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Slug redefines time of origin of complex life

Tiny creature 585M years old

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EDMONTON -- University of Alberta scientists say they have uncovered the fossilized trails of an ancient slug that proves complex life evolved 30 million years earlier than established by previous discoveries.

The tiny tracks, found in a South American rock formation, date back 585 million years. The fossil find, reported Thursday in the journal Science, confirms what scientists had theorized was possible -- but not previously proved -- allowing for an important addition to the evolutionary chronology.

"We now have the oldest physical evidence that multicellular life that could move was around between 600 million and 585 million years ago," said U of A geomicrobiologist Kurt Konhauser, one of the article's seven co-authors.

Before these animals evolved, there was single-celled life such as bacteria and eventually other creatures such as sponges, which could not move on their own.

"Then by 585 million years ago, we have evidence something bigger was in existence," Konhauser said.

Bigger, of course, is a relative term.

The slug-like animal that created these three-millimetre-wide burrows through the sand would have been tiny, a millimetre wide and three mm long, explained Ernesto Pecoits, a post-doctoral geology fellow at the university and the report's lead author.

Pecoits and Natalie Aubet, a PhD student, discovered the burrows in 2007 as they were studying a unique cluster of rocks known as the Tacuari formation in eastern Uruguay.

As the two Uruguayan geologists worked to analyze and describe the different rocks, Pecoits noticed the lines in the stone. It was a chance discovery, but "he knew he was onto a winner," Konhauser said.

The team eventually found the trails, extending up to 40 centimetres in length, in six different locations.

From the ridges left behind along the burrows, the researchers believe the unnamed slug, known as a bilaterian because of its symmetrical sides, moved through the sands with wavelike movements. Indents along the trail show the slug had primitive feet of sorts that could help it move.

At different points in the trail, researchers said it appears the rice grain-sized slug would abruptly leave. They speculate that it was surfacing from the sands below the ocean floor into the bacterial slime above to eat or to get oxygen.

"From an evolutionary point of view, this is quite complex behaviour," Pecoits said. "Once the organism starts moving, that's a huge stage of evolution."

To put this discovery in perspective, it helps to have a sense of where it fits into the geological time scale. The Earth is about 4.6 billion years old. Single-cell life forms, starting with bacteria, began to appear about 3.5 billion years ago. Other studies have shown that creatures like sponges appeared about 700 million years ago. By comparison, the earliest dinosaurs are relative newcomers, first appearing in the fossil record about 245 million years ago.

Prior to finding these slug burrows, researchers say the oldest confirmed discovery of a creature that could move dated back 555 million years ago.

The researchers say the discovery shows life evolved quite quickly -- geologically speaking -- going from sponges to these moving slugs in just tens of millions of years.

-- Postmedia News

Republished from the Winnipeg Free Press print edition June 29, 2012 A21

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