Wednesday, January 26, 2005

Climate change and extinctions

This is interesting:
Like a chaotic pendulum, earth's climate swings, at uneven intervals, between warm and chilly ages lasting from thousands to millions of years. New research suggests that about 251 million years ago, one of those swings jolted the world so violently that oxygen became scarce, the planet's thermostat went awry and nearly all life fell into oblivion in the greatest of mass extinctions. ...

Dr. Ward proposes that the climatic changes were wrought by geological ones. As the supercontinent Pangea pulled apart, the rearrangement of land lowered sea levels. That exposed decaying plants in the sediments to air, producing chemical reactions that dropped oxygen levels from 21 percent to 16 percent or lower. Trying to breathe at sea level then was as hard as breathing at 14,000 feet today.

The breakup of Pangea may have also set off widespread volcanic eruptions that flooded what is today Siberia with hundreds of thousands of cubic miles of lava. Carbon dioxide released by the eruptions created a greenhouse effect, and the hotter temperatures killed plants. "It's simply a world that's going climatically screwy very fast," Dr. Ward said.
It's amazing to consider how variable and unstable the earth's climate is and has been. One of the most fascinating concepts in geological history is the idea of the "snowball earth," in which increasing ice cover creates a runaway albedo effect (ice reflects light, so the earth absorbs less heat from the sun, so it gets colder, so there's more ice, etc.) so bad that glaciers covered almost the whole earth. The earth escaped from the snowball state because of carbon dioxide: as ice covered the land and oceans, it would block the carbon cycle by preventing carbon dioxide from getting locked up in rock and sediment (through weathering reactions with rock that normally convert carbon dioxide to calcium carbonate, or through photosynthesis). Meanwhile volcanoes kept pumping out carbon dioxide at the same rate they always do; with the carbon cycle blocked, carbon dioxide would continue accumulating in the atmosphere. After millions of years, enough carbon dioxide accumulates to reverse the snowball effect and the earth swings suddnely to a very warm climate. Apparently this occurred several times between 580 and 750 million years ago.

The snowball earth also suggests a highly speculative, but fascinating idea: Could the snowball earth explain why there was such a sudden flourishing of multicellular animals in the Cambrian explosion 525-600 million years ago, with representatives of all modern phyla appearing so quickly in the fossil record?
A series of global "freeze-fry" events would cause population "bottlenecks and flushes", observed to accelerate evolutionary rates in some species. The crash in population size accompanying a global glaciation would be followed by millions of years of comparative genetic isolation in high-stress environments. This is a favorable scenario for genomic reorganization and the evolution of new body plans.
I have no idea how plausible this idea is, but it's certainly an attractive explanation...

17 Comments:

Anonymous Anonymous said...

"body plans" is going too far. bilateral symmetry isn't even that heritable, it's a genetic trait that can be disrupted developmentally. it seems that isolation can result in changes, but they might be sexual in character.

but i'm not up on the latest literature either. 

Posted by razib

1/26/2005 02:30:00 AM  
Anonymous Anonymous said...

Again, wild speculation - but perhaps it could be that the emergence of body-plan-specifying Hox genes and their sequential organization on the genome (and other similar features) could have been due to the "genomic reorganization" that might be induced by the stress of a snowball earth? 

Posted by Andrew

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