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article imageMassive geographic event caused Cambrian explosion of species

By Robert Myles     Nov 2, 2014 in Science
Austin - Newly published research may help solve the puzzle of the Cambrian explosion, a period in Earth’s ancient past that saw a massive diversification of animal life.
The Cambrian explosion started 542 million years ago, during the Cambrian Period in Earth’s prehistory. In the space of just 20 million years — the blink of an eye in geological terms — most major animal phyla appeared, the species strands that would later evolve into dinosaurs, and, much later, larger mammals. It’s one of the most significant events in Earth’s 4.5-billion-year history.
Before the Cambrian explosion, most organisms consisted of simple, single cell forms of life that occasionally organized into colonies.
Since the days of Charles Darwin, just why there should have been such a rapid and widespread diversification of animal life in such a short period of time is a riddle that has baffled scientists.
Now, a new analysis of geologic history from the Jackson School of Geosciences at the University of Texas at Austin's (UTA) suggests the movement of Earth’s tectonic plates acted as the detonator for the Cambrian explosion.
Earth’s tectonic plates are in a constant state of movement; sometimes drifting apart from each other, at other times scrunching into each other. The San Andreas Fault and the Himalayas are two prime geologic and geographic examples of plate tectonics in action.
The Jackson School’s research paper, authored by Ian Dalziel, a research professor at the Institute for Geophysics and a professor in UTA’s Department of Geological Sciences is published in the November issue of Geology, a journal of the Geological Society of America. Dalziel suggests a major tectonic event may have triggered sea level rises and other environmental changes that accompanied the apparent burst of life.
The Cambrian explosion amounted to a surge of evolution leading to the sudden appearance of almost all modern animal groups. Such rapid evolution has been well documented from fossil records of the period but just why the Cambrian explosion happened at all has remained a mystery.
So much so that the Cambrian explosion is sometimes called "Darwin's dilemma," often mentioned by creationists, since paleontological records seem to show animal life, broadly as we know it, having begun all at once, roughly half a billion years ago.
Such an animated, animalistic burst appears to be at odds with Darwin's hypothesis of gradual evolution by natural selection.
"At the boundary between the Precambrian and Cambrian periods, something big happened tectonically that triggered the spreading of shallow ocean water across the continents, which is clearly tied in time and space to the sudden explosion of multi-cellular, hard-shelled life on the planet," said Dalziel.
But it wasn’t just a rise in sea levels that caused life to blossom. According to Dalziel, these ancient geologic and geographic changes probably led to a build-up of oxygen in the atmosphere and a change in ocean chemistry. These changes meant that Earth’s environment became more conducive to allowing more complex life-forms to evolve.
The UTA paper is the first to integrate geological evidence from five present-day continents — North and South America, Africa, Australia and Antarctica — and examine paleo-geography during the Cambrian explosion, such a critical time for the evolution of life on our planet.
Dalziel advances the proposition that what constitutes present-day North America was still attached to the southern continents until some way into the Cambrian period.
Until now, many reconstructions of the globe's geography during the early Cambrian Period postulate that the ancient continent of Laurentia — the progenitor of what is, for the most part, now North America — had already separated from the supercontinent known as Gondwanaland.
But Dalziel suggests the development of a deep oceanic gateway between the Pacific and Iapetus — what would later become the Atlantic — oceans isolated Laurentia in the early Cambrian Period.
In Dalziel’s revisionist view of Earth’s proto-geography, Laurentia’s isolation immediately preceded the global sea level rise and the apparent explosion of life.
"The reason people didn't make this connection before was because they hadn't looked at all the rock records on the different present-day continents," said Dalziel.
He highlighted the rock record in Antarctica, for example, sourced from the very remote Ellsworth Mountains.
"People have wondered for a long time what drifted off there, and I think it was probably North America, opening up this deep seaway," Dalziel said, adding, "It appears ancient North America was initially attached to Antarctica and part of South America, not to Europe and Africa, as has been widely believed."
Although this new analysis adds to the body of evidence suggesting a massive tectonic shift caused Earth’s sea levels to rise more than half a billion years ago, Dalziel concedes more research is needed to determine whether this new chain of paleo-geographic events can truly explain the sudden rise of multi-cellular life in the shape of the Cambrian explosion.
“I'm not claiming this is the ultimate explanation of the Cambrian explosion, but it may help to explain what was happening at that time," he concluded.
More about Cambrian period, Cambrian explosion, Laurentia, gondwana, Iapetus
 
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