The Language of the Genes

by Steve Jones

  The battle of the sexes is often seen as regrettable but unavoidable, but most people assume that the bonds between mother and child.nv driven by mutual devotion. To tin- cold eye ot the biologist rhc transaction between generations is also filled with conflict, with many chances for i he two parties to exploit each other. It is in the child's interest to gain as much attention as possible from its mother. The mother's concern is to provide as little as will allow her progeny to survive. If she is too generous to one, the next may suffer.

  Such confrontations, deplorable as they seem, are the commonplace of the animal world. There has grown up in biology the comforting supposition that nature is not really red in tooth and claw and that animals rarely do much harm to other members of their own species. The battle for reproductive success shows how wrong this is. Eagles lay several eggs. If food is plentiful all the chicks are fed, but any shortage means that the last to hatch is allowed to starve or is killed by its sibs. Rats, mice and other mammals often eat all their young when food gets short (a habit known as kronism, after the Greek deity Kronos, who devoured his own children).

  Any mother is certain that all her children (first, second or third born) carry her own genes, but it is quite possible (and in many animals almost guaranteed) that the father of her first child will not be the same as that of her later offspring. As Aristotle put it: 'This is the reason why mothers are more devoted to their children than fathers: it is that they suffer more in giving them birth and are more certain that they are their own'. The conflicts of interest involved, and the differences in the investment of each sex in their young, may help to explain certain strange patterns of inheritance.

  Much to the surprise of geneticists the effects of a particular gene sometimes depends on whether it is passed on by mother or by father. This effect, 'genomic imprinting' as it is known, is quite different from sex linkage, as the genes involved may be on any chromosome. Each sex seems to stamp its personality on what it transmits. The DNA itself is not permanently altered, but its effects on those who inherit it depend on which parent it came from. A gene passed on by a father to his daughter differs in its impact from that of the same gene passed by her to her own children. The DNA is 'marked' as it is conveyed through sperm or egg and the mark reversed when tin- line of transmission changes from one sex to another.

  Each embryo contains both maternal and paternal DNA. If we use the (rather dubious) metaphor that every gene acts in its own interests, it pays those that come from the father to extract as much as possible from the mother in which they find themselves, irrespective of any damage which this does to her or to subsequent children, as later offspring may well carry genes from a different father. The first father loses nothing by exploiting his mate as much as he can as she will bear no more of his children. The mother, in contrast, needs to ensure that further attempts to pass on her own heritage are not jeopardised by the avarice of her firstborn. The difference in behaviour of the same gene when transmitted through fathers or through mothers hence arises from paternal greed.

  In mice, the genes that make the membranes through which the foetus feeds are more active if they come from the father; and the effect is so strong that the placenta itself has been described as a parasite forced on the mother by the father. Those passed on by the male parent also tend to increase the size of the tongue (which is, of course, used in suckling). Genes for human disease show the same effect. Some foetuses by accident inherit two copies of a gene that promotes growth. They become abnormally large only if both copies come from the father. In normal foetuses just the paternal copy is switched on as further evidence of the father's interest in his child extracting the most it can from its mother. Two rare genetic diseases (the Prader-Willi and Angelman syndromes) were once thought to he different as their symptoms are distinct. In fact they arc duo to the same mutation, a deletion of a short segment of chromosome fifteen which damages the gene involved in imprinting paternal identity. The differences depend on whether it is passed on by father or by mother. Children with Prader-Willi syndrome (whose abnormal chromosome comes from their father) suckle hard, become obsessively interested in food and are fat, while Angelman children (who receive the same structure from their mother) are thin or of normal weight, but have quite severe nervous symptoms, such as epilepsy, a tendency towards constant laughter, and a fascination with water. In the latter disease, the paternal copy of the gene is silenced in parts of the brain, perhaps explaining the mental illness. Enthusiasts for conflict suggest that even a baby's cries are an attempt to manipulate its mother to provide more food and that the mother retaliates by secreting in her milk substances similar to those used as sedatives by doctors. Whether or not this is true, it is clear that once sex has evolved it has some unexpected effects on the lives of the creatures who practise it. Without sex there would be almost no evolution and no genetics. Our universal fascination with the subject may, one day, solve the most important sexual problem of all — why we bother in the first place.

  Chapter Eight CLOCKS, FOSSILS AND APES

  The boundary between apes and humans w;i once far from clear. Lord Monboddo, a friend of Dr Johnson's, was convinced that The Orang Utan is as ardent for women;is it is for its own females' and that the Malayans cut the tails off the offspring of such matings and took them as their own. 'From the particulars mentioned' — he wrote -(it appears certain that they are of our species… though they have not come to the lengths of language.' Dr Johnson was not impressed: 'It is a pity to see Lord Monboddo publish such notions… in a fool doing it, we should only laugh; but when a wise man does it, we are sorry.1

  A complementary foolishness is around today. Millions of Americans do not believe that humans are related to apes at all or even that the human species is more than a few thousand years old. Creationists are determined to stay ignorant. They deny that we evolved and are hence connected through our genes to the rest of life. President Reagan himself once said that 'Evolution is only a theory which is not believed in the scientific community to be as infallible as it once was.. Recent discoveries have pointed up great flaws in it.' The creationist dogma bores, when it does not exasperate, biologists. As a result they have been less active in opposition than they should and the bigots have had some success, in the USA at least.

  The written documents of history stop at the day before yesterday. The earliest texts come from the Sumerians. Records go back a iittle further in mythic form. Gilgamesh was King of the Sumerian city-state of Uruk in 2700 BC. The Kpic which hears his name has some familiar features. It has a Ci.irdcn of 1'dcii, a Hero's descent to the Underworld (witli.1 safe return) and a Flood. In the same way, the best tH;ill evidence that humans did evolve and that they arc members of the animal kingdom comes from our own ancient records. Without fossils, the portrait of our forebears can never be complete. A historian who knows only the modern world would find it impossible to infer the progress of, say, Turkey and the United States just from what exists today. All historians need documents from the past. To have confidence in their theories evolutionists must have the same thing.

  Fossils authenticate the past. At one time, they were defined to be the work of the Devil, placed in the rock to mislead the faithful. Later came a last-ditch attempt to fit them into the Bible. Some fossil mammals seemed to be standing on tiptoe with their noses in the air when they met their end; proof that they had been overwhelmed by Noah's Flood.

  Darwin was well aware of the power of the past. About one page in six of The Origin of Species deals with the fossil record of animals and plants. The fragments of their ancestors were central to his theory. For humans, Darwin had an enormous gap in his evidence. He knew little about the remains of our ancestors and there is scarcely a mention of them in his other great work, The Descent of Man, published in 1871. Although we now know a little more about the bones of our predecessors, the record of our own evolution is still very incomplete.

  The first fossil to be recognised as a possible human ancestor was Neande
rthal man, found in the Neander Valley in Germany in 1856. Such was the power of belief in those days that some dismissed the bones as those of an arthritic cripple or of a cossack who had died in the retreat from Moscow. A hundred years ago, a skull intermediate between humans and other primates was found. This was Pithecanthropus erectus, Java Man. The search for our birthplace was on and has continued ever since.

  Palaeontologists still do not agree about where modern humans came from and where they went. The fossil record is so incomplete that a cynic might feel thai the main lesson to be learned is thar evolution usually takes place somewhere else. The origin of humanity has been claimed to be Asia, Africa and even the whole world at the same rime. The human record has been studied as hard as any, but still has enormous holes. Even the best known deposits are sketchy. The area around Lake Turkana in East Africa is almost never off the television screen. Guesses about population size suggest that perhaps seventy million people lived there over its two and a half million year history. Remains of just a few hundred have been found, most as small fragments. The fossil record will never give us the complete history of our past, but it can give dates and places which genes can only hint at. It is worth glancing at the bones before staring at the molecules.

  Just as with the genome, the biggest problem with the preserved record of the past is one of scale. Life began about four thousand million years ago. The journey from Land's End to John o'Groat's can again be used as a metaphor. Everywhere south of Birmingham is covered with primeval slime about which we know little. The first primitive land animals crawl ashore near Edinburgh. There are frogs in the Cairngorms and for thirty miles north of Inverness the landscape is infested with dinosaurs. The first primates appear near Wick, while our own species can look over the icy waters of the Pentland Firth from its birthplace a hundred yards from the northernmost shore of Britain. Recorded history begins on the beach, at high tide mark.

  The journey through time needs milestones. Fossils can be dated in many ways. Some depend on the decay of radioactive materials as time passes. Others are more ingenious. Ostrich eggs were favoured as containers in the ancient world. The structure of their amino acids, like that ot all tissues, is biased towards the left. Over the years, the amino acids decay into a mixture of left- and right-handed forms. To measure the ratio of left to right dates the shells and the people who used them. The oldest ostrich-shell containers are at Klasies River Mouth, a site in South Africa occupied by humans whose skulls are much like those of today. They are dated at a hundred and twenty thousand years old. The oldest outside Africa, found in the Israeli cave of Qafzeh, are twenty thousand years younger; and fifty thousand years ago the shells were used to make the first of all ornaments, some beads unearthed in Tanzania.

  Our history stretches back to the same dawn as all other creatures; and, like them, we are improbable survivors from a past that has almost disappeared. There were several nasty moments on the road to Homo sapiens. For much of history, several species of pre-humans lived at once. Most went nowhere. As apes, we belong to the less successful branch of our family, for most of our kin disappeared fifteen million years ago as monkeys flourished. As primates humans claim allegiance to a group that thirty million years earlier left only a few twigs on a flourishing family tree; and as mammals we are the descendants of a rare and insignificant band of mouse-like creatures that cowered beneath the once thriving dinosaurs.

  The history of mankind's earliest ancestors is obscure. Bones that look like those of primates — apes, monkeys and humans — appear around sixty million years ago. The first fragment of what may be an anthropoid (the group which evolved into monkeys, apes and humans) is about forty million years old. This creature was not much bigger than a rat; but within ten million years the group was well established from North Africa to China and Burma. The animals were small, nor much larger than today's tarsiers and, like them, they probably ate insects and fruit rather than the leaves favoured by many of their modern descendants. A jawbone half that age from an early hominoid (the group which includes humans and apes) has been found in Kenya. By fifteen million years ago several species of ape roamed Africa and Asia. None was larger than;i seven year-old boy and all had small brains and pointi*d faces.

  One of the earliest direct precursor of modern humans appeared between three and four million years before today in the Laetoli beds of Kenya. Australopithecus afar-ensis is named after the Afar region of Ethiopia, the Biblical Ophir referred to in the story of Solomon and Sheba. The most famous specimen is 'Lucy', so named because the discoverers were playing the Beatles' 'Lucy in the Sky with Diamonds' at the time. She was less than four feet tall, with a small skull. Australopithecus bones show that, like today's chimpanzees and gorillas, they went in for knuckle walking. The earliest bones which look as if they belong to our immediate ancestors, the genus Homo% come from Kenya and are dated at about two and a half million years old. The first stone tools appear at about the same time.

  It is as difficult to classify fossils as to define artistic styles. Because they evolve one into another it is pointless to draw a line to show exactly when, for example, impressionist painting changed into post-impressionism. A certain arbitrariness is bound to creep in. In palaeontology things are even worse, as few specimens are found and their discoverers have a natural tendency to grace each with its own name. Even what is needed for promotion to that august genus, Homo is in dispute. A pint or so of brain, a grasping hand and a few simple stone tools are the minimum entrance requirements; but the pass mark depends on the whim of the examiner (some of whom are choosy enough to exclude the oldest member, Homo habilis> and to downgrade him to;i mere australopithecine).

  However, mosl experts agree that there were at least tour species of Homo: the first. Homo habilis, ('handyman') from around 2.3 million years ago to its disappearance some seven hundred thousand years later; the second, Homo erectus^ emerging about 1.9 million years before the present, with the youngest reliably dated specimens at about 200,000 years old, and — in the end — our own species, Homo sapiens, which began to emerge about half a million years ago. Homo habilts is sometimes divided into two distinct species, habilis itself and Homo rudolfensis. Homo erectus, too, has been subdivided into two or more species; and the history of our species may be one of a series of close relatives who became extinct, leaving no trace of their presence. Habilis had a larger brain than its predecessors, its face jutted out less and for the first time there was a noticeable nose and a perceptible chin. An almost complete skeleton of an erectus boy has been found near Lake Turkana in Kenya. He had a brow ridge and a massive jaw, with long arms and legs. For much of the time more than one species of man-like beast existed at once. In Africa beasts which looked much like Lucy and her relatives lived for thousands of years alongside Homo habilts. For much of the time, several species of Homo may have lived together, a situation hard to conceive today.

  Homo erectus was the first to escape from Africa and did so soon after it appeared. Erectus bones almost two million years old, mixed with those of sabre-toothed tigers and elephants, have been found in the the Republic of Georgia and this species soon spread, in modified form, to the Middle East, China, Java and Europe. Its remains include 'Java Man1 and 'Peking Man' (whose bones disappeared during the Japanese invasion of China) which had a static existence with almost no change in the skull over its long history.

  The first Homo sapiens — some of which look rather like erectus — emerged in Africa more than four hundred thousand years ago. They were robust and would appear distinctly hostile to modern eyes, although some had brains larger than the average today. A half-million-yrnr-oid skull that may belong to this group was found M Knxgrove in Sussex. Within a couple of hundred of thousand years such populations of 'archaic Homo sapiens' wen* (omul throughout Europe.

  The Neanderthals, who flourished for a hundred thousand years before they sank beneath a wave of modern humans, had larger brains than our own (albeit on a heavier body), with large no
ses and teeth. Their remains have turned up all over Europe and the Middle East, and have been found as far east as Iraq, but not in Africa or elsewhere. Once, they were assumed to be on the direct line to ourselves; but fragments of DNA extracted from mitochondria show them to have been so distinct that Neanderthals mark yet another dead end on the road to humankind.

  Around a hundred and thirty thousand years ago, the first humans of modern appearance (light build, thin skull, large brain and small jaw) appear in Africa. Their remains have been found from Omo-Kibish in Ethiopia to the southern tip of Africa, thousands of miles south. Quite soon after they emerged, modern humans began a relentless expansion. Many of the earliest sites are near the coast: but in those distant days the sea was lower than it is today and broad coastal plains stretched in front of those ancestral caves. Stone tools have been found on coral reefs in the Red Sea (which was dry a hundred thousand years ago) so that perhaps our early ancestors walked along a now-drowned shoreline from Africa to Indonesia, leaving their archaic relatives, who were still around at the time, inland.