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HUMAN EVOLUTION | Walking the Path to Humanity:

The Origins of Bipedalism

Walking on two feet is one of our defining features as hominins

Having had some experience of walking myself lately, it seems appropriate in this month's blog to address the origins of our peculiarly upright way of moving, which is one of our defining traits as humans. Unlike any other member of the primate order, we humans habitually walk on two feet, using legs that are far longer than our arms, which is the reverse of the condition found in all of our fellow apes. This upright, two-footed (bipedal) walking can be witnessed in its myriad expressions on any given day in Ireland, from the long lope of a farmer crossing a muddy field, to the short-stepping gait of a high-heeled urbanite, or the side-to-side waddling of a toddler. But no matter where these Irish people walk, or in what fashion, their form of upright locomotion has roots going back millions of years, although recently new evidence has challenged long-held views about where and why the members of our line rose to walk on two legs.

The traditional view regarding the origins of upright walking on two legs (bipedalism) relates it to the shift in Africa within the last 10 million years from a landscape dominated by woodlands to one dominated by grasslands, which forced our ancestors to adapt to these changing conditions by becoming less arboreal and more terrestrial – in essence, to become 'ground apes'.

Among the myriad theories regarding the advantage of evolving a more upright posture, some have related it to being able to see over tall grass (to guard against predators, etc.) or being a more efficient way of walking between trees, while others have proposed that it might be linked to feeding, whether foraging in wetland habitats by wading in rivers and lakes, or some other feeding behaviour. Others have argued that the roots of our bipedalism lie in the possibility that it conferred some reproductive advantage, maybe as a form of blatant sexual advertisement, while still others have held that it was more related to some aspect of threat displays or gestures of appeasement.

Many of these theories have one thing in common, though: they seem to envisage the transition to upright walking as occurring on the ground, with an increasingly terrestrially-adapted quadrupedal ape eventually rising up to walk on two legs. For a long time, this was perhaps the only sound conclusion one could arrive at from a study of the fossil record as well as our closest living relative, the chimpanzee, but fossil discoveries in recent decades have lobbed a hand grenade into this bastion of orthodoxy and ignited a furious debate amongst experts in what is already a volatile field of study.

Part of the reason why the origins of bipedalism in our line has produced so many theories is that, up until the 1990s, there was not a great deal of fossil evidence to prove or disprove any claim. The most ancient fossil which was anywhere near complete was the skeleton of Lucy (Australopithecus afarensis), which was found in Ethiopia in 1974 and dated to around 3.2 million years ago. As undoubtedly spectacular a find as Lucy was, in terms of finding out more about the origins of bipedalism there was really not much to go on.

This is because, although the shoulders and arms of Lucy's skeleton still retained features consistent with a certain amount of time being spent in the trees, the rest of the body had already evolved into a fully upright posture. Even the big toe was lined up alongside the other toes rather than being splayed out from them in the more hand-like feet of chimpanzees. The origins of bipedalism, then, lay somewhere in the unknown two-to-three-million-year gap book-ended by Lucy and our last common ancestor with the chimpanzees which lived around 6 million years ago.

'Lucy': Life reconstruction (L) & Skeletal elements, posture, and locomotion (R)

Thus, it was reasonably surmised that our last common ancestor with the chimpanzees would have walked on all fours and have certainly been more like chimpanzees than us, while somewhere along the line to Lucy our ancestors became more terrestrial and, eventually, rose to walk on two feet. However, three fossil discoveries in the last 20 years have greatly upset this tidy and linear picture and forced us to look anew not just at ourselves but at our cousins the chimpanzees too.

The greatest of these discoveries, and certainly the most complete, is the fossil of Ardipithecus ramidus, or 'Ardi', which was first found in 1994, like Lucy, in Ethiopia. In 2009, Ardi's discoverers published the full findings on the skeleton, which is even more complete than that of Lucy. Ardi, dating to around 4.4 million years ago, is over a million years older than Lucy, and bears a number of fascinating features with, according to its finders, revolutionary implications.

Although a tree-dweller that lived in a wooded environment, with a splayed big toe for grasping branches, features of Ardi's pelvis and the possession of a rigid foot show that she could move bipedally, and in a manner quite different to the occasional bipedal behaviour seen in other primates.

This means that our ancestors need not have become more terrestrial first, in response to the disappearance of their forest habitat, and then evolved to walk bipedally, but may have evolved to move bipedally in the trees, possibly as an adaptation in terms of posture which aided certain types of feeding. Ardi, then, was a tree-dwelling ape that moved both on four limbs and two limbs – it was a quadruped and a biped.

'Ardi' (Ardipithecus ramidus): full skeletal reconstruction (L) and graphic of actual fossils (R). Note the still fully divergent, ape-like big toe

Also, according to Ardi's discoverers, other features of this ancient hominin reveal much about the possible appearance of our last common ancestor (LCA) with the chimpanzees, and much about chimpanzees themselves.

Our LCA with the chimpanzees is often pictured as, more or less, a chimpanzee itself – a notion which has been strengthened in popular culture, most famously in the opening scene of 2001: A Space Odyssey, where ancient members of the human line are depicted as essentially chimpanzee-like. However, in the light of their ground-breaking find, Ardi's discoverers argue that this is very far from the truth.

For one thing, Ardi's face does not protrude like a chimpanzee's but is quite flat – more like ours. For another, features of Ardi's wrist show that when moving on all fours she walked on her palms like a monkey, not on her knuckles as chimpanzees do. It is therefore possible that our LCA with chimpanzees also possessed a more human-like face and walked on its palms and that knuckle-walking and a protruding face are not preserved ancient traits but derived features in chimpanzees which evolved after they split from our line (this does, however, mean that the same features observed in gorillas would have to have evolved independently).

If this were to be true, it should not surprise us terribly, as we must remember that chimpanzees have had just as much time to evolve new features as we have since our lines split around 6 million years ago.

Our picture of the LCA we share with chimpanzees could be quite a bit different than we once supposed, then. It could, in fact, be radically different if we take a look at some of the interpretations of the two other important fossils unearthed recently.

Orrorin was discovered in the year 2000 in Kenya and dated to six million years ago, while a year later in Chad a form called Sahelanthropus was found and dated as slightly older, to between six and seven million years ago. The discoverers of each of these fossils claim theirs as the earliest hominin on record, which would extend the fossil record right back to the beginning of our line, when we split with the ancestors of the chimpanzees. What is even more revelatory is that each of these fossil hominins is claimed to have walked upright.

Life reconstruction of Sahelanthropus tchadensis, the earliest known hominin

These claims have sparked great controversy and the truth is that the exact relationship of these fossils to our own line is unresolved. If they are closely related to us and truly bipedal, though, they throw up a world of fascinating possibilities regarding the origins of our line and the origins of bipedalism.

One possibility, and undoubtedly the most mind-blowing, is that Sahelanthropus, whose venerable age by some measures predates our split with the chimpanzees, could actually represent a common ancestor of both ourselves and the chimpanzees. The glaring implication of this, then, is that the ancestors of chimpanzees may also have walked upright and only reverted back to walking on all fours after they split from our line!

What a stratospherically fascinating possibility that is. And yet, we must always be careful to keep our feet on the ground when we tread the path of discovery relating to our own origins, fringed as it is with conflicting evidence, bold claims, and high emotion.

For instance, getting back to the Ardi fossil, as undoubtedly incredible a find as this is, some experts have poured cold water over some of the claims made by its discoverers. One, David Begun, a leading authority on fossil apes, although accepting Ardi to be a hominin, contends that the implications drawn about our LCA with chimpanzees are not supported by other evidence.

Principally, he points to the numerous features of the wrist, arms, shoulders, and torso in humans, chimpanzees, and gorillas that indicate their common ancestor was not a palm-walker, as Ardi's discoverers have suggested, but an ape that hung from branches in the trees and knuckle-walked on the ground.

Ultimately, this picture will only become clearer when the earth surrenders more fossils of our ancient relatives for us to study. What can be said with certainty, though, is that the Irish and every other human on Earth today are very lucky to be living at a time unparalleled in human history in terms of understanding our deepest origins. And though there are undoubtedly more twists and turns on the road ahead, it is an exciting time to be walking in the footsteps of our ancestors.

----- update 2019 -----

As promised above, a new twist in the tale of how our ancestors rose to walk on two feet emerged in November 2019 with the announcement of the discovery of an ape called Danuvius guggenmosi which lived in the forests of Bavaria 11.6 million years ago that combined African ape-like arms and hands for hanging from branches with more human-like legs and feet for walking upright upon them.

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