[biomed] Cambrian Explosion even more dramatic

[Eugen Leitl]

http://richarddawkins.net/article,3361,Single-Celled-Giant-Upends-Early-Evolution,Discovery

Thursday, November 20, 2008 | Science : Evolution and Biology |  print
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Single-Celled Giant Upends Early Evolution

by Discovery

Thanks to BanJoIvie for the link.

Reposted from:

http://dsc.discovery.com/news/2008/11/20/gromia-cambrian.html

Nov. 20, 2008 -- Slowly rolling across the ocean floor, a humble
single-celled creature is poised to revolutionize our understanding of how
complex life evolved on Earth.

A distant relative of microscopic amoebas, the grape-sized Gromia sphaerica
was discovered once before, lying motionless at the bottom of the Arabian
Sea. But when Mikhail Matz of the University of Texas at Austin and a group
of researchers stumbled across a group of G. sphaerica off the coast of the
Bahamas, the creatures were leaving trails behind them up to 50 centimeters
(20 inches) long in the mud.

The trouble is, single-celled critters aren't supposed to be able to leave
trails. The oldest fossils of animal trails, called 'trace fossils', date to
around 580 million years ago, and paleontologists always figured they must
have been made by multicellular animals with complex, symmetrical bodies.

But G. sphaerica's traces are the spitting image of the old, Precambrian
fossils; two small ridges line the outside of the trail, and one thin bump
runs down the middle.  At up to three centimeters (1.2 inches) in diameter,
they're also enormous compared to most of their microscopic cousins.

"If these guys were alive 600 million years ago, and their traces got
fossilized, a paleontologist who had never seen this thing would not have a
shade of doubt attributing this kind of trace to the activity of a big,
multicellular, bilaterally symmetrical animal," Matz said.

"This is a very important discovery," Shuhai Xiao of Virginia Polytechnic
Institute said. "The fact that protists can make traces has important
implications for how we interpret many trace fossils."

The finding could overturn conventional thinking on a mysterious time in the
evolution of early life known as the Cambrian Explosion. Until about 550
million years ago, there were very few animals leaving trails behind. Then,
within ten million years an unprecedented blossoming of life swarmed across
the planet, filling every niche with hard-bodied, complex creatures.

"It wasn't a gradual development of complexity," Matz said. "Instead these
things suddenly seemed to burst out of a magic box."

Charles Darwin first noticed the Cambrian Explosion and thought it was an
artifact of a poorly preserved fossil record. The precambrian trace fossils
were left by multicellular animals, he reasoned, so there must be some gap in
fossils between the nearly empty Precambrian and the teeming world that
quickly followed. But if the first traces were instead made by G. sphaerica,
it would mean the Explosion was real; it must have been a diversification of
life on a scale never before seen.

Genetic analysis of the water-filled G. sphaerica cells also reveals
tantalizing clues that it could be the oldest living fossil on the planet.

"There's a 1.8 billion-year-old fossil in the Stirling formation in Australia
that looks just like one of their traces, and with a discoidal body
impression similar to these guys." Matz said. "We haven't proved anything,
but we might be looking at the ultimate living macroscopic fossil."