For more than a century, scientists used the fossil record to look back in evolutionary history and believed that the ancestors of modern mammals originated and flourished around 65 million years ago, at the extinction of nonavian dinosaurs known as the Cretaceous/Tertiary (K/T) boundary.
But about 30 years ago, researchers began to estimate when species diverged by comparing the number of differences in their DNA (called molecular clock dating) and made their own evolutionary phylogenies in addition to the phylogenies based on the fossil record. By the turn of the 21st century, however, scientists began noticing that dates on the various mammalian phylogenies weren't adding up: molecular data suggested that modern mammals originated during the Cretaceous period, more than 100 million years ago—a time period without fossils that resemble modern mammals.
In this month's Hot Paper a group of researchers from Europe and the United States combined more than 2,500 ...
