Their findings -- which were based on measurements of the spatial distribution and concentrations of isotopes in carbonate ions -- are being reported this week in the early online edition of the Proceedings of the National Academy of Sciences (PNAS). "When you measure the temperature of the ground, you learn a lot about the environment above it," says John Eiler, Robert P. Sharp Professor of Geology and professor of geochemistry at Caltech. In fact, he says, soil temperature tells you not just about air temperature, but about whether there were trees and plants to shade the soil, keeping temperatures cooler during the hottest part of the day.It is suggested that these kinds of conditions, in which the forests retreated led to the earliest hominids trying to minimize their heat exposure. This would have been done two ways: sweating and walking upright.
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