250-year-old statistical technique has prompted researchers to rewrite the evolutionary history of placental mammals, which include humans and all other mammals that give birth to live, fully developed young. many paleontologists think that these animals arose 65 million years ago, this new work suggests that the two most ancient groups appeared a little more than 100 million years ago during the breakup of the giant southern continent called Gondwana.
Traditionally, researchers have built evolutionary trees by evaluating the degree of change in a given trait--limb length, for instance--or a given DNA sequence between supposed relatives. But the trees tend to differ depending on the data and analytical techniques used. So William Murphy and Eduardo Eizirik of the National Cancer Institute (NCI) in Frederick, Maryland, joined forces with Mark Springer of the University of California, Riverside, to combine DNA evidence with Bayesian inference. The technique was developed in the 1700s for assessing how new information influences the chances that a current belief continues to be correct.
In the 14 December issue of Science , the researchers report that that placental mammals all trace their roots to what is now Africa, not to an ancient northern landmass. Their analysis shows that mammals fall neatly into four groups as others had suggested, with some modifications. The oldest, Afrotheria, includes elephants, aardvarks, and hyraxes. Another, the Xenarthra, covers armadillos, sloths, and anteaters. Euarchontoglires includes some of the more common mammals: rodents, rabbits, and primates, for example--a grouping "that's new and is strongly supported by the data," points out NCI's Stephen O'Brien, one of Murphy and Eizirik's collaborators. And carnivores, whales, cows, and horses make up the Laurasiatheria. By clearing up ambiguous relationships between major groups of placental species, the scientists show that the current effort to sequence genomes of humans, rats, and mice merely skims the surface of mammalian diversity.
The findings are controversial, and the team got mixed reviews from their colleagues. "What they write about higher level [evolutionary] relationships seems perfectly reasonable," comments Philip D. Gingerich, a paleontologist at the University of Michigan, Ann Arbor. "But on the other hand, what they write about the timing of [mammalian] divergences seems completely unreasonable," as in his view fossil evidence points to a completely different scenario.