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 a few exceptions[a] male animals in the clade have a scrotum which serves the function of cooling the testicles to improve the production of sperm. The sub-clade Scrotifera was named after this feature.
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 a 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.
^O'Leary, M. A.; Bloch, J. I.; Flynn, J. J.; Gaudin, T. J.; Giallombardo, A.; Giannini, N. P.; Cirranello, A. L. (2013). "The placental mammal ancestor and the post–K-Pg radiation of placentals". Science. 339 (6120): 662–667. doi:10.1126/science.1229237. PMID23393258.
^Drew, Liam (8 July 2013). "Why are testicles kept in a vulnerable dangling sac?". slate.com. Between these branches, however, is where it gets interesting, for there are numerous groups, our descended but a-scrotal cousins, whose testes drop down away from the kidneys but don't exit the abdomen. Almost certainly, these animals evolved from ancestors whose testes were external, which means at some point they backtracked ... , evolving anew gonads inside the abdomen. They are a ragtag bunch including hedgehogs, moles, rhinos and tapirs, hippopotamuses, dolphins and whales, some seals and walruses, and scaly anteaters.
Waddell, PJ; Kishino, H; Ota, R (2001). "A phylogenetic foundation for comparative mammalian genomics". Genome Inform Ser Workshop Genome Inform. 12: 141–154.
Murphy, William J.; Eizirik, Eduardo; Springer, Mark S.; et al. (2001). "Resolution of the early placental mammal radiation using Bayesian phylogenetics". Science. 294 (5550): 2348–2351. doi:10.1126/science.1067179. PMID11743200.