The Fossil Evidence for Human Evolution
4 million to 1.5 million years ago.
Most paleoanthroplogists now believe that East Africa was the main crucible of early human evolution, largely because this area which yielded the greatest diversity of primordial hominids. Five to four million years ago, the now desertic-regions of Ethiopia and northern Kenya were open grasslands, teeming with herds of antelope and other mammals. Both predators and our remote hominid ancestors hunted these creatures, and it is here that the earliest known hominids are.
Molecular biology tells us the last common ancestral hominoid stock split into two main lineage's', which evolved into apes and hominids, between 9 and 4 million years ago. The details of this split are still a complete mystery, largely because fossil beds dating to this critical period are very rare in Africa. The fossil record increases after about 5 million years ago, but is still very fragmentary and controversial.
The summary of fossil hominids which follows is certain to be outdated within a few years. Between 5 and 2 million years ago, the East Africa savanna was populated by a great variety of hominids. Paleoanthropologists divided them into two broad groups: the australopithecines and Homo.
What is Australopithecus?
This is Latin for “southern ape.” Anatomist Raymond Dart in the Taung site in South Africa first identified it in 1925. He described small, gracile primate that displayed both human and ape like features. Dart named his find Australopithecus africanus, a much lighter creature than other, more robust form of australopithecine that subsequently turned up at other South African sites and, later, in East Africa.
For years, Paleoanthropologists thought that Australopithecus africanus was the direct ancestor of humankind, that human evolution proceeded in a relatively linear way through time. More recent finds from East Africa have muddied the picture and revealed far earlier primates on the human line.
The Earliest-Known Hominid
The earliest-known hominid was a small creature, which stood upright, with thin-enameled teeth and a skull closer to those of apes, suggesting close links with ancestral chimpanzees. We know little of this remote, small-brained ancestor, who was found by Paleoanthropologists Tim White in a 4.5 million -year-old layer at Aramis in the arid Awash region of Ethiopia. White and his colleagues named their find Ardipithecus ramidus , to distinguish it from later, and different, Australopithecines. Fragments of about 17 individuals are known.
Ardipithecus ramidus apparently lived in more wooded terrain than many of its successors and must lie close to the first hominids to diverge from the African apes. This stills little known, probably bipedal hominids were related to, or even ancestral to, two later East African forms: Australopithecus anamnesis and Australopithecus afarensis.
Australopithecus anamnesis and Australopithecus afarensis
About 4 million years ago, another form of upright-walking hominid flourished at Allia Bay and Kanapoi, the shores of Lake Turkana, northern Kenya. The fossil fragments, found by Meave Leakey and Alan Walker and named Australopithecus anamnesis (“anam” is “lake” in Turkana), come from a hominid with large teeth and a mosaic of what appear to be apelike and more evolved features. The hind limbs are thick enough to support the extra weight of walking on two feet, but anamnesis was not such an efficient walker as modern humans. Measurements of the hind limb suggest the hominid weighted 104 to 121 ponds. The anatomical relationship between Aramis fossils and this later find still remain highly uncertain.
Australopithecus ramidus and Australopithecus anamnesis were the ancestors of a much better known and somewhat later Australopithecus, found at Hader, about 45 miles north of Aramis, also on the Middle Awash River. It was Hadar on north Ethiopia that Maurice Taieb and Donald Johanson discovered a remarkably skeleton of a small primate (the famous “Lucy”), together with fragments of at least 13 males, females, and children. Lucy, who dates to about 3.2 million years ago stood 4 feet tall and was 19-21 years old. She was a powerful, heavily muscled primate, fully bipedal, with arms slightly longer for their size than those of humans were. Lucy and her contemporaries had humanlike hands and brains about the size of chimpanzees. Johanson and Tim White classified Lucy as Australopithecus afarensis, implying that the Hadar hominids were direct ancestors of later Australopithecines.
A nearly intact Australopithecus afarensis skull and arm bones from several other males have come from another Awash location about 1 mile upstream and date and date about 3 million years ago 200,000 years later than Lucy. This important find confirms that all the Australopithecus africanus fragments found over the past 20 years are from a single australopithecine species. It also confirms that Australopithecus afarensis displays considerable size variations. Some individuals stood 5 feet tall and weighed 150 pounds a far cry from the small slender Lucy.
Even more remarkable evidence of bipeadlism comes from fossil-bearing beds at Laetotil in Tanzania, where Mary Leaky uncovered not only human fossils like those at Hadar, but the actual 3.6 million year old foot prints of big games and some fairly large bipedal primates. The footsteps are those of an adult male and female, the later carrying a child. “The tracks indicate a rolling and probably slow-moving gait, with the hips swiveling at each step, as opposed to the freestanding gait of modern man.” Some scientists believe that Laetoli prints are from Australopithecus africanus, flourishing about 1000 miles south Hadar.
The Hadar and Laetoli confirms that the fundamental human adaptation of bipeadlism predates the first evidence of tool–making and the expansion of the brain beyond the level found in our nearest living relatives, the African apes. Bipeadlism also implies that later hominids were preadapted (had evolved sufficiently to utilized their hands for tool–making.
Originally, experts thought Australopithecus afarensis was confined to East Africa. French paleontologist Michel Brunet discovered a 3 to 3.5 million years old fossilized Australopithecus afarensis with seven teeth at Koto Toro in Chad in the southern reaches of the Sahara Desert. The Chad hominid flourished in a savanna–woodland–environment, much wetter than the arid landscape of today. Koto Toro is the first Australopithecine find west of East Africa's Rift Valley and debunks a long-held theory that the great valley formed a barrier separating ape population and causing those in more open country to move from the trees on to the ground. The evolutionary picture was much more complex than that.
Many scientists consider Australopithecus afarensis a primitive form of the Australopithecines, one, which displayed considerable anatomical variation yet, was hardly enough to adapt to harsh, changing savanna environments and survive for nearly a million years. Without question, there were several, as yet largely unknown hominid forms in eastern Africa before 3 million years ago. A 3.5 million-year-old hominid with a much flatter face was recently found near Lake Turkana. Named Kenyapithecus platyops (“the flat-faced person of Kenya”), it has no obvious relationship to Australopithecus afraensis. The new find is evidence of a more complex story of human evolution than once suspected, marked by geographical and biological diversification.
About 3 million years ago, the descendants of afraensis split into different lines. At this pint, the evolutionary plot really thickness. One line comprises the more gracile Australopithecus afraensis, first identified by Raymond Dart in 1925 and known entirely from South Africa, far from the putative East Africa cradle of humankind. The second line held at least three species of robustly built Australopithecines, somewhat later than afraensis, which became extinct about 1 million years ago. There are probably other, still undescribed lines. With this diversification, we emerge into a more complex chapter of human evolution, marked by geographic and biological diversification and many competing theories.
All Kinds of Australopithecines (3 million to 2.5 million years ago)
Gracile Australopithecines: Australopithecus afraensis
Australopithecus afraensis was a gracile, highly mobile hominid, marked by fossil form by small, almost delicate skulls and prognathous (jutting-out) faces. Found entirely in South Africa, africanus is an evolutionary mystery, for no one has yet found this form in East Africa, where Australopithecus afraensis flourished, even if it ultimately evolved from this widely distributed ancestor. It could be an evolutionary experiment that went nowhere or even has been among the first of a doomed line of robust hominids.
Robust Australopithecines: A. aethiopicus, A. boisei, and A. robustus
The robust Australopithecines, known by several taxonomic labels, lived between 3 million and 1 million years ago. Found in both eastern and southern Africa, they are remarkable for their heavy build. These hominids had large teeth and brains and were specialized for the chewing course fibrous plant food. As a group, these squat, heavy built hominids were very diverse.
Australopithecus garhi
A recent discovered, large tooth, small–brained, hominid with an ape like face defies classification within either gracile or robust Australopithecine lines. Working the arid washes of the Ethiopia's Awash desert, a team of 40 researches from 13 countries recently unearthed teeth and skull fragments from yet another hominid from, dating about 2.5 million years ago. The new hominid, named Australopithecus garhi (garhi means “suprise” in the local dialect) stood about 4 feet 10 inches and had protruding features, not unlike those of a chimpanzee. The lower molars are three times the size of modern humans, the canines almost as large. A. Garhi's brain is only a third the size of a modern human. The legs were long and humanlike, while the arms were long and more like an ape's. The hominid was a efficient scavenger. Bones of antelope and other large animals found only a few feet away display cut marks from stone tools, the earliest known instance of hominid butchery of animals.
Unfortunately, no stone tools were found close to the fossil remains, but surface finds of crude stone flakes and cobbles come from a nearby lake bed dating to about 2.5 million years ago.
A. garhi is a remarkable find, which will renew debate over the identity of the very first human tool maker. That this hominid was eating meat suggests that a switch to a high energy, high–fat meat diet was under way. This, in turn may have led to an increase in brain size among some hominids, which occurred only a few thousand years later.
The latest player on the evolutionary field is an enigma. With its apparent toolmaking and meat eating propensities, A. garhi could conceivably be the exclusive ancestor of the Homo family tree and technically the first humans. No one, least of all Tim White is prepared to make such a claim on so little fossil evidence. What we do know that is a far from robust–Australopithecine derived from A. afarensis survived until at least 2.5 million years ago. However, whether this form participated in a rapid evolutionary transition, or series of transitions into an early form of Homo remains a complete mystery. What we do know is that major changes to the hominid skull and face occurred after 2.5 million years ago. Many of them as a direct consequence of brain enlargement. New behavior patterns connected with obtaining more meat and marrow using stone tools may have been a short, highly critical period of human evolution.
