The Accidental Species: Misunderstandings of Human Evolution

by Henry Gee

  The variety of these finds should have been evidence enough that the human family was diverse, and that more than one kind of hominin existed at any one time: the first evidence that human evolution was uncertain and bushy, far from the single lineage I caricatured in chapter 1. At the time, however, the geological ages of these fossils was not known with any certainty. All the australopiths came from cave deposits, which are invariably a jumble of things that have fallen in, or were brought in by predators at various times. Even today, getting reliable dates for fossils found in caves is a difficult business, and that’s with the battery of modern techniques for dating that hadn’t been invented in Broom’s day.

  As more and more fossils came to light, it became clearer that the australopiths did not conform to the George Clinton model of evolution. They had rather small brains, but would have walked erect—the precise opposite of the model espoused by the Piltdown committee.

  Further evidence came from China in the late 1920s and early 1930s, at a site called Dragon-Bone Cave (Chou Kou Tien, modern-day Zhoukoudian), where a Canadian called Davidson Black and his colleagues reported fossils of a creature they called Sinanthropus.31 “Peking Man” had a larger brain than Australopithecus, but smaller than modern humans.32 Sinanthropus was associated with stone tools, and perhaps even the controlled use of fire—hallmarks, it was thought, of technology, and therefore of humanlike activity.33 Sinanthropus was later shown to be very similar to Dubois’s Pithecanthropus from Java,34 and the two were united into one species, Homo erectus—the Man who stands upright.

  By the late 1930s, a picture of early human evolution was beginning to emerge that has remained intact, more or less, ever since. The earliest members of the human family evolved in Africa, and were typified by forms such as Australopithecus—rather apelike, with small brains, but which nevertheless walked upright. Later on, hominins dispersed into Eurasia, acquired tools and a certain stature, and became Homo erectus—with a brain larger than those of australopiths, but smaller than in modern humans. Homo erectus walked upright. His name said as much.

  The steady accumulation of evidence made Piltdown Man look like an increasingly anomalous side issue, ever harder to fit into evidence that challenged the preconceptions of the experts. Big-brained Piltdown might have had some support from Neanderthal Man, which had, if anything, a larger brain than seen in modern humans—an inconvenient fact that is usually brushed aside in the canonical picture of acquisition and improvement. Neanderthals were also seen as stooped and shambling, as Piltdown was meant to have been. But the skulls of Neanderthals, while large, are very distinctive, and quite different from those of modern humans. And the picture of Neanderthals as stooped comes from the interpretation of just one skeleton, of an elderly male crippled with arthritis. In reality, Neanderthals stood as erect as any healthy modern human. Piltdown stood alone.

  Eventually, the penny dropped. As the years wore on, it became ever clearer that Piltdown was not so much anomalous as embarrassing. In 1953, proof came of what many had already come to suspect, that Piltdown Man was a fraud.35 The skull looked like that of a modern human because it was one. The apelike jaw had come from an orangutan. The joint where the jawbone would have attached to the skull had been broken, so nobody could have seen that the two didn’t fit together. The teeth in the orangutan jaw had been filed down so that they didn’t look so apish to have given the game away, and the whole arrangement had been stained to make the bones look very old. The gravel pit at Piltdown had been salted with bones of archaic mammals from elsewhere.

  The identity of the hoaxer remains unknown to this day. There has been some suggestion that it was one Martin Hinton,36 an expert on fossil rodents at the Natural History Museum, who had the means and the technical knowledge, and also a motive: a grudge against Arthur Smith-Woodward, his boss, a prominent paleontologist—and a leading light on the Piltdown committee, and a critic of Dart’s Australopithecus.

  Whoever was responsible, the joke went far too well—perhaps so well that there was no possibility of a safe confession for the hoaxer. Smith-Woodward and his cronies bought the story without question. Further “finds” at Piltdown relating to the “First Englishman” included a hunk of bone deliberately carved into the shape of a cricket bat, clearly meant to be so ridiculous that someone, surely, would have suspected something. This, too, was treated as genuine.

  At the risk of laying it on with a trowel, the moral is that it is very easy to see fossil evidence (or, indeed, any scientific evidence) through the highly selective and distorting lenses of one’s deeply held preconceptions, rather than for what it plainly is. If I have gone on about it at some length, that’s because it can be seen as the message for this whole chapter, that the discoveries made in the course of shedding light on human prehistory have a habit of challenging preconceptions—and indeed for this whole book, that when looked at dispassionately, many if not all the attributes we think of as distinctly human are in fact nothing of the kind, and even if they are unique to humans, this uniqueness is in itself nothing special.

  With the final unraveling of Piltdown the focus moved back to Africa, and the name of Leakey. Louis Leakey, the son of a missionary who came to preach the gospel to the Kikuyu, soon became fascinated with the search for what he called in a later book “Adam’s ancestors.”37 The search was long, hard, and, for thirty years, mostly fruitless. In 1959, however, Leakey’s wife, Mary, discovered the skull of a fossil hominin at Olduvai Gorge in what is now Tanzania.38 This creature was Zinjanthropus boisei, a robust australopith, similar to Australopithecus robustus from South Africa. (These days many paleoanthropologists prefer to group all robust australopiths together in a separate genus, Paranthropus, so that A. robustus is Paranthropus robustus and Leakey’s “Zinj” is Paranthropus boisei.)

  In 1964, Leakey announced the discovery of what he claimed to be the earliest evidence for the genus Homo. This was Homo habilis (handy man).39 The name was attached to a second hominin discovered at Olduvai, less robust than “Zinj.”

  Although Leakey’s 1964 paper describing Homo habilis was very careful, laying out technical statements on the anatomy of the genus Homo in general and the species Homo habilis in particular, the presumption was clear: if Homo habilis and Zinjanthropus were at the same site, associated with stone tools, then habilis, with its larger brain, was probably the toolmaker. “When the skull of Australopithecus (Zinjanthropus) boisei was found on a living floor at F. L. K. I,” wrote Leakey and colleagues,

  no remains of any other type of hominid were known from the early part of the Olduvai sequence. It seemed reasonable, therefore, to assume that the skull represented the makers of the Oldowan [stone tool] culture. The subsequent discovery of remains of Homo habilis in association with the Oldowan culture at three other sites has considerably altered the position. While it is possible that Zinjanthropus and Homo habilis both made stone tools, it is probable that the latter was the more advanced tool maker and that the Zinjanthropus skull represents an intruder (or a victim) on a Homo habilis living site.

  Because you cannot reliably infer the behavior of an extinct creature from its bones, the whole definition of that creature, if encountered as a fossil but classified according to its presumed behavior, becomes debatable. The fossils we have are fragmentary. One debate centers on whether there is one species of early Homo—Homo habilis—or two, the other being Homo rudolfensis, a name attached to a skull discovered in 1972 by Bernard Ngeneo—a member of the research team led by Leakey’s son, Richard—at Koobi Fora on the eastern shore of Lake Turkana in Kenya.40 A third species of early Homo was recently added to the mix. This is a skull discovered in 1977 from Sterkfontein, a site famous for its australopiths. The skull was originally assigned to Homo habilis but has now been renamed Homo gautengensis.41

  The problem is that the type specimen of Homo habilis—that is, the fossil used to name the species—is a jawbone with teeth, and the type specimen of Homo rudolfensis is a skull, complete in most r
espects except that it lacks a jaw and teeth. This means that there is no way to compare the two species directly, so any decisions about the status of these species has to be made in a roundabout way, by comparing them with other fossils that might very well have their own problems of interpretation.42 The confusion deepens with the possibility that any and all early Homo species might really be australopiths, and not Homo at all. The murk is thickened by a general fogginess about what features make a hominin Homo—but mostly by the general lack of fossil evidence. Even after almost a century of sustained effort, the fossil record of hominins is too slender for us to say anything definite about the origins and general characteristics of the earliest members of our own genus, Homo.

  Some scientists, notably Bernard Wood of George Washington University in Washington, DC, have suggested that these early species of Homo look sufficiently archaic—and too much like australopiths—that to include them in the genus Homo makes defining our own genus even more difficult than it is already.43 Wood prefers to cast these hominins as australopiths, similar in many ways to Australopithecus africanus—a species that has also been hard to define, given that its name was originally coined to refer to a baby, rather than an adult in which the full expression of a species’ distinctive traits might be seen. The situation has been complicated further with the detailed description of two partial skeletons of an australopith from Malapa, a cave near Sterkfontein.44 This hominin, named Australopithecus sediba, had a small brain, but features of its skeleton are reminiscent of early Homo.

  What of the tools? Evidence for tools now goes back at least 2.5 million years—for tool use, perhaps as long as 3.39 million years, if the scratches seen on animal bones excavated from a site in Ethiopia were deliberately made by hominins.45 This makes toolmaking far more ancient than the genus Homo, even if Homo habilis is admitted to the club. It is possible, indeed likely, that australopiths made tools. If Homo floresiensis is the descendant of an australopith, rather than Homo erectus, then the case is made, given that tools have been found associated with Flo.

  The error is to construct an argument that is both circular and spurious. If we assume that only members of the genus Homo can make tools, then anything associated with stone tools must be in the genus Homo. This ignores the possibility that the tools might have been made by Zinj or indeed any other hominin, including species as yet undiscovered. Now that we have good reason to think that australopiths indeed made tools, the necessary restriction to the genus Homo becomes nonsense. The alternative is to admit australopiths to Homo, which would then make Homo even harder to define than it is already.

  Worse, though, is the conceit that toolmaking necessarily accompanies a bigger brain, such that when a brain becomes big enough, facilities such as technology become possible. As I show later in this book, many animals with brains much smaller even than that of Homo floresiensis make and use tools. Conversely, organisms as simple as bacteria can make structures at least as elaborate as stone tools, but nobody would accuse such creatures of having any brains at all. Leakey, like the Piltdown committee before him, was in danger of making unwarranted assumptions about the progressive evolution of hominins where no such assumptions were justified.

  After Leakey, paleoanthropology split into two streams, one going further forward in time, the other, backward.

  Forward first. With Homo erectus in Asia and Neanderthals in Europe, but Australopithecus and very early Homo in Africa, the consensus view emerged that Homo migrated out of Africa with, or soon after, the evolution of Homo erectus, around 1.8 million years ago. Further hominins found in Eurasia appeared to confirm this view.

  Recent discoveries include several spectacular specimens of hominin skulls and skeletons in caves in the Sierra de Atapuerca in northern Spain.46 These finds, while remarkable, are not alone. Remains of hominins have been found across Eurasia from Britain to China. Some appear to belong to Homo erectus, whereas others are much harder to place, and are conventionally lumped into a kind of dustbin called Homo heidelbergensis, named after a mandible discovered in Germany in 1907, and conventionally regarded as a generalized Eurasian form whence descended the Neanderthals, and possibly also Homo sapiens, if enigmatic finds from Africa such as Homo rhodesiensis (a distinctive skull found in 1921 in what is now Zambia) belongs to this increasingly inclusive transitional form. Another species, Homo antecessor, comes from deposits at Atapuerca thought to be older than those yielding the bones of supposed H. heidelbergensis.47

  This conventional view has run into problems. The first is the nature of Homo erectus itself. Did this species really originate in Africa? Finds assignable to this species have indeed turned up in Africa, notably a near-complete skeleton of a youth discovered at Nariokotome on the shores of Lake Turkana in Kenya in 1984, by the legendary fossil hunter Kamoya Kimeu, one of Richard Leakey’s “hominid gang.”48 The youth clearly belonged to Homo, based not just on the features of the skull, but on the skeleton, which had the cylindrical ribcage and long legs seen in Homo, rather than the more conical ribcage and shorter legs typical of Australopithecus. Some researchers, though, found sufficient differences between the Nariokotome skeleton and some other African finds and later Homo erectus to create a new species, Homo ergaster, to encompass early erectus-like hominins from Africa.49 Very early examples of hand axes, a style of stone tool very much associated with Homo erectus, have also been found in Ethiopia and Kenya50—with, of course, the usual health warnings about linking tools and their makers.

  So, what’s the problem? Here’s the deal: Homo erectus evolves in Africa around 1.8 million years ago; evolves advanced hand axes rather than the simpler pebble tools of Homo habilis (and maybe Australopithecus); colonizes Rest of World. To me, this narrative seems rather too biblical to be credible.

  The first crack in the story was the discovery of a remarkable collection of hominin skulls and bones from rocks beneath a medieval monastery at Dmanisi in the Republic of Georgia, associated with primitive tools, and dated to between 1.85 and 1.78 million years ago.51 The Dmanisi hominins are arguably the oldest known hominin fossils outside Africa—but if they are the descendants of the first bold exiles from Africa, they seem to have taken a step backward. They are sufficiently primitive to have drawn comparisons with Homo ergaster rather than Homo erectus but have also, lately, acquired their own species name, Homo georgicus.52

  It’s very tempting to view the Dmanisi hominins as primitive members of Homo erectus (or something closely related to it) caught in the act of migrating out of Africa. Such temptations should be resisted. To be sure, there are bits and pieces of Homo erectus, and of stone tools indicative of their passing, recovered from all over the Old World that could quite plausibly be strung together to make a story of migration from Africa, but that would be to ignore the gaps in time and space that must be bridged. Rather than Homo erectus having evolved from an earlier form of Homo in Africa and moved into Eurasia, it is perfectly possible for Homo erectus to have evolved in Asia from some even earlier form and migrated back into Africa, replacing Homo habilis. The Dmanisi hominins might therefore be creatures caught in the act of coming home, not venturing forth. This scenario might seem a little contrived, especially as no fossils of hominins older than 1.7 million years are currently known from outside Africa. But the dates for many early hominins in Europe are constantly being pushed backward toward the 2-million-year mark. And the existence of Homo floresiensis, which looks arguably more like Australopithecus than a dwarfed Homo erectus, suggests that hominins left Africa much, much earlier than had been thought possible.

  The recent discovery of early hand axes together with pebble tools from the western shore of Lake Turkana in Kenya further blurs the picture.53 It suggests that the first hominins to have left Africa might have fled without their distinctive hand axes. It also raises the possibility that the first exiles were more primitive than Homo erectus—and the even more remarkable possibility that Homo erectus didn’t evolve in Africa at all, but having evolved
in Asia—perhaps from even earlier African roots—went back to Africa again.54 Everyone knows that the Israelites crossed the Red Sea, but nobody said anything about them coming back.

  Rolling the tape forward, from about 1.8 million to 200,000 years ago, we see in the fossil record the first signs of behavior that seems distinctively human, as opposed to just hominin. The earliest known remains attributable to Homo sapiens are almost 200,000 years old and come from Ethiopia.55 At about the same time, cave sites in South Africa show signs of new things, such as shells pierced to make ornaments, the use of a natural pigment called ocher in decoration, and the extensive exploitation of seafood.56 When Homo sapiens evolved, the first thing it did was head for the beach.

  The story goes that once modern humans evolved in Africa, they spread throughout the world, displacing any other hominins they might have come across, most notably Neanderthals in Europe. This “out-of-Africa” tale was vigorously countered by another view, called “multiregional continuity,” that Homo sapiens evolved several times, independently, from various forms scattered throughout the world, including Neanderthals. At this point I shall point you to figures 5 and 6 in chapter 1 and ask you whether you think, on the basis of the fossil evidence, either view stands up.

  The out-of-Africa idea received a huge boost in 1987 with a study on human evolution that broke new ground by not being based on sparse, fragmentary fossil evidence, but on comparisons between people alive today. Our inheritance is encoded in the genetic material, DNA, almost all of which is found in the nucleus of each cell. But cells also contain other bodies, called mitochondria, which have DNA. It so happens that this mitochondrial DNA (or mtDNA) is passed strictly down the female line. Writing in Nature, the late Allan C. Wilson and his colleagues described how they analyzed the mtDNA from 147 people of diverse origins and used the pattern of similarities and differences between the samples to sketch a kind of evolutionary genealogy of humanity.57 The results showed that the greatest diversity of mtDNA was to be found in Africa, and that mtDNA from everywhere else seemed to have been an offshoot of an ancient African form. This idea gave credence to the view that modern humans evolved in Africa and spread throughout the rest of the world. Calculations of the rate at which mtDNA would acquire new variations suggested that humans left Africa roughly 200,000 years ago. Given that mtDNA is passed down exclusively from mothers to daughters, the authors wrote that “[a]ll these mitochondrial DNAs stem from one woman who is postulated to have lived about 200,000 years ago, probably in Africa.” The significance was not lost on the author of an accompanying commentary in Nature entitled “Out of the Garden of Eden,” which described the Wilson paper as reporting that “Eve was alive, well, and living in Africa around 200,000 years ago.”58 With Genesis in your PR department, you can hardly go wrong. It’s perhaps fortunate that subsequent work has largely borne out the idea that modern humans originated in Africa.