Gondwana split sorts out mammalian evolution

Placental mammals are a diverse group, with nearly 4000 described species (e.g., rodents, bats, elephants, humans) that bear live young and are nourished before birth in the mother's uterus through the placenta. In contrast, marsupials are commonly thought of as pouched mammals. While the latter also give live birth, they do not have long gestation times; the early development is completed instead in the pouch.

Although independent studies have resolved placental mammals into four major groups, it is not clear what the hierarchical relationships within the groups are, thus hampering the understanding of the early biogeographic history of placentals. The four major groups are:

(1) Afrotheria [elephants, hyraxes, manatees and dugongs, aardvarks, golden moles, tenrecs, and elephant shrews],

(2) Xenartha [armadillos, anteaters, and sloths],

(3) Laurasiatheria [carnivores (e.g., bears, cats, dogs), pangolins, whales and dolphins, even-toed ungulates (e.g., hippos, cows, pigs), odd-toed ungulates (e.g., horses, rhinos), bats, and insectivores (e.g., shrews, moles, hedgehogs)], and

(4) Euarchontoglires [rodents, rabbits, tree shrews, flying lemurs, and primates (e.g., humans, monkeys, lemurs)].

In the 14 December 2001 issue of Science, a team of scientists discuss alternative positions for the root of the placental tree. They report results based on Bayesian and other statistical methods and use a data set that comprises approximately 16,400 base pairs for each of 44 mammals and that includes segments from 22 different genes. "We have resolved the interordinal relationships almost entirely," says Mark S. Springer of the University of California, Riverside, a member of the team. "Based on molecular clocks, we found that the deepest split occurs between Afrotheria and other placentals at ~103 million years, a date that coincides with a major plate tectonic separation."

The result is controversial. Some research

Contact: Iqbal Pittalwala
University of California - Riverside

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