Another example of using the tree to understand history can be seen with ancestral characters in whales.

The hypothesis

Reconstructing the characters of ancestors can be a useful tool. For example, scientists used to think that whales’ ancestors were now-extinct carnivores called mesonychids. However, based on recent findings, scientists have hypothesized that whales are actually more closely related to hoofed mammals like hippos and ruminants such as cows and giraffes.

The prediction

Our hypothesis leads us to predict that ancient whales should share some characters with their close relatives.

The evidence

And in fact, recent fossil discoveries have borne out that prediction. Scientists found ancient whales with hind legs and pelvises: these whales had the same kind of ankle bone that modern pronghorns, camels, cows and hippos have (right).

Compare the ankle bones of the two ancient whales on the left and right (this specimen is missing some bones) and that of a modern pronghorn (center). Notice the double pulley structure present in all three.