Just when did our human ancestors come down from the trees to become permanent ground-dwelling apes? Did the evolution of our upright posture and two-footed (bipedal) locomotion mark the end of a life in the trees?
If our ancestors were terrestrial bipeds, why are we capable of amazing sporting feats such as gymnastics, using muscles and joints that evolved for swinging in the trees?
A new study of ancient shoulder bones from Ethiopia sheds light on these questions by showing our early ancestors actually continued to live in the trees millions of years after they first stood upright.
The findings of the paper – published by US researchers David Green and Zeresenay Alemseged in Science on October 26 – have important implications for understanding the course of our evolution and even offer some fascinating insights into the way we behave today.
Events such as these are a delight for anthropologists as they provide us with a rare glimpse of the physical capabilities of our species at its peak.
As a student of human physical form and the evolutionary processes and history that shaped it, gymnastics events are some of my favourites because they showcase our capabilities in a way that makes our primate heritage all too real.
What other human activity sees us swinging between wooden beams (branches?) using our remarkably mobile ape shoulders as though our species never left the trees?
A shoulder to lean on
The human shoulder joint is very shallow and surrounded by a stabilising cuff of muscles, making it capable of a remarkably wide range of movements, but also prone to injuries such as dislocation.
Our shoulders are ape-like, in many ways resembling those of chimpanzees and gorillas, but our upper limb muscles are puny compared to theirs, suggesting our ancestors used them little for climbing.
Despite some important similarities, our shoulder is certainly not identical to other apes, as our hominin ancestors at some point stopped living in the trees and adopted a fully terrestrial lifestyle, much as we have.
When they did, their shoulder anatomy changed. Shoulder blades became larger and joints faced towards the sides of their bodies like ours, not angled towards their heads – as seen in chimpanzees – who do much climbing with their arms above their heads.
A lofty lifestyle
Green and Alemseged analysed the shoulder blades (scapulae) of one of the most complete fossil hominins ever found – the more-than-3-million-year-old Australopithecus afarensis juvenile skeleton from Dikika, Ethiopia: one of Lucy’s kind.
Although the skeleton was originally described and published more than six years ago, the scapulae remained encased in sediment, and only after years of toil was their anatomy revealed.
The Dikika child’s shoulders show it had a rotated (head-facing) shoulder joint, a lot like that of living apes, confirming its tree-dwelling lifestyle.
At the same time, the child, like other fossils of afarensis, provides clear evidence of bipedalism.
So, despite the fact our ancestors were ground-dwelling bipeds from the time immediately after we shared a common ancestor with living chimpanzees, they retained a very ape-like lifestyle, spending a great deal of their time climbing in trees.
In fact, for the first two-thirds of our evolution, our ancestors seem to have had a mixed lifestyle of ground-walking bipedalism and tree-living arm-swinging.
They probably fed, slept and escaped predators in trees, and moved between galleries of forests, foraged and drank water on the ground.
Despite the obvious similarities to living apes, they were bipeds not quadrupeds (four-footed) apes, and seem to have mostly lived in open and lightly wooded environments. This is in contrast with chimpanzees, which live in more densely wooded habitats and walk on the ground using their feet and knuckles.
Until now, we only had the shoulder bones of adult afarensis individuals to study, and anthropologists debated whether their features truly reflected a life in the trees.
The alternative was the ape-like features of Lucy’s kind were simply evolutionary baggage and not used for serious climbing at all.
The Dikika child convincingly shows that these early human ancestors were actually not very human-like in their shoulders and employed above-head climbing – or were tied to the trees – throughout their lives.
All of this changed with the arrival of the human genus Homo some 2.5 million years ago with its human-like shoulder joint and commitment to a fully terrestrial locomotion and lifestyle.
With Homo we see the beginnings of the hunting and gathering lifestyle heavily reliant on animal foods, stone tools and probably fire, and occupying vast territories covered on foot and extending into the challenging climates of Ice Age Asia and Europe.
Instead of a life in the trees, they probably spent virtually all of their time on the ground, and may even have relied heavily on the vast herds of animals on the African savannah as the big cats do today.
Yet, the members of Homo have retained some rather remarkable capabilities – on show recently in London – which must have served our ancestors well even after they became obligate ground-dwelling bipeds.
There was always fruit to be picked from the trees and the occasional predator to be avoided.
Not required, but still entertaining
We possess some truly remarkable athletic capabilities which were honed by natural selection and reached their peak with Lucy’s kind.
While these abilities are no longer required for survival, today they provide us with a remarkable source of entertainment.
The Dikika child shows us again it’s wrong to think of early human ancestors like Lucy as just transitional forms from ape ancestor to us.
They had a unique and remarkable set of physical adaptations, ecology and lifestyle that differs in important ways from our own as well as gorillas and chimpanzees.
These physical adaptations are worthy of investigation in their own right.