Our grasping ancestry
Dramatic new finds at the Cradle of Humankind at Sterkfontein near Krugersdorp might change well- established ideas about human evolution, says one of the worldâ€™s most prominent palaeoanthropologists.
Ron Clarke of Wits University, who leads the team that has been laboriously excavating the Little Foot skeleton at the site since 1997, says the fossil indicates that man did not descend from knuckle-walking apes. The most recent finds lead Clarke to draw unexpected conclusions about how our ancestors moved about, how we might have evolved, and critical differences between us and apes such as gorillas, chimpanzees, orangutans and gibbons.
Clarke, Stephen Motsumi and Nkwane Molefe uncovered the lower legs of the 3,3-million-year-old Australopithecus in 1997, the skull in 1998, its left arm and hand in 1999, and more recently the upper part of the right thigh bone, the crushed pelvis, ribs and vertebrae of the lower back. The team estimates it might take another year to expose the rest of the skeleton.
Little Foot is the most complete fossil of its kind anywhere in the world, and the only one ever found where the hands and feet were associated with the same individualâ€™s arms and legs.
The combined evidence from the skeleton makes Clarke question the common view that humans derive from a forebear that walked on all fours, using its knuckles as a chimpanzee does.
Clarke believes our ancestor is more likely to have walked upright from the start and to have been a slow climber when in the trees.
“This is our first-ever hand, the first time weâ€™ve got a hand with an arm, and the first time ever weâ€™ve got a complete arm with a complete leg that belong to the same individual,” he says. “So for the very first time we can talk about ratios [of one body part to another]. Prior to this, itâ€™s all been surmise.”
The important thing about Little Footâ€™s hand, he reports, is that the palm is short and the thumb is well developed and long compared with the fingers. “The shape and proportions are the same as those of our modern human hand and different from the apes with their relatively much shorter thumbs, longer palms and longer curved fingers.”
Hand shape and proportions, he says, tell us how different animals move about in trees. They can either run along branches, as many small creatures do, they can climb around slowly in a vertical position like a koala bear, or they can hang beneath the branches by their arms.
“If youâ€™re suspended from the branches itâ€™s advantageous to have a long hand with curved fingers that you can use as a hook. But a long thumb gets in the way, especially if, like a gibbon, you move very fast through the trees, swinging beneath the branches with arm-over-arm movements.
“At some time, the ancestors of chimps, gorillas and orangs spent more time suspending themselves from branches and they developed long hands with curved fingers and relatively short thumbs. Their long arms relative to their short legs also didnâ€™t evolve for nothing — theyâ€™re advantageous in that kind of locomotion because the longer the arm the more distance you can cover in a shorter time through the branches.”
When these apes moved along the ground, says Clarke, they could either run on the ground as the gibbon does, with arms extended in the air to keep them out of the way, or they could use arms as props, knuckle-walking on all fours and using their hands with the long curved fingers for support.
The proportion of arm to leg length of the Little Foot hominid is quite different, says Clarke. Instead of the long arms and short legs characteristic of apes, its arms and legs are about the same length.
Clarkeâ€™s conclusions are sure to arouse debate. The hand, he contends, tells us that our ancestors were probably never knuckle-walkers and that when they were in the trees they were slow branch-climbers.
“They would grasp branches powerfully, with fingers around one side and thumb around the other. Theyâ€™d have moved the way children move about on a jungle gym today. Itâ€™s in our genes. We see gymnasts and circus performers climbing by grasping with their hands — they can go straight up a vertical pole, and youâ€™ll see them gripping with their feet and their knees as they shimmy up.”
We cannot yet say that one or other form of Australopithecus was ancestral to modern humans, says Clarke. “But when you look at a fossil you see features that are held over from an ancestry, and you see specialised features that separate out from other creatures and make it part of a new species: it very seldom happens that a specialised feature reverts back to the unspecialised form.”
The Little Foot skeletonâ€™s hand, argues Clarke, shows how “early, primitive [equally long] digits — such as those of a crocodile, for instance — can evolve in different directions. My contention is that the ancestor of the Australopithecus with the human-like hand always had this kind of hand, and retained it. The thumb is useful for climbing and for gripping branches in this pincer-like way.
“Descendants of Australopithecus in the form of Homo [the human genus] had this sort of hand too, which they found useful for making and using tools. Thatâ€™s why weâ€™re so adept — everything we have we owe to our slow-climbing arboreal ancestry.
“My argument is that chimps may not, as has previously been supposed, be our closest relatives after all. If the ancestors of humans were specialised knuckle-walkers like chimps and gorillas, they would have kept the long arms, palm, and fingers as well as the short thumbs that these apes had developed.
“In contrast with Little Footâ€™s more or less equally proportioned arms and legs, however, modern humans have developed longer legs than arms, because leaving the trees to become fully terrestrial and bipedal made longer legs advantageous for covering greater distances on the ground in a single stride. You can see how the greatest runners have the longest legs.”
Still in the ground and, Clarke hopes, waiting to be found are the skeletonâ€™s right arm and shoulder-blade, the rest of the spinal column, kneecaps, and perhaps a collarbone, which is rare for early hominids.
“But oneâ€™s got to be patient,” he says. “This work can be frustrating and very disheartening. Every time we put in the smallest chisel and weâ€™re tempted to work a bit faster, we have to remember that if we go too aggressively we can go through bone before we know it. But at the back of our minds â€¦ we know that the rest has to be there. Thatâ€™s what keeps us going.
“When people ask ‘How long until we get it all out?â€™ I have to say, ‘Itâ€™s been in the ground three and a half million years — a year or two is as nothing to it.â€™ The important thing is to find everything, and not to damage it.”
Ron Clarkeâ€™s findings are published in the latest South African Journal of Science