Chapter Thirteen
In which Darwin considers classification and shows how his theory can be used to organise the living world along evolutionary lines
As the past is the key to the present, so is the infant the key to the adult. Darwin realised that animals adopt the form which is adapted to their own evolved way of life as they develop. As a result, the deep relatedness between organisms might more readily be seen by comparing embryonic forms.
Darwin spent eight years working on barnacles, then an entirely obscure group but known to be related to crabs, insects and other jointed-limbed creatures. Adult barnacles vary in form from the familiar seashore kind to a sinister version that is an internal parasite of crabs and resembles a giant fungus. Whatever the divergence among the adults, though, the embryos are remarkably similar. They also resemble, to a lesser extent, the embryos of lobsters and crabs, and other jointed-legged creatures of the seas. All those in turn show affinities to the embryos of insects, a hint that butterflies have relatives on wave-battered shores, while that great group is in turn united in its earliest development with tapeworms and their relatives – creatures entirely different in their adult form. This shared identity, lost as the animals grow, shows how the embryo can reveal deep patterns of descent with modification, hidden as development goes on.
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Darwin’s own classification of life – of groups within groups as evidence of a shared hierarchy of descent that encompasses not just barnacles and butterflies but birds and bananas – turned only on what he could see with the naked eye, or down the microscope. Now genes – the units of evolution – have come to the rescue, and they reveal whole kingdoms of existence quite unknown only a few decades ago.
The new tree of life is a direct descendant of the single sketch of shared descent that appears in The Origin. It is based on comparing the billions of DNA sequences now available from across the whole of existence. The affinity of whales with giraffes is a minor surprise when compared with discovery of the relatively close affinity of all animals to mushrooms, and the emergence of a whole new kingdom of single-celled creatures, the Archaea, which have a structure and a way of life entirely different from the bacteria that they superficially resemble. They may even have cooperated with them, in the early days of life, to provide the nucleated cells that build all plants and animals of today.
Read more: On the Origin of Species, Revisited