by Peter Watson
It is a plausible and original idea, but there are problems. The next step in the chain of life was the creation of cellular organisms, bacteria, for which a skin was required. Here the best candidates are what are known as lipid vesicles, tiny bubbles that form membranes automatically. These chemicals were found naturally occurring in meteorites, which, many people argue, brought the first organic compounds to the very young Earth. On this reasoning then, life in at least some of its elements had an extraterrestrial beginning. Another problem was that the most primitive bacteria, which are indeed little more than rods or discs of activity, surrounded by a skin, are chiefly found around volcanic vents on the ocean floor, where the hot interior of the earth erupts in the process that, as we have already seen, contributes to sea-floor spreading (some of these bacteria can only thrive in temperatures above boiling point, so that one might say life began in hell). It is therefore difficult to reconcile this with the idea that life originally began as a result of sunlight acting on clay-crystal structures in much shallower bodies of water.10
Whatever the actual origin of life (generally regarded as having occurred around 3,800 million years ago), there is no question that the first bacterial organisms were anaerobes, operating only in the absence of oxygen. Given that the early atmosphere of the earth contained very little or no oxygen, this is not so surprising. Around 2,500 million years ago, however, we begin to see in the earth’s rocks the accumulation of haematite, an oxidised form of iron. This appears to mean that oxygen was being produced, but was at first ‘used up’ by other minerals in the world. The best candidate for an oxygen-producer is a blue-green bacterium that, in shallower reaches of water where the sun could get at it and with the light acting on chlorophyll, broke carbon dioxide down into carbon, which it utilised for its own purposes, and oxygen – in other words, photosynthesis. For a time the minerals of the earth soaked up what oxygen was going (limestone rocks captured oxygen as calcium carbonate, iron rusted, and so on), but eventually the mineral world became saturated, and after that, over a thousand million years, billions of bacteria poured out tiny puffs of oxygen, gradually transforming the earth’s atmosphere.11
According to Richard Fortey, in his history of the earth, the next advance was the formation of slimy communities of microbes, structured into ‘mats,’ almost two-dimensional layers. These are still found even today on saline flats in the tropics where the absence of grazing animals allows their survival, though fossilised forms have also been found in rocks dating to more than 3,500 million years old in South Africa and Australia. These structures are known as stromatolites.12 Resembling ‘layered cabbages,’ they could grow to immense lengths – 30 feet was normal, and 100 metres not unknown. But they were made up of prokaryotes, or cells without nuclei, which reproduced simply by splitting. The advent of nuclei was the next advance; as the American biologist Lynn Margulis has pointed out, one bacterium cannibalised another, which became an organelle within another organism, and eventually formed the nucleus.13 A chloroplast is another such organelle, performing photosynthesis within a cell. The development of the nucleus and organelles was a crucial step, allowing more complex structures to be formed. This, it is believed, was followed by the evolution of sex, which seems to have occurred about 2,000 million years ago. Sex occurred because it allowed the possibility of genetic variation, giving a boost to evolution which, at that time, would have speeded up (the fossil records do become gradually more varied then). Cells became larger, more complex – and slimes appeared. Slimes can take on various forms, and can also on occasion move over the surface of other objects. In other words, they are both animate and inanimate, showing the development of rudimentary specialised tissues, behaving in ways faintly resembling animals.
By 700 million years ago, the Ediacara had appeared.14 These, the most primitive form of animal, have been discovered in various parts of the world, from Leicester, England, to the Flinders Mountains in south Australia. They take many exotic forms but in general are characterised by radial symmetry, skin walls only two cells thick, with primitive stomachs and mouths, like primitive jellyfish in appearance, and therefore not unimaginably far from slime. The first truly multicellular organisms, the Ediacara did not survive, at least not until the present day. For some reason they became extinct, despite their multifarious forms, and this may have been ultimately because they lacked a skeleton. This seems to have been the next important moment in evolution. Palaeontologists can say this with some confidence because, about 500 million years ago, there was a revolution in animal life on Earth. This is what became known as the Cambrian Explosion. Over the course of only 15 million years, animals with shells appeared, and in forms that are familiar even today. These were the trilobites – some with jointed legs and grasping claws, some with rudimentary dorsal nerves, some with early forms of eye, others with features so strange they are hard to describe.15
And so, by the mid- to late 1980s a new evolutionary synthesis began to emerge, one that filled in the order of important developments and provided more accurate dating. Moving forward in geological time, we can leap ahead from the Cambrian Explosion by more than 400 million years, to approximately 65 million years ago. One of the effects of the landing on the Moon, and the subsequent space probes, was that geology went from being a discipline with a single planet to study to one where there was suddenly a much richer base of data. One of the ways that the moon and other planets differ from Earth is that they seem to have far more craters on them, these craters being formed by impacts from asteroids or meteorites: bodies from space.16 This was important in geology because, by the 1970s, the discipline had become used to a slow-moving chronology, measured in millions of years. There was, however, one great exception to this rule, and that became known as the K/T boundary, the boundary between the Cretaceous and Tertiary geological periods, occurring about 65 million years ago, when the fossil records showed a huge and very sudden disruption, the chief feature of which was that many forms of life on Earth suddenly disappeared.17 The most notable of these extinctions was that of the dinosaurs, dominant large animals for about 150 million years before that, and completely absent from the fossil record afterward. Traditionally, geologists and palaeontologists considered that the mass extinctions were due to climate change or a fall in sea level. For many, however, this process would have been too slow – plants and animals would have adjusted, whereas in fact about half the life forms on Earth suddenly disappeared between the Cretaceous and the Tertiary. After the study of so many craters on other moons and planets, some palaeontologists began to consider whether a similarly catastrophic event might not have caused the mass extinctions seen on earth 65 million years ago. In this way there began an amazing scientific detective story that was not fully resolved until 1991.
For a meteorite or asteroid to cause such a devastating impact, it needed to have been a certain minimum size, so the crater it caused ought to have been difficult to overlook.18 No immediate candidate suggested itself, but the first breakthrough came when scientists realised that meteorites have a different chemical structure to that of Earth, in particular with regard to the platinum group of elements. This is because these elements are absorbed by iron, and the earth has a huge iron core. Meteorite dust, on the other hand, would be rich in these elements, such as iridium. Sure enough, by testing rocky outcrops dating from the Cretaceous/Tertiary border, Luis and Walter Alvarez, from the University of California at Berkeley, discovered that iridium was present in quantities that were ninety times as rich as they should have been if no impact had taken place.19 It was this discovery, in June 1978, that set off this father- and-son (and subsequently daughter-in-law) team on the quest that took them more than a decade. The second breakthrough came in 1981, in Nature, when Jan Smit, a Dutch scientist, reported his discoveries at a K/T boundary site at Caravaca in Spain.20 He described some small round objects, the size of a sand grain, called spherules, which he said were common at these sites and on analysis were shown to have crystals
of a ‘feathery’ shape, made of sanidine, a form of potassium feldspar.21 These spherules, it was shown, had developed from earlier structures made of olivine – pyroxene and calcium-rich feldspar – and their significance lay in the fact that they are characteristic of basalt, the main rock that forms the earth crust under the oceans. In other words, the meteorite had slammed into the earth in the ocean and not on land.
This was both good news and bad news. It was good news in that it confirmed there had been a massive impact 65 million years ago. It was bad news in the sense that it led scientists to look for a crater in the oceans, and also to look for evidence of the massive tsunami, or tidal wave, that must have followed. Calculations showed that such a wave would have been a kilometre high as it approached continental shorelines. Both of these searches proved fruitless, and although evidence for an impact began to accumulate throughout the 1980s, with more than 100 areas located that showed iridium anomalies, as they were called, the actual site of the impact still remained elusive. It was not until 1988, when Alan Hildebrand, a Canadian attached to the University of Arizona, first began studying the Brazos River in Texas, that the decade-long search moved into its final stage.22 It had been known for some time that in one place near Waco the Brazos passes over some rapids associated with a hard sandy bed, and this bed, it was recognised, was the remnant of a tsunami inundation. Hildebrand looked hard at Brazos and then went in search of evidence that would link it, in a circular fashion, with other features in the area. By examining maps, and gravity anomalies, he finally found a circular structure, which might be an impact crater, on the floor of the Caribbean, north of Colombia, but also extending into the Yucatán Peninsula in Mexico. Other palaeontologists were sceptical at first, but when Hildebrand brought in help from geologists more familiar with Yucatán, they soon confirmed the area as the impact site. The reason everyone had been so confused was that the crater – known as Chicxulub – was buried under more recent rocks.23 When Hildebrand and his colleagues published their paper in 1991, it caused a sensation, at least to geologists and palaeontologists, who now had to revise their whole attitude: catastrophic events could have an impact on evolution.24
The discovery of Chicxulub produced other surprises. First, it turned out that the crater was to an extent responsible for the distribution of cenotes, small, spring-fed lakes that provided the fresh water that made the Mayan civilisation possible.25 Second, three other mass extinctions are now recognised by palaeontologists, occurring at 365, 250, and 205 million years ago. The disappearance of the dinosaurs also proved to have had a liberating effect on mammals. Until the K/T boundary, mammals were small creatures. This may have helped their survival after the impact – because they were so numerous – but in any event the larger mammals did not emerge until after the K/T, and in the absence of competition from Tyrannosaurus rex, Triceratops, and their brothers and sisters. There would probably have been no humans unless the K/T meteorite had collided with Earth.
So far as the origins of humanity were concerned, the 1980s provided one or two crucial excavations, but the period was really a golden age of interpretation and analysis rather than of discovery.
‘Turkana Boy,’ discovered by the Leakeys near Kenya’s Lake Turkana in August 1984, was much taller than people expected and quite slender, the first hominid to approach modern man in his dimensions.26 He had a narrow spinal canal and a thorax that tapered upward, which suggested to anatomists that Turkana Boy had only limited nerve signals being sent to the thorax, giving him less command of respiration than would have been needed if he were to speak as we do. In other words, Turkana Boy had no language. At the same time the tapered thorax meant that his arms would be closer together, making it easier to hang in trees. Assigning him to Homo erectus, the Leakeys dated Turkana Boy to 1.6 million years ago. Two years later their archrival Don Johanson discovered a skeleton at Olduvai, attributed to Homo habilis and only 200,000 or so years older. This was very different – short and squat with long arms very like those of an ape.27 The idea that more than one hominid type was alive at the same time around 2 million years ago was not accepted by all palaeontologists, but it did seem plausible that this was the time when the change occurred that caused hominids to leave the forest. Elisabeth Vrba, from Yale, argued that around 2.5 million years ago other changes induced evolutionary developments.28 For instance, polar glaciation reduced the temperature of the earth, lowering sea levels and making the climate more arid, reducing vegetation. This was supported by the observation that fossils of forest antelopes become rare at this time, to be replaced by a variety that grazed on dry, open savannahs.29 Stone tools appeared around 2.5 million years ago, suggesting that hominids left the forests between, say, 2.5 and 1.5 million years ago, growing taller and more graceful in the process, and using primitive tools. More ‘prepared’ tools are seen at about 200,000 years ago, roughly the time when the Neanderthals appeared. Opinions on them changed, too. We now know that their brains were as large as ours, though ‘behind’ the face rather than ‘above’ it. They appeared to bury their dead, decorate their bodies with ochre, and support disabled members of their communities.30 In other words, they were not the savages the Victorians imagined, and they coexisted with Homo sapiens from about 50,000 to 28,000 years ago.31
These and other varied finds, between 1975 and 1995, consolidated in Ian Tattersall’s compilation of fossils, therefore suggested the following revised chronology for hominid evolution:
4–3 million years ago Bipedalism
2.5 million years ago early tool-using
1.5 million years ago fire (for cooking food, which implies hunting)
1 million years ago emigration of hominids from Africa
200,000 years ago more refined tools
Neanderthal Man appears
50,000–100,000 years ago Homo sapiens appears
28,000 years ago Neanderthals disappear
And why did the Neanderthals disappear? Many palaeontologists think there can be only one answer: Homo sapiens developed the ability to speak. Language gave modern man such an advantage in the competition for food and other resources that his rival was swiftly wiped out.
There are within cells organelles known as mitochondrial DNA. These organelles lie outside the nucleus and are in effect cell batteries – they produce a substance known as adenosine triphosphate or ATP. In January 1987 in Nature, Allan Wilson and Rebecca Cann, from Berkeley, revealed a groundbreaking analysis of mitochondrial DNA used in an archaeological context. The particular property of mitochondrial DNA that interested Wilson and Cann was that it is inherited only through the mother – it therefore does not change as nuclear DNA changes, through mating. Mitochondrial DNA can therefore only change, much more slowly, through mutation. Wilson and Cann had the clever idea of comparing the mitochondrial DNA among people from different populations, on the reasoning that the more different they were, the longer ago they must have diverged from whatever common ancestor we all share. Mutations are known to occur at a fairly constant pace, so this change should also give an idea of how long ago various groups of people diverged.32
To begin with, Wilson and Cann found that the world is broken down into two major groups – Africans on the one hand, and everyone else on the other. Second, Africans had slightly more mutations than anyone else, confirming the palaeontological results that humanity is older in Africa, very probably began there, and then spread from that continent to populate the rest of the world. Finally, by studying the rate of mutations and working backward, Wilson and Cann were able to show that humanity as we know it is no more than 200,000 years old, again broadly confirming the evidence of the fossils.33
One reason that the Wilson and Cann paper attracted the attention it did was because its results agreed well not only with what the palaeontologists were discovering in Africa, but also with recent work in linguistics and archaeology. As long ago as 1786, Sir William Jones, a British judge serving in India at the High Court in Calcutta, discovered that Sansk
rit bore an unmistakable resemblance to both Latin and Greek.34 This observation gave him the idea of the ‘mother tongue,’ the notion that there was once, many years ago, a single language from which all other languages are derived. Joseph Greenberg, beginning in 1956, began to re-examine Sir William Jones’s hypothesis as applied to the Americas. In 1987 he concluded a massive study of native American languages, from southern South America to the Eskimos in the north, published as Language in the Americas, which concluded that, at base, the American languages could be divided into three.35 The first and earliest was ‘Amerind’, which covers South America and the southern states of the US, and shows much more variation than the other, northern languages, suggesting that it is much older. The second group was Na-dene, and the third Aleut-Eskimo, covering Canada and Alaska. Na-dene is more varied than Aleut-Eskimo, all of which, says Greenberg, points to three migrations into America, by groups speaking three different languages. He believes, on the basis of ‘mutations’ in words, that Amerind speakers arrived on the continent before 11,000 years ago, Na-denes around 9,000 years ago, and that the Aleuts and Eskimos diverged about 4,000 years ago.36