Animal Interaction Behind Cambrian Explosion? 'Missing' Ancestors Of Today's
Animals May Not Be Missing After All
ScienceDaily (May 8, 2008) - An event as simple as the world's first bite
may have sparked an ancient "explosion" of life 500 million years ago that
led to the rise of the broad groups of animals that are still alive today.
The cause of what is known as the "Cambrian Explosion" - which occurred
during the Cambrian Period 542 million to 490 million years ago - has
puzzled scientists for years. Theories about the event's cause include an
increase in the amount of atmospheric oxygen, a recovery from a global
glaciation, and key genetic changes in precursor animals that allowed the
development of bilateral symmetry, hard shells and bones, and rapid
locomotion.
Harvard Professor of Biology and of Geology Charles Marshall presented his
alternate theory Tuesday (April 29), suggesting that it was an increase in
interactions between species, such as predation, that drove an escalating
evolutionary process that led to the development of teeth and claws and the
wide variety of characteristics that we see among Earth's animals today.
The Cambrian Explosion was unique, Marshall said, because, though there have
been mass extinctions - such as that of the dinosaurs - and recoveries
since, there has never been another event as sweeping as that which occurred
in the Cambrian seas 500 million years ago. It was during that time when all
the modern phyla of animals first arose. Phyla are major classifications of
life that include broad groups of creatures. The phylum Chordata, for
example, includes all vertebrates, such as mammals, reptiles, amphibians,
and birds.
Marshall, who spoke in the Geological Lecture Hall Tuesday evening as part
of the Harvard Museum of Natural History's annual lecture series, started
off his talk by disputing whether the event commonly termed an "explosion"
was rapid enough to earn that moniker. The rise of modern animal groups
happened over millions of years. Only looking back over 500 million years of
history could the expansion of life that occurred be termed an "explosion,"
Marshall said.
Prior to the Cambrian Explosion, life on land was unknown and life in the
sea was made up of soft-bodied multicellular creatures that strained food
from the seawater around them or fed on mats of bacteria on the ocean floor.
Though these animals virtually disappeared at the beginning of the Cambrian,
the ancestors of the new modern phyla that replaced them haven't been found.
Marshall reviewed other theories explaining the explosion and said that
though each of them has merit - an increase in atmospheric oxygen, for
example, would be needed to fuel more active lifestyles - each also falls
short in some way.
As Marshall pondered alternatives, he began to think that it was possible
that the creatures in the pre-Cambrian seas during the Ediacaran Period didn
't entirely disappear. Though they were very different from what followed,
they may have been genetically complex enough to hold the genetic seeds of
the explosion.
Marshall cited recent findings from genetic studies that indicate even
creatures as diverse as flies and fish share many of the same genes. They
differ, he said, more in how the genes are used - whether they're switched
on or off - than in the genes' presence or absence.
"It's not new genes that create new morphological innovation, but rather the
way they're wired together," Marshall said. "[Different-looking creatures]
are not apples and oranges."
If the precursors to the creatures that arose during the Cambrian Period
were swimming in the Ediacaran seas, something had to spark the dramatic
change.
Marshall said that computer modeling of the forms that plants would take
under different environmental conditions provided a clue. The models showed
that widely divergent plants can result from a simple ancestor whose
descendants are subjected to different environmental conditions. The model
started with a simple primitive plant form and applied six basic genetic
rules. It then added four selective pressures to drive evolutionary change -
reproductive success, mechanical stability, light interception, and
minimized surface area. The model produced 20 widely different body types.
When researchers checked the fossil record, they found all types
represented.
Applying that lesson to animals, Marshall began to search for an
environmental force that might have driven such dramatic change in the
fleshy animals that populated the oceans before the Cambrian began. Marshall
realized that those creatures had no organs of interaction - no eyes, no
antennae, no jaws or claws - and began to think that the new force on the
scene was the ability of animals to interact with each other.
"Ediacarans were not interacting with each other as animals do today,"
Marshall said. "I think what drove the Cambrian Explosion was ecological
interactions."
The other factors that have been cited as playing a role in the Cambrian
Explosion very well may have had a hand, Marshall said, but they made the
conditions ripe for the change driven by interactions among animals. Just
what the trigger was that sparked those changing interactions, Marshall didn
't know, but, in a world populated by what he described as fleshy
"beefsteaks" lying on the ocean floor, it may have been something as simple
as the evolution of jaws with toothlike projections that allowed the world's
first painful bite.
"I believe . the explosion was driven by the onset of adult-adult
interactions," Marshall said. "Maybe the evolution of jaws or a large enough
gut, or the evolution of something like chitin so they could bite rather
than just giving a nasty suck."
Adapted from materials provided by Harvard University. Original article
written by Alvin Powell.
Harvard University (2008, May
. Animal Interaction Behind Cambrian
Explosion? 'Missing' Ancestors Of Today's Animals May Not Be Missing After
All. ScienceDaily. Retrieved May 8, 2008,
fromhttp://www.sciencedaily.com/releases/2008/05/080506195605.htm
Posted by
Robert Karl Stonjek
