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NYTimes.com Article: With an Evolutionary Milestone, the Race for Survival Began



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With an Evolutionary Milestone, the Race for Survival Began

February 18, 2003
By YUDHIJIT BHATTACHARJEE 




 

More than three billion years ago, when the earth was still
a lonesome ball with no habitation, life made its first
appearance on the planet in the form of single-celled
organisms. 

>From these evolved more complex manifestations of life:
multicelled organisms like algae that floated about in the
seas; then wormlike, soft-bodied creatures with a more
elaborate organization of cells; and finally, millions of
years later, animals like flies and reptiles that had
well-defined heads. 

Scientists studying this gigantic, densely branched tree of
evolution believe that the appearance of the head was a
major milestone in the history of animal life. It marked
the beginning of active feeding and predatory behavior,
setting off a survival race that accelerated the pace of
evolution. 

Through genetic studies on some of the oldest living
species on the planet, researchers are now piecing together
an account of this evolutionary watershed. 

Their findings suggest that the earliest head appeared
about 700 million years ago in a hydra-like organism that
may have been a common ancestor to species from snails to
human beings. 

In the picture emerging from these studies, the early head
was simply a net of nerve cells at the mouth of the
organism. 

Some scientists believe it was similar to the cluster of
nerves present around the oral opening in cnidarians - a
family of stinging aquatic creatures that includes the
modern-day hydra, the sea anemone and the jellyfish. 

Beachgoers who have been stung by jellyfish will appreciate
why cnidarians (pronounced nih-DARE-ee-yens) have that
name, which is derived from the Greek word knidos, for
"stinging nettle." 

In hydra and jellyfish, as in other cnidarian species, the
mouth is surrounded by a ring of tentacles that the animal
uses to poison and numb its prey. The nerve cluster in this
region of the cnidarian's body, researchers say, could be
the most primitive head in the animal kingdom. 

In their quest for the origin of the head, scientists have
identified genes in corals, sea anemones and hydra that are
similar to genes responsible for head development in higher
animals like flies and mice. Studying such homologous genes
across species, which have closely matching protein
sequences, is a standard technique used by researchers to
trace the ancestry of physical and behavioral traits. 

In experiments on hydra, Dr. Brigitte Galliot and her
colleagues at the University of Geneva studied genes that
were similar to head-development genes in the fruit fly.
They chopped off the top of a hydra and monitored the
expression of specific proteins regulated by these genes as
the organism regenerated its lost part. From the pattern of
proteins expressed during the regeneration process, the
researchers concluded that the genes were involved in
forming the hydra's upper region, including the
organization of nerve cells. 

Researchers have reported similar results for other sets of
hydra genes that are homologous to head-development genes
in higher animals. Others have shown that corals and
jellyfish have basic versions of head-patterning genes that
are involved in the development of neurons, and are
possibly active in forming the body's top region. 

These findings suggest that the head in higher animals may
have evolved from a mouthlike structure similar to the
cnidarian's upper body, consisting of a nerve net around an
oral opening. A larger conclusion, according to Dr.
Galliot, is that the head's origin may have been driven by
the need for active feeding. 

"It is as if the important thing at that stage of evolution
was to organize cells in a way that supported autonomous
and active feeding behavior," Dr. Galliot said. 

That goal could have been achieved by having a high density
of neurons around the mouth. The neurons density would have
given the organism some maneuvering ability to trap food
floating by. 

The nervous system may have further evolved to allow more
active feeding. The need to catch prey using controlled
tentacle movements, for instance, may have fueled the
evolution of multiple kinds of sensory cells and neurons. 

For an organism determined to find food more efficiently,
it made sense to develop the nervous system largely at one
end of the body, which later evolved into the head, said
Dr. Luis Puelles, a biologist at the University of Murcia
in Spain. 

"Especially once you had a free-swimming organism with a
feeding pole, it was convenient to put the best sensors and
computing units at the front of the animal where it would
most efficiently meet prey," Dr. Puelles said. 

Over millions of years, the sensory and neural apparatus of
the hydra-like ancestor flowered into a central nervous
system. 

Scientists believe that this rudimentary brain was similar
to that of the present-day worm. It marked the next big
step in the head's evolution. 

"A centralization of the nervous system provided many
advantages," said Dr. Ulrich Technau, a researcher at the
University of Darmstadt in Germany. "It provided for
shorter connections between neurons, allowing quicker
reaction to changes in the environment. Also, it was
possible to have a higher number of connections between
neurons, which enabled more complex behaviors." 

As their behavioral repertory expanded, organisms could
feed more actively than ever before. Over time, they
evolved elaborate organs for sight, smell and taste.
"Active feeding also spurred the evolution of head
structures such as jaws and gills," said Dr. Volker
Hartenstein, a researcher at the University of California
at Los Angeles. 

As nature molded the head from a primitive nerve cluster to
a well-organized unit for sensing and computation, the head
itself became a driving force in the evolution. Its
sophistication was accompanied by a rise in predatory
behavior, which led to an explosive diversification of
species in the animal kingdom. 

"Predation set off an evolutionary arms race," said Dr.
Peter Holland, a zoologist at the University of Oxford in
England. "The game went like this: `I'll eat you unless you
defend yourself or swim away or hide. But if you do that, I
then need to be even better at sensing, chasing and
catching.' And so it went on." 

http://www.nytimes.com/2003/02/18/science/18HEAD.html?ex=1046585030&ei=1&en=4a156852f438c7ce



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