Modern Mind

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, wasfollowed 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