The Accidental Species: Misunderstandings of Human Evolution

by Henry Gee

  The picture has, perhaps inevitably, become more complicated. More recent studies on nuclear DNA, including DNA recovered—remarkably—from Neanderthals, shows that modern humans didn’t completely replace earlier species of hominin. If you are of European descent, then around 4 percent of your genes came from Neanderthals.59 If you’re from New Guinea, you might boast an even more remarkable heritage, for some of your genes come from the still-obscure Denisovans.60 In his book The Origin of Our Species, Chris Stringer suggests that some archaic hominins might have survived even in Africa until a few tens of thousands of years ago. This idea is supported by genetic work showing traces of interbreeding with ancient hominins as yet unknown in some modern African populations.61 Everywhere you look, we all bear some genetic traces of hominins past.

  In any case, it must be remembered that “mitochondrial Eve” was not the only woman around at the time, only the one whose mitochondrial DNA appears to have survived until the present day. It is, perhaps, no more than luck that it was she, rather than any other female, living earlier or later, who turned out to have been the ancestor of all the mtDNA found (so far) in modern humans.62

  The tale of out-of-Africa makes a piquant contrast with the story of Piltdown. With Piltdown, a fossil that accommodated prevailing prejudices about the course of human evolution was found to be a fake. When more (indeed, when any) evidence surfaced, it was shown that hominins walked upright before they got bigger brains. This remained true despite the squabbles about which of the fossils should be called Homo and which Australopithecus, and indeed over which one was related to whom.

  Out-of-Africa based its imagery on biblical narrative, and turned out to have been correct. This needn’t have been the case. The Wilson study was based on rather few samples, and, as it happened, the African sample came from African Americans, some of whose ancestries might not have been purely African. One of the functions of science is to test new results and, if possible, extend them with new data. It so happened that further work tended to support rather than refute Wilson’s conclusions. Importantly, we should not take that outcome as a given. In which case, the invocation of Eve and the Garden of Eden was brave indeed, and could be interpreted as grandstanding ahead of the evidence.

  The Piltdown committee looked at the evidence and asserted that it supported their particular view of how human evolution ought to have been. That they had been fooled by a forgery shows only how strong such received notions can be, and how hard they are to shake. The Wilson paper came out in support of the out-of-Africa view. Although it received pretty much immediate, universal approbation, the same might be said of Piltdown. Science is not a democracy: public acclaim is itself no guarantee that any view is the correct one. The co-option of the biblical narrative was in its way as prejudicial as the Piltdown committee’s view that brains came before bipedality. In the end it all comes down to chance. The Piltdown committee was (completely) wrong; the out-of-Africa view was (mostly) right. But as I showed in chapter 1, the “true” story of human evolution has no obligation to cleave to any story we might imagine, no matter how informed our guesswork.

  And now, backward.

  For a long while, the most ancient known hominin was Australopithecus africanus, reckoned to have lived from about 3 million years ago. However, it was clear that the hominin line was much more ancient than told by the fossils. First, australopiths share more traits with humans than they do with apes: there was still room for hominins more primitive than australopiths, between A. africanus and the latest common ancestor of chimps and humans. Second, estimates of evolutionary rate based on fossils suggested that the human lineage split from that of chimps 10–15 million years ago. This estimate was shortened dramatically when it became possible to estimate evolutionary rate directly from DNA differences between modern humans and chimps. The consensus now is that humans and chimps diverged between 5 and 7 million years ago.

  Before that was an uncomfortable gap. Between around 5 and 10 million years ago, the fossil record was until recently almost completely blank. This was particularly frustrating, as it is in this interval that the lineage leading to hominins is thought to have diverged from that leading to chimps. Yet the fossil evidence for this most epochal, most defining event in human evolution, in which many generations of the earliest chimps and hominins lived, died, and eventually went their separate evolutionary ways, might be fitted into one rather small box.

  In the 1970s the focus of fossil exploration moved northward from Kenya to Ethiopia, where rocks of the right age came to light.63 Deposits exposed by the evolving Awash and Omo rivers, and in the Afar depression, provide a rich history of the Rift Valley older than Olduvai Gorge, the Turkana Basin, and the caves of South Africa. After much work, hominins began to turn up, the most famous being a partial skeleton of a diminutive female hominin known to her discoverers as “Lucy” and officially described as Australopithecus afarensis (southern ape from the Afar region).64 Lucy was around 3.6 million years old. She was clearly more primitive than other hominins so far known, but she was definitely a biped. There was as yet space for even older hominins to be found.

  An even older and more primitive creature, Australopithecus (now Ardipithecus) ramidus was discovered in the early 1990s, again in Ethiopia.65 The remains were fragmentary—mostly scraps of teeth and jaws—but there was just enough to suggest that the fossil record of hominins stretched back 4.4 million years. The fragments were accompanied by a skeleton, so fragile and crushed that the fossils took more than a decade to be prepared from the concrete-like rock matrix.66 Ardi was like Lucy, only more so: very likely a biped, but small, primitive, and somewhat apelike.

  Ardipithecus ramidus is now joined by a number of other very early east African hominins known from fragmentary remains—Australopithecus anamensis, Ardipithecus kadabba, and Orrorin tugenensis extend the hominin record back beyond the 5-million-year mark.67

  The further one travels back in time, the more fragmentary the fossils become—and the harder it is to distinguish them as hominin. This is only to be expected when one traces an evolutionary lineage backward. First one distinctive human trait disappears, then another, until one is left with a blank canvas of an ancestor on which one might paint any picture one likes. This exercise reminds me of one of my favorite playground jokes, which, like all such things, is more profound than it seems at first.

  Q: Why is an elephant large, gray, and wrinkled?

  A: Because if it were small, round, and white it would be a Ping-Pong ball.

  To complicate matters, the very earliest hominins did not exist for our retrospective convenience. They, like every other creature, played their part on the tangled bank of Darwin’s imagination, living alongside and competing with many other species. If the latest common ancestor of chimps and hominins had no features pointing to either one or the other in particular, it would certainly have had traits all its own that were lost in both subsequent lineages.68 Even if you held in your hands the skull of the latest common ancestor of apes and humans, you could never know that you had done so.

  In 2001 I had just such an experience. When I heard that Michel Brunet and his colleagues from the University of Poitiers in France had discovered something interesting, I invited myself to Poitiers to see the evidence in person. A few days later, Brunet put a fossil in my hands and left me alone in a room with it to contemplate the long evolution of humanity.

  The fossil was a complete skull excavated from what was once a lush lakeshore, but now a blasted desert in the central African country of Chad. About the size and weight of a house brick, the skull was somewhat crushed, but it was far more complete than most hominin fossils of any age. The age of this skull was uncertain, but based on the many fossils of archaic mammals accompanying the find, it was believed to be somewhere between 6 and 7 million years old—right around the time when the human and chimpanzee lineages are thought to have diverged. Brunet and colleagues later described the skull in Nature as Sahelanthropus tchadensis (Sa
hel Man from Chad),69 asserting its hominin status based on details of the dental anatomy and the position of the foramen magnum—the hole in the base of the skull that admits the backbone and spinal column. A foramen magnum at the back of the skull suggests that the owner was a quadruped, like a chimp. A foramen magnum set well beneath the skull, in contrast, suggests that the owner walked upright, like a hominin. The situation in Sahelanthropus seemed to suggest at least a tendency toward bipedality.70

  Criticism of Sahelanthropus was swift,71 and very reminiscent of the early comments aimed at Dart after his publication of Australopithecus africanus. Sahelanthropus couldn’t be a hominin, but an ape, more specifically a female gorilla. Further work has gone on to show that Sahelanthropus, while apelike in many ways, is quite distinct. Of course, trying to work out whether a creature so close to the common ancestry of chimps and humans is more closely related to humans or to chimps will be practically impossible, as any distinctive features it has might be purely idiosyncratic, and have nothing to say about later evolutionary history. It could even be that Sahelanthropus branched off the evolutionary tree before the ancestors of humans and chimps became distinct. If Sahelanthropus was a biped, it could be that the common ancestor of humans and chimps was a biped, and that chimps later lost this facility.

  If Sahelanthropus was destined to become an elephant, it made a very fine Ping-Pong ball.

  To hold a fossil such as Sahelanthropus in one’s hands is not to experience Schliemann’s joy, when excavating a Mycenaean death mask, of coming face to face with Agamemnon. One does, however, find oneself, looking into those blank and squinting eye sockets, and asking questions of the boundaries of knowledge. What are we to make of this skull, almost the only evidence for the existence of a hominin (if that’s what it is) to have been unearthed from the otherwise yawning void that stretches between around 10 million years ago (when the world teemed with fossil apes) and around 5 million years ago, when Orrorin and Ardipithecus start to appear? As the nineteenth-century Scottish preacher and geologist Hugh Miller wrote in his book The Old Red Sandstone (1841), the questions we would most like to ask fossils might forever remain unanswered: “We cannot catechise our stony ichthyolites, as did the necromantic lady of the Arabian Nights the coloured fishes of the lake, which had once been a city, when she touched their dead bodies with her wand, and they straightway raised their heads and replied to her queries. We would have many a question to ask them if we could—questions never to be solved.” Instead, one ends up talking only to oneself, mostly about how little we know about anything, and of how vast is the ocean of ignorance in which we flounder.

  It says something about the hominin fossil record that the discoveries of Orrorin, Sahelanthropus, and Ardipithecus have all been made very recently, in the past twenty years, and thanks to titanic efforts from scientists from many different countries, not least from the countries in Africa where the fossils are found.

  Given the extent of our ignorance, and how it increases with every passing day, one can only be surprised—retrospectively, of course—about how loudly and certainly paleoanthropologists of the past asserted that the course of human evolution went this way or that, when the evidence was so sparse that practically any course of evolution might have been possible. In which case it is no surprise that they were often completely wrong, and if they were right, it was as much by luck as by judgment.

  6: The Human Error

  We human beings have at least two remarkable abilities.

  One of them is the ability to recognize patterns. This facility is, in fact, the basis of all science: pattern recognition is the foundation of all knowledge and understanding, for without some ability to compare and contrast the properties of objects, no order can ever be discerned. Whether mystical nature philosophers or hard-headed Darwinians, all biologists recognize that within the apparent riot of biological diversity one can see a clear pattern, that of a tree, and from that, one can begin to ask how such a pattern might be generated.

  Pattern recognition is likewise the basis of paleontology. The best fossil hunter in the Gee family is Gee Minima, who from an early age has been able to pick out fossil sea urchins from the jumble of rocks and detritus on Cromer beach. More than any other family member, she can see the distinctive double rows of dots that signal the positions of the tube feet of an animal that lived here more than 70 million years ago. Now she applies her keen eye for design and detail to the arts, fashion, and textiles, and can pick out details of shape and line that her sartorially challenged father plainly misses.

  It is easy to see why pattern recognition is such an asset. Without some ability to categorize objects, the complex and crowded world in which we live would indeed be a dangerous place. In earlier times, an ability to recognize and quickly infer the nature of an approaching object, without taking time to explore it first, might have been a lifesaver. Those early hominins unable to tell the difference between a dead branch and a black mamba, or who misinterpreted the growl of an approaching leopard as the purr of a cuddly kitten, would stand less chance of passing on their genes to the next generation than those who saw the patterns, sorted these objects into the right categories, made the right choices.

  And so we use that ancient circuitry today, and every day, when trying to make sense of our world. In his book Us and Them (which is probably the best anthropology book I have ever read, and if you haven’t read it, I implore you to do so), David Berreby notes how this ability prompts us to leap to snap judgments about our fellow humans that on closer inspection they might not deserve. That rowdy crowd of tattooed and pierced bikers hanging around in your favorite restaurant? Instinct and experience—if only perhaps vicarious—might make you turn on your heel and walk out for fear of being mugged. The stories you’ve heard . . . Your instincts might well be right, and could save your life. Except that further investigation might have revealed that these particular bikers are all college graduates devoted to their mothers, have congregated to celebrate their charity bike ride to raise money for a sanctuary for abandoned puppies, and have chosen this restaurant because they’ve heard that the chef cooks up a crème brûlée that’s to die for.

  First impressions can save your life—or tell you lies.

  So, while we are very good at recognizing objects, our talent is so refined that we are inclined to see patterns where there aren’t any. Almost everyone who looks at the surface of the moon sees a human face, even though we know quite well that the features responsible for the illusion are in fact gigantic plains of ancient lava, and nothing to do with faces at all. We are perfectly aware of our tendency to make nonexistent connections, to spot nonexistent patterns. In such error lies much of importance and interest in our cultural heritage, in images of all kinds from classical trompe l’oeil to surrealism, in the comedies of errors in Shakespeare’s plays and Mozart’s operas to the cheapest farces, and in just about every joke you can think of. Here is an example (I have better ones, but they are too rude for a family audience).

  A: I say, I say, I say, how do you tell the difference between a postbox and the back end of a cow?

  B: I don’t know, how do you tell the difference between a postbox and the back end of a cow?

  A: Well, if you don’t know the answer to that, I won’t send you to post the letters.

  “Ba-boom,” and, moreover, “tish.”

  So, much as we might indulge children who see elephants and railway trains in passing clouds, not to mention scoff at people who see images of Jesus in pieces of toast, everyone is at it—even scientists. One immediately brings to mind the story of the Italian astronomer Schiaparelli, who unwittingly joined the barely visible dots of craters on distant Mars into “channels” or, in Italian, canali—an illusion (compounded by mistranslation) that led American astronomer Percival Lowell to posit the existence of a globe-spanning system of canals, moving quantities of water from the martian poles, dug by a technologically advanced civilization under threat of extinction by drought. Fro
m this very human error comes H. G. Wells’s stirring tale of Earth’s invasion by Mars in The War of the Worlds; Orson Welles’s notorious radio adaptation that had terrified crowds flocking into the streets, watching the sky; Edgar Rice Burroughs’s Barsoom fantasies; and much else. Even today, when we know perfectly well that John Carter of Mars is a character in a pulp fantasy, and that tripods squirting death rays are unlikely to be found in New Jersey, nor, as it may be, suburban Surrey, scientists with their intellects vast, overheated, and oversympathetic look at Mars with eyes perhaps overwelcoming of the possibility of life.