Expansionary Institute

TIME: Humans Split From Chimpanzees 6 Million Years Ago, Much Earlier Than Thought PART TWO (Expansionary Theory)

It was the detailed anatomy of these fragmentary fossils, especially the teeth, that convinced Haile-Selassie that he had discovered a new human ancestor. Although apelike, the lower canines and upper premolars, in particular, display certain traits found only in the teeth of later hominids—the term scientists use to describe ourselves and our non-ape ancestors. They also differ in shape from the teeth of all known fossil and modern apes. Even the way in which the teeth had been worn down was telling. Explains Haile-Selassie's thesis adviser, Berkeley paleontologist Tim White: "Apes all sharpen their upper canines as they chew. Hominids don't." The new creature's back teeth are larger than a chimp's too, while the front teeth are narrower, suggesting that its diet included a variety of fibrous foods, rather than the fruits and soft leaves that chimps prefer.

When Haile-Selassie compared the newly discovered bones and teeth with those of Ardipithecus ramidus, a 4.4 million-year-old hominid found in the Middle Awash in the early 1990s that was the previous record holder, he realized that the two creatures were very similar. But the older one's teeth, while different from an ape's, do have a number of characteristics that are decidedly more apelike than those of the younger hominid.
On the basis of these minor but distinctive differences, Haile-Selassie decided to classify the new human ancestor as a subspecies, or variant, of ramidus and has given it the name Ardipithecus ramidus kadabba. (The name is derived from the local Afar language. Ardi means ground or floor; ramid means root; and kadabba means basal family ancestor. In accordance with the sometimes bizarre nomenclature of science, the younger creature now gets renamed Ardipithecus ramidus .)

Haile-Selassie and his colleagues haven't collected enough bones yet to reconstruct with great precision what kadabba looked like. But they do know it was about the size of modern common chimpanzees, which when standing average about 4 ft. tall. That makes it roughly the same size as its close relative A. ramidus ramidus and about 20% taller than Lucy, the famous 3.2 million-year-old human ancestor discovered about 50 miles away in 1974 that is even further along the evolutionary track. The size of kadabba's brain and the relative proportions of its arms and legs were probably chimplike as well.

But unlike a chimp or any of the other modern apes that amble along on four limbs, kadabba almost certainly walked upright much of the time. The inch-long toe bone makes that clear. Two-legged primates (modern humans included) propel themselves forward by leaving the front part of their foot on the ground and lifting the heel. This movement, referred to as toeing off, causes the bones in the middle of the foot to take on a distinctive shape—a shape that is readily apparent in the ancient toe bone. "If you compare a chimp's foot bones with its hand bones, they look the same because they're used for the same thing"—that is, for grasping—Haile-Selassie explains. "Hominid fingers and toes don't look alike at all."

Exactly how this hominid walked is still something of a mystery, though with a different skeletal structure, its gait would have been unlike ours. Details of kadabba's lifestyle remain speculative too, but many of its behaviors undoubtedly resembled those of chimpanzees today. It probably still spent some time in trees. It probably lived in large social groups that would include both sexes. And rather than competing with one another for mates, the males may well have banded together to defend the troop against predators, forage for food and even hunt for game.

But that kadabba walked upright at all is hugely significant. Paleontologists have suspected for nearly 200 years that bipedalism was probably the key evolutionary transition that split the human line off from the apes, and fossil discoveries as far back as Java Man in the 1890s supported that notion. The astonishingly complete skeleton of Lucy, with its clearly apelike skull but upright posture, cemented the idea a quarter-century ago.

What's been much tougher to pin down is just why two-leggedness arose. The conventional wisdom has long focused on the fact that eastern Africa became significantly dryer about the time that humans first evolved. The change would have tended to favor grasslands over forests, and, so went the theory, our ancestors changed to take advantage of the new conditions. We learned to walk upright so that we could see over the tall grasses to spot predators coming; an upright posture, moreover, would offer a much smaller target for the oppressive heat of the grassland sun, and a larger target for cooling breezes.

The only trouble with this theory is that it's wrong. The earliest humans, it turns out, didn't live in grasslands. Dry climate or not, a companion paper published last week in Nature shows on the basis of the other fossilized flora and fauna, as well as the chemistry of the ancient soil, that Ardipithecus ramidus kadabba lived in a well-forested environment. That's also the case with other extremely ancient hominids found during the past several years, including Ardipithecus ramidus and a species called Orrorin tugenensis , announced last December by French and Kenyan researchers. And while the ability to walk on two legs probably started out as an increasingly frequent behavior, evolution demands an explanation for why it persisted. On first blush, bipedalism just doesn't make much sense. For our earliest ancestors, it would have been slower than walking on all fours, while requiring the same amount of energy. Says Lovejoy bluntly: "It's unnatural. It's bizarre."

Yet the advantages of walking upright were somehow so great that the behavior endured through thousands of generations. Indeed, the anatomy of our ancestors underwent all sorts of basic changes to accommodate this new way of moving. Many of the changes help the body stay balanced by stabilizing the weight-bearing leg and keeping the upper torso centered over the feet. Lovejoy, who studies the anatomy and biomechanics of locomotion, thinks the changes may have improved coordination as well. "To walk upright in a habitual way, you have to do so in synchrony," he says. "If the ligaments and muscles are out of synch, that leads to injuries. And then you'd be cheetah meat."

By far the most crucial changes, according to Lovejoy, were those in the spine. The distance between chest and pelvis is longer in humans than in apes, allowing the lower spine to curve, which locates the upper body over the pelvis for balance. The pelvis grew broader, meanwhile, and humans developed a hip joint and associated muscles that stabilize the pelvis. Explains Lovejoy: "That's why a chimp sways from side to side as it walks upright and humans don't."

Changes also had to take place in the femur, or thighbone. For example, the femoral
neck—the bent portion at the top of the bone—is broader in humans than it is in apes, which improves balance. The human knee is specialized for walking upright too: to compensate for the thighbone's being at an angle, there's a lump, or groove, at the end of the femur that prevents the patella from sliding off the joint. "A chimp doesn't have this groove because there is no angulation between the hip and the knee," Lovejoy says. "This change says you're a biped."

Finally, there's the foot. "What's important here is the arch," Lovejoy says. "It's a really important shock absorber. It's like wearing a good pair of running shoes." In order to create that arch, the chimp's opposable great toe became aligned with the others, and the toe's muscles and ligaments, which had been used for grasping and climbing, were repositioned under the foot. "The shape of the big toe is indicative of this. You can see it in Lucy's species," Lovejoy says, but not in the bone Haile-Selassie found, because it's from a different toe. "What we can see [in the new discovery's foot] is that the base of the bone adjacent to the knuckle has a distinct angle, showing that the creature walked step after step after step with its heel off the ground, using the front of its foot as a platform."

That's how it walked. Why it walked is tougher to understand, since motivation leaves behind no physical remains. But armed with knowledge about our ancestors' physical attributes and the environment that surrounded them, scientists have come up with several theories. Anthropologist Henry McHenry, of the University of California, Davis, for example, champions the idea that climate variation was part of the picture after all. When Africa dried out, say McHenry and his colleague Peter Rodman, the change left patches of forest widely spaced between open savannah. The first hominids lived mostly in these forest refuges but couldn't find enough food in any one place. Learning to walk on two legs helped them travel long distances over ground to the next woodsy patch, and thus to more food.

Meave Leakey, head of paleontology at the National Museums of Kenya and a member of the world's most famous fossil-hunting family, suspects the change in climate rewarded bipedalism for a different reason. Yes, the dryer climate made for more grassland, but our early ancestors, she argues, spent much of their time not in dense forest or on the savannah but in an environment with some trees, dense shrubbery and a bit of grass. "And if you're moving into more open country with grasslands and bushes and things like this, and eating a lot of fruits and berries coming off low bushes, there is a hell of an advantage to be able to reach higher. That's why the gerenuk [a type of antelope] evolved its long neck and stands on its hind legs, and why the giraffe evolved its long neck. There's strong pressure to be able to reach a wider range of levels."

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Tue Jul 24