Who knows what will be discovered next?

For over 120 years, the origin of whales vexed paleontologists. They were among the strangest of all mammals, creatures completely adapted to the sea with more in common with us than any fish (although at the beginning of the 19th century "common sense" said otherwise), and it was difficult to imagine how they evolved. If Charles Darwin was right and all life had evolved, different evolutionary paths diverging through time, then whales must have had some sort of traceable ancestry.

The discovery of fossil whales like Basilosaurus and Squalodon illustrated that the evolution of whales may have been preserved somewhere in the fossil record, yet they were fully aquatic forms that did little more than point the way towards a terrestrial origin. Seals and sea lions helped to fill the functional gap showing how a terrestrial mammal might have become adapted to the sea, but for decades evolutionists could do little more than speculate on the earliest origin of whales. In 1890, the Neo-Lamarckian paleontologist E.D. Cope was forced to remark, "The order Cetacea is one of those of whose origin we have no definite knowledge."

As the years passed a few other ancient fossil whales, like Protocetus, were discovered, but the creatures that represented the transition from land to water were still missing. During a reorganization of ancient carnivorous mammals in the 1960's the evolutionist Leigh Van Valen proposed that a group of hoofed carnivores called mesonychids made the closest terrestrial approach to whales, but even then there was a yawning gap in the series.

By 1976 things were getting unbearable. It had been over 100 years since T.H. Huxley had proposed that whales had evolved from land-dwelling carnivores in an address before the Geological Society of London, and paleontologists began to doubt whether the transitional whale fossils might ever be found. Jere Lipps and Edward Mitchell proposed (perhaps inspired by the recently announced concept of punctuated equilibira) that the transition from land to water happened so fast that no fossils had been preserved. It was either that or paleontologists weren't looking in the right places. It was the latter proposition that would turn out to be true.

In 1981 the first fragments of a truly transitional whale were announced. Dubbed Pakicetus, this "whale from Pakistan" possessed a feature of the skull that unites all whales. More complete skeletons of this genus proved elusive (finally being announced in 2001), but its discovery pinpointed a new place to search for fossil whales. The strata of Pakistan did not disappoint.

Within my lifetime (25 years) the study of the earliest whales, or archaeocetes, has exploded. Presently there is a wide array of transitional forms, and while they cannot be placed into a single linear series, each fossil form represents different levels of adaptation to life in the water. What's more, the archaeocetes exemplify the branching pattern of evolution; at the same time that some forms were small and lived at the water's edge others were almost entirely aquatic. What's more, the fossil record has confirmed the prediction from molecular evidence that whales evolved from artiodactyls, and studies of development have allowed researchers to begin to understand how whales became adapted to life at sea. Indeed, 150 years after On the Origin of Species was published, our current understanding of the earliest whales resoundingly confirms Darwin's theory.

Our understanding of whale evolution is not the only area of paleontology that has undergone drastic changes. The gap between lobe-finned fish like Eusthenopteron and early tetrapods like Acanthostega is being rapidly filled-in by new discoveries like Tiktaalik (and, once again, there is branching diversity as evidenced by Ventastega). Documentaries when I was young hinted that birds might be closely related to dinosaurs, but now we can confidently say that birds are dinosaurs. Even in terms of our own evolution, numerous discoveries made in the past thirty years have given us a more complete view of our diverse ancient relatives.

I'm sure none of this is particularly shocking, but it is important to keep in mind how much evolutionary paleontology has changed in recent years. As I discussed yesterday, there was a time when some paleontologists rejected evolution by natural selection (or confined it to a minor role) because it did not seem to match what was seen in the fossil record. As we approach the 150th anniversary of the publication of On the Origin of Species, however, things are quite different. Major transitions that long perplexed evolutionists are now evidenced by graded transitional forms of just the type Darwin's theory predicted.

I apologize if this post is old hat for those of you who are regular readers. Given the massive influx of visitors I have received from Digg, I wanted to make clear that much has changed in the 99 years since H.F. Osborn pronounced his preference for orthogenesis in Fifty Years of Darwinism. There are so many fascinating discoveries it is nearly impossible to recount them all, and I would certainly discourage anyone from taking Osborn's critique of natural selection as the consensus among paleontologists.

Where there once was a dearth of fossil evidence in direct support of evolution by natural selection, now there is almost an overabundance. I say "almost" because it can be difficult to keep up with new finds and discoveries made concerning so many different lineages. There is no guarantee that the flood of transitional fossils for some of the lineages I mentioned above will continue unabated, but I am certainly impressed by how quickly our understanding of ancient life has evolved.