Born in Africa_ The Quest for the Origins of Human Life - Martin Meredith [72]
Orrorin consists of a collection of skeleton bones from four sites in the Tugen Hills—parts of a couple of thigh bones, part of an upperarm bone, two jaw fragments and teeth, but no cranium. Its claim to hominid status rests mainly on its leg bones, which show features associated with upright walking. But too few fossils of Orrorin have been found to make any classification certain.
The claim of Ardipithecus kadabba to hominid status is based largely on the size and shape of its teeth, which are said to be similar to those of early australopithecines. The anatomy of an inch-long toe bone provides some indication that it was able to move its feet like a hominid. But, as with Orrorin, too few fragments have been found to advance its claim much further.
The evidence suggests that from an early stage, the transition from ape ancestors to hominids involved a bout of evolutionary experimentation played out over the course of several million years. Hominids emerged not just as a single species but as a collection of similar species displaying a mixture of primitive and derived features. As rain forests broke up into more open woodlands, they were forced to explore and adapt to new forest-edge and woodland habitats. Whereas ape ancestors had made occasional use of standing upright to obtain food or walking bipedally—as modern apes such as gorillas and chimpanzees do—hominids began to rely increasingly on bipedalism as a principal method of locomotion on the ground, moving over greater distances in search of food, while retaining their tree-climbing habits for safety. Bipedalism became the key physical adaptation that set the hominid line in motion.
Modern experiments have shown that humans walking on two legs use only one-fourth of the energy of chimpanzees knuckle-walking on four legs. As well as saving energy, bipedalism had the advantage of freeing hands for purposes other than supporting body weight, such as carrying food or objects. Standing upright also enabled hominids to see further over distance to spot predators. Furthermore, it may have helped reduce exposure to heat from the rays of the tropical sun in a more open environment.
The first definite evidence of upright walking comes from Ardipithecus ramidus, a hominid species that lived 4.4 million years ago, first discovered by Tim White’s team in 1992 in the Afar region of northeast Ethiopia. A partial skeleton of a female named ‘Ardi’ and other fossils reveal that ramidus possessed a primitive walking ability using flat feet while retaining certain anatomical features such as long arms, large hands and opposable big toes that allowed it to continue a tree-living existence at the same time.
Next comes the first of the australopithecines, hominids that were better adapted to walking on the ground. Australopithecus anamensis is a collection of fossils ranging in age from 4.2 to 3.8 million years ago, found by Meave Leakey’s team at two sites in northern Kenya, Allia Bay to the east of Lake Turkana, and Kanapoi, to the southwest of the lake. The sample of fossils is small, but it includes pieces of tibia—the lower-leg bone—that give a firm indication of upright posture. Anamensis fossils have also been found in Middle Awash deposits in Ethiopia.
Anamensis is now regarded as the likely progenitor of Australopithecus afarensis, a species dating from 3.9 million to 3.0 million years ago, found mainly at Awash Valley sites in Ethiopia. Its most famous member is Lucy, a tiny individual standing at little more than three feet tall which lived 3.2 million years ago. Another notable example is the partial skeleton of a 3.6-million-year-old male, discovered in the Woranso-Mille area of Ethiopia’s Afar region in 2005, which stood at about five feet tall. Afarensis possesses a mixture of apelike and humanlike features. Its face, teeth and small braincase are similar to those of ape ancestors. But the shape of its pelvis and its knee joint clearly indicate that it walked erect. Locomotion experiments show that although it had not yet achieved the full