Lyell described Earth as a planet which was constantly changing, raising mountains where there had been plains, and land where there had been sea, eroding peaks, giving birth to new islands in the oceans, grinding rocks into sand. He argued that these changes were due to natural causes which were still operative and that therefore the world must still be changing. If the changes were imperceptible it was because they took place very slowly. Lyell’s geology required his readers to discard the Church’s teaching that only a few thousand years had elapsed since the world was created. He opened people’s minds to the idea of earth-time stretching far into the past over millions of years of gradual change, and Darwin adopted the same time-scale when he wrote his own book.
It is impossible to understand the processes of the evolution of living things without reference to the geology, geography and climate of the places where they live. Within fairly narrow limits, these factors dictate the kinds of life-forms which can exist on the world’s surface, in the thin outer layer of land and water now christened the biosphere. It supplies the chemical elements of which all living things are made, and has determined the range of temperatures within which they can survive.
If the earth’s axis were not tilted at an oblique angle to the sun, there would be no oak trees and no dormice – because there would be no seasons to give rise to leaf fall and hibernation. If the earth were not circled by a satellite, there would be no tidal beaches and none of the creatures specialised to exploit them.
In popular accounts of evolution there is a tendency to devalue the dynamic planet as a factor in evolution, and replace it with an inner life force. For example, the great evolutionary landmark 350 million years ago, when the first backboned fishes came onto the land, is often described in terms of aspiration and conquest, as if the fish climbed up the beach as John Hunt climbed Everest – because it was there.
It cannot have happened like that. No advantage could possibly accrue to the species that would even begin to outweigh the pains and perils of the first few hundred generations of being a fish out of water. It happened because the earth is not static. Lakes and pools shrink or dry up; rivers change their courses; land sinks below sea level; estuaries silt up. The first land vertebrates, like the little mud skippers found in the tropics today, were creatures who found themselves stranded for at least part of the time by movements of the tide or by seasonal droughts. They did not move out of their natural habitat to advance upon the land and colonise it. They stayed where they had always been. It was the earth that moved.
Since Lyell’s day we have learned things about the movements of the earth that he never dreamed of. He introduced the concept of the earth’s fabric on each of the great continents rearing up, contorting, being compressed or pulverised, fractured by ice or molten by volcanic fires. But he thought the continents themselves were firmly anchored in positions they had occupied since the original formation of the earth’s crust.
In 1912 Alfred Wegener published two articles outlining the hypothesis of Continental Drift – the idea that continents have moved horizontally relative to one another and to the ocean basins, and that the movement represents the splitting and drifting apart of land masses which had formerly been contiguous. He supplied a mass of evidence based on geological data, measurements of gravity, facts about climate in earlier ages, and the eccentric and unexplained distribution of fossil plants and animals throughout the continents as they are today.
When the physicist W. Lawrence Bragg organised the first British discussion of Wegener’s theories in Manchester, he was staggered by the response. ‘The local geologists were furious; words cannot describe their utter scorn of anything so ridiculous as this theory.’ Wegener’s opponents spent little time addressing his evidence, and none at all in attempting to find alternative explanations of the phenomena he described.
They stressed that they could not see how Continental Drift worked – with the implied corollary that if they could not see the method, then the method did not exist. They outlined theoretical proofs that it could not possibly work. Lecturers defending Wegener’s ideas were often greeted with the kind of boos and catcalls usually reserved for a third-rate vaudeville act at the Glasgow Empire. Wegener died in 1930 during an expedition to Greenland while this hooligan phase of geological analysis was at its height. He would have had to live another thirty years to see the tide of opinion swing in his favour, and Continental Drift transmuted from heresy to orthodoxy.
The Wegener hypothesis about moving blocks of the earth’s crust has particular relevance to the story of man’s origins. In north-east Africa there is a unique and extraordinary geological situation where not two but three crustal blocks converge. One line of rifting continues 280 miles to the north to form the Wadi Aqaba-Dead Sea-Jordan rift. Another runs east 600 miles to form the Gulf of Aden. The third runs south along the whole length of the African Rift Valley, marked by signs of continuous volcanic activity lasting over eleven million years.
This convergence of geological forces and tensions has its focus in the district of Ethiopia known as Afar, which gave its name to Lucy’s scientific title: Australopithecus afarensis. The region has become as much a Mecca for tectonic, volcanological and geological researchers as for fossil-hunters, and they have made equally remarkable discoveries.
As with the fossil-hunters, the information gathered by the geologists has given rise to excitement and controversy in about equal measure. Paul Mohr, the geologist responsible for reviving the old name of Afar for the region, summed up what had been discovered, and the debates over interpretations, in a paper bearing the one-word title ‘Afar’.
He made it clear that despite disagreement over some aspects of the new data, there was virtual unanimity on the one point which chiefly concerns us here. He wrote as follows: ‘What is more or less agreed upon is that by the late Miocene [seven million years ago] a marine basin had become established over northern Afar … and these conditions persisted until the isolation and desiccation of the Salt Plain arm of the sea some 70,000 years ago (CNR-CNRS team, 1973).’
The sea flooded into Afar, but it never flooded out again. Further movements of the jostling crustal blocks sealed off the Sea of Afar from the ocean, and over millions of years the water evaporated. A similar thing happened, at the northern end of the rift, to the Dead Sea in Israel and the end result will be the same – it will become brinier and brinier until nothing is left but a salt plain. The Afar Depression is now one of the hottest and most nearly impassable deserts in the world. The salt deposits are thousands of feet deep. For the tribesmen who sometimes load the salt and take it on the long trek to the cities, it is the only marketable product the territory affords.
At the eastern end of the salt plain is a highland region called the Danakil Alps. Geologically these highlands are described as a ‘horst’ – a chunk of the earth’s crust torn away from one of the crustal blocks and tilting down at the western end. Danakil was never covered by the ancient Afar sea. Geologists can tell, from the rocky debris, precisely how far up the side of it the water rose. Some of the plants on it have become differentiated from their counterparts to the west of the salt plain, as normally happens when an island is separated from a mainland.
These things were happening around the time suggested by molecular biologists as the beginning of the ape/man split. They were happening close to the region where the very earliest of the hominid fossils have been discovered. They took place in latitudes where a great unbroken band of forest formerly extended from the west of Africa across the continent and on throughout most of Asia. The region affected by these geological disturbances would therefore have been the kind of habitat in which apes flourished and still flourish.
It is hard to escape the conclusion that some of the apes must have found their environment suddenly and radically changed. Some may have been cut off from the rest of the population by an arm of the sea as it flowed into the terrain which now lies far below the great Salt Plain. The American Le
on P. LaLumiere, of the Naval Research Laboratory in Washington, was the first to suggest Danakil Island (now the Danakil Alps) as a place where one such group may well have been marooned.
Geographic separation of this kind provides ideal conditions for rapid evolutionary change. In 1963 Ernst Mayr, in his classic work on this subject, Animal Species and Evolution, regarded it as the commonest cause of species divergence. He wrote: ‘That geographic speciation is the almost exclusive mode of speciation among animals, and most likely the prevailing mode even in plants, is now quite generally accepted. The theory of geographic speciation is one of the key theories of evolutionary biology.’
Figure 1 (left). The generally supposed relationship of the Nubian plate of the African continent to the Arabian plate during the late Miocene.
Figure 2 (right). The configuration of the same region as displayed in Figure 1, but as it may have been at the beginning of the Pliocene.
Figure 3 (left). The African Rift Valley showing the principal sites where hominid fossils have been discovered.
Figure 4 (right). The location of the Tertiary deposits called the Red Series that may yield hominoid fossils if this hypothesis is correct.
Figure 5. Cross section through northern Afar, Ethiopia, looking north-northwest. After Hutchinson and Engels (1970, 1972). Detritus deposits vary in age, and can range from Late Miocene to the present. This hypothetical view is neither exact nor to scale.
An alternative mode sometimes occurs where part of a species remains in the same shared habitat but occupies a different niche in it – ‘sympatric’ or ‘in-the-same-place’ speciation. But no one has suggested that as the mode of our own origin.
Savannah theorists agree that there must have been geographic separation, but they suggest that a voluntary move from the trees to the savannah was sufficient to account for the split. In theory it could be so, but it would have taken much longer to result in species divergence. The isolation would be partial and intermittent in the early stages. The apes who ventured onto the grassland would sometimes return to the trees for shade or safety, or encounter individuals who were still arboreal but making a brief sortie. Any such encounter could lead to interbreeding, and the genes would once again be mingled in the common pool. The savannah version looked more convincing in the old days when fifteen or twenty million years were allowed for a gradual speciation.
The more absolute the isolation, the quicker the change. R. R. Miller in the ’50s and ’60s researched speciation of fish in the freshwater springs and creeks of the western North American deserts, and demonstrated that speciation in small isolated bodies of water might proceed one thousand times as fast as in the ocean.
The aquatic theory has been criticised on the grounds that the ape/man divergence took place so rapidly that ‘… there would not have been time’ for a species to go through an aquatic phase during the period of the fossil gap. But time is on the side of the aquatic theory. Really rapid speciation calls for stricter isolation and a more abrupt change in the environment than the savannah theory provides for.
It is impossible to deduce exactly the kind of environment in which the apes lived. Nowadays we publish maps with clear black lines defining the boundaries between land and water, and try to separate the two by draining bogs and fens and turning wetlands into marinas. But the earth is not quite like that, even today. All over the world there are areas like the Everglades of Florida, the Sundarbans of the Ganges delta, the mangrove swamps of the East Indies, and the Okavango in Botswana which are hard to define either as land or water. In the Amazon basin there are tracts of territory – of a total surface area equal to that of Great Britain – which are forested but for more than six months of every year are under water and inhabited by fish and dolphins swimming among the tree tops.
If we wish to know what happens to an ape in an aquatic environment, we can turn to one species whose aquatic credentials are less controversial than those of Homo. There was a species – no ancestors of ours and long extinct – called Oreopithecus. These apes have been dubbed ‘swamp apes’ because their bones were laid in mud, which preserved them so well that their fossil remains have been found in large numbers. Some of the skeletons found in Italy were nearly intact. They were once hotly fancied as our earliest ancestors, and one enthusiastic palaeontologist, J. Hürzeler, claimed to be able to trace eighteen resemblances between the skeleton of Oreopithecus and that of Homo.
Since Oreopithecus was expunged from our family tree, interest in the species has waned. It is now recognised that any resemblances between the swamp apes and man are not due to a close genetic relationship. The only other possible explanation of them, if we rule out blind coincidence, is convergent evolution. This is the mechanism by which species inhabiting similar environments and with a similar life-style often grow to resemble one another though they are genetically unrelated and may be separated by half the width of the world.
The idea of convergent evolution providing a link between man and Oreopithecus is an illuminating one. Because one of the interesting things about Oreopithecus is its pelvis. It is characterised by the short iliac bones which are seen in most aquatic mammals, and in man and his ancestors. The evidence suggests that through living in marshes Oreopithecus had become either a good swimmer or a bipedalist – or both.
5
The Cost of a Naked Skin
‘There are one hundred and ninety-three living
species of monkeys and apes. One hundred
and ninety-two of them are covered with hair.
The exception is a naked ape self-named Homo
sapiens. The zoologist now has to start making
comparisons. Where else is nudity at a
premium?’
Desmond Morris
Darwin, as we have seen, believed he could accommodate human bipedalism within the basic framework of his theory. He thought it possible that walking upright made our ancestors fitter to survive. But human nakedness was a different matter. He had no illusions about that.
In his book The Origin of Species, published in 1859, he gave copious examples of how natural selection worked, but he did not discuss the human species in relation to it. After stating his belief that his theory would open the way to new avenues of future research, he contented himself with thirteen words on Homo sapiens. ‘Much light,’ he wrote, ‘will be thrown on the origin of man and his history.’
This reticence is sometimes ascribed to prudence, or caution, a desire to minimise the hostility he knew he was going to encounter. If that was his hope it was certainly not fulfilled, and he cannot seriously have expected it to be. In the famous Oxford debate of 1860, all the most heated arguments raged not around the theory in general, but around the one omitted topic – man’s relationship with the apes.
There could have been a different reason for the omission. Perhaps he was not ready to give an account of human evolution while there were things about it he could not understand. He could not, for example, convince himself that being naked could possibly make a primate fitter to survive. This is what he had to say about it:
‘The loss of hair is an inconvenience and probably an injury to man, for he is thus exposed to the scorching of the sun and to sudden chills, especially due to wet weather. No one supposes that the nakedness of the skin is any direct advantage to man; his body therefore cannot have been divested of hair through natural selection’.
The most striking thing about that statement is that most modern evolutionists – some of them more Darwinist than Darwin himself – would find it impossible to accept. They start with the axiom that man evolved on the savannah, they observe that he has lost his body hair, and they conclude that nakedness must, therefore, have been an efficient adaptation for life in that milieu.
Practically all the contemporary theories about nakedness begin with the concept of a torrid savannah and an overheated ape. That is an oversimplified picture of a savannah environment. It is perfectly true that the days are hot there, som
etimes very hot; but in the nights the temperature can sometimes drop as low as 11°C, and the indigenous animals have to live there for twenty-four hours a day.
In 1989 the BBC sent a series of outside broadcast crews to spend an entire day on the savannah, sending back live pictures of the wildlife to be slotted into the viewing programme at intervals during the day. Some magnificent shots of wildebeest, lions and elephants were obtained, but in some ways the most memorable shot was the final one of a couple of broadcasters muffled up to the eyebrows trying to keep their teeth from chattering. As one reviewer summed it up, ‘The weather broke, the light went, and Julian Pettifer nearly died of exposure.’ It would not have been a good time to explain to Mr Pettifer that his ancestors had shed their fur to make them better fitted to survive in these conditions.
If they had kept their coat of fur they would have been better insulated not only against the cold of the night but also the heat of the day. All desert animals have retained their fur. In hot countries, shaving off even a portion of wool from a sheep’s back leads to an immediate rise in its internal temperature and the rate of panting. Desert-dwelling humans like the Bedouin keep their heads and bodies covered when out in the sun, just as northern peoples cover themselves with clothes when out in the wind, in both cases attempting to replace the natural protection that our species has lost.
A covering of either fur or feathers has been the standard equipment for warm-blooded creatures ever since they emerged over a hundred million years ago, and ultimately took over from the reptiles as the dominant form of animal life.
One of the many advantages of such a covering is the ability to adjust the thickness of insulation within seconds. Hairs and feathers have erectile muscles attached to them, so that robins, for instance, can puff out their feathers when the temperature drops; animals can erect their hairs either in response to cold or danger; cats and dogs bristle the hair on their backs when confronted with an enemy in order to look larger and more intimidating.
The Scars of Evolution Page 6