Updated Friday, February 8, 2013 at 06:47 AM
Humankind’s common ancestor with other mammals may have been a roughly ratsize animal that weighed no more than a half a pound, had a long furry tail and lived on insects.
In a six-year study of the mammalian family tree, scientists have identified and reconstructed what they say is the most likely common ancestor of the many species on the most abundant and diverse branch of that tree: creatures that nourish their young in utero through a placenta. The work appears to support the view that in the global extinctions some 66 million years ago, all nonavian dinosaurs had to die for mammals to flourish.
Scientists had been searching for a common genealogical link and have found it in a lowly occupant of the fossil record, Protungulatum donnae, that until now has been so obscure that it lacks a colloquial nickname. But as researchers reported Thursday in the journal Science, the animal had several anatomical characteristics for live births that anticipated all placental mammals and led to some 5,400 living species, from shrews to elephants, bats to whales, cats to dogs and, not least, humans.
A team of researchers described the discovery as an important insight into the pattern and timing of early mammal life and a demonstration of the capabilities of a new system for handling copious amounts of fossil and genetic data in the service of evolutionary biology. The new technology is expected to be widely applied to similar investigations of plants, insects, fish and fowl.
Given some belated stature by an artist’s brush, the animal hardly looks the part of a progenitor of so many mammals (which does not include marsupials, such as kangaroos and opossums, or monotremes, egg-laying mammals such as the platypus).
Maureen O’Leary of Stony Brook University on Long Island, principal author of the journal report, wrote that a combination of genetic and anatomical data established that the ancestor emerged within 200,000 to 400,000 years after the great dying at the end of the Cretaceous period. At the time, the meek were rapidly inheriting the Earth from hulking predators such as T. rex.
Within an additional 2 million to 3 million years, O’Leary said, the first members of modern placental orders appeared so profusely that researchers have started to refer to the explosive model of mammalian evolution. The common ancestor itself appeared more than 36 million years later.
The research team drew on combined fossil evidence and genetic data encoded in DNA in evaluating the ancestor’s standing as an early placental mammal. Among characteristics associated with full-term live births, the Protungulatum species was found to have a two-horned uterus and a placenta in which the maternal blood came in close contact with the membranes surrounding the fetus, as in humans.
Evidence of the common ancestor was found in North America, but the animal may have existed on other continents as well.
The publicly accessible database responsible for the findings is called MorphoBank, with advanced software for handling the largest compilation yet of data and images on mammals living and extinct. “This has stretched our own expertise,” O’Leary, an anatomist, said.
The project was financed primarily by the National Science Foundation as part of its Assembling the Tree of Life program. Other scientists from Stony Brook, the American Natural History Museum and the Carnegie Museum participated, as well as researchers from the University of Florida, the University of Tennessee at Chattanooga, the University of Louisville, Western University of Health Sciences, in Pomona, Calif., Yale University and others in Canada, China, Brazil and Argentina.
S. GOLDBERG / M. NOVACEK / The New York Times
A Cretaceous-age animal found in 1994, is one of many used to study the mammalian tree of life.