By Jennifer Viegas, Discovery News
 Scientists
have sequenced DNA extracted from leg bone fossils of four gigantic
extinct birds and have determined not only that the ancient ancestors
of flightless birds competed with dinosaurs over food, but that
these birds evolved over land bridges that once connected many
of the world's continents.
The DNA came
from one extinct Madagascan elephant bird and three moas, an ostrich-like
bird that stood up to 10 feet tall and died out around 1300-1400
A.D. The moa sequence, which looked at mitochondrial, or cell
energy, DNA is the most complete gene sequence determined for
any extinct species, the researchers say.
Alan Cooper,
a Natural Environmental Research Council Advanced Fellow at the
University of Oxford, and his colleagues compared the extinct
birds' DNA with that of current ratites, birds with flat breastbones
unsuitable for flight muscles. This family includes the ostrich,
emu, cassowary, rhea and kiwi.
Findings are
published in the current journal Nature.
The DNA information
enabled the scientists to piece together a history of flightless
bird evolution and southern continent formation.
"(Our)
data indicates that the ratites evolved very early on- in fact
the speciation events took place before the K-T boundary (65 million
years ago) when dinosaurs became extinct," said Cooper. "The
pattern of speciation is consistent with the geological break-up
of Gondwana, the Cretaceous super-continent that existed in the
Southern Hemisphere from about 145 million years ago to after
80 million years ago."
Cooper and
his team believe ancestors of today's flightless birds evolved
into different species as Gondwana slowly broke into South America,
Africa, peninsular India, Australia and Antarctica. He theorizes
land bridges once connected Australia, Antarctica, India and Madagascar.
For example, Cooper said ostriches received "a free ride
on India up into the northern hemisphere about 65 million years
ago" when a land link probably existed between India and
Eurasia. Kiwis, on the other hand, evolved from birds that wound
up on Australia, New Caledonia and New Zealand.
Cooper said,
"Other land-based forms (plants, animals and certain birds)
around the same time may well have used similar routes as the
ratites."
David Penny,
professor of theoretical biology at Massey University in New Zealand,
agreed with the findings and added, "It looks to me that
the earliest modern birds must have been feeding on the ground.
They would have been in direct competition for food with the smallest
dinosaurs, but had better escape mechanisms."
Perhaps crossing
eroding land bridges into newly formed continents provided the
big birds with the ultimate escape.
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