The Facts

What would constitute evidence for evolution in the fossil record? There are several types. First, the big evolutionary picture: a scan through the entire sequence of rock strata should show early life to be quite simple, with more complex species appearing only after some time. Moreover, the youngest fossils we find should be those that are most similar to living species.

We should also be able to see cases of evolutionary change within lineages: that is, one species of animal or plant changing into something different over time. Later species should have traits that make them look like the descendants of earlier ones. And since the history of life involves the splitting of species from common ancestors, we should be able to see this splitting—and find evidence of those ancestors—in the fossil record. For example, nineteenth-century anatomists predicted that, from their bodily similarities, mammals evolved from ancient reptiles. So we should be able to find fossils of reptiles that were becoming more mammal-like. Of course, because the fossil record is incomplete, we can't expect to document every transition between major forms of life. But we should at least find some.

When writing The Origin, Darwin bemoaned the sketchy fossil record. At that time we lacked transitional series of fossils or "missing links" between major forms that could document evolutionary change. Some groups, like whales, appeared suddenly in the record, without known ancestors. But Darwin still had some fossil evidence for evolution. This included the observation that ancient animals and plants were very different from living species, resembling modern species more and more as one moved up to more recently formed rocks. He also noted that fossils in adjacent layers were more similar to each other than to those found in layers more widely separated, implying a gradual and continuous process of divergence. What's more, at any given place, the fossils in the most recently deposited rocks tended to resemble the modern species living in that area, rather than the species living in other parts of the world. Fossil marsupials, for instance, were found in profusion only in Australia, and that's where most modern marsupials live. This suggested that modern species descended from the fossil ones. (Those fossil marsupials include some of the most bizarre mammals that ever lived, including a giant 10-foot kangaroo with a flat face, huge claws, and a single toe on each foot.)

What Darwin didn't have were enough fossils to show clear evidence of gradual changes within species, or of common ancestors. But since his time, paleontologists have turned up fossils galore, fulfilling all the predictions mentioned above. We can now show continuous changes within lineages of animals; we have lots of evidence for common ancestors and transitional forms (those missing ancestors of whales, for instance, have turned up); and we have dug deep enough to see the very beginnings of complex life.

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