| EVOLUTIONARY 'ARMS' RACE
By Sean Henahan, Access Excellence
 Madison,
WI (August 14, 1997)- Molecular biological back-tracking is begnning
to provide clues to the appearance of a possible worm-like common ancestor
of all current animal life.
The absence of fossils before the Cambrian period precludes the
use of comparative anatomy to estimate what the earliest forms of animals
looked like. Now, developmental geneticists are developing new methods
to go beyond the fossil record, starting with genes preserved in and common
to modern animals, and working back.
"The fossil record prior to the Cambrian is so scant nobody knows the
origin of animal life. But now we're drawing a picture of something no
one has ever seen." explained Sean Carroll of the Howard Hughes Medical
Institute at the University of Wisconsin-Madison.
Using techniques developed in molecular biology, Carroll and colleagues
have determined that the genes used to grow appendages - legs, arms, claws,
fins and antennas - were operational at least 600 million years ago,
and that the genetic machinery is very similar in all animals past and
present. The thing that differentiates a crab from a mouse or a fruit fly
from an eagle, is simply how those genes are expressed, he said.
"This is stunning," said Carroll. "Nobody thought that this animal was
so sophisticated. We're talking about the common ancestor of all
the most successful animals on Earth."
Prior to recent embryonic research indicating that animals share the
same genetic machinery that governs body architecture, the best guesses
about functional evolution were based on comparative anatomy. Indeed, appendages
were often cited as examples of independent evolution. Carroll and
colleagues beleive the problem of developing limbs, be they claws or wings,
was solved just once a very long time ago, and that the genetic mechanism
is still at work.
"Everybody thought the wheel was invented again and again and again,"
Carroll said, "but there was a single solution and everything is a modification
of that."
That argument is supported, said Carroll, by the discovery of the same
appendage-making genes in six broad divisions of the animal kingdom, including
vertebrates, insects and fish.
"We found the same mechanism in all of these divisions of the animal
kingdom. The architecture can vary tremendously, but the genetic instructions
are the same and have been preserved for a very long period of time," he
said.
The idea that a common set of genes is responsible for building appendages
not only simplifies evolutionary history, but helps explain the great burst
of evolutionary activity known as the Cambrian explosion, he said.
This "evolutionary big bang" took place in the world's oceans more than
500 million years ago when new animals appeared at breakneck speed.
"The reality is that animals with appendages took off and dominated
the Earth," at that time, Carroll said. "It was like an arms race" with
animals that could swim faster, grab tighter and fight with greater effect
dominating the ocean environment and conquering new ones like the land.
During the Cambrian, animals got bigger and more diverse, but those changes
did not require new genes.
The new research appears in the August 14, 1997 issue of in the
journal Nature. Carroll reported related research in the May 1997
issue of the Proceedings of the National Academy of Sciences.
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