Grassland theory uproots story of human ancestors’ first steps

Analysis shows open landscapes may have existed 10 million years earlier than thought, challenging the idea that shrinking forests led to humans walking upright.

An artist’s recreation of how Morotopithecus would have needed to come down from the trees. Illustration: Corbin Rainbolt/The Times

The story of how our distant ancestors first came to stand upright may have to be rewritten after a reappraisal of when Africa gained its earliest grasslands.

It was widely thought ancient apes first adopted a vertical posture while living high in the canopies of dense forests, where the limbs of one tree would have met those of the next.

Being upright, it was reasoned, would have made it easier for these animals to climb and to reach for fruit while balancing on branches. It was thought that only rarely, if ever, would they have set foot on the ground.

Researchers scoured sites across east Africa, including the Napak region in Uganda, for clues of what vegetation had grown there millions of years ago. Picture: J. Kingston/The Times

This idea has been challenged by research that paints a different picture of what the east African home of these creatures would have looked like 21 million years ago. Part of the work focused on a group of plants known as the C4 grasses, found on modern savannahs.

Researchers examined nine sites in east Africa as part of a 10-year project. They searched for clues of what vegetation had grown in these places long ago, including the remnants of ancient phytoliths – microscopic particles of silica that form within plants and help to give them their structure.

From the shape of the phytoliths that were found, along with other evidence, they were able to show that C4 grasses were an important part of the ecosystem about 21 million years ago. This suggests that substantial areas of open grassland existed at least 10 million years earlier than was thought.

“For the first time, we’re showing that these grasses are widespread,” Robin Bernstein, of the US National Science Foundation, said.

A fossil specimen shows part of the Morotopithecus face and upper jaw. Picture: The Times

The results, which left the researchers amazed, suggest that a very early ape known as Morotopithecus lived not in a dense jungle, but in a grassy landscape with scattered areas of woodland.

“Morotopithecus is regarded as one of the best representatives of the ancestors to all of the living apes and humans,” Professor Kieran McNulty of the University of Minnesota, one of the senior authors of the research, said.

Morotopithecus held its torso upright – a key ape characteristic – but would not have walked on two legs like a human. Instead, it would probably have scurried along the floor like a monkey.

“People have assumed that these upright postures evolved in what we think of as a closed canopy forest – where the branches of trees are touching the branches of other trees, and that these apes wouldn’t necessarily have to come down to the ground,” McNulty said.

“What we’re showing here is that some of these apes, including Morotopithecus, lived in a much more open space. There would be trees around but to get from one to the next, you’d have to come down.”

That Morotopithecus lived in an open habitat may challenge ideas about what drove the development of our own species millions of years later.

Roughly speaking, researchers have often assumed that humans came to walk on two legs because dense forests receded and grassland environments opened up. Being upright would have allowed us to see for relatively large distances across a flat savannah and would have given us an efficient running gait.

However, the idea that “shrinking forests made us human” seems too simplistic, McNulty said.

Professor Laura MacLatchy of the University of Michigan, one of the lead authors of the work, said: “These open environments have been invoked to explain human origins, and it was thought that you started to get these more open, seasonal environments between 10 and seven million years ago.

“Now that we’ve shown that such environments were present at least 10 million years before bipedalism evolved, we need to really rethink human origins, too.”

The findings appear in the journal Science.

The Times