed as an oddity: it looked archaic, but its owner lived in geologically recent times. Now Chris Stringer of the Natural History Museum in London and his colleagues think it might have represented a hitherto unknown kind of archaic human, surviving well into the era of Homo sapiens.9 Meanwhile, a number of skulls of ancient hominins from China have defied categorization.10 Early Homo sapiens? Not quite. Homo erectus? Not that, either.

  The incredible growth of research into ancient DNA is beginning to shed some light on such matters. The sequencing of the genome of several Neanderthals shows that around 4 percent of the DNA in modern Europeans comes from that rugged acme of Ice Age cave life. More startling still was the sequencing of DNA from an otherwise unremarkable hominin finger bone preserved in Denisova Cave in southern Siberia.11 The DNA signaled the arrival of a hitherto unknown hominin species, distinct from both Neanderthals and modern humans, which had lived in eastern Asia until as recently as 30,000 years ago. The latest known occurrence of a species as a fossil is never its last, so the Denisovans must have been around more recently than that. In a way they are still with us, because these archaic hominins have left traces of their genes in modern human populations throughout New Guinea and the western Pacific Ocean.12 The discovery allowed a whole host of questions to be asked, questions whose framing had not hitherto been possible—were some of the enigmatic Chinese skulls from Denisovans? What about some of the strangely archaic-looking skulls of the earliest-known colonists of Australia?13 Were they Homo sapiens, or perhaps Denisovans—or a mixture of both, or something else altogether? Largely thanks to Flores, the world of paleoanthropology (the study of fossil hominins) has, in the past decade, learned to appreciate the stark magnitudes of the unknown.

  Now, to the third and perhaps most controversial of the three topics I raised above—is it possible that hominins other than Homo sapiens might still be living in the modern world, or, if extinct, perished only in historical times? The discoveries of Homo floresiensis and the Denisovans suggest that the question is not quite so outlandish as it might appear at first. After all, we know that the last known appearance of a species in the fossil record might antedate by some margin the actual date of a species’ extinction.

  The Denisovans are known—so far—from just one locality, so their time range is hard to estimate, but we can get some idea of the likelihood of Homo floresiensis persisting into the modern age. To recap: the skeleton of Homo floresiensis from Liang Bua cave, the best and most informative specimen of the species so far known, has been reliably dated to around 18,000 years ago. Other specimens of isolated bones, all from different layers in the same cave but attributable to the same species, range from 14,000 to perhaps as old as 95,000 years. Extensive evidence from elsewhere on Flores shows that hominins were making tools on the island for at least a million years. At the top end of the sequence, a layer of volcanic rock speaks of a massive volcanic eruption around 12,000 years ago. Layers deposited more recently show no sign at all of Homo floresiensis or of other creatures endemic to the island. Leaving aside local Floresian folk wisdom of the ebu gogo, the Little People who lived in the mountains (tales found pretty much everywhere), it is very likely that the volcanic eruption did for Homo floresiensis just as random, localized disasters have almost certainly tipped other, isolated hominins into extinction. It is a teasing thought, however, that had the eruption not happened, Homo floresiensis might have lasted into modern times, and, because of all the chances and mischances of life, death, and fossilization I’ve discussed in this book, it shouldn’t be so surprising were other species of hominin to be found living obscurely on Earth with us.

  Even today, when the earth teems with Homo sapiens wherever you look, and you’d think that scientists had shaken every tree and peered behind every bush on the planet, creatures as yet unknown to science emerge blinking into the light. Not just small, obscure creatures—insects, microbes, nematodes—but creatures large enough to do you an injury if they stepped on your foot.

  Southeast Asia—home of Homo floresiensis—seems especially prone to this phenomenon. In 1937, a species of wild ox called the kouprey (Bos sauveli) was described for the first time, living in what is now Cambodia.14 A species of the archaic fish known as the coelacanth was discovered living near the island of Sulawesi in 1998, a full 10,000 kilometers from the only other known species, in the Comoro Islands just off Madagascar,15 itself a relic of an ancient lineage thought to have perished in the days of the dinosaurs until a recently dead one was brought ashore off South Africa in 1938.

  In 1993, a report hit my desk in Nature of a large and very peculiar antelope, unknown to science, and described from horns and skins found in the huts of hunters working in the Annamite mountains in the remote border region between Laos and Vietnam.16 The description contained no account of living animals, for none had yet been seen by scientists. It took several more years before the animal, by then known variously as the saola, the Vu Quang ox, or Pseudoryx nghetinhensis, was filmed, and very few have been captured and studied.17 There can be few other cases of a creature that goes so far out of its way to go out of its way.

  Such new discoveries do not always toil in
obscurity, however—the okapi, a familiar zoo animal and friend of every player of Scrabble,18 was discovered in what is now the Democratic Republic of the Congo as recently as 1901.19 The discovery was not entirely a surprise, as rumors had been circulating among Africa hands for some time, particularly after the explorer Henry Morton Stanley (the same who presumed to have met Dr. Livingstone) described an as yet scientifically unknown ox-like beast in the 1880s.

  The story of the okapi shows that it is possible for creatures of myth, rumor, and folklore to emerge into the light. Might the same be true of any of the several as-yet-mythological varieties of hominin believed by some to roam various corners of the earth? There is no reason in principle why it should not. However, virtually all the several species of large creatures described by scientists over the past century have been surprises—they were found without anyone setting out to look for them on purpose: in other words, rather in the manner of the coelacanth, the saola, and Homo floresiensis. It is perhaps significant, therefore, that the fabled Sasquatch or “Bigfoot” of North America, the Yeti of Tibet, the Orang Pendek of Malaysia, and other famous beasts (the Loch Ness monster being perhaps the most notorious) have failed to materialize despite decades of directed effort, often derailed by fairground hucksterism and not a few deliberate hoaxes.20 The likelihood of an unknown animal being found appears to be inversely proportional to the efforts devoted to its pursuit. This doesn’t mean that Homo sapiens is necessarily the only hominin to survive—only that news of any others will come suddenly and quite unexpectedly. Homo floresiensis was unexpected—and much the same can be said of almost every other discovery made concerning human evolution. Such findings usually challenge deeply held beliefs about human uniqueness that are very hard to shift. It is perhaps not surprising, therefore, that almost the only discovery of a fossil hominin that immediately convinced all the experts turned out to have been a fraud.

  The first fossil hominin to be recognized as such was dug up in Gibraltar in 1848, but the finding that really marks the start of paleoanthropology was made in 1856, three years before the publication of The Origin of Species. This was a skullcap and bones collected in a cave high in the Neander valley near Düsseldorf in Germany.21

  The findings were immediately a source of controversy. Those who thought that the creature was an extinct kind of human were brave indeed, given that “extinction” as a concept was still quite new. Nowadays the findings are regarded as having belonged to Neanderthal Man (Homo neanderthalensis), perhaps the closest extinct relative of Homo sapiens. The Gibraltar skull is also regarded as Neanderthal. At the time, though, the authorities of the day thought that the Neanderthal finds belonged to a modern human, if of perhaps a primitive sort, and possibly suffering from rickets, a vitamin deficiency that results in deformities of bone growth. It was also suggested that the finds belonged to a Mongolian Cossack who’d been involved in the Napoleonic Wars. These views brushed aside the obvious problem that the poor soul who gave up his bones to posterity, injured and perhaps deformed, scaled the sheer walls of a cliff to find a convenient cave in which to expire. In that way, paleoanthropology started as it was to go on—by loud proclamations from establishment alpha males that the latest discovery is in fact that of a diseased or deformed member of modern humanity. Either that, or it’s an ape. Critics rarely allow that the new discovery might be anything that might genuinely expand our vision of what is known.

  Darwin had the percipience (in his book The Descent of Man) to suggest that as the closest still-living (as opposed to extinct) relatives of modern humans were chimpanzees and gorillas, which lived in Africa, then the deepest roots of humanity lay in that continent. This seems entirely logical to us, in hindsight, given almost a century of African fossil discovery, but it was not always so. At the end of the nineteenth century and well into the twentieth, it was commonly thought that Asia, not Africa, was the cradle of humanity. One man, a Dutch doctor named Eugène Dubois, was so convinced of this that he staked his career on it, traveling to Java in the (then) Dutch East Indies to search for fossil evidence that might bear on human evolution. By the most amazing luck, he found it. In 1891 a skullcap, and later, limb bones, came to light, which Dubois named Pithecanthropus (ape man). Further finds of this creature were made in subsequent years.22

  Pithecanthropus showed an interesting combination of somewhat humanlike limb bones but a skull rather smaller than that of modern humans. Pithecanthropus could be seen as an intermediate between apes and humans, but ran against the prevailing theoretical view that as modern humans are distinguished by big brains, then it must have been the case that big brains evolved first, before humans learned to stand fully erect. As the musician George Clinton once memorably put it in another context—free your mind and your ass will follow. Pithecanthropus was therefore generally seen as an apish side issue, perhaps a giant gibbon, but not anything especially close to human ancestry. Thus was a mark made for the second of the two conventional reactions to new discoveries of members of the human family: if it’s not a diseased human, then it must be some kind of ape.

  The most important and influential fossil hominin discovery ever made was a skull and jaw of a fossil human from a gravel pit in Piltdown, in southern England, in 1912.23 The skull was undeniably old, as shown by fossils of ancient mammals found in the same gravels, but it looked remarkably modern. The jawbone, assumed to have been associated with the skull, looked very apelike, with a receding chin. The dynamite combination of modern-looking skull and primitive jaw showed that the brain had, indeed, led the way in human evolution, dragging the brutish body after it.

  It might be no coincidence that the heyday of Piltdown Man coincided with the nadir of Darwinism. Piltdown came at just the right time to fall victim to the vacuous storytelling as condemned by scientists such as William Bateson. If distinguished anthropologists, who ought to know, assure us that natural selection drove the evolution of the brain before all else, then, in short, it did, and that was that. Eoanthropus was orthodoxy for a generation, the fossil against which all others would be judged.

  Eoanthropus had had a clear run for more than a decade when the first evidence for human evolution in Africa turned up. The discovery threatened to overturn the now-established view of the “Piltdown committee”—the London-based group of grandees whose opinions on human evolution went unchallenged.24 The report came from Raymond Dart, an Australian-born medical scientist at the fledgling University of the Witwatersrand in South Africa. Dart had been sent a crate full of fossils blasted out of a lime works at Taung in the Transvaal. One of the fossils was the cast of a brain of a small, apelike creature. Another was the skeleton of the face that hafted onto the brain cast, as snugly as a cricket ball in a wicket keeper’s mitt.25 Dart, who had been taught neuroanatomy in London by another expat Australian, Grafton Elliot Smith—one of the Piltdown committee—immediately grasped the significance of the find. This was not an ape, but a child of a new species, intermediate between apes and modern humans. Dart sent a preliminary description of the find to Nature, where Australopithecus africanus (southern ape from Africa) was published.26

  The reaction to the Taung “baby” was immediate and negative. Letters sent to Nature from the various members of the Piltdown committee, including Dart’s mentor, Grafton Elliot Smith, damned Dart’s finding with faint praise.27 Yes, the finding was important, but fossils of juveniles are always hard to judge, they said. Humans and chimps look far more alike as babies than they do as adults, so the Taung baby could be an infant ape just as well as an infant ape-man. The Piltdown committee effectively suppressed the publication of Dart’s subsequent monograph, which contained much evidence in favor of the ape-man hypothesis that was either incompletely considered or not available when Dart sent his short communiqué to Nature. In 1929, the Royal Society in London rejected the monograph for publication—the referees certainly included members of the Piltdown committee—and the manuscript lay ever afterward buried with Dart’s papers at the Unive
rsity of the Witwatersrand. Would paleoanthropology have been changed had the monograph been published? It’s impossible to say.

  Dart’s salvation came in the form of more fossils to back up his ideas. Robert Broom—a paleontologist and a rather more intrepid character than Dart—saw the Taung fossil for himself, and reported to Nature that it was precisely as Dart had said it was.28 Although Broom had had the advantage over the Piltdown committee of actually having seen the fossil for himself, it took Broom’s discoveries of several more fossil hominins29 from cave deposits in South Africa for the idea of ape-men to take hold.

  Broom’s hominins were a varied lot. Although no more fossils were forthcoming from Taung, fossils of what looked like adult versions of Australopithecus africanus came from an ancient cave called Sterkfontein, whereas fossils of a rather different creature, eventually called Australopithecus (or Paranthropus) robustus, emerged from other sites, Swartkrans and Kromdraai. A. africanus was slightly built, with somewhat humanlike teeth not specialized for any diet in particular. A. robustus, in contrast, had massive jaws and big, blocky teeth, perhaps more suitable for a diet of tough vegetation such as roots, seeds, and nuts. Broom’s almost single-handed barrage of papers on these creatures to Nature from the mid-1930s onward was largely responsible for rehabilitating Dart’s reputation.30