From What Remains to Be Discovered: Mapping the Secrets of the Universe, the Origins of Life, and the Future of the Human Race
(pp. 264-268)
by
© 1998 by John Maddox.
Used with permission of The Free Press.
(
Editor's note: As we embark on a new year fraught with significance, we find ourselves in a time of stock-taking, a time when milestones are being carefully noted and some fundamental questions asked about what lies beyond them. In the realm of science, it is difficult to imagine anyone better qualified than John Maddox to mark those milestones and ask those questions. Twenty-three years as editor-in-chief of Nature have given him a unique perspective on scientific progress thus far, and a better-than-average sense of where scientists' endeavors will tend in the future. In What Remains to Be Discovered, Maddox gives us a lucid, detailed, and highly readable accounting of the history, current status, and likely future of fields ranging from the origin of the universe to evolution and genetics to neuroscience, in sweeping categories called "Matter," "Life," and "Our World."
Here Maddox delves into the mystery that concerns us most intimately - the process by which humans began to be human. Who were our ape and hominid cousins; how and when did the group emerge that would become us? Maddox describes the complexities attending these questions, and tells us that the key to unraveling the tangle will be found in genetics.
The Human Confusion
The problem of human evolution is simply stated. All species of the great apes have very similar genes to those in people: the globin genes are all essentially the same, for example. The DNA structures of corresponding genes differ slightly from each other, but by no more than can be accounted for by the mutations accumulating in 5 or 6 million years. To be sure, the catalogs of genes are still incomplete - there is no Great-Ape Genome Project yet. Yet the genomes of the species differ in a dramatic way: people have only 46 chromosomes while the great ape species all have 48 (23 and 24 distinct pairs respectively).
Whatever happened to the "missing" chromosome of the great apes? The chromosome has not disappeared, but is substantially incorporated at the end of the long arm of the human chromosome called chromosome 2. It is a "translocation" resembling those responsible for many types of cancer, as described in the previous chapter. In this case, the translocation is the most conspicuous difference between Homo sapiens and the great apes, although it is not yet known why the effect of the translocation should be so profound. It may be possible to make educated guesses when the Human Genome Project is complete; the great-ape genes on human chromosome 2 may turn out to be regulated differently. It is also possible that some of them (or some other genes) have been disrupted or duplicated. Only detailed comparison will tell that - and only when there is a fuller catalog of the functions of human genes than there is now. Nevertheless, one of the mechanisms of species formation seems to consist predominantly of a chromosome translocation. It remains to be seen how that can be squared with Darwin's gradualism.
The timing of the divergence between Homo sapiens and the great apes agrees well with the evidence of the fossil record. In 1995, a group of paleontologists from the United States (from Berkeley, California) and Ethiopia described their finding of a hominid [1] skull and part of a femur (thigh-bone) in central Ethiopia that was dated with reasonable confidence to 4.3 million years ago. The fossilized remains are by far the most ancient hominid fossils yet found, while the creature concerned (called Australopithecus afarensis) seems to have resembled the chimpanzee more closely than any of the other great apes [2], which again agrees with the equivocal evidence the geneticists marshal.
Paleontologists have always held the habit of walking upright on two feet to be the hallmark of Homo sapiens; great apes use their knuckles, but are better adapted for climbing trees. With the continuing discovery of fossils (particularly of skulls), it became plain in the 1930s that there was a progressive threefold increase in the size of the skull in the course of human evolution. Now the faculty of using language (understanding spoken speech, reading written words, making sense of them and articulating a reply) would be taken as an even more specifically human attribute. Can a simple translocation of a chromosome account for all that? Not by itself.
Confusion about the origin of Homo sapiens arises because the fossil record of the great apes is meager or almost nonexistent, for reasons that are not well understood. Fossils of Australopithecus from 4.3 million years ago appear to be much like modern chimpanzees: the great apes have not changed much, but Homo sapiens has had a much more checkered history. The direct lineage to Homo sapiens appears to run through the hominids called Australopithecenes up to Homo erectus (from about 2.3 million years ago), but along the way, there seem to have been many occasions where more than one potentially hominid species coexisted. Several different Australopithecene fossils are classified as belonging to distinct species, but it is impossible to be sure that part of a single skeleton can be representative of mutually infertile species.
What is, however, clear is that in attempts to reconstruct a Darwinian pathway for the evolution of Homo sapiens, there are many exceptions to any neat and tidy scheme. What is to be made of the species named by the late Louis Leakey as Zinjanthropus boiseii from 1.2 million years ago (now renamed Australopithecine boisei), of Java man (800,000 years ago), of Peking man and Boxgrove man (both about 500,000 years ago) and even the Neanderthals, whose often well-preserved remains date from as recently as 30,000 years ago? All these creatures seem to be separate from the line of descent to modern human beings. During the past four million years, it is plain that there were long periods when different forms of hominids were alive at the same time - H. erectus and H. robustus at similar locations in east Africa, H. erectus and Peking man at separate locations and H. sapiens together with the Neanderthals in Central Europe as recently as 30,000 years ago.
Confusion can however be dispelled. The human lineage plainly runs from the Australopithecenes (typified by Australopithecus afarensis) found in Ethiopia to Homo erectus (at 2.1 million years ago), which is known from the anatomy of the skeletons to have walked upright. Homo erectus was the genetic source of several forms of hominids, such as H. robustus (found alongside H. erectus in Kenya) as well as the forms of early man found elsewhere, from Boxgrove in the United Kingdom to Peking and Java in the Far East; presumably they were the first of many waves of emigration from Africa.
The remaining uncertainty concerns the age at which Homo sapiens appeared. Wilson's estimate of 125,000 years for the age of the most recent common ancestor of all the human groups is based on the analysis of mitochondrial DNA in modern populations. The stretch of DNA concerned has no effect on people's well-being; it is simply a marker in which mutations signify the passage of time. The implication is that all the people now alive are descendants of a group of people living 125,000 years ago who shared essentially the same stretch of mitochondrial DNA. How many people were in the group? A few hundred, perhaps, or even a thousand or so. The group must have been so small that there would have been no significant variations of the marker DNA among them. And what is the basis on which the group was afterward able to grow so quickly? They must have shared a powerful selective advantage: the faculty of language is the obvious candidate.
There is no proof that the people who came out of Africa 125,000 years ago were the first human beings able to speak to each other, but that fits in well with much else that is known from the fossil record. Why, for example, did the Neanderthals disappear from Central Europe? In the fossil record, they predate the most recent migration out of Africa; presumably they could not speak. Indeed, a remarkable study of mitochondrial DNA recovered from the type specimen of the Neanderthals during 1997 shows that the genetic lineage of these creatures diverged from the lineage of modern man roughly 600,000 years ago.[3] The inference must be that they lost the competition for resources once their articulate successors had occupied their territory.
The best place to look for confirmation of this surmise is not in fossil skeletons, but in the human genome. Soon, there will be a listing of all the human genes. In due course it will be possible to identify those whose chief function is in the development of the brain. Eventually it should be possible to tell which are particularly involved with the development of language and to glean from them something about their origin. It will be a task comparable in difficulty with the old biblical task of making bricks without straw. Every scrap of information from paleontology and ecology will be required to ensure success. But that is how the history of the human race will be reconstructed in the next century or so.
Sir John Maddox is editor emeritus of Nature. He was knighted in 1994 for services to science, and became a Fellow of the American Academy of Sciences in 1996. His other books include Beyond the Energy Crisis (1974), The Doomsday Syndrome (1972), and Revolution in Biology (1963).
Endlinks
talk.origins Archive: Fossil Hominids - descriptions of hominid species and summaries of recent research.
Neanderthal: No Relation - a summary of the mitochondrial-DNA sequencing project that proved Neanderthals were not ancestors of Homo sapiens. From Access Excellence.
Archaeology on the Net - a searchable database of online resources, annotated and organized by topic.
Introduction to Biological Anthropology - John H. Relethford's site for his course at the State University of New York at Oneonta includes annotated and well-organized links to other sites.
In Search of Human Origins - transcript from the three-part series. See also parts 2 and 3. From PBS's Nova, hosted by Don Johanson.
Co-Evolution of Neocortex Size, Group Size, and Language in Humans - preprint of a 1993 paper in behavioral and brain sciences. By R.I.M. Dunbar.
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