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Discovery of miniature bilaterians rewriting evolutionary history
(China Daily)
Updated: 2004-06-14 08:44
A team of Chinese and USs scientists ay they have discovered 10 fossils of
the world's earliest bilaterian animal that lived some 580 million years ago.
The fossils, which they say provides valuable clues on the origin of early
life, are only 0.2 millimetre long.
They reveal under microscope a life-form 580 million years old, with one pair
of coeloms - internal cavities - and symmetrically arranged sensory pits,
according to Chen Junyuan, chief scientist of the research team.
He said bilaterians refer to those animals that are symmetric on a central
axis, with a body divided into equivalent right and left halves, and with an
anterior-posterior polarity. This includes worms, ants, and ranges up to humans.
The emergence of baliterians represents a crucial point in evolutionary
history, as their structure allows them to develop various new bodily functions
that are necessary for the evolution of more complex life forms.
"When they first appeared and what they looked like are two of the most
intriguing issues biologists worldwide are now addressing," Chen said.
Chen, a researcher with the Nanjing Institute of Geology and Palaeontology,
said this find has advanced the recorded existence of bilaterian fossils by 40
to 55 million years.
Their findings were published in the latest issue of the scientific journal
Science.
The fossil, officially named "Vernanimalcula guizhouena," has paired coeloms
extending the length of the gut and paired external pits that could be sense
organs.
Based on the fossils Chen's team recovered, the animal appears to have had a
thick multi-walled pharynx and a triploblastic structure. And its mouth points
to the belly. The structural complexity suggests it is an adult rather than
larval form.
"Its structure is surprisingly complex given its small size," Chen said.
He said it may suggest that the genetic tools required for the development of
the multi-layered, bilateral body structure precede those required for the
development of its large size.
"It shows that bilaterian animals did not have to grow big to be complex,
which runs against conventional views," He added.
Given that the fossil creature came into being during the "Snow Ball" period
when the snow covering the Earth began to melt due to rising temperatures, Chen
also calls the creature the "Small Spring Animal."
The fossils, numbering 10 in all, were discovered last November among the
Weng'an Biota of Guizhou Province in Southwest China. Each piece was identified
out of from 5,000 to 10,000 microfossils.
Over the past few years, interesting new animal fossils have been discovered
among the Weng'an Biota on a regular basis, keeping rewriting evolutionary
history.
Chen said the newly published discovery is the result of investigations that
have been going on for six years.
Six years ago, Chen Yuanjun and his colleagues, for the first time, unveiled
their world-stunning discovery of fossils unearthed from the Weng'an Biota.
These fossils encase remains of a variety of multicellular animals, as well
as their embryos, that were identified as having lived before the Cambrian
Period (543-490 million years ago).
That discovery, also published by Science, was hailed by the Washington Post
as "one of the greatest findings in evolutionary biology of this century." It
lauded the discovery as having opened the window on a magic and crucial point in
the evolutionary history of life on the Earth.
This is important because scientists had previously focused on the Cambrian
era, which begins some 40 to 60 million years later as the period in which
living forms evolved. That period featured the so called "Cambrian Explosion,"
which saw an explosive emergence of a broad spectrum of life forms that
challenged the theories of orthodox Darwinism.
This unconventional event in evolutionary history was confirmed by fossil
finds worldwide, particularly in the world-renowned Chengjiang Biota in Yunnan
Province neighbouring Guizhou.
The Weng'an Biota, however, existed in a strata earlier than that of the
Chengjiang Biota and may boast as many, if not more, novel species than the
latter.
Chen's team's new discoveries from the Doushantuo Formation in the Weng'an
Biota provide the first evidence confirming the hypothesis that bilaterian
animals appeared well before the Cambrian.
"It is exciting because fossils of bilaterian animals have been found to be
common in the strata of Cambrian Period, but little trace has been found prior
to that time, although researchers worldwide tend to believe that they may lie
somewhere else in the world," Chen said.
"We had a bit of luck in finding the trace."
Chen noted that the "Small Spring Animal" normally lived in running water
under small tides. Faced with the risk of being swept away by the tides, it
might hide itself in the sediment just as microorganisms do today.
Research results also showed it would suck in nutrition from the sediments
through its mouth on the belly with the help of pharyngeal muscle, which Chen
claimed could be a more primitive life form than grass eaters and cave dwellers.
Judging from its structure, Chen said, the life form had entered its
adulthood and acquired reproductive capability.
Chen said the discovery of the "Small Spring Animal" has brought us a step
closer to the origin of bilaterian animals, which he termed in the Science
article as the last common bilaterian ancestor (LCBA).
Scientists have generally agreed that the LCBA may have emerged and diverged
sometime between 600 million year ago and the Cambrian boundary.
The fossil of its ancestors may well lie deep in the phosphorite in the
Weng'an Biota, where Chen and his colleagues are now continuing their digging.
"What we have found may just be the tip of the iceberg," he said.
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