fossil record

"In biological anthropology, we're always interested in finding new ways to extract behavior from the fossil record, and this is a great example," said Rebecca Ferrell, a program director at the National Science Foundation who helped fund this portion of the research.

The team assembled the largest-ever dataset of ancient human fossils spanning 7 million years and used advanced computational and statistical methods to account for gaps in the fossil record.

According to the fossil record, cetaceans -- whales, dolphins and their relatives -- evolved from four-legged land mammals that returned to the oceans beginning some 50 million years ago.

But until now, no one had found those earlier forms in the fossil record.

"Like many modern-day animal groups, ecdysozoans were prevalent in the Cambrian fossil record and we can see evidence of all three subgroups right at the beginning of this period, about 540 million years ago," said Ian Hughes, a graduate student in marine biology at Harvard University and the paper's first author.