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February 07, 2013

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Small insect-eater gave rise to today’s mammal diversity, study finds

Feb. 7, 2013
Courtesy of University of Toronto Scarborough,
American Museum of Natural History
and World Science staff

New re­search traces the family tree of most mam­mals back to a hypo­thetical small, scam­per­ing, insect-eater that first lived a few hun­dred thou­sand years af­ter the di­no­saurs died out.

Sci­en­tists re­con­struct­ed the family tree of pla­cen­tal mam­mals – a di­verse group that in­cludes cats, dogs, hors­es and hu­mans. Pla­cen­tal mam­mals are those that bear live young.

An artist’s ren­der­ing of the hypo­the­tical pla­cen­tal an­cestor. (Cred­it: Carl Bu­ell)

The re­search­ers used the world’s larg­est da­ta set com­bin­ing ge­net­ic and phys­i­cal traits to re­con­struct the tree. A ma­jor find­ing is that pla­cen­tal mam­mals di­ver­si­fied much lat­er than pre­vi­ous the­o­ries had sug­gested, with all of the ma­jor groups alive to­day orig­i­nat­ing af­ter the ex­tinc­tion of the di­no­saurs. 

The work is fea­tured in this week’s is­sue of the jour­nal Sci­ence.

Ge­net­ic ev­i­dence alone had sug­gested that pla­cen­tal mam­mals were al­ready a di­verse group in the Late Cre­ta­ceous pe­ri­od, be­fore the event that drove the di­no­saurs and 70 per­cent of oth­er then-existing spe­cies ex­tinct.

But “anal­y­sis of this mas­sive da­taset shows that pla­cen­tal mam­mals did not orig­i­nate dur­ing the Meso­zoic,” or di­no­saur era, said the lead au­thor, Mau­reen O’Leary of the School of Med­i­cine at Stony Brook Uni­vers­ity in New York and the Amer­i­can Mu­se­um of Nat­u­ral His­to­ry.

“In the field of mammal research, there had been a big div­ide be­tween people working with DNA and others working on morph­ol­ogy,” or phys­ical struc­ture, added John Wible, cur­ator of mam­mals at Car­negie Mu­seum of Na­tural His­tory in Pitts­burgh and a co-author. “They just weren't work­ing with each other until now.”

O’Leary said the findings show “species like ro­dents and pri­ma­tes did not share the Earth with non-avian di­no­saurs but arose from a com­mon an­ces­tor—a small, insect-eating, scam­per­ing an­i­mal—shortly af­ter the di­no­saurs’ demise.”

The re­search­ers built a da­tabase that recorded phys­i­cal traits for 86 pla­cen­tal mam­mal spe­cies, in­clud­ing 40 spe­cies that are ex­tinct and known only from fos­sils. More than 4,500 traits, in­clud­ing the pres­ence or ab­sence of wings, teeth, and cer­tain bone types, were recorded in the da­tabase and used to make the tree, in com­bina­t­ion with ge­net­ic da­ta.

The da­ta set is 10 times larg­er than those that had pre­vi­ously been used to study mam­mal rela­t­ion­ships, and is pub­licly avail­a­ble on­line, il­lus­trat­ed with over 12,000 im­ages, the researchers said. The work will serve as a mod­el for fu­ture pro­jects that will give us a bet­ter idea of how spe­cies evolved and are re­lat­ed to one anoth­er, added Mary Sil­cox, a Uni­vers­ity of To­ron­to Scar­borough re­searcher who worked on the proj­ect.

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New research traces the family tree of most mammals back to a small, scampering, insect-eating creature that first lived a few hundred thousand years after the dinosaurs died out. Scientists reconstructed the family tree of placental mammals – a diverse group that includes cats, dogs, horses and humans, and which bear live young. The work is featured in this week’s issue of the journal Science. The researchers used the world’s largest dataset combining genetic and physical traits to reconstruct the placental mammal tree of life. A major finding is that placental mammals diversified much later than previous theories had suggested, with all of the major groups alive today originating after the extinction of the dinosaurs. Genetic evidence alone had suggested that placental mammals were already a diverse group in the Late Cretaceous period, before the event that drove the dinosaurs and 70 percent of other then-existing species extinct. But “analysis of this massive dataset shows that placental mammals did not originate during the Mesozoic,” or dinosaur era, said the lead author, Maureen O’Leary of the School of Medicine at Stony Brook University in New York and a research associate at the American Museum of Natural History. “Species like rodents and primates did not share the Earth with non-avian dinosaurs but arose from a common ancestor—a small, insect-eating, scampering animal—shortly after the dinosaurs’ demise.” The researchers built a database that recorded physical – or phenomic – traits for 86 placental mammal species, including 40 species that are extinct and known only from fossils. More than 4,500 traits, including the presence or absence of wings, teeth, and certain bone types, were recorded in the database and used to construct the tree of life, in combination with genetic data. The phenomic dataset is 10 times larger than those that had previously been used to study mammal relationships, is publicly available online, and illustrated with over 12,000 images. The work will serve as a model for future projects that will give us a better idea of how species evolved and are related to one another, said Mary Silcox, a University of Toronto Scarborough researcher who worked on the project.