Science has long known that the elevation change in Africa was a driver for human ancestors descending from the trees and walking upright. But when, exactly, that happened has remained a mystery – at some point the East African plateau was a lush, moist lowland area, until it wasn’t. Now, a beaked whale fossil found in inland Kenya provides an exact date for when the elevation change occurred.
“The whale was stranded up river at a time when east Africa was at sea level and was covered with forest and jungle,” said vertebrate paleontologist Louis L. Jacobs of Southern Methodist University. “As that part of the continent rose up, that caused the climate to become drier and drier. So over millions of years, forest gave way to grasslands. Primates evolved to adapt to grasslands and dry country. And that’s when — in human evolution — the primates started to walk upright.”
The fossil is the only one of its kind found so far inland on the African continent. Dated to 17 million years ago, the whale would have been swimming upstream in a river with a gradual gradient ranging from 24 to 37 meters over more than 600 to 900 kilometers.
Beaked whales are still one of the ocean’s top predators, and are the deepest-diving marine mammals known to man. As such, they’re typically found in deep ocean waters, and it’s likely this whale would have called the Indian Ocean home. Given that the fossil was found in the bed of the ancient Anza River, it’s likely that the whale got there by becoming severely disoriented. Not unreasonable, considering that when doing so the whale was on the cusp of death.
“You don’t usually find whales so far inland,” Jacobs said. “Many of the known beaked whale fossils are dredged by fishermen from the bottom of the sea.”
The finding, though rare, is particularly useful in that it precisely identifies the period when the East African plateau began to rise far above sea level. Prior to the change in elevation, it thrived on moisture from the neighboring Indian Ocean and was heavily forested. At some point, though, the land began to ascend, at which point moisture from the ocean could no longer reach it. It’s about then when our earliest ancestors, previously content to thrive in the trees, descended to the new savannah and learned to walk on two legs.
Particularly helpful is the fact that the fossil (which was lost in storage at Harvard University for more than 30 years) was found under a 17 million year old lava flow, making the latest date for the elevation change unambiguous.