CHAPTER 15
PROTOTYPES
TEN MILLION years ago, apes were the lords of creation. Originating in Africa, more than fifty different species roamed the world during the Miocene age. Miocene apes flourished in particular in the tropical forests of eastern Africa. Among them were Proconsul and various contemporaries, thought to include the last common ancestor of both modern apes and hominids. Fossils found in the volcanic highlands of Kenya and Uganda reveal them as tailless, fruit-eating quadrupeds, about the size of a female baboon, which lived in trees and moved on the forest floor on all four legs.
This thriving ape community then mostly died out, probably as a consequence of global climate change affecting their environment. Climate records tell of a dramatic cooling around the world between 6.5 million and 5 million years ago. Polar ice caps expanded; sea levels plunged so low that the Mediterranean was repeatedly drained; icy winds blew off cold oceans. Africa escaped the icy conditions, but in the dry, cold climate, the Miocene forests shrank, fragmenting in places into more open woodland. Whole ape populations perished. Only a few survivors emerged: the ancestors of orang-utans and gibbons in Asia, the ancestors of gorillas and chimpanzees in Africa—and hominids.
The fossil record of this period is sparse, almost non-existent. According to molecular-clock calculations, hominids and the chimpanzee line split from a common ancestor between 8 and 6 million years ago. But the identity of this last common ancestor, its exact nature and the dates at which it lived remain obscure.
Recent discoveries have produced a number of contenders said to have crossed the hominid threshold: Sahelanthropus from Chad, dated between 7 and 6 million years old; Orrorin from Kenya, dated at 6 million years old; and Ardipithecus kadabba from Ethiopia, dated between 5.8 and 5.2 million years old. None of them provides conclusive evidence of habitual upright walking—the principal characteristic used to define the human lineage.
Sahelanthropus consists of a skull—an intact cranium, some lower jaw fragments and several teeth. It is the only exhibit of a skull recovered from a 5-million-year-long period. It shares several facial features with later hominids—a short face with a massive brow ridge and a mouth and jaw that protrude less than in most apes. The positioning of its foramen magnum suggests a posture similar to bipeds, with the skull balanced atop a vertically held spine. But other aspects of the skull are decidedly more apelike than other hominids. Moreover, no foot or leg bones have been found to help ascertain on which side of the divide between apes and hominids it belongs.
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 potential of bipedal gait, it was nevertheless able to travel some distance upright. While it continued to use trees to forage for fruits and leaves and to seek safety from predators, it spent much of its time on the ground. Kenya’s flat-faced platyops , dated at 3.5 million years ago, shares many features with afarensis.
A variety of other australopithecines then appear on the scene, including several species from southern Africa. Cave sites in Sterkfontein Valley, to the west of Johannesburg, have produced a plethora of specimens. Among the oldest is Little Foot, the Sterkfontein australopithecine found by Ron Clarke, dating back 3.3 million years, regarded as a possible antecedent to Australopithecus africanus. Africanus itself prospered for a period of a million years, from 3 to 2 million years ago. First named by Raymond Dart upon discovering the Taung child which lived about 2.7 million years ago, africanus shares more features with the later Homo species than any previous australopithecine. Although its brain size was similar to that of apes—about 400 cubic centimetres—its braincase was shaped like that of a Homo. It also had a relatively flat face and short jaw.
Another branch of the australopithecines—the ‘robust’ variety—crops up at sites in both southern and eastern Africa. The first evidence of their existence was discovered in 1938 by Robert Broom, who coined the name ‘Paranthropus robustus’ to account for their massive chewing teeth. A later generation of palaeontologists took up the name when sorting out classification. Louis Leakey’s Zinjanthropus became known as Paranthropus boisei, along with other Paranthropus discoveries in eastern Africa. The size of their jaws and teeth is attributed to changes in diet brought on by the effects of a drier, cooler climate. They became specialised plant-eaters, adapted to eating hard nuts and seeds and large quantities of coarse, fibrous roughage low in nutritional value. Although their brain size increased to about 500 cubic centimetres, their fate was to head into an evolutionary cul-de-sac. In eastern Africa, Paranthropus boisei survived for nearly 1 million years, from 2.3 to 1.4 million years ago, before becoming extinct. In southern Africa, Paranthropus robustus is estimated to have lived between 2 and 1.5 million years ago.
As a group, the australopithecines were remarkably successful. They persevered on earth for a period of 3 million years, exploring and adapting to new habitats, developing their bipedal gait and use of hands, and expanding the range of their food sources. It was from their ranks that the first species of Homo emerged. Despite their small brains, they may also have been responsible for producing the first primitive stone tools—the world’s first technology.
The oldest recognised stone tools—consisting of sharp flakes chipped off small cobbles and hand-sized stone hammers used to hit the cores—date back to 2.6 million years ago. They come from a site on the Gona River in Ethiopia’s Awash Valley, five miles to the west of Hadar, where Lucy was found in 1974. What is notable about them is the degree of skill involved in their making. The toolmakers were selective about what types of stone they chose, collecting particular kinds of cobbles they knew would flake more easily, sometimes from distant locations. Cut marks found on animal bones at the site indicate that sharp-edged flakes were used for cutting meat from carcasses and for cracking open bones for edible marrow.
Further evidence of skilful tool-making comes from Lokalalei, a site to the west of Lake Turkana in Kenya dating back to 2.3 million years ago. Researchers there have found what they termed ‘a tool factory’ where toolmakers turned out hundreds of flakes, striking cobbles at the right angle and with the right force to give them effective results. The toolmakers appeared to prefer phonolite as their raw material, producing as many as thirty flakes from a single core; other types of rock were found with only a few fragments missing, suggesting they had been tested and then discarded. Archaeologists examined more than 2,000 stone fragments at the site. By piecing together some 285 flakes, they were able to reconstruct thirty-five of the original stones from which the tools had been made. The evidence indicates that small-brained hominids living about 2.5 million years ago were capable of mass production that required forward planning and learning.
The makers of these stone tools remain unidentified. No hominid fossils have been found at the sites, though parts of a 2.5 million year-old australopithecine—Australopithecus garhi—have been recovered near Gona. But in view of the skill the toolmakers at Gona applied, the origin of toolmaking clearly lies further back within the australopithecine world. Cut marks on bones found at Dikika in Ethiopia provide evidence of possible tool use as far back as 3.4 million years ago. Although simple, this pioneer technology was effective enough to last for nearly 1 million years, being used long after the first Homo had arrived on the scene.
Toolmaking opened up a vast new range of possibilities. It enabled hominids to dramatically expand their sources of food. Hitherto, they had depended on fruits, roots, herbs and insects. Now they could use stone flakes to cut up animal carcasses so they could carry the meat to safe locations. Meat and marrow fat provided a nutritious and concentrated food, nourishing brain development. And brain development was the stimulus propelling the evolution of Homo.