The Accidental Species: Misunderstandings of Human Evolution

by Henry Gee

  After all, what choice was there? It seemed far simpler to admit a new member to our own select genus, no matter how weird the entrant, than to defy everything we thought we knew we knew about the human story: to open the floodgates of the unknown unknown.

  Conventional wisdom suggests that the entire tale of human evolution had taken place exclusively in Africa until around 1.8 million years ago, when Homo erectus became the first hominin to leave that continent and colonize much of the rest of the Old World. The fossils from Georgia might have represented this wave of emigration. There is no compelling evidence that earlier hominins, such as Australopithecus or the earliest members of our own genus such as Homo habilis (somewhat more like Australopithecus than Homo erectus in many ways) had ever left Africa.23 Homo floresiensis just might be that first piece of evidence.

  Perhaps some hominin left Africa long before Homo erectus had evolved, migrating across the Old World, evolving into all sorts of diverse and unimagined forms, the only trace of such an adventure being a single, late-surviving relic marooned on remote and distant Flores.

  When the researchers unearthed Homo floresiensis from its long home, they opened the door to things we not only didn’t know, but didn’t even suspect, so wedded were we to the canonical out-of-Africa picture: not just to a remarkable, almost unbelievable testament to the power of evolution to shape living matter into unexpected shapes; but to a hitherto unknown and unsuspected chapter in human evolution, a vista far greater and more varied than anyone had dreamed possible.

  I have chosen to highlight the case of Homo floresiensis as it’s the best example I can think of, from my own experience, of a new discovery that challenges our expectations, our restricted notions of evolution based on human exceptionalism, and with it an idea of progressive improvement.

  The tale of the Hobbit is the book in microcosm. It shows that new discoveries often challenge deeply held notions of how we think evolution really ought to have happened, such that we humans are the culmination of some cosmic striving for order and perfection. It also shows us that stories we tell based on fossils are often easily bruised by the sheer scale of our ignorance. Fossilization is rare—so rare that there could well have been an entire episode of human evolution, a pre-Homo exodus from Africa, that has left no trace in the geological record other than the Hobbit.

  If there is one lesson that science holds for us, it is this—that our special estate, based either on a progressive scheme of evolution leading to its inevitable human culmination, or on a narrative reading of prehistory, is never justified. It was Charles Darwin himself who put it best. Right at the end of The Origin of Species, he presented the idea of the “tangled bank,” his vision of evolution in action: “It is interesting to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent upon each other in so complex a manner, have all been produced by laws acting around us.” From this, it’s evident that Darwin saw evolution not as progressive or improving, but as an activity that happens in the continuous present, as creatures interact with one another, moment by moment. From this it is clear that evolution has no plan. It has neither memory nor foresight. No vestige of cosmic strivings from some remote beginning; no prospect of revelatory culmination in some transcendent end.

  Rather than being at the pinnacle of creation, human beings are just one species on the tangled bank of Darwin’s imagination. Human beings are special in many ways—of course we are—but so is each and every other species, from the insects flitting above the bank to the birds perching on the branches to the worms struggling through the damp earth beneath.

  The idea of progress is, however, deeply pervasive. Our culture is drenched in it: our politics, our economics, and our science, including (and perhaps especially) evolution. It is always assumed that things advance unerringly upward as if motivated by some inherent force. Progress is unstoppable. What’s more, we are told that we need it, that we are reliant on it, and that the stagnation or reversal of progress is a Bad Thing.

  Progress is destiny. The only way is up.

  To be sure, if you look back from the viewpoint of the present day to any period in the past, progress seems natural and inevitable. But such a perspective is limited, because it denies that any other course might have been possible, and edits out any promising side branches that went nowhere.

  An important concept to take away from this discussion is that of loss. Stories of progress are written by history’s victors—or at least its survivors. Such tales tend to talk of increasing complexity and sophistication, and ignore the perhaps different perspectives of creatures that have become extinct.

  The concept of loss is vital to a proper understanding of evolution. This is especially so for human evolution, a subject that is often deficient in perspective: understandably so, because we, the story tellers, are only human. The history of life told by other organisms might have different priorities. Giraffe scientists would no doubt write of evolutionary progress in terms of lengthening necks, rather than larger brains or toolmaking skill. So much for human superiority. If that’s not ignominy enough, bacterial scientists would no doubt ignore humans completely except as convenient habitats, the passive scenery against which the bacterial drama is cast. Now, ask yourself—which of these stories is any more valid than any other, at least as a narrative?

  The late Stephen Jay Gould punctured the idea of inevitable progression in his book Wonderful Life, by introducing the concept of “contingency.” That is, creatures need to be more than fitted to their lives and lifestyles by evolution: they also need a generous dollop of luck. Once luck has been stirred in, the whole idea of progress driven by some innate striving, or superiority, or destiny, becomes nonsense.

  Gould started Wonderful Life by showing how our idea of human evolution as a matter of inevitable progress is so deeply ingrained in our culture that admen use it as a way to sell products. Admen use the metaphor of human evolution so frequently that it’s become a cliché. You’ll no doubt have seen a progression of apelike beings, walking from left to right, each one following the next, each more upright and humanlike than the last. Figure 1 is my own modest contribution to the canon.

  Figure 1

  Admen complete this familiar parade with the latest computer or washing machine. The subtext is that the consumer product we’re being urged to buy is the result of successive improvements in a kind of mechanical evolution, each better than the one before. Some commercials even exploit popular notions of evolution explicitly. The TV commercial that presents evolution as a device to produce a creature sufficiently evolved to appreciate Guinness beer was especially memorable. My favorite variation on this theme concerns a car. “It’s Evolved” purrs the voice-over.24

  The idea of human evolution as a tale of inevitable progress is, however, a travesty, and has nothing much to do with Darwin. The bastardized view of evolution that’s become so much a part of the general consciousness—so much so that it’s so much low-hanging fruit for admen—owes much to Ernst Haeckel, Darwin’s number one fan in Germany.

  Haeckel took Darwinian natural selection and bolted on to it older ideas of progress popular among nineteenth-century German thinkers. Take a series of forms, each one more advanced than the last—according to whatever criterion you desire, be it larger brains, longer necks, more prominent plumage, whatever—and simply draw arrows between them, representing some innate striving toward cosmic perfection.

  And that’s evolution—or, at least, evolution as most people think of it—a kind of cartoon, infused more by our prejudices, desires, and innate self-regard than any actual evidence. Figure 2 is my version—my parade of likely characters, linked by arrows, pointing in the direction of progress.

  Figure 2

  The arrows represent natural selection, or evolution, as
essentially and inherently an agency of inevitable progression, with—perhaps—the aim of producing, in its final form, the perfection that is Man (with a capital M).

  Yes, to be sure, I’m having a lot of fun at everyone’s expense, but how can this cartoon be in any way wrong as a picture of human ancestry, at least in a general sense?

  No sensible, informed person would doubt that we all had ancestors, and the further back you look in time, the more apelike they’d have looked, right?

  This cartoon picture of human evolution doesn’t really represent our actual ancestors, but metaphors, right?

  Well, yes, up to a point. That we all had ancestors is true—emphatically so. That our ancestors would have been more apelike the further back in time we look is also highly likely, given that our closest living relative in the animal kingdom is the chimpanzee, and it remains reasonable to suggest that the chimpanzee has evolved less far from our common ancestral state than we have.25

  But if the figures in this parade are metaphors, what do these metaphors represent? The figures, surely, represent idealized evolutionary stages, between ancient ape and modern human, rather than specific individuals or even species. That’s fair enough.

  But my argument is less with the figures themselves than the arrows between them, arrows that seem to represent inevitability and progress, of evolution leading, inexorably, through one lineage and one lineage alone, to its culmination, the latest model human (or washing machine or car), more refined, more sophisticated, and crucially, more perfect than the one before.

  Another problem pointed out by Darwin in the Origin was what he called the “imperfection” of the fossil record. The record of life preserved as fossils is immediate evidence for evolution having happened. It is, however, rarely good enough for us to be able to trace the evolution of one particular species from another with any confidence. It is important to remember that fossils, on their own, are remnants of creatures toiling on some tangled bank of the past. They do not, of themselves, represent coherent statements about evolutionary history—still less, evolutionary progress. If a fossil is a statement, it is not a sentence, such as

  because fossils are not buried with their pedigrees, nor prognostications on the future of their progeny, if any. No, fossils are not statements. Nor are they phrases, or words, but exclamations, from which we, the finders, are invited to make what we can.

  Piecing together the tale of evolution from fragmentary fossils is a hard business. Because fossils are so rare, and because an unknowably large proportion of the history of any lineage will have been erased, what fossil hunters can never do with confidence is look at a fossil and assert that it is the actual ancestor of any creature now living (or of any other fossil). To be sure, the fossil might be such an ancestor—because, after all, we do have ancestors—but the chances of this in any particular case are unknowable, and in any case vanishingly small. There is, happily, a way out of this apparently blind alley, and that is the fact of evolution.

  Evolution is demonstrated by the existence of fossils and the community of all life. By this, I mean that the chemistry that animates you is virtually identical to that which animates every other living creature. Because of this, there is very good reason to suspect that all life shares a single common ancestor.26 This is more than a supposition—the notion of a single common ancestry has been tested, formally and rigorously, and has been found to support the pattern of extant life far better than any model positing independent origins.27 It follows, therefore, that any fossil we find will be a cousin, in some degree, of any other creature, living or extinct, discovered or undiscovered—even if we can never show that anyone was anyone else’s ancestor.

  In short, this approach to reconstructing the story of evolution as a matter of degrees of relationship, which can be inferred and tested, is far superior as a scientific approach to evolution than suppositions about of ancestors, descendants, and “missing links”—which can be inferred but never falsified. My task here, though, is to show how the sparseness of the fossil record is sufficient to mislead us, were we bent on thinking of evolution as an onward march of progress and improvement.

  Let’s try this thought experiment. Imagine that by some divine grace (because you’d get it no other way) you were granted knowledge of the complete history of every individual creature that ever lived—its offspring, its ancestors and descendants—and could draw the true tree of life, that is, what actually happened. Just to make things simpler (and more relevant to our current concerns), you restrict yourself to drawing the true tree of hominin evolution, back to our common ancestor with chimpanzees. I’ve drawn what this might look like in figure 3.

  Time moves from left to right—the left is long ago, the right is more recent. This, the true tree, is very bushy, as you can see, and most of the branches lead nowhere—that is, to extinction. At first glance it’s impossible to select any one branch as especially important.

  But because of your divinely granted complete knowledge, you’d be able to pick out the line that leads, uniquely, to modern humans. Figure 4 shows the “true tree” with the ancestry of modern humans indicated by a thick line.

  Figure 3

  Figure 4

  It’s important to realize that in reality you could have no absolutely certain knowledge of the true line of human evolution—what really happened so surely that you’d know you knew it—unless you had a record of every single hominin that ever existed, and full details of their ancestry back to our last common ancestor with chimpanzees.

  Back in the real world, we are left with what few scraps time and chance have left us, and that’s very few indeed. Primatologist Robert D. Martin estimated that we knew perhaps as much as seven out of every hundred primate species that have ever existed, given a few assumptions about the known diversity of fossil primates, and the number of primate species currently living. Martin made his estimate twenty years ago.28 Given that the amount of ignorance expands with the gain of knowledge (that the more scientists discover, the more they know that they don’t know),29 that proportion might well have decreased, even though paleontologists have discovered quite a few extinct primate species since then.

  In figure 5 I show what remains of the “true tree” once the majority of its branches and twigs have been pruned, leaving only a few fossils. The deletions are, I confess, not totally random. I have been particularly careful to erase any branching points, as fossils will never come complete with that information.

  From this scatter, one could arrange many different sequences of fossils from older to younger, and suggest that this sequence might represent a probable evolutionary sequence between apelike ancestor and modern humanity. However, given the evidence at hand, you could link up more or less any sequence of fossils from this scatter and make other assertions of equal validity—without divine grace, who would know which was more likely to be correct?

  Figure 5

  In figure 6 I show three of the very large number of possible trajectories between ancestor (left) and human (right). All use the same scatter of fossils, but the trajectories are different from one another.

  The first is closest to the “truth,” but we could never “know” this. The three plots would, I think, have something in common and that’s this: the fossils used as links in the chain would, when arranged in the order the arranger assumes to be correct, show a progressive increase in those features assumed (retrospectively) to be characteristic of humans—more erect posture, larger brain, and so on. At no point would there be a reversal, such that a descendant would be more stooped than an ancestor, or have a smaller brain. Not that such things are not possible—the case of the Hobbit shows that they are—but because the assumption of progress is so ingrained that it would not occur to most people that there might be any other course besides onward and upward.

  Perhaps the most important thing to take away from this chapter is that new discoveries challenge our idea of progress—that matters are subject to a continual improv
ement, the refinement of each stage building on that of the one before in seamless progression. What the conceit of progress tends to ignore is the idea of loss—that many experiments in life were made that subsequently went extinct, and so are left out of the canonical tale of improvement.

  More than this, the idea of progress tends to be based on criteria that we decide after the fact according to our prejudices, and which need not be the most important or relevant ones. Because we seem to have larger brains and a more erect gait than earlier essays in the human condition, we always assume that the evolution of humanity is a story that must be told in terms of progressive increases in brain size and stature. This reasoning, however, is circular. We have larger brains than our presumed ancestors, so evolution must be couched in terms of brain size, so the discovery of creatures living in the past that had smaller brains will naturally confirm our prejudices. For all we know, our picture of human evolution might be better told in terms of, say, changes in the number of kinds of bacteria that live in our small intestines. After all, your body probably contains around ten times as many bacterial cells as human ones.30

  Figure 6

  To really get a grip on why evolutionary arguments about human exceptionalism are wrong, you need to have a good understanding of what evolution is—and what it is not.

  The next chapters offer a handy cut-out-and-keep guide to evolution by natural selection. You might be surprised to learn that evolution by natural selection is far less—and far more—than you thought it was. After that I’ll discuss the concept of loss in more detail, showing how the stories on which we base our fragile suppositions about human exceptionalism are based on very little evidence at all.