Oceanic records paint a more complex picture of human evolution

by Jessica Orwig
Tuesday, May 28, 2013

It has long been hypothesized that human ancestors evolved the ability to walk upright — a feature that appeared about 6 million years ago — in response to African landscapes changing from forests to grasslands. Now, a group of scientists has assembled the most continuous timeline of landscape evolution and grassland development over the last 12 million years near the African Rift Valley in northeastern Africa — and the timeline contradicts conventional hypotheses.

To examine the landscape in northeastern Africa, Sarah Feakins, a geologist at the University of Southern California, and her colleagues turned to oceanic cores. The Deep Sea Drilling Project extracted samples of ocean floor from the Gulf of Aden off the coast of northeastern Africa. Over time, leaf wax and pollen from the land surface are carried to the ocean by wind and runoff, eventually accumulating on the seafloor. The history of the nearby landscape is encapsulated in layers of sediment built up over time. The leaf wax and pollen the team found showed that grasslands were already extensive by 12 million years ago, Feakins says. Moreover, she and her colleagues reported in Geology, the data suggest that the average extent of grasslands actually decreased from the Miocene through today.

The results were surprising, Feakins says. Previous research based on isotopes in ancient soil samples had focused on a certain group of grasses called C4s that are prevalent in Africa today. So earlier ancestral grasslands had been overlooked, she says.

“This paper is the most intensive, high-resolution study of vegetation change relevant to the African continent that I’ve seen,” says Richard Potts, a paleoanthropologist at the Smithsonian National Museum of Natural History who was not involved in the study. Previous studies that relied on soil data were less complete, he says, because only some of the area’s geological history preserves soils. On the other hand, “deep-sea cores [like Feakins' team used] show continuous deposition, so you can study them at very fine intervals.” In addition, Potts adds, “what’s so exciting about this long, continuous record is that it provides an opportunity for more substantial ties between the fossil and environmental record on land with the environmental record in the deep sea.”

However, Potts notes, “early humans did not live in the oceans, so it’s going to be very important to find ties between the deep-sea record and the terrestrial records of where early humans lived.” The next step, he says, is to understand how this change in vegetation may have affected ancient hominins and other animals.

Another way to determine what kinds of vegetation were present is to look at fossilized teeth. The new data correspond nicely with fossilized horse teeth from this period, which show that horses started eating C4 grasses about 10 million years ago, Feakins says. A next step might be to analyze hominin teeth for diet.

One thing is clear from the new research, says Bernard Wood, a professor of human origins at George Washington University: “Whatever scenario scientists want to come up with for hominin evolution will now have to be consistent with what Feakins and her colleagues found.” Feakins' team’s evidence throws cold water on hypotheses that suggest hominins in this area evolved by adapting to the emergence of open grasslands, he says.

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