Long before evolution on Earth kicked in with a vengeance, it seemed to stall completely. From 1.7 billion years ago, for a billion boring years, Earth remained a slimy, near-static world of algae and microbes. The pace picked up 750 million years ago: glaciers spread, complex animals appeared, and by 520 million years ago the Cambrian revolution – an explosion of varied life – was under way. The reason for that long stasis has been a mystery.
|Too hot to handle [Credit: Frans Lanting/Corbis]
We may now have the answer: the gradual cooling of the planet’s interior. Just as turning down a stove burner slows the boiling of a stew pot, cooling of the mantle allowed the “scum” on top to thicken, says Peter Cawood at the University of St Andrews, UK. The resulting surface stability slowed geological change, seemingly stalling evolution for a billion years, until the planet was cool enough for tectonic activity to shift up a gear.
Cawood and Chris Hawkesworth, also at St Andrews, analysed studies of continental motions and geologic processes to see how they lined up with the boring period.
About 1.8 billion years ago, the cores of modern North America, Baltic Europe and Siberia collided and went on to form part of a supercontinent called Rodinia, which accounted for most of the planet’s land mass. They found that Rodinia was surprisingly stable, and that it stayed largely in tropical and temperate zones before breaking up 750 million years ago.
What caused such a long period of tectonic stability? Taras Gerya at the Swiss Federal Institute of Technology in Zurich reviewed studies that modelled Earth’s early formation and found that the process changed as the mantle cooled.
On the hot young Earth, the outer layer was too weak and soft for plate tectonics to operate until the upper mantle cooled enough to allow sections of crust to slip under each other, or subduct, at collision zones some 3.2 to 2.5 billion years ago. However, the mantle remained so hot that it softened the subducting oceanic crust too much for it to pull large areas of continental crust down behind it, as it does today. Only when the mantle cooled further did that modern-style subduction start, about 750 million years ago. Rodinia was duly ripped apart and the boring billion ended.
Cawood and Hawkesworth also found more big differences between the boring billion and other times. Major ice ages occurred before and after but not during the boring billion. Oxygen levels were also stable during, but varied widely before and after.
Cawood says all these systems are linked. “The atmosphere, the oceans, and the crust of the Earth were acting as a stable, interlinked system.” The start of modern-style plate tectonics that tore up Rodinia also brought other changes, and complex life evolved to meet the new challenges.
Martin Brasier at the University of Oxford says the stable period may also have been vital for the evolution of eukaryotic cells – cells with a nucleus of genetic material. “I argue that the boring billion was the anvil on which the eukaryote cell was forged. If so, then modern eukaryote cells could be the product of geologically rare conditions.”
Author: Jeff Hecht | Source: New Scientist [May 01, 2014]