Andrew Knoll

Life on a Young Planet

We can explain the similarities between humans and chimps by descent from a common ancestor that possessed the various features the two groups share. Their differences have arisen since they diverged. This makes the paleontological prediction that the oldest fossils of humanlike primates should resemble the last common ancestor of chimps and humans more closely than modern people do; the features that make us distinctly human should appear only in younger fossils of our lineage. The fossil record of human ancestry is notoriously sketchy, but skeletal remains unearthed in Africa and Asia confirm this prediction. (Note that there is no expectation that successively older members of our lineage should close in on chimp morphology. Humans didn't descend from chimpanzees; humans and chimps both diverged from a common ancestor that was neither Homo nor Pan.)

Not all shared features are equally helpful in determining 'propinquity of descent' (another delightful Darwinism). For example, birds, bats, and the extinct pterosaurs all sport wings, but their wings have different skeletal structures, and many other features show that these air-borne animals are not closely related. Wings evolved independently in each group as an adaptation for flight; in the parlance of systematic biology, these features are convergent. Only features that are shared because of common ancestry (homologies, in evolution-speak) can be used to assess evolutionary relationships. In practice, we don't always know whether similar features are convergent or homologous and so rely on sophisticated computer algorithms to sort out large sets of comparative biological data.

It is relatively easy to see how morphological characteristics might be used to articulate a hypothesis of evolutionary relatedness, or phylogeny, for all primates, all mammals, or even all vertebrate animals. We can also grant that an expert, at least, could do the same for mollusks or arthropods. But how can we place mollusks, arthropods, and vertebrates within a greater evolutionary tree of all animals? And, much harder, how can we reconstruct the whole of Darwin's great Tree of Life, a phylogeny that encompasses all living things?