Sunday, December 17

Human Origin

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Human is in biological terms a mammal belonging to the primates and closely related to the African apes, but nevertheless, we have unique traits which are far outside the characteristics of any other animal of the world among which the most spectacular ones are the erect manner of walking and the bipedal stride, the peculiar behaviour in relation to the two sexes, the power of speech, and the very peculiar kind of inward activity, the consciousness or ego-feeling which some may call “the soul”.  Our species has been so successful in spreading over the world and changing it, sometimes more to our liking, that it is difficult to see ourselves in relation to other forms of life, to see ourselves “in nature.” We have made ourselves different and set ourselves apart from the rest of the natural world. Perhaps for that reason alone, among many others, it is useful and instructive to try to understand our origins and to reconstruct our relationship to other life on the planet.  Since the great task of classifying the world’s life forms according to their anatomical similarities got well under way during the late 18th century, the affinities between humans and the great apes have been recognized and pondered. In the mid nineteenth century, Charles Darwin postulated that apes and humans had a common ancestor. Until recently, however, we classified the apes as very distant cousins of human beings. The living great ape species include the gibbon; the orangutan; the gorrilla; the chimpanzee; the pigmy chimp or bonobo. The pigmy chimp was only identified as a separate species in the 1930’s.

More recently, genetic evidence from comparing our DNA with that of apes has led to new views of our degrees of relation. Asian apes (gibbons and orangutans) are, according to this line of evidence, not closely related to us or to the African apes. But human DNA is surprisingly similar to, and almost identical with, the DNA of chimps and gorillas; both ape species have 48 chromosomes, we have 46. Chimpanzees even share the same ABO blood types with humans, and thus appear to be very close kin to us.  As we approach the end of the twentieth century, new discoveries continually force us to re-examine our origins, and to search for clues about our own nature, and what we share with other forms of life that exist on the Earth with us.

It is useful to locate ourselves on the “family tree” or classification chart used by biologists to show relationships among our planet’s species. Human beings belong to the large mammalian “Order of Primates, which has many members, living and extinct, and includes lemurs, monkeys, apes, and humans. Within that Order, humans belong to the Family of Hominidae (or Hominids) which includes all two-legged, man-like species, extinct or living. Within the Hominid family are a number of extinct erect walking apes, ourselves and our immediate ancestors.  We are members of the Genus Homo (“Man”), and our species name is Homo sapiens–“wise man.” Sometimes we are further classified as a subspecies, Homo sapiens sapiens. All living humans are members of this subspecies and, although regional populations exhibit some obvious differences from one another, all of us are quite closely related.

A nineteenth-century Englishman, Charles Darwin, was the first to present a coherent scientific theory of human origins. His famous book, On the Origins of Species by Means of Natural Selection, published in 1859, advanced the theory that species came into existence by evolution, or gradual changes over a long period of time (Browne, pgs 4-6). Darwin suggested two basic “mechanisms” at work in this process: first, variation” continually occurred within species. That is, individuals of the same species vary slightly from one another. He noted that members of the same litter of pups or the thousands of offspring of a pair of salmon are not identical. Darwin was not certain why variation occurred; he only observed that it, in fact, happened that way. Individual offspring, he noted, because of their different characteristics, were not equally successful in the struggle to survive and reproduce. He called this weeding out process with the consequent differences in reproductive success “natural selection–which is the second of his two “mechanisms.”

Darwin concluded from these observations that those individuals whose characteristics fitted them to the circumstances in which they found themselves would have more offspring. Over time, the characteristics which gave them a reproductive advantage would be fully expressed in their descendants. Given more time, these small accumulating changes would result in a new species. The theory is, in essence, very simple: new species have emerged on earth because of variation andnatural selection.  Darwin’s theory is thus one of the central ideas of modern biology and its fundamental organizing concept.

In his second book, The Descent of Man, Darwin speculated that because of their many anatomical similarities, apes and humans must have a remote common ancestor. Darwin suggested in 1871 that the most likely site of human origins was Africa, home of the gorilla and chimpanzee, which he regarded as our closest living relatives.  Subsequent fossil discoveries have borne out Darwin’s early observations. A remarkably well preserved skull of Proconsul africanus (sometimes called Dryopithicus africanus or “woodland ape”) was discovered in 1948 in east Africa (Browne, pg 15). This gibbon-sized animal roamed the forests of the Miocene period roughly 18 million years ago. This creature appeared after monkeys and apes had diverged into separate families of species. Structurally, Proconsul appears to be the best candidate yet discovered to be the origin point of our large “family” of species. That is, Proconsul is probably the distant ancestor from which all modern species of apes and all hominids (human beings included) evolved.

The emergence of our distinctive species characteristics must have proceeded in gradual stages over an extended period of time. This line of thought led to a search for the expected intermediate forms between apes and humans–the famous concept of a “missing link” popular in nineteenth-century journalism.  Influenced by Darwin’s work, Dutch naturalist Eugene Dubois set out in 1891 to confirm the theory and to find the “missing link.” Dubois differed from Darwin in assuming that humans had evolved in tropical Asia, so he searched in Java. He found a fossil skull cap and a thigh bone near the Solo River in Java which seemed to him mid-way between apes and humans. He named his find “Pithicanthropus erectus” or “erect ape-man.” The public named it “Java Man”2. As more information accumulated, the species was re-named “Homo erectus” (“erect or upright man”). The earliest specimens of this species have since been found in Africa.  According to available evidence, it was the first human species to migrate out of Africa and adapt to a wide variety of environments in Europe and Asia.  Subsequent discoveries in the twentieth century have confirmed Darwin’s guess. We now have a rough picture of the human family tree as far back as 4.4 to 5 million years ago.

Every year adds significant discoveries to our accumulating knowledge of our past. But because the evidence about human origins is much more complex and interesting and full of surprises than anyone guessed, each year has added to the number of questions as well.  There were, as it turned out, many ancestral species rather than a single one, and the human family tree appears tangled and bushy rather a simple trunk-and-branch. Below are typical specimens of hominds that have been discovered since 1891.  There is plenty of room for argument and informed speculation about the implications of fossil finds. Even the best fossil record provides largely random snapshots of points in time rather than a complete record. Much of what we would urgently like to know is simply not preserved in the fossil record. However, the broad outlines of human descent are remarkably well established and, in a sense, the arguments are about details.

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