Saturday, December 09, 2006

 

Finding an answer to Darwin's Dilemma

The sudden appearance of large animal fossils more than 500 million years ago - a problem that perplexed even Charles Darwin and is commonly known as 'Darwin's Dilemma' - may be due to a huge increase of oxygen in the world's oceans, says Queen's University (Canada) paleontologist Guy Narbonne (homepage), an expert in the early evolution of animals and their ecosystems.

In 2002, Dr. Narbonne and his research team found the world's oldest complex life forms between layers of sandstone on the southeastern coast of Newfoundland. This pushed back the age of Earth's earliest known complex life to more than 575 million years ago, soon after the melting of the massive 'snowball' glaciers. New findings reported today shed light on why, after three billion years of mostly single-celled evolution, these large animals suddenly appeared in the fossil record.

In a paper published on-line in Science Express, Dr. Narbonne's team argues that a huge increase in oxygen following the Gaskiers Glaciation 580 million years ago corresponds with the first appearance of large animal fossils on the Avalon Peninsula in Newfoundland.

Now for the first time, geochemical studies have determined the oxygen levels in the world's oceans at the time these sediments accumulated in Avalon. 'Our studies show that the oldest sediments on the Avalon Peninsula, which completely lack animal fossils, were deposited during a time when there was little or no free oxygen in the world's oceans,' says Dr. Narbonne. 'Immediately after this ice age there is evidence for a huge increase in atmospheric oxygen to at least 15 per cent of modern levels, and these sediments also contain evidence of the oldest large animal fossils.'

Continued at "Finding an answer to Darwin's Dilemma"
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Based on "Late-Neoproterozoic Deep-Ocean Oxygenation and the Rise of Animal Life"
Don E. Canfield, Simon W. Poulton , Guy M. Narbonne

Abstract

Animals have an absolute requirement for oxygen, and an increase in late Neoproterozoic oxygen concentrations has been forwarded as a stimulus for their evolution. The iron content of deep-sea sediments show that the deep ocean was anoxic and ferruginous before and during the Gaskiers glaciation 580 million years ago, becoming oxic afterward. The first known members of the Ediacara biota are found shortly after the Gaskiers glaciation, suggesting a causal link between their evolution and this oxygenation event. A prolonged stable oxic environment may have permitted the emergence of bilateral motile animals some 25 million years later.

(The original press release from Queen's University says "PLEASE NOTE: A PDF of the research findings published today in Science Express is available upon request." - A contact email address is given on the Abstract page)
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Also see the open access June 2000 PNAS paper "Solution to Darwin's dilemma: Discovery of the missing Precambrian record of life" by J. William Schopf:

Abstract

In 1859, in On the Origin of Species, Darwin broached what he regarded to be the most vexing problem facing his theory of evolution - the lack of a rich fossil record predating the rise of shelly invertebrates that marks the beginning of the Cambrian Period of geologic time (approx 550 million years ago), an "inexplicable" absence that could be "truly urged as a valid argument" against his all embracing synthesis. For more than 100 years, the "missing Precambrian history of life" stood out as one of the greatest unsolved mysteries in natural science. But in recent decades, understanding of life's history has changed markedly as the documented fossil record has been extended seven-fold to some 3,500 million years ago, an age more than three-quarters that of the planet itself. This long-sought solution to Darwin's dilemma was set in motion by a small vanguard of workers who blazed the trail in the 1950s and 1960s, just as their course was charted by a few pioneering pathfinders of the previous century, a history of bold pronouncements, dashed dreams, search, and final discovery.
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And yesterday's post "Geologists Provide New Evidence for Reason Behind Rise of Life in Cambrian Period":

"Geologists have uncovered evidence in the oil fields of Oman that explains how Earth could suddenly have changed 540 million years ago to favor the evolution of the single-celled life forms to the multicellular forms we know today.

Reporting in the December 7 issue of the journal Nature, researchers from MIT, the California Institute of Technology, and Indiana University show that there was a sudden change in the oxygenation of the world's oceans at the time just before the 'Cambrian explosion,'* one of the most significant adaptative radiations in the history of life. With a increased availability of oxygen, the team speculates, single-celled life forms that had dominated the planet for the previous three billion years were able to evolve into the diverse metazoan phyla that still characterize life on Earth..."

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