| |
| C. Human Origins (4 Million to 1.8 Million Years Ago) |
| |
| All humans are members of the order Primates. There are two suborders: Anthropoids (apes, humans, and monkeys) and Prosimians (lemurs, tarsiers, and other “premonkeys”). We also belong in the family Hominidae, which includes modern humans, earlier human subspecies, and their direct ancestors. Our closest primate relatives are the Pongidae, the so-called anthropoid apes, including the chimpanzee and the gorilla. The research of more than a century has shown that the many similarities in behavior and physical characteristics between Hominidae (hominids) and Pongidae can be explained by identical characteristics that each group inherited millions of years ago from a common ancestor. Nearly all scientists agree that tropical Africa was the cradle of humankind, simply because this is where our closest living primate relatives still live. | 1 |
| According to biochemists Vincent Sarich and Alan Wilson, apes and Old World monkeys diverged about 23 million years ago, and the chimpanzee, the gorilla, and humans last shared a common ancestor about 7 to 6 million years ago. Unfortunately, this divergence occurred at a moment in geological time for which fossil-bearing beds in Africa are very rare. There is what has been called a “black hole” in primate evolution between about 15 and 4 million years ago. But this was a critical time when the Africa forest and savanna were densely populated by many primate species. Some of these primates were flourishing in small groups, probably walking upright, and adopting daily behaviors different from their tree-dwelling relatives nearby. | 2 |
| To understand early human evolution means identifying the ecological problems faced by our earliest ancestors at a time of constant climatic change. The first humans were descended from a yet unidentified nonhuman primate, who lived both in forests and, increasingly, on the open African savanna. Early hominids adapted to open country not only by adopting a more mobile, wide-ranging lifeway, but also by walking upright. | 3 |
| An upright posture and bipedal gait are characteristic of hominids. Walking upright frees the hands for other tasks like toolmaking, while bipedalism favors endurance and the covering of long distances, vital for hominids living in open country. Our remote ancestors may have first “come down from the trees” about 10 million years ago. Relying as they now did on food supplies scattered over large areas, they not only became more mobile, but began scavenging meat from predator kills, even hunting down small animals when the opportunity arose. Among mammal species, these characteristics are associated with a trend toward larger brain size. | 4 |
| The earliest fossil evidence for these anatomical and behavioral changes comes from the Middle Awash and Hadar areas of Ethiopia. Here physical anthropologists—Don Johanson, Tim White, and others—have uncovered primates dating back to close to the time, between 4 and 5 million years ago, when the ancestors of living apes and humans split off from one another. Australopithecus ramidus, from Hadar, was a small, upright-walking primate who displayed many apelike features and might have been the ancestor of later hominids. Another small primate, which Johanson and White nicknamed “Lucy,” lived during a somewhat later time than ramidus. She was between three and a half and four feet tall and was 19 to 21 years old when she died. A gracile, lightly built hominid, she was fully bipedal, with arms slightly longer than the arms of modern humans. Johanson and White also recovered the remains of other contemporary hominids. All had ape-shaped heads, brains the size of chimpanzees, and forward-thrusting jaws. Potassium-argon dates for the Hadar fossil beds are between 3.75 and 3 million years ago. Johanson and White believe that all the Hadar specimens are members of the species Australopithecus afarensis (“southern ape-man of the Afar”) and that they are the common ancestor of all later hominids, including the first humans. | 5 |
| Australopithecus (“southern ape-man”) was first identified by anatomist Raymond Dart in 1924, when he described the fossil skull of an immature primate from the Taung limestone quarry in South Africa. Dart realized that the Taung skill was not that of an ape; it had more humanlike teeth and other features that set it apart from chimpanzees. He named it Australopithecus africanus, a small, graceful creature that stood upright, and announced it was an ancestor of modern humans. Dart's claim was greeted with derision by the fossil experts of the day. Only one scholar, Robert Broom, realized Dart was right. He found more Australopithecus fossils in caverns in northern South Africa. These included not only Australopithecus africanus, but also a much more robust form, massively built with a crested skull. Broom named this australopithecine Australopithecus robustus. The South African Australopithecus finds date to between 3 million and 800,000 years ago. | 6 |
| The South African finds showed there was great variability among early hominids, but no stone tools or other artifacts were found with these fossils. Most of them have come from the sites of ancient hyena kills. Who, then, was the ancestor of humanity? In 1959, Louis and Mary Leakey announced the discovery of an Australopithecus robustus fossil (named Zinjanthropus boisei) in the lowermost bed of Olduvai Gorge in northern Tanzania. | 7 |
| Olduvai is a great natural gash in the Serengeti Plain, where hundreds of feet of ancient lakebeds have been exposed by earthquake action. Olduvai transects the shores of a long-dried-up, shallow lake where game and hominids came to drink. The Leakeys found small scatters of stone tools and debris, also animal bones, in the lakebeds. The robust australopithecine came from one such scatter and was potassium-argon dated to about 1.75 million years ago. A year later, the Leakeys found the skull bones of a much lighter, more human-looking hominid at a slightly lower level in the gorge, again associated with animal bones and flaked-stone artifacts. Louis Leakey did not believe this was an australopithecine, so he named it Homo habilis (“handy person”) and claimed that this was the earliest toolmaking human being. | 8 |
| Even earlier than Olduvai are the rich fossil beds at East Turkana in northern Kenya. They have yielded fragments not only of gracile and robust australopithecines, but also of Homo habilis as well. These discoveries showed that there was great variability among the hominid populations of eastern Africa between 4 and 2 million years ago. This complicates the study of early human evolution greatly, but makes it certain that there was no direct, linear progression from Australopithecus afarensis at Hadar to Homo habilis at East Turkana and Olduvai Gorge. Instead, it is best to think in terms of a branching bush, with numerous parallel and highly varied evolutionary lines that reflect a great diversity of hominid populations on the savanna between 4 and 1.5 million years ago. | 9 |
| Most likely, however, Homo habilis was the earliest toolmaking hominid, and, in general terms, the ancestor of all humankind. Homo habilis was a graceful, fairly human-looking primate that stood about 4 feet 3 inches tall and weighed about 80 pounds. Homo habilis looked less apelike around the face and skull, with a taller forehead and a large brain size. These hominids walked upright, but their upper and lower arm bones were of almost equal length and their powerful hands were more curved than ours. These features enabled them to grip branches and swing in the tress, a sign that our earliest ancestors spent considerable periods of time off the ground. | 10 |
| The earliest archaeological evidence for toolmaking, the appearance of rudimentary human culture, comes from the Koobi Fora area of East Turkana and is potassium-argon dated to about 2.5 million years ago. Excavations at both Koobi Fora and Olduvai Gorge give us a tantalizing portrait of very early human behavior, tantalizing because it is incomplete. | 11 |
| Originally, the Leakeys and others assumed that Homo habilis was a hunter and a plant food forager, who behaved just like today's hunter-gatherers. But microscopic examination of the stone artifacts and food remains from the scatters at Koobi Fora and Olduvai paint a picture of a much more apelike lifeway. | 12 |
| Archaeologist Nicholas Toth has replicated the simple Oldowan (named after Olduvai Gorge) stone technology used by Homo habilis so thoroughly that he has even been able to show that some of the toolmakers were left-handed. He discovered this was an opportunistic technology. The hominids could carry around convenient lumps of lava with them, then strike off simple stone flakes with a hammerstone when they needed them to split tough hide, to cut up an animal carcass, or to break open a bone for its marrow. | 13 |
| Homo habilis lived a highly mobile life, ranging over large territories in search of meat and plant foods. Undoubtedly, fruit, seeds, and tubers played a vital role in the diet, for they come into season on the savanna throughout the year. Our earliest ancestors also scavenged meat from the kills of lions and other predators, chasing away the animals, seizing limbs and other body parts, and running away. They may also have run down and killed some smaller antelope themselves. | 14 |
| Once the precious meat was in hand, they would find a convenient spot in the shade to butcher and eat the flesh and marrow. These places comprised the bone and stone scatters uncovered at Olduvai and Koobi Fora. One Koobi Fora band camped in a dry watercourse about 1.8 million years ago, where they found the carcass of a hippopotamus. Carrying in some of their tools from nearly 9 miles away, they gathered around and then removed bones and meat from the carcass with small stone flakes. | 15 |
| Much of this life is not that different from that of chimpanzees, who hunt small game and break open bones and nuts. Two important differences separated the first humans from their other primate relatives in Africa. First, they were fully bipedal, a posture far more efficient for carrying objects of all kinds. Second, they were adapted to savanna living, where they had to organize and cover far larger territories in open country than their relatives in the forest. At the same time, our ancestors became more and more dependent on technology and on one another, a development that led to better communication skills and, eventually and much later, to fully articulate speech. | 16 |
| By a million years ago, the hominid line had been pruned to the extent that one lineage, Homo, remained. Judging from the abundance of hominid fossils at Koobi Fora, these highly varied populations were about as common as baboons on the East African savanna. The adoption of a wider-based diet with a food-sharing social group would have placed many more acute demands on the hominids' ability to cope with the complex and unpredictable. They had to become more and more socially adept, living as they did in a world far more complex and demanding than that of Australopithecus. The increased complexity of human social interactions was a powerful force in the evolution of the hominid brain and led to the appearance of more advanced human forms after 1.8 million years ago. | 17 |
| |
| |
