The fossils say yes: the discovery of transitional forms has filled in some of the most talked-about gaps in the fossil record

It has been asserted over and over again, by writers who believe in the immutability of species, that geology yields no linking forms. This assertion ... is certainly erroneous.... What geological research has not revealed, is the former existence of infinitely numerous gradations ... connecting together nearly all existing and extinct species.

--Charles Darwin, the Origin of Species

When Darwin first proposed the idea of evolution by natural selection in 1859, the fossil record offered little support for his ideas. Darwin even devoted two entire chapters of the Origin of Species to the imperfection of the geologic record, because he was well aware it was one of the weakest links in his arguments. Then, just two years after his book was published, the first specimen of Archaeopteryx was discovered, hailed by many as the "missing link" between birds and reptiles. By the late nineteenth century, fossils helped demonstrate how the modern thoroughbred horse evolved from a dog-size, three-toed creature with low-crowned teeth. (The understanding of those fossils has since been much refined.)

Fossil evidence supporting evolution has continued to mount, particularly in the past few decades. DNA analysis, moreover, has helped make sense of how the evidence fits together in the family tree of life on Earth. Unfortunately, many people still think, quite erroneously, that the fossil record shows no "transitional forms." In large part, that misconception is the product of the campaign of misinformation--or disinformation--spread by the creationist movement.

The fossil record is far from perfect, of course. By most estimates, less than 1 percent of all the species that have ever lived are preserved as fossils. The reason for the scarcity is simply that the physical conditions needed to turn a dead organism into a fossil lasting millions of years are unusual.

Nevertheless, there are numerous excellent specimens that reflect transitional stages between major groups of organisms. Many more fossils exhibit how "infinitely numerous gradations" connect the species. The one caveat is that when a sequence of fossils appears to follow a direct line of descent, the chances are slim that they actually bear such precise interrelations. Paleontologists recognize that when one fossil looks ancestral to another, the first fossil is more safely described as being closely related to the actual ancestor.

The classic story of the evolution of the horse is a good example. The various known fossils were once arranged--simplistically, it turns out--into a single lineage leading from "Eohippus" to Equus. When more fossils became available, paleontologists revised that simple lineage. The fossils now give a branching and very bushy picture of equine evolution, with numerous now-extinct lineages living side by side. One quarry in Nebraska has yielded a dozen distinct species of fossil horses, in rock about 12 million years old. The earliest horses, such as Protorohippus (from early in the Eocene epoch, about 53 million years ago), are virtually indistinguishable from Homogalax, the earliest member of the lineage, which also gave rise to tapirs and rhinoceroses. Very early in my career, when I was taking an undergraduate paleontology class, I discovered just how tough it is to sort out those two ancient genera.

Perhaps the most remarkable recent discoveries are the numerous fossils that connect whales with their four-legged terrestrial ancestors. If you look at dolphins, orcas, and blue whales, all fully aquatic animals, you would have a hard time imagining them walking on land. Yet even living whales retain vestiges of their hips and thighbones, deeply buried in the muscles along their spines. Paleontologists have known for a long time, on the basis of detailed features of the skull and teeth, that whales are closely related to hoofed mammals. But creationists long touted the absence of transitional fossils for whales as evidence against evolution.

The balance has now changed. In 1983 specimens of Pakicetus were discovered Pakistan in early Eocene beds about 52 million years old. Although the body of Pakicetus was primarily terrestrial, it had the skull and teeth of the ancient archaeocetes, the earliest family of whales--which swam the world's oceans in the Middle Eocene epoch, about 50 million years ago.

Then, in 1994, Ambulocetus natans (literally, the "walking whale that swims") was discovered, also in Pakistan [see illustration below]. The animal was the size of a large sea lion, with broad webbed feet on both fore--and hind limbs, so it could both walk and swim. Yet it still had tiny hooves on its toes and the primitive skull and teeth of the archaeocete. Ambulocetus apparently swam much like an otter, with an up-and-down motion of the spine, the precursor to the motion of the flukes of a whale's tail. In 1995 yet a third transitional creature was discovered, Dalanistes, with shorter legs than Ambulocetus, webbed feet, a longer tail, and a much larger and more whalelike skull.