High above the forest floor, I feel anxious stepping out onto a shaky platform strung between the trees. I’m not risking life and limb to retrieve a stray kitten, but getting back to my ancient arboreal roots at an adventure site in the UK called Go Ape, where travelling through the treetops is the way to go.
Rather than swinging along Tarzan-style, most of the connections between trees require walking across narrow beams or wobbly wooden platforms. Even with a safety wire it’s a vertiginous experience: it’s so much easier when there are cables above my head to hold onto as I move along.
This kind of hand-assisted tree-walking makes sense for us humans, adapted as we are to walking on the ground with two feet. You might assume that our primate ancestors had better ways of moving in the treetops, but some researchers now think that the common ancestor of the great apes often moved around in the trees in just this way, and that this form of movement set the stage for the evolution of human bipedalism. Simply put, we might have started walking on two legs before we came down from the trees, rather than afterwards as most theories assume.
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Our upright posture and two-footed mode of locomotion is one of our defining characteristics. Fossils suggest that early hominins might have evolved this trait as early as 6 million years ago, not long after the common ancestor of chimpanzees and hominins split into their different lineages (see “These bones were made for walking”). By freeing our ancestors’ hands to carry tools as well as use them, bipedalism may have aided the later explosion in brain size.
The big question is why did we start walking upright? There is no shortage of ideas: a dozen or more hypotheses have been proposed. Many readers will be familiar with the idea that as our ancestors moved out from trees and onto the savannah, they took to walking upright because it is a more energy-efficient way of travelling long distances than knuckle-walking like chimpanzees and gorillas. Or perhaps a changing climate made some forests more open, forcing our ancestors to cover increasing distances across the ground to find food. Standing upright would also have reduced the area of the body exposed to sunlight, which might have allowed our ancestors to forage in the open during the hottest part of the day.
These savannah-related ideas have fallen out of favour with palaeoanthropologists, however. Fossil discoveries have revealed that although early hominins had legs adapted to walking upright, they still retained long forearms adapted for climbing. What’s more, the earliest fossils have been found in sites that were forested, rather than in grassland.
Another possibility is that bipedalism began as an adaptation for feeding on the forest floor rather than for finding food. Feeding in a squatting position might have led to the development of flatter feet, for maintaining balance while resting on the haunches. Squat-feeding could also have led to a more flexible waist and a spine held more vertically, all of which would have primed our ancestors for standing and walking upright. Yet another idea is that bipedal displays evolved as a way to resolve conflicts, or as a form of sexual signalling.
Despite this diversity of hypotheses for why humans took to standing upright and walking on two feet, the vast majority assume that it had something to do with living on the ground. It seems natural to imagine that the common ancestor of chimps, gorillas and humans was a chimp-like creature able to move through the trees by climbing and swinging, as well as moving on the ground by getting on all fours and knuckle-walking – and occasionally walking upright for brief periods.
Why walk upright?
In fact, we don’t know exactly how our ancestors moved before they started walking upright, which makes working out why bipedalism evolved even more complex. Over the years, a few researchers have challenged the orthodoxy, speculating that bipedalism actually evolved in the trees. One idea is that it emerged in a small ape that moved on all fours through the trees ().
Such speculation received little attention until last year, though, when a new twist on the tree-walking idea was proposed, based largely on studies of a living ape. Not the ground-dwelling chimpanzee, our closest relative, but a tree specialist: the orang-utan.
Orang-utans live almost entirely in the trees. They clamber around the forest canopy and from tree to tree mostly by hanging from branches – not quite the swift swinging perfected by gibbons, but elegant in its own way. They also use their feet, which are fashioned more like hands, to clasp branches as they move through the rainforest.
However, observations of orang-utans in the wild by Susannah Thorpe and Roger Holder of Birmingham University, and Robin Crompton of the University of Liverpool, all in the UK, have revealed another element of the orang-utan’s repertoire. From time to time, orang-utans “walk” along branches, grasping the support with their prehensile toes, while holding onto the branches above to aid balance.
Thorpe and her colleagues call this “hand-assisted bipedalism” and it has occasionally been observed in other apes too. But why should orang-utans, in particular, have anything to tell us about humans?
“Orang-utans, in particular, may have something to tell us about humans”
Well, they have the most human-like (that is, upright) posture of all primates when they stand or walk using hand-assisted bipedalism. Their hip and knee joints are more extended, unlike the chimp’s bent-knee, bent-hip version. This, says Thorpe, makes the orang-utan’s hand-assisted walking a more plausible precursor to exclusive bipedality than knuckle-walking along the ground.
The team’s studies also reveal why a tree-dwelling specialist like the orang-utan resorts to tree-walking. “Orang-utans use extended-leg, hand-assisted bipedalism to move on the really thin, flexible branches at the periphery of trees,” says Thorpe, which is where the tastiest fruits often hang. Being able to negotiate thinner branches also allows orang-utans take a more direct route from tree to tree, saving time and energy.
For a large animal, hand-assisted walking is much safer than relying only on a single branch. “The ability to move bipedally in the forest canopy offers very clear benefits to orang-utans – benefits which we would expect to be subject to natural selection,” says Thorpe. Yet until this study, the adaptive value of arboreal bipedalism had eluded researchers. At first glance, Crompton admits, the whole idea seems counter-intuitive.
“Hand-assisted walking is much safer than relying only on a single branch”
The similarities that Thorpe, Holder and Crompton see between the upright postures of humans and orang-utans led them to propose a new evolutionary scenario for the emergence of bipedalism – what might be called the tree-walking view, since it all began among the branches ().
According to this view, the ancestor of all the great apes was, like the orang-utan, an arboreal specialist that used hand-assisted bipedalism among other ways of getting around. When, around 13 million years ago, the lineage leading to the African apes split from that leading to orang-utans in Asia, both groups inherited the skill of tree-walking. In Asia, the ancestors of orang-utans kept doing what they had been doing, remained committed to life in the trees and eventually became restricted to closed forests.
Meanwhile, the forests in which the ancestors of the African apes evolved were fragmenting and becoming more open as a result of climatic cycles. Instead of moving from tree to tree, this ancestor was increasingly forced to move along the ground to get between trees and to find food. These animals had to do a lot more vertical climbing up and down trees – holding on with their arms and “walking” up the tree trunk.
According to Thorpe and her colleagues, adaptations to vertical climbing would favour knuckle-walking while on the ground, because vertical climbing involves a similar posture, with knees and hips bent. Two groups of apes – the ancestors of chimpanzees and gorillas – evolved along these lines.
Another group of apes, however, took a different path. The ancestors of humans, Thorpe suggests, retained the extended-leg bipedalism of their arboreal forebears as they descended from the forest branches. They never went through a knuckle-walking stage, according to this view. Instead, their bodies evolved for two-legged walking between the trees, while retaining the long arms that were so useful in the treetops. Only much later did these hominins abandon the trees altogether.
While certainly plausible, many are unconvinced by this story. One issue is Thorpe and her colleagues’ choice of subject for looking into our evolutionary past. “Theirs is a good study of how orang-utans walk and climb in trees, but it is not a study of our distant ancestors,” says Brian Richmond, an anthropologist at George Washington University in Washington DC. “Evidence from the fossil record is needed to tell us what actually happened in our past.”
Richmond thinks that the fossils paint a clear picture of our ancestors: “They more closely resemble chimpanzees than orang-utans or any other living primates, showing that they had a skeleton adapted to the kinds of tree-climbing that chimpanzees practise today, but not the highly specialised anatomy and behaviour seen in orang-utans.”
In 2001, Richmond, along with David Begun of the University of Toronto, Canada, and David Strait of the University of Albany, New York, published a study suggesting that the fossilised hand bones of early humans have features that show they descended from knuckle-walkers (). Last year, in a riposte to Thorpe and her colleagues, the trio claimed that their findings rule out the tree-walking theory ().
Others are also impressed by this evidence. “I wouldn’t exclude the possibility that the immediate ancestors of the first humans were arboreal, but the evidence for a knuckle-walking stage is strong,” says Henry McHenry, an anthropologist at the University of California, Davis.
This evidence includes anatomical features in the early hominin fossils that lock the wrist so that it can’t bend back too far. “This is a feature that chimps have, but that doesn’t necessarily mean that the common ancestor was a knuckle-walker,” says Kevin Hunt of Indiana University in Bloomington. “While the knuckle-walking scenario seems most likely, the fossils surprise us so often I wouldn’t bet a month’s pay on it.”
Thorpe, Holder and Crompton dismiss the knuckle-walking interpretation. They point out, for instance, that wrist features in one fossil that Strait and Richmond claim are characteristic of knuckle-walking actually fall within the range of shapes seen in orang-utans, which are not knuckle-walkers.
Straight-legged gait
Another vexed issue is vertical climbing, which the ancestor of apes and humans almost certainly engaged in, along with orang-utans. In the tree-walking scenario, vertical climbing is linked to knuckle-walking. Yet in the 1980s, primatologist John Fleagle of Stony Brook University, New York, showed that vertical climbing involves muscle-use patterns that closely match those of bipedalism. This supports the view that bipedalism evolved on the ground, Richmond argues.
Thorpe sees it differently. There is no convincing evidence as to how the gait of a chimp-like ancestor – which would have involved flexed limbs both when moving bipedally and climbing vertically – could lead to straight-legged, erect bipedality, she says.
Begun disagrees: “There is this odd preconception that knuckle-walking is too specialised to have been a precursor to human bipedality. This is nonsense; we cannot predict the path of evolution. It just makes so much sense to envision a stage in which ancestors went from almost exclusively arboreal, to effective both in the trees and on the ground, to almost exclusively terrestrial.”
To Thorpe and her colleagues, however, it is far simpler to imagine that the two-legged walking evolved from hand-assisted walking in trees, rather than our ancestors going through a stage of inefficient knuckle-walking. They think this also fits in better with the fossil evidence suggesting bipedalism evolved relatively early, as going through a knuckle-walking phase would require more time than if our ancestors started walking upright as soon as they started coming down from the trees.
Others in the field are keeping an open mind. Sarah Elton of the Hull York Medical School in Hull, UK, thinks studies of living apes are relevant. “The strength of Thorpe’s work is that it is grounded in behavioural observations of living primates, as well as referring to the fossil record,” she says.
“We are still not sure how our ancestors moved from the trees to the ground,” adds Paul O’Higgins at the University of York, UK. “While a knuckle-walking ancestor for humans has been strongly argued, Thorpe’s proposal neatly deals with several key issues, such as how and why straight legs evolved.”
In the end, the debate will only be settled by digging up more fossils. Let’s hope someone strikes lucky soon.
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These bones were made for walking
When the remarkably complete skeleton of Lucy was found in 1974, she was the oldest “missing link” yet discovered. The arms of this 3.2-million-year-old Australopithecus afarensis are adapted to climbing, but the leg bones show she undoubtedly walked upright, suggesting that bipedalism evolved around 4 million years ago.
Recently, even older fossils have been discovered. In 1994, a 4.4-million-year-old hominin named Ardipithecus ramidus was found in Ethiopia. Fragments of the skull indicate that it sat on top of the spine, as it does in modern humans. Toe bones also hint that this creature was an upright biped.
Another fossil found in Kenya in 2000 dates to 6 million years ago. An analysis of the femur bone of Orronin tugenensis suggests this species walked on two feet.
The remains of Sahelanthropus tchadensis, found in Chad in 2002, are even older. The fossils are between 6 and 7 million years old, and the position of the foramen magnum – the hole in the skull through which the spinal cord passes – suggests its head perched on a vertical spine.
The interpretation of these latest finds as bipedal is highly controversial but, if correct, it means walking on two feet evolved much earlier than previously supposed.
“The recently discovered fossils push the acquisition of bipedality back in time, close to or before DNA-based dates for the genetic separation of humans and chimpanzees,” says Susannah Thorpe at the University of Birmingham in the UK.
This fits well with the tree-walking theory she and her colleagues have proposed (main story), as does the fact that all of these early fossils came from wooded environments – but it certainly does not prove it.
