100 million years of mammals: Genomic study of 241 species provides ‘definitive’ revision of evolutionary timeline

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Research led by a team of scientists from the Texas A&M School of Veterinary Medicine and Biomedical Sciences puts to bed the heated scientific debate regarding the history of mammal diversification as it relates to the extinction of the non-avian dinosaurs.

Their work provides a definitive answer to the evolutionary timeline of mammals throughout the last 100 million years. 

The study, published in Science, is part of a series of articles released by the Zoonomia Project, a consortium of scientists from around the globe that is using the largest mammalian genomic dataset in history to determine the evolutionary history of the human genome in the context of mammalian evolutionary history. Their goal is to better identify the genetic basis for traits and diseases in people and other species. 

The Texas A&M University research — led by Dr. William J. Murphy, professor in the Department of Veterinary Integrative Biosciences, and Dr. Nicole Foley, associate research scientist in Murphy’s lab — is rooted in phylogeny, a branch of biology that deals with the evolutionary relationships and diversification of living and extinct organisms.

“The central argument is about whether placental mammals (mammals that develop within placentas) diverged before or after the Cretaceous-Paleogene (or K-Pg) extinction event that wiped out the non-avian dinosaurs,” Foley said. “By performing new types of analyses only possible because of Zoonomia’s massive scope, we answer the question of where and when mammals diversified and evolved in relation to the K-Pg mass extinction.”

The research — which was conducted with collaborators at the University of California, Davis; University of California, Riverside; and the American Museum of Natural History — concludes that mammals began diversifying before the K-Pg extinction as the result of continental drifting, causing the Earth’s land masses to drift apart and come back together over millions of years. Another pulse of diversification occurred immediately following the K-Pg extinction of the dinosaurs, when mammals had more room, resources, and stability.

This accelerated rate of diversification led to the rich diversity of mammal lineages — such as carnivores, primates, and hoofed animals — that share the Earth today. 

Murphy and Foley’s research was funded by the National Science Foundation and is one part of the Zoonomia Project led by Elinor Karlsson and Kerstin Lindblad-Toh, of the Broad Institute, which also compares mammal genomes to understand the basis of remarkable phenotypes — the expression of certain genes, such as brown vs. blue eyes — and the origins of disease.