New geologic evidence supports the theory that the transition from forest to grassland encouraged key adaptations during early human evolution.

Genetic analysis suggests the first hominins split from chimpanzees roughly 6 to 7 million years ago. During that time, scientists believe climate change precipitated a shift in the vegetation of East Africa, from dense forest to savanna.

The thinking goes that fewer trees and new wide open spaces forced the first hominins to take to the ground and adapt to a new environment. They developed a larger brain for problem solving, bipedalism for covering greater distances and more sophisticated social structures as a result of increased interaction.

A new study, published in the journal PNAS, offers the most extensive evidence yet of a climatic shift around the time of the appearance of the first hominins. The study focuses on paleobotanical evidence from Ethiopia and Kenya, thought to the birthplace of humans.

The new evidence comes from the sediment cores collected from the bottom of the Red Sea and the western Indian Ocean. They contain evidence of ancient plant life that blew out to sea and sank millions of years ago.

Scientists analyzed carbon-based chemicals called alkanes found in the plant remains. These chemical signatures can be matched to different plant types. Samples older than 10 million years feature higher concentrations of alkanes associated with the types of photosynthesis carried out by woody plants like trees, while younger samples yielded larger amounts of alkanes linked to grasses.

"The entire evolution of our lineage has involved us living and working in or near grasslands," lead study author Kevin Uno, a postdoctoral research scientist at Columbia University's Lamont-Doherty Earth Observatory, said in a news release. "This now gives us a timeline for the development of those grasses, and tells us they were part of our evolution from the very beginning."

Researchers say the grasslands were likely patchy at first, and that a number of other ecological factors likely shaped the evolution of the first hominins. But grasslands played a role, and as they expanded, they continued to influence the competition and interactions among the earliest humans.

"Lots of people have conjectured that grasslands had a central role in human evolution," said co-author Peter deMenocal, a climate scientist at Lamont-Doherty. "But everyone has been waffling about when those grasslands emerged and how widespread they were. This really helps answer the question."