The Accidental Species: Misunderstandings of Human Evolution Read Online Free PDF Page B

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