Human evolution: Thus we began to walk on two legs
The day primates looked up and stopped relying on their upper limbs
The first time Owen Lovejoy saw the female who would occupy his thoughts for the next 14 years, he felt no emotion. It was 1995, and the comparative anatomy specialist at Ohio’s Kent State University had the privilege of taking a look at the newly excavated skeleton of Ardipithecus ramidus at the National Museum of Ethiopia in Addis Ababa. Some of the bones were crushed and in poor condition.
“My first thought was: ‘Did they bring us here to see that crushed thing?’ Lovejoy recalls. “It took me about ten minutes to realize that all the important parts were there. My second thought was, ‘My God, who would have thought that!'”
Over the years, as researchers separated Ardi ‘s bones from the rocky matrix and reconstructed them, Lovejoy’s amazement only increased. It had long been believed that the further back we go in the human evolutionary past, the more our ancestors would resemble our closest living relatives, the chimpanzees. At 4.4 million years old, Ardi had lived more than a million years before Lucy, and he didn’t look like her, but he didn’t look like a chimpanzee either. Instead, he exhibited a strange combination of very primitive features that had only been seen in monkeys. and in the extinct Miocene, anthropomorphic primates and traits were observed only in our hominid lineage.
Consider Ardi’s foot. Later hominins, including Lucy, have the great toe aligned with the other toes, helping to provide the propulsive force necessary for bipedal locomotion, which has long been the defining trait of our lineage. Its big toe, on the other hand, is divergent, like that of apes, which is more useful for holding onto branches when climbing trees. But Ardi’s foot also contains a small bone called the os perineum (present in monkeys and ancient apes and preserved in the hominid lineage, but almost always absent in chimpanzees and gorillas ), which keeps the sole of the foot more rigid. Lovejoy and his colleagues believe that this rigidity allowed Ar.ramidus to walk upright, using all four fingers in a row to provide the force that propels a bipedal stride.
Ardi’s pelvis is also testimony to the transition from a primitive primate evolving toward human features. The human pelvis has undergone profound remodeling to accommodate bipedalism, a true locomotor juggling maneuver that requires keeping one limb suspended in the air while the other pushes forward. As early as Lucy’s time, 3.2 million years ago, the hip bones had widened and shortened to widen the attachment areas for the gluteal muscles, which stabilize the hip-bearing joint. In contrast, chimpanzees’ pelvises are narrow and elongated, giving them firmer support for climbing but forcing them to sway from side to side when walking upright.
The upper part of Ardi’s pelvis is short and broad and exhibits other features almost unique to hominins, such as a bulge on the inner edge that offers further support to the bipedal stride. However, the lower part of the pelvis is completely apelike, with attachments for the huge muscles of the lower extremities, necessary for effective climbing.
Ardi’s amazing hand
Then there is the surprising hand of Ardi. African great apes have long fingers and palms adapted for climbing trees, and the joints of the hands are stiff and strong to support the entire body weight on the knuckles when walking on the ground. Since this knuckle-walking adaptation is not only seen in chimpanzees but also in gorillas, which diverged from our lineage earlier, it was long believed that it represented the primitive form of locomotion that our ancestors also passed through in its evolution towards an upright gait. But Ardi’s hand calls that assumption into question. Although his fingers are long, the palm is short and very flexible, which would have allowed him to walk on the branches of trees supporting the palms (more like monkeys do than any of the great apes alive),
That hand is typical of monkeys, together with other very primitive features that Ar presents. ramidus has great repercussions for the knowledge of our origins. If the discoverers of Ardi are correct, our ancestors never went through a knuckle-supporting locomotion phase, as chimpanzees do, on their way to bipedalism. To hold otherwise would be to accept that our lineage developed a number of chimpanzee-like adaptations at a very early stage, and then lost all of them and returned to the primitive condition around the time Ar. ramidus wandered around Africa. It is highly unlikely.
Still, in view of the extremely primitive features, some researchers argue that Ar. ramidus is not really a hominid. For example, Terry Harrison of New York University points out that there was an enormous diversity of anthropomorphic primate species throughout most of Africa and Eurasia during the Miocene, between 23 and 5 million years ago. “Perhaps it was simply one of the many apes that were out there and not the one that gave rise to hominins”, he points out. In response, Lovejoy points to more than 20 distinctive features that link Ar. ramidus only and exclusively with more recent hominids. If Harrison is right, one would have to think that all those traits were concentrated by chance in an extinct ape completely unrelated to us. But even accepting that Ar. ramidus is a hominid, was it really bipedal? Before discovering Ardi, the question would have been unthinkable. Among the higher primates, only hominids are bipedal; therefore, all hominids must be bipedal. Many scientists are not clear about Ardi’s bipedal locomotion, especially with such a divergent big toe.
“That foot isn’t bipedal!” comments William Jungers, a specialist in evolutionary morphology at Stony Brook University. log? Flying? Jungers wonders why an animal fully adapted to quadrupedal locomotion in the trees would opt for bipedal locomotion on the ground.
Bipedalism is the consequence of a radical transformation in social behavior
Lovejoy has an answer to Jungers question: sex. In his opinion, bipedalism is the consequence of a radical transformation in social behavior. His theory of him is based not on a trait acquired by our lineage, but on a lost one: the dagger-like tusks of male great apes, so effective against other males seeking opportunities to copulate. The males of almost all living and extinct species of great apes have pointed canines, which sharpen themselves by rubbing against their lower teeth. The canines of male hominids are much smaller, and more similar to those of females. In the middle of Awash sediments was Ar. ramidus, canines corresponding to 21 individuals have been found, which were presumably male and female. They all share the hominid pattern.
Instead of gaining access to females through conflict with other males, according to Lovejoy, the male of Ar. ramidus supplied a particular female and her offspring, de Ella, with high-fat, high-protein foods in exchange for the exclusivity of her sexual favors de Ella. In this way, he ensured that the young he was feeding was his. But to transport food, the male’s hands had to be freed from the role they played in quadrupedal locomotion. Perhaps bipedalism was an inefficient way of moving for Ar. ramidus, but because of his contribution to the “sex for food” contract it must have been a good way to have more offspring. And in evolution, it all comes down to having more offspring.
Whatever the reason for Ardi’s incipient bipedalism (if there was one), barely 200,000 years later the fully bipedal genus Australopithecus, Lucy’s, made its appearance in the same region, like all the hominids that followed. Was the primitive Ar. ramidus, with its divergent big toe, the creature that changed over the course of those 200,000 years to become the ancestor of all subsequent hominids? Or was theirs a dead end, maintaining its strange mosaic of primitive and advanced traits until it reached extinction?
“These finds are extremely important, and given the state of preservation of the bones, what their discoverers have done is nothing short of heroic,” says Jungers. “But this is only the beginning of the story.”
