Dense forests in eastern Africa started to give way to open woodlands 10 million years earlier than previously thought, driving the evolution of upright apes that later gave rise to humans. That is the conclusion of a team that has been analysing everything from ancient soil to fossil ape bones at several sites in the region.
“Part of the reason why we feel very confident in this story is that it’s based on multiple lines of evidence,” says Laura MacLatchy at the University of Michigan.
It was thought that dense forests in eastern Africa only began to turn into grasslands from around 10 million years ago, and that this change is what made our ancestors come down from the trees and take to running across the savannah.
But MacLatchy and her colleagues have now done analyses of fossil soils from several sites in Kenya and Uganda, revealing that C4 grasses were present as far back as 21 million years ago. C4 grasses, which are more productive and drought-resistant than other grasses, are the main type found in grasslands.
“We found grasses at almost every site we looked at,” says team member Daniel Peppe at Baylor University in Texas.
The findings point to very open woodland rather than pure grassland, says Peppe, with around 10 to 30 per cent of the land covered in trees at that time. There were also wet and dry seasonal changes, meaning animals couldn’t rely on fruiting trees all year around, as occurs in tropical rainforests.
“We’re saying these variable environments were around a lot longer ago, twice as far back as we thought,” says MacLatchy. “So we really need to rethink origins of apes as well as origins of humans.”
The predecessors of apes walked on branches on all fours like many animals still do today, limiting the use of their hands. But around 20 million years ago, some became bigger.
This meant that, to reach the ends of small branches, they had to find other ways of moving, such as swinging by the arms or standing on branches while holding on to others. “You have to distribute your body weight over multiple supports. You can’t get there if you are big by walking on top of branches,” says MacLatchy.
Crucially, these changes resulted in apes with an upright posture, paving the way for upright walking to evolve later on.
The conventional view is that it was fruit-eating apes living in unbroken forests that evolved this upright posture. But finds by MacLatchy and her team, including the teeth, jaw and femur of an ape called a Morotopithecus that lived at this time, challenge this idea.
The teeth suggest that this ape was a leaf-eater, not a fruit-eater, while the shortness of the femur relative to body size – like those of chimps and gorillas – and a vertebra previously found by another team point to an upright posture. MacLatchy thinks these animals climbed to the top of trees to reach young leaves and then moved across ground to reach other trees – in other words, that the upright posture came about as a result of the change to open, seasonal woodland.
“MacLatchy and colleagues’ habitat reconstruction looks ironclad, yet I remain cautious,” says Kevin Hunt at Indiana University, Bloomington. Mandrills also have relatively short femurs but walk on all fours, including on branches, he says.
Hunt is especially sceptical about the idea that Morotopithecus was predominantly a leaf-eater, although it may well have eaten leaves when times were lean, he says.