Reprinted from ScienceDaily Magazine ...

Source: University Of California - Riverside 
Date Posted: Monday, January 21, 2002

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 researchers cite fossil evidence that suggests
that mammals diversified only ~65 million years ago. But Springer and colleagues
argue that the separation of South America and Africa around 100 million years
ago during the Cretaceous in Gondwana (the southern hemisphere
super-continent that incorporated Africa, Antarctica, Australia, India,
Madagascar and South America before it broke apart) explains the split. "We
suggest that the
common ancestor of living placental mammals occurred not in the northern
hemisphere, as is commonly believed, but in the southern hemisphere instead, in
Gondwana," says Springer. "Furthermore, our study provides the first convincing
molecular evidence that flying lemurs and tree shrews are the closest relatives
to primates." 

The scientists find that Afrotheria is the oldest group, with some of its orders
never having left Africa. The Xenartha, which populate South America, constitute
the next group. Because these oldest groups are southern groups, the placental
mammals originated in the south, the scientists contend. They also determine
that Laurasiatheria and Euarchontoglires are sister taxa (i.e., taxa derived from a
common ancestral node), and together constitute a clade (i.e., an organism and
all of its descendants) named Boreoeutheria, with a northern hemispheric origin. Deeper
in the placental tree, the authors find that Xenartha and Boreoeutheria are
sister taxa. 

Such deciphering of higher level relationships among mammalian orders is
important because of its ramifications for evolutionary biology and genomics. "A
well resolved phylogeny offers a good framework for performing other studies," notes
Springer. "It allows for better predictions on what fossils may look like and
where they might be found in different parts of the world. Our findings are also
likely to assist genomicists in determining which organisms they should sequence
genomes for in future." 

Reprinted from ScienceDaily Magazine ...

Source: University Of California - Riverside 
Date Posted: Monday, January 21, 2002 
Web Address: http://www.sciencedaily.com/releases/2002/01/020121090546.htm

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