New study of mammalian evolution reveals one of our earliest ancestors

Jennifer Viegas - Discovery News

This small and furry scampering insect-eater lies at the base of the family tree for humans and most mammals, according to the largest-yet study of mammalian evolution.

The agile animal is the earliest among placental mammals — the largest branch of the mammal tree consisting of more than 5,100 living species. Only marsupials, such as kangaroos, and monotremes (egg-laying mammals including the platypus and echidna) fall outside of that huge group.


The mammal "had a diet of insects, a fleshy nose, a light underbelly in its fur, and a long tail," said Maureen O'Leary, an associate professor in the Department of Anatomical Sciences in the School of Medicine at Stony Brook University. O'Leary was lead author of a study about the mammal in the latest issue of Science.

"It was larger than a mouse, but smaller than a rat," added O'Leary, who is also a research associate at the American Museum of Natural History

"As for its bones, it lacked special bones that are found near the pelvis, and in its ear had a small bone for hearing that was shaped like a stirrup."

Such a detailed description was made possible due to an unprecedented combining of both DNA and anatomical data for placental mammals. Humans again fall into that mammal group, which is distinguished by certain reproductive features and skeletal traits.


Recording of the data employed a system called MorphoBank. Its data set, which includes more than 4,500 traits detailing characteristics such as hair types and teeth structure, is 10 times larger than what was previously used for studies of mammal relationships.


In addition to revealing what the "mother" of most mammals looked like, the study shows that placental mammals arose 200,000 to 400,000 years after the extinction of non-avian dinosaurs about 65 million years ago.

"This is about 36 million years later than the prediction based on purely genetic data," said co-author Marcelo Weksler of the Museu National-UFRJ in Brazil.


Somehow the distant relative of mammals that led to the "mother" of our species and others survived the asteroid crash, dramatic climate change, and other happenings that did in the dinos.

"Other species that survived are some crocodiles, turtles and flowering plants," O'Leary said.


The newly constructed mammal family tree indicates that the fragmentation of Gondwana- one of two supercontinents formerly part of Pangaea- came well before the origin of placental mammals, co-author John Wible of the Carnegie Museum of Natural History shared.

This negates an earlier theory, which held that mammal diversification was tied to the breakup of the supercontinent. It previously was thought that this breakup put the early mammals in different environments, leading to their different evolutionary paths and the diversity that we see today among mammals.


It now could be, however, that the earliest mammals traveled far distances of their own accord and just evolved their broad diversity over long periods of time.

Yet another finding of the extensive study is that a branch of the placental mammal tree called Afrotheria (a group ranging from elephants to aardvarks that today lives in Africa) did not originate in Africa, but rather in the Americas.


"Determining how these animals first made it to Africa is now an important research question, along with many others, that can be addressed using MorphoBank and the phylophenomic (physical traits) tree produced in this study," said co-author Fernando Perini of Minas Gerais Federal University in Brazil.

Mary Silcox, who years ago famously helped put humans on the tree of life, agreed and said, "This project is not exhaustive, but exposes a way forward to collect data on other phenomic systems and other species."


This post originally appeared at Discovery News. It has been republished here with permission.


Top image by Carl Buell; fossil image courtesy AMNH

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