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Careful Climbing in the Miocene: The Forelimbs of Ardipithecus ramidus and Humans Are Primitive

C. O. Lovejoy et al.

A grasping hand and highly mobile forelimb are defining characteristics of primates. The special ability to pick things up and manipulate them has probably been a central selective force in making primates so unusually intelligent. It's something that porpoises can't do at all and crows can't do very well. It may also be one reason why humans alone eventually evolved cognition.

The hands of African apes are specialized in a number of ways that make them dramatically different from our own. Apes must support their large body mass during climbing to feed and nest, especially in the middle and higher parts of the tree canopy. Their hands must therefore withstand very high forces, and this is facilitated by their elongated palms and fingers. Our palms are much shorter and our wrists more mobile. This allows us to grasp objects and compress them with great dexterity and force—something often called a “power grip.” The differences between ape and human forelimbs become less pronounced going from the hand to the shoulder. Ape and human elbow joints, for example, diverge only moderately in their manner of load transmission.

The high loads that apes bear during locomotion have required them to greatly stiffen the joints between their fingers and palms. Because their thumb has not been elongated in the same way as their palms and fingers have, thumb-to-palm and thumb-to-finger oppositions are more awkward for them. We are therefore much more adept at making and using tools. All of these forelimb characteristics in apes have led them to adopt an unusual form of terrestrial quadrupedality, in which they support themselves on their knuckles rather than on their palms. Only African apes exhibit this “knuckle-walking.” Other primates, such as monkeys, still support themselves on their palms.

It has long been assumed that our hands must have evolved from hands like those of African apes. When they are knuckle-walking, their long forelimbs angle their trunks upward. This posture has therefore long been viewed by some as “preadapting” our ancestors to holding their trunks upright.

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Two views of the left hand of Ar. ramidus showing primitive features absent in specialized apes. (A) Short metacarpals; (B) lack of knuckle-walking grooves; (C) extended joint surface on fifth digit; (D) thumb more robust than in apes; (E) insertion gable for long flexor tendon (sometimes absent in apes); (F) hamate allows palm to flex; (G) simple wrist joints; (H) capitate head promotes strong palm flexion. Inset: lateral view of capitates of Pan, Ar. ramidus, and human (left to right). Dashed lines reflect a more palmar capitate head location for Ar. ramidus and humans, which allows a more flexible wrist in hominids.

Until now, this argument was unsettled, because we lacked an adequate fossil record. Even Lucy, the most complete Australopithecus skeleton yet found, had only two hand bones—far short of the number needed to interpret the structure and evolution of the hand. The Ardipithecus skeleton reported here changes that. Not only is it more than 1 million years older than Lucy (4.4 million versus 3.2 million years old), its hands are virtually complete and intact. They show that Ardipithecus did not knuckle-walk like African apes and that it lacked virtually all of the specializations that protect great ape hands from injury while they climb and feed in trees.

Ardipithecus hands were very different from those of African apes. Its wrist joints were not as stiff as those of apes, and the joints between their palms and fingers were much more flexible. Moreover, a large joint in the middle of the wrist (the midcarpal joint) was especially flexible, being even more mobile than our own. This would have allowed Ardipithecus to support nearly all of its body weight on its palms when moving along tree branches, so that it could move well forward of a supporting forelimb without first releasing its grip on a branch.

This discovery ends years of speculation about the course of human evolution. Our ancestors' hands differed profoundly from those of living great apes, and therefore the two must have substantially differed in the ways they climbed, fed, and nested. It is African apes who have evolved so extensively since we shared our last common ancestor, not humans or our immediate hominid ancestors. Hands of the earliest hominids were less ape-like than ours and quite different from those of any living form.

Ardipithecus also shows that our ability to use and make tools did not require us to greatly modify our hands. Rather, human grasp and dexterity were long ago inherited almost directly from our last common ancestor with chimpanzees. We now know that our earliest ancestors only had to slightly enlarge their thumbs and shorten their fingers to greatly improve their dexterity for tools-using.

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