More than one explosive evolutionary event occurred during the early evolution of animals

January 4th, 2008 - 4:39 pm ICT by admin  

Washington, January 4 (ANI): While it is believed that most major groups of complex animals appeared in the fossils record during a rapid evolutionary event called the Cambrian Explosion, which occurred 542 million years ago, Virginia Tech palaeontologists have now identified another explosive evolutionary event that occurred about 33 million years earlier among macroscopic life forms unrelated to the Cambrian animals.

The researchers have named this event the Avalon Explosion.

Writing about their discovery in the journal Science, the researchers said that their findings indicated that more than one explosive evolutionary event might have taken place during the early evolution of animals.

The Cambrian explosion event refers to the sudden appearance of most animal groups in a geologically short time period between 542 and 520 million years ago, in the early Cambrian Period. While there were not as many animal species as in modern oceans, most living animal groups were represented in the Cambrian oceans.

“The explosive evolutionary pattern was a concern to Charles Darwin, because he expected that evolution happens at a slow and constant pace,” said Shuhai Xiao, associate professor of geobiology at Virginia Tech.

Darwins perception could be represented by an inverted cone with ever expanding morphological range, but the fossil record of the Cambrian Explosion and since is better represented by a cylinder with a morphological radiation at the base and morphological constraint afterwards, he added.

He further said that Darwin reckoned that there should be long and hidden periods of animal evolution before the Cambrian Explosion.

However, palaeontologists have not found any such evidence, and the scientists have learnt that biological evolution has not been moving on a smooth road.

Accelerated rates may characterize the early evolution of many groups of organisms, said Michal Kowalewski, professor of geobiology at Virginia Tech.

With a view to testing whether other major branches of life also evolved in an abrupt and explosive manner, the researchers analyzed the Ediacara fossilsthe oldest complex, multicellular organisms that had lived in oceans from 575 to 542 million years ago, that is, before the Cambrian Explosion of animals.

“These Ediacara organisms do not have an ancestor-descendant relationship with the Cambrian animals, and most of them went extinct before the Cambrian Explosion. And this group of organisms most species seems to be distinct from the Cambrian animals,” said Virginia Tech graduate student Bing Shen.

The main question before the researchers was how did the Ediacara organisms first evolve. They wanted to determine whether they also appeared in an explosive evolutionary event, or the Cambrian Explosion was truly an unparalleled event.

They found that major types of Ediacara organisms appeared during the Avalon Explosion.

Kowalewski said that their research team had not anticipated the discovery.

Using the scientific literature, we were trying to create a more rigorous reconstruction of the morphological history of Ediacara organisms, he said.

The process involved adapting quantitative methods that had been used previously for studying morphological evolution of animals, but never applied to the enigmatic Ediacara organisms.

Scientists, however, are still unsure as to what were the driving forces behind the rapid morphological expansion during the Avalon explosion.

“But, one thing seems certain — the evolution of earliest macroscopic and complex life also went through an explosive event before to the Cambrian Explosion. It now appears that at the dawn of the macroscopic life, between 575 and 520 million years ago, there was not one, but at least two major episodes of abrupt morphological expansion, Xiao said. (ANI)

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