Today, northern Kenya — where the Turkana Basin is found — is among the warmest areas on earth. It has little canopy forest, leaving the ground exposed to sunlight. The question is, was the ground here ever cooler than it is today? And if it was, why? Was it because the air was cooler, or because of more forest shading?

“When you measure the temperature of the ground, you learn a lot about the environment above it,” says John Eiler, professor of geochemistry at California Institute of Technology (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.

East Africa's Turkana Basin has been a hot savanna region for at least the past 4 million years — including the period of time during which early hominids evolved in this area.

These findings may shed light on the evolutionary pressures that led humans to walk upright, lose most of our body hair, develop a more slender physique, and sweat more copiously than other animals.

Their findings were based on measurements of the spatial distribution and concentrations of isotopes in carbonate ions.

The team examined the spatial organization (or ‘clumping') of rare, naturally occurring isotopes of carbon and oxygen — specifically, carbon-13 and oxygen-18 —in the form of carbonate ions that are constituents of minerals found in buried soils from northern Kenya. The clumping of these isotopes, Eiler and his colleagues have demonstrated in previous papers, is dependent on temperature: Hot temperatures lead to less clumping; cold temperatures, more.

“These carbonates are a common constituent of these soils,” Eiler explains. “If you have the ability to measure their isotopes, then you have a ground-temperature thermometer.”

When the researchers applied that thermometer to various layers of buried soils from East Africa, they found that the Turkana Basin region — one of the key places where hominid fossils documenting human evolution are found — has been a really hot place for a really long time. But why does it matter how hot Africa was millions of years ago? “This is the area where we find the occurrence of some of the earliest hominid species,” notes Eiler. “It tells us that this environment, though harsh, was a place where our ancestors could thrive.”

The findings also shed some light — and heat — on a longstanding debate over the origin of bipedalism in early humans, according to a Caltech press release.

“For a long time, anthropologists have hypothesized that bipedalism and other unique human traits would be advantageous to life in hot savanna environments,” says Benjamin Passey, one of the researchers.

“For example, by standing upright, we intercept less direct sunlight than if we were on all fours, and in hot, open environments, the ground and near-surface air can be appreciably hotter than the air a few feet above the ground. So, by standing upright, we are avoiding a high-temperature environment.”

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