The Accidental Species: Misunderstandings of Human Evolution by Henry Gee Page B

the change in form of entire species, usually (though not necessarily) from simpler forms to more complex ones.
    The two processes—evolution and transformation—were analogous, but distinct. Today, though, they have become conflated. When most people today talk of “evolution,” what they mean is “transformation.” This conflation has had the consequence of conferring a sense of direction and choreography onto the idea of Darwinian evolution. This is why people, when they think of “evolution,” imagine (for example) a series of individuals, each one an improvement on the one before, and if there are gaps in the series, they are “missing links”—pieces in a metaphorical chain whose beginning, end, and intermediate progress are already known.
    There are deeper roots to this conflation, however, but before I get to that, I must tempt you into a little digression about the nature of and evidence for Darwin’s descent with modification.
    Earlier I mentioned that the “community of descent” provides much evidence for descent with modification. By this I mean that all forms of life are organized in fundamentally the same way, down to the minutest detail, supporting the view that all life shares a common heritage.It’s worth considering this in a little more detail. As far as we know, all organisms owe their structure to the peculiar chemistry of the element carbon. Carbon atoms readily bind with one another and with atoms of other elements (notably oxygen, hydrogen, nitrogen, phosphorus, and sulfur) to produce highly elaborate molecules, sometimes disposed in long chains of smaller, similar units strung together. So it is that all organisms so far discovered carry genetic information in the form of long, carbon-based, chain-like molecules called nucleic acids, either DNA (deoxyribose nucleic acid) or the related form RNA (ribose nucleic acid). This information specifies the structure of a different set of chain-like, carbon-based molecules called proteins, and does so using a code that’s the same (albeit with minor variations), irrespective of the organism concerned. All organisms more complicated than viruses have cells, bounded by a membrane constructed of two layers of carbon-based, chain-like molecules called lipids, sometimes bound in an extracellular matrix made of chain-like carbon-based molecules such as cellulose or collagen. The contents of the cells are pretty much the same, irrespective of the organism in which they occur.
    The similarities between creatures at this most detailed level are so great that it’s a wonder that organisms as a whole come to look so different—from the worms burrowing beneath Darwin’s tangled bank to the birds and insects flitting above it. This underlying sameness is such compelling evidence for descent with modification that it would, according to Richard Dawkins (in his book
The Greatest Show on Earth
), stand alone, even had no fossils ever been discovered.
    Why is the evidence so strong? Because life needn’t have been arranged like that. It is possible to imagine systems that have some of the properties of life that use only some of the above features, or none. It is also possible to imagine a situation in which different living organisms sharing the same planet have fundamentally different constitutions. The fact that all life, no matter how various in form, is specified so minutely according to the same recipe suggests that all living creatures descend, ultimately, from a creature that had all these same fundamental features of inheritance and construction.
    So much for descent: what of modification? Darwin supposed that the pattern of inheritance might vary, the offspring of parents becoming sorted by natural selection, so that the offspring would come to look different from their parents. These differences would accumulate, and the offspring would spread and diversify. As with offspring andparents, so, eventually, with new species arising from existing ones.