The Record of Human Evolution
Eric Delson, Herbert H. Lehman
College and the Graduate School and University Center of the City University of New York
The story of human evolution is long, extending far back in time. Anthropologists and other scientists consider that people are part of the zoological group called the Order Primates, which also includes the apes, the monkeys (of Africa, Asia, Central and South America), and the prosimians, such as the squirrel-like lemurs of Madagascar. In turn, primates are part of the great group of the mammals, animals that bear their young alive and feed them with mother's milk. Fossils, the petrified remains of once-living creatures, tell us that mammals originated hundreds of millions of years ago, from animals similar to lizards. By the time when dinosaurs finally disappeared, about 65 million years ago, some mammals had evolved into the earliest primates. These later gave rise to monkeys, apes, and humans.
What does it means to say that early mammals evolved into primates or that humans evolved from nonhuman primates? Evolution is the concept of biological change through time. Most scientists today understand it as a complex process involving genetic change from one generation to the next, under the control of natural selection. This term, devised by Charles Darwin in 1859, means that in a group of individuals of any one species, in a given environment some individuals will be better able than others to find food, escape from predators, protect their living space, find a mate, and care for their offspring. These more successful individuals are said to be better adapted to their environment, or more "fit" than their fellows of the same species. The best adapted individuals must also produce offspring who can carry the genetic heritage of their well-adapted forebears onward to successive generations. Many people refer to the "Theory of Evolution," in the common sense of "theory," as an unproven idea. Scientists, however, have a more restricted concept in mind when they use this term -- a theory is a well-tested idea, one that has been confirmed by a number of experiments or investigations. Thus the concept that most of us think of as the "Law of Gravity" is more technically known as the "Theory of Gravitation"; similarly, we could talk colloquially of the "Law of Evolution."
Early primates began to evolve into the main groups we know today by about 60 million years. Such ancient dates are not guesses, but the result of numerous chemical and physical tests applied to geological samples from around the world. When several different kinds of tests agree on the same date, then the date is accepted by scientists, and the time scale used here is a result of this process. For the first 35 million years of primate history, known fossils are related to the living prosimians. The "higher" primates, the monkeys, the apes, and humans, appear later but have their ancestry among groups known by fossils 30 million years old.
In the time period between 30 and 15 million years ago, the ancestral apes and monkeys diverged in their basic adaptations. Between 15 and 8 million years ago, one group of so-called "ground apes" spread outward from Africa into the open plains and less dense forests of Eurasia, where they seem to have begun to adapt to life on the ground and a diet that included nuts, seeds, and other tough objects requiring heavy chewing. Asian ape fossils from this time period are called Sivapithecus, an animal now thought to be close to the ancestry of the living orangutan of Indonesia. Similar fossils are known in Africa that may have been close to the common ancestry of the chimpanzee, gorilla, and humans. Based on both fossils and studies of the genetic structure of living apes and humans, most scientists think that the two African apes split off from the human line around 8 to 10 million years ago.
At Laetoli in Tanzania, Mary Leakey and co-workers discovered some fossil jaws and teeth and two footprint trails of creatures who walked upright on two legs; this ability is a primary characteristic that sets humans apart from apes. These 3.5 million-year-old fossil footprints bear a close resemblance to those that would be made by a small human walking on soft ground. From the Hadar region of Ethiopia, Donald Johanson and his colleagues have recovered the more complete remains of the walkers themselves: most of the skeleton of a 3.5 foot tall female nicknamed Lucy and parts of the skeletons (jaws, teeth, legs, hands, and feet) of 20-30 other early humans. These bones confirm the ability of this type of human, known as Australopithecus afarensis, to walk upright, although not as well as do living people. The fossils also show that these early humans had brains that were quite large for their body size, although small by comparison to our own. For example, the 50-pound "Lucy" probably had a brain larger than that of a 250-pound female gorilla. On the other hand, the jaws and teeth of this creature indicate that it was adapted to heavy chewing and grinding, probably of coarse vegetable foods, in much the same way as its ground-ape ancestors. They may also have eaten small quantities of meat, either scavenged from carnivore kills or as a result of killing defenseless (sick or young) individuals found almost by chance.
Several new types of early humans appear in the fossil record between 2 and 3 million years ago. This diversity has led researchers to disagree among themselves about the exact path of human evolution in this time interval. This is not a matter of argument over the mechanism or existence of evolution, merely which of the many known fossils might be on or closest to the branch which led to ourselves. In South Africa, the form known as A. africanus differs from A. afarensis only in relatively minor details. In neither of these species is there any evidence for the use of stone tools, although wooden or bone implements were likely to have been employed. Some workers place A. africanus in the line of human descent, between A. afarensis and later forms, while other authorities consider it the beginning of a major side branch that led to the so-called "robust" species, A. robustus and A. boisei. The latter pair were larger and more heavily built, with huge grinding teeth but perhaps a smaller brain compared to body size.
The next major step in the course of human evolution is seen in the fossil record by 2.0 million years ago, with the appearance of a new creature, H. habilis. This species is the oldest one placed in the same small zoological unit as living humans, the genus Homo. By comparison with Australopithecus, fossils of H. habilis have a larger brain compared to body size and a more modern type of walking ability. They also made stone tools to a reasonably fixed pattern, which implies such features as greater hand-eye coordination and forethought in choosing and shaping raw materials. Homo habilis showed an increased hunting adaptation, as evidenced by the finding of tools in association with skeletons of large animals such as hippopotamus. For the most part, however, these people probably ate mainly wild plant foods and meat scavenged from animals that died or were killed by carnivores. It appears that H. habilis of eastern and southern Africa was well on the way toward modern humanity.
Up to this point, the entire story of human evolution has taken place in Africa. During the next phase, the people known as Homo erectus evolved and spread out into the tropics and eventually the temperate zones of Eurasia. The earliest known representatives of this group appeared in East Africa about 1.75 million years ago; they were descendants of H. habilis, with whom they briefly coexisted. Homo erectus continued to develop the features that set Homo apart from Australopithecus: large brain and modern body size and an increased cultural ability, as seen in part by their manufacturing of larger and more complex stone tools. As they increased the scope of their adaptations, H. erectus competed directly for the first time with A. boisei, which had long coexisted with Homo habilis: by about l.25 million years ago, the more "primitive" Australopithecus had finally become extinct. About the same time, there is evidence for H. erectus having expanded into southern Asia, where it is popularly known as "Java Man." Soon afterward, H. erectus may have entered China, where a late variety is sometimes called "Peking Man."
As currently understood, H. erectus lived for almost 1.5 million years in Africa and Asia. These people had control of fire from an early date, made a variety of tools, and probably were active hunters of both small and large game. They lived in small huts and in caves (especially in colder climates). In Europe, they are known only from tools found without associated human fossils, suggesting that they may have been less numerous there. Throughout much of its long existence, the species H. erectus seems to show little definite evolutionary change, suggesting that it was well adapted to the range of environments in which it lived.
Then, about 0.5 million years ago, fossils suggesting a new form of human turn up in Europe and then Africa. These fossils look much like those of H. erectus, but differ from them in ways which more closely resemble our own species, H. sapiens. These features include slight relative increase in brain size but decrease in tooth size, coupled with expansion of the back end of the skull and opening of massive facial sinuses or air pockets. The latter are especially prominent inside the large brow ridges, which had been made of solid bone in H. erectus. Most scientists today consider that these fossils represent an archaic or ancient variety of H. sapiens, which may have arisen as a result of some H. erectus populations becoming geographically isolated by the increasingly more extreme climatic fluctuations of this period. They coexisted for a time with the most recent of the true H. erectus in northern Africa and China (Peking Man itself, known from dozens of fossils and tens of thousands of tools from a huge cave inhabited for hundreds of thousands of years). Eventually, archaic H. sapiens seems to have succeeded at the expense of its ancestor, H. erectus, so that by some 250,000 years ago, early H. sapiens was the only human species left.
The most famous and best known of the local varieties descended from archaic H. sapiens were the Neanderthal peoples of Europe and the Near East. All living humans are usually placed in the single subspecies Homo sapiens sapiens to show that they are extremely similar to each other. The greater distinctiveness of the Neanderthals has lead anthropologists to class them in a different subspecies of the same species. Homo sapiens neanderthalensis. Beginning about 300,000 years ago, European archaic H. sapiens were already on the way to becoming Neanderthals. The earliest fossils that appear to be fully Neanderthal in structure are now known about 150,000 years ago, suggesting that their special features developed over a period of time, perhaps as successive populations adapted to the rigors of living in glaciated western Europe. During the warmer interval between 125,000 and 100,000 years ago, the Neanderthals spread east and south, and they are known in northern Africa and from France through southern Europe into the southern U.S.S.R. and Iraq. When the great ice sheets returned to northern Europe and the southern mountains after 100,000 years ago, the Neanderthal inhabitants of Europe evolved farther along their own special road, becoming the more extreme or "classical" variety of this group, while the earlier type persisted to the east and south.
Neanderthals are also among the most misunderstood of our extinct relatives, the chief example of this being the cartoon and low-grade movie stereotypes of a shuffling, grunting savage pulling his mate by the hair and stopping only to fight the local dinosaurs. Today, even children realize that the dinosaurs died out over sixty million years before Neanderthals, or any extinct human, walked the earth, and current evidence has shown that the Neanderthals were not very different from ourselves in many important ways. Certainly, they had a distinctive skeletal structure, but few differences clothes would not hide. On the average, Neanderthals had longer and lower skulls than living humans do, with larger face and teeth, but no chin, and massive brow ridges in front of a brain as large as our own, but differently shaped. Their bodies were stockier and more muscular than ours, which, combined with their facial features, gave them greater resistance to the fierce cold of glacial climate. Neanderthals were capable craftsmen. As compared to the general-use artifacts of earlier humans, their stone tools were made in a variety of well-defined shapes, often for specific purposes. There is also clear evidence that they had control of fire, lived in caves or open-air structures of stone and vegetation, hunted large game from which they made clothing, cared for their sick or weak, and even buried their dead with some "religious" ceremonies.
What of the contemporaries of Neanderthals in other parts of the world? Unfortunately, less is known of Asian and African early H. sapiens and their descendants, but there is clear evidence that they spread and replaced late H. erectus. Recent discoveries in Africa reveal the widespread presence of what may be termed yet another subspecies of humans Homo sapiens rhodesiensis, often called "Rhodesian Man" after an early find. These people were much like the European ancestors of Neanderthals, living perhaps between 400,000 and 100,000 years ago between Morocco and South Africa. The roundness of their skulls and lack of Neanderthal-like specializations have suggested to some scientists that the Rhodesian group was a possible ancestor for Homo sapiens sapiens.
This view is supported by several fossil finds dating between 150,000 and 100,000 years ago, during the time that the Neanderthals were spreading across the Mediterranean area. Fossils from Ethiopia, Tanzania, and South Africa seem to link the Rhodesian group to the earliest fully modern humans, which first appear in South Africa before 90,000 years ago and soon afterward in the Near East. The most interesting aspect of these early Homo sapiens sapiens fossils is that they can be linked in some cases to the living racial groups: the South African specimens are definitely African, while those from North Africa and Israel appear to represent a Eurasian race which no longer exists -- perhaps they split up into the ancestors of later Europeans and Asians. Fossils of fully modern humans become more common worldwide after about 40,000 years ago. By that date their distinctive tool-kits appear in Yugoslavia and Hungary, but they may have taken a long time to reach as far west as France, where the latest Neanderthals persisted until nearly 30,000 years ago. Although some scientists disagree, most accept that the Neanderthals did not evolve directly into later Europeans or any other living people, but were replaced by the dispersing moderns, who nonetheless interbred and mixed culturally with the Neanderthals and other contemporaneous more archaic humans until by around 30,000 years ago, only Homo sapiens sapiens remained.
During the next twenty thousand years, humans continued to increase in number, leading to further migration following herds of game animals. In this way, early northern Asian peoples crossed the Bering straits (over land exposed by glacial conditions and a drop in sea-level) to reach North America (between 30,000 and 13,000 years ago) and eventually wander on to the far south. Similarly, Australia was reached by simple boats at least 30,000 years ago, and the less hospitable regions of Africa and Asia were inhabited as time passed. With major changes in climate caused by the nearly complete retreat of the most recent glaciers by 10,000 years ago, the stage was set for the development of agriculture, pottery, settled life, and finally urban societies and space travel by creatures whose ancestors had lived in huts or damp, soot-blackened caves only 15,000 years ago.
We have seen that humans are members of the Order Primates, along with apes, monkeys, and prosimians. Our last common ancestor with chimpanzee and gorilla lived about 8 to 10 million years ago. Between 4 and 2 million, early humans known as Australopithecus lived in Africa, walked upright on two legs, had brains larger than those of apes of equal body size and ate mainly plant food. The first members of our own biological genus Homo appeared in Africa before two million years ago, differing from Australopithecus in having a larger brain and more modern walking ability, as well as by making stone tools that allowed them to prepare vegetable foods and cut up mainly scavenged meat. These H. habilis people were succeeded by H. erectus, who lived between 1.75 and 0.25 million years ago, had still larger brains, tamed fire, hunted large game animals, and spread throughout much of tropical and temperate Africa and Eurasia. By half a million years ago, the earliest member of our own species, H. sapiens, appeared and spread outward from Europe and Africa, replacing the last H. erectus. In Europe and western Asia, these early H. sapiens people evolved into the Neanderthals, while an African variety may have evolved into the earliest fully modern people like ourselves. By 90,000 years ago, early Africans were living in the southern part of the continent, and moderns appeared in Eurasia somewhat later, finally replacing (and probably mixing with) the Neanderthals. Asian peoples continued to spread outward as populations increased, reaching both Australia and the Americas. In the geologically short time since then, Homo sapiens sapiens has taken advantage of cultural rather than biological means to conquer all the climatic regions of the world and begin the exploration of the universe.
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