Charles Darwin theorized that walking upright (bipedalism) probably began when early apes climbed down from the trees to spend more time on the ground, where they learned to walk on two legs. Was he right? Speculation on the origins of bipedalism has always been handicapped by far too many unknowns. That is, until now.
A new study, just published in the June issue of Science, turns traditional theories on their heads by suggesting the opposite. That apes may have been walking around upright in trees for millions of years. The study is based on a full year of direct observation, watching and recording the movements of wild orangutans in the trees of Sumatra, Indonesia.
Orangutans are fruit eating, tree dwelling apes that spend much of their lives among the branches foraging for food. And the best fruit is usually out hanging from the thinner branches of the tree. After analyzing close to 3,000 observations of orangutans moving through the trees, the researchers discovered that when orangutans moved along the thinner branches, they walked upright and used their arms only for balance (and picking the fruit).
When they navigated medium size branches, the orangutans still moved upright in a bipedal posture, but used their arms more for hanging than balance. Only when the apes moved to the heaviest branches did they drop to all fours.
This study provides solid evidence that walking in trees had very strong benefits for adapting to environmental changes. Susannah Thorpe, the researcher who spent a year in the field, said that “…we don’t need to explain how our ancestors could have gone from being quadrupedal to being bipedal.”
The authors agree with other researchers regarding the evolutionary timeline (5 to 24 millions years ago) along with the climatic changes in East and Central Africa that caused gaps in the rainforest canopy that the apes couldn’t cross.
The Science authors propose an alternate to the prevailing theory that apes learned to walk upright after they abandoned the trees. The Science article suggests that early human ancestors took to the ground and stayed there. They already knew how to walk upright. On the other hand, the ancestors of chimps and gorillas adapted to the changed environment by specializing in vertical climbing and learning how to walk on all fours between trees.
While this study sheds new light on the origins of bipedalism, it also suggests that bipedalism is no longer a reliable way to differentiate between humans and other ape ancestors. It muddies the waters a bit. We might never understand our true origins, but then again, we may already know.
AAAS News release