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Did humans cause ecosystem collapse in ancient Australia?

the nucleus. By far the most abundant variety is in the lightest, 12C. About 1% is 13C, a heavier sibling with an additional neutron. There is even less 14C, the unstable, radioactive heavyweight of them all.

"13C is the key," Fogel explained. "There are two main ways plants metabolize 13C . About 85% of plants belong to what is known as C3 photosynthesis group. This group incorporates less 13C than plants belonging to the second most common class, C4. Thus, the differences in 13C that we detected in the eggshells and teeth pointed to the type of plants that were consumed. Although, both types of plants were present before the extinctions, we saw a quick shift in dominance from C4 drought-resistant trees, shrubs and grasses, to C3 desert plants. "

Why did the scientists point to humans as the instigators of the ecological changes? First, they ruled out the other usual suspect--global climate change. They looked at their data in 15,000-year intervals back to 140,000 years ago. The period included dramatic climate changes, but no changes in the diet and environment until an abrupt transition was noted that corresponded to the arrival of humans. "Humans are the major suspect," commented Fogel. "However, we don't think that over-hunting or new diseases are to blame for the extinctions, because our research sees the ecological transition at the base of the food chain. Bands of people set large-scale fires for a variety of reasons, including hunting, clearing, and signaling other bands. Based on the evidence, human-induced change in the vegetation is the best fit to explain what happened at that critical juncture."


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Contact: Dr. Marilyn Fogel
m.fogel@gl.ciw.edu
202-478-8981
Carnegie Institution
7-Jul-2005


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