The earliest known fossils belonging to this group date to about 65 million years ago, shortly after the K-Pg extinction event, though molecular data suggest they may have originated earlier, during the Cretaceous period.
With the exceptions of moles, hedgehogs, pangolins, some seals and walruses, rhinoceroses, tapirs, hippopotamuses and cetaceans, male members of the clade share the distinction of external testicles which serve the function of cooling the testicles.
The common ancestor of Boreoeutheria lived between 100 and 80 million years ago. The boreoeutherian ancestor gave rise to species as diverse as giraffes, dogs, mice, bats, whales, and humans. The concept of boreoeutherian ancestor was first proposed in 2004 in the journal Genome Research. The paper’s authors claimed that the genome sequence of the boreoeutherian ancestor could be computationally predicted with 98% accuracy, but would “take a few years and a lot of money”. It is estimated to contain three billion base pairs.