PARIS – The rugged landscape created by volcanic eruptions and tectonic plate shifts in eastern and southern Africa millions of years ago may be what prompted our human ancestors to start walking on two legs, a study said Friday.
The research, published in the journal Antiquity, challenges the commonly held theory that early hominins (members of the broad human family) were forced onto two feet on the ground because climate change reduced the number of trees they lived in.
According to the new hypothesis, the explanation for their evolution lies not in why they left the forests but in where they went.
“Our research shows that bipedalism may have developed as a response to the terrain, rather than a response to climatically driven vegetation changes,” said study coauthor Isabelle Winder from the University of York’s archaeology department.
Between 2 million and 6 million years ago, our ancestors lived exclusively in Africa — mainly in the east and south, where much tectonic activity happened.
Winder and her team compared geological changes with evolution of hominin anatomy over millions of years, and concluded it was likely that our early tree-living ancestors were attracted not to flat plains as widely thought, but to rocky outcrops and gorges. These would have offered shelter from predators and made it easier to corner prey.
But rugged terrain also required more upright postures for scrambling and climbing, prompting the emergence of bipedalism.
“For an animal moving on rough ground, the land is made up of lots of small, broken surfaces at different heights and angles. If you use four limbs to carry your weight, the chances are higher that you will be unable to position yourself effectively or that one of your hands or feet will slip,” Winder said by email. “It is to your advantage if you can balance on just two or three limbs and use the others to steady yourself.”
Thus our ancestors’ legs came to carry most of their weight, and their hands would have been used to stabilize and pull the body up rock faces and would have become better at grasping as a result — ultimately enabling the evolutionary leap to tool-making.
“The varied terrain may also have contributed to improved cognitive skills such as navigation and communication abilities,” Winder said.
She said the new finding answers a question that has stumped scientists for decades: How did our ancestors survive the many predators of Africa when they moved from the trees to the ground? As it turns out, the rugged terrain where they emerged made it easier to hide.
The move to flatter ground probably only started a few million years later, Winder said.
“This study is the first to successfully explain how our ancestors lived during this period and why they evolved as they did,” she said.
Climate change catalyst
Early humans living in South Africa made cultural and industrial leaps in periods of wetter weather, said a study that compared the archaeological record of man’s evolution with that of climate change.
Anatomically modern humans, Homo sapiens, first made their appearance in Africa during the Middle Stone Age, which lasted from about 280,000 to 30,000 years ago.
Some of the earliest examples of human culture and technology are found in southern Africa, with fossil evidence of innovative spurts that have left scientists puzzled.
The record reveals that a notable period of human advancement occurred about 71,500 years ago, and another between 64,000 and 59,000 years ago.
Examples of such innovation include the use of symbols, linked to the development of complex language, in engravings, the manufacture and use of stone tools, and personal adornment with shell jewelry.
“We show for the first time that the timing of . . . these periods of innovation coincided with abrupt climate change,” study coauthor Martin Ziegler of the Cardiff University School of Earth and Ocean Sciences said of the study in the journal Nature Communications.
“We found that South Africa experienced wetter conditions during these periods of cultural advance. At the same time, large parts of sub-Saharan Africa experienced drier conditions, so that South Africa potentially acted as a refugium for early humans.”
Ziegler and a team reconstructed the South African climate over the past 100,000 years using a sediment core drilled out from the country’s east coast. The core shows changes in river discharge and rainfall.
Coauthor Chris Stringer of London’s Natural History Museum said the findings supported the view that population growth fueled cultural advancement through increased human interactions.