What’s bigger than a shrew and smaller than a squirrel?
It’s not a riddle for an international research team but rather part of the answer to a question the team explored. With the 5,100 species of placentals alive today--from animals that walk, or fly or swim and, of course, humans, what could their common ancestor possibly look like.
A team of 23 researchers has just completed a six-year groundbreaking study of the evolution of placental mammals, as they tried to construct “The Tree of Life” --well, at least the mammalian branches of that tree.
Researchers from Carnegie Museum of Natural history including Curator of Mammals John Wible and Michelle Spaulding were part of the project that utilized two disciplines: molecular (DNA) and morphological (anatomy) data to reconstruct the family tree of placental mammals.
"There are some ancient DNA studies, they've got sequences out of extinct animals like the wooly mammoth but that's fairly limited," Spaulding said. "I think 30,000 years is the oldest DNA they've obtained, and we were looking at animals from about 120 million years ago."
There are 5,100 living species of placental mammals. Researchers looked at 83 species--living and extinct. "What we did was sample at least one animal from every order," Spaulding said.
The molecular team collected DNA sequences of living mammals and the morphology team examined the anatomy of both living and extinct mammals and thereby including fossils in the study to take into account such features as bone length and types of teeth. Spaulding said combining the two disciplines reveals a more complete tree of life.
"It's called the total evidence study. It's the idea that you're taking all that you have and putting it together because you find sometimes you get different 'trees of life' when you combine them [DNA and anatomy] than you do from when you run them alone."
According to Spaulding one of their findings was that placental mammals "radiated" or diverged into different branches of the tree of life around the end of the Cretaceous Period when dinosaurs became extinct.
"It was directly the extinction of the reptiles that led to the origination of the modern groups of mammals," Spaulding said.
"What also came out of this project was to be able to actually reconstruct what the ancestor of all [modern] placental animals could have looked like."
Hypothetically? "Yes, hypothetically" confirmed Spaulding. "We can say it weighed between six and 245 grams (0.2 ounces and 8.6 ounces). So it's fairly small--smaller than a squirrel, bigger than a shrew. It probably lived on both trees and the ground so it could run around on the ground quite well but it could also run up to the trees to escape predators and look for food." Spaulding added that their construction of the placental ancestor had a full complement of teeth, probably ate insects, and it's tail was nearly as long as its body.
The research team's paper appears in the journal Science.
