to reduce their chances of being eaten.
Of all the technical novelties evolution called into existence, from scales to jaws, perhaps the most interesting is the development of sight. The eye may have been the innovation sparking this intense burst of Cambrian competition, for both predators and prey would have had an equally powerful reason to evolve vision. The fossil record demonstrates that sight evolved independently in different groups of animals, though in a remarkably similar way. The octopus, for example, has an eye much like ours, with a lens and a retina behind it, yet our common ancestor was probably some kind of sightless worm. All the higher animals that survived the Cambrian could see.
The oceans now had a fully developed food web, and it may have been to escape the marine killing fields that some of the less well-armoured fish first ventured onto land – already colonised, from about 450 million years ago, by plants and insects. Fins gradually morphed into limbs, though the hybrid water–land transition is still repeated in the life cycles of today’s amphibians, hundreds of millions of years later. As some of these early amphibians grew more accustomed to onshore life, they evolved into reptiles, with leathery skins to hold in moisture and eggs with watertight shells that could be laid on dry land rather than in ponds.
We are now up to 300 million years ago in geological time – nearly to the appearance of mammals, for our mammalian line is surprisingly ancient, if rather insignificant for most of its existence. The sail-back reptile Dimetrodon displayed many mammal-like features: its sail was probably a way to regulate temperature, perhaps demonstrating an early attempt at warm-bloodedness. Its teeth had differentiated into molars and canines, just as ours still do. Its descendants developed fur, modified – like the feathers of birds – out of reptilian scales, also as a way to control its body temperature. By the late Triassic, true mammals appeared, and were present on Earth throughout the entire age of the dinosaurs, though as very junior partners indeed. For the next 135 million years – during the entire Jurassic and Cretaceous periods – our ancestors stayed in the shadows, living furtive existences as the dinosaurs dominated the planet.
Mammals then were tiny, most no bigger than rats. They could dart out under the cover of darkness, snatching insects and worms as Tyrannosaurus slept. But there was an evolutionary tradeoff. Without the luxury of laying masses of eggs, and confined to burrows and crevices, mammals evolved sophisticated ways of nurturing their young: live births and lactation. Their specialised teeth enabled them to chew and grind up food, yielding more energy. In contrast the bulky dinosaurs wolfed their meals down whole. But the most outstanding adaptation of the mammals to their subordinate status was far more important than milk or molars. It was the evolution of intelligence. Contrary to popular myth, dinosaurs had big brains – not because they were smart, rather because they were big animals. But it is not brain size per se that counts for intelligence; more important are the relative proportions of brain and body, and in the diminutive mammals, this relationship was beginning to change. As one evolution textbook puts it: ‘The pint-sized mammal was the intellectual giant of its time.’6
So why did selective pressures force this shift? Most likely, the shadowy existence of mammals demanded very different skills from those of the daytime excursions of dinosaurs. The mammalian world was one of sound and smell as much as sight, demanding more subtle skills of deduction and reasoning. The smell of a predator, for instance, could mean danger if the killer is soon to return – or safety if it is gone. All would need to be kept in memory for retrieval later. Similarly, to interpret sound on a dark night would require consulting a mental map of some complexity, adding further evolutionary pressure for larger brains. The result was the neocortex, a completely new brain structure found only in mammals. This is our ‘grey matter’ – vital for all higher functions that we collectively define as ‘intelligence’, such as sensory perception, spatial reasoning and conscious thought.
The age of mammals dawned, with spectacular suddenness, 65 million years ago. Perhaps aggravated by extensive volcanic eruptions and consequent global warming, a mass extinction tore through the planetary biosphere when a large asteroid ploughed into the sea off modern-day Mexico. Once the dust had settled, the dinosaurs were gone – along with half of life on Earth. Why mammals made it through the bottleneck, no one knows. Perhaps they were better protected from the environmental holocaust thanks to their furry, furtive existences. Either way, the end-Cretaceous extinction cleared the way for the explosive evolution of mammals into all the ecological niches previously occupied by dinosaurs. Some took to the water, losing their four legs and re-evolving the fins they had lost over 300 million years earlier to become dolphins and whales. Others joined birds in the air, the fingers of their ‘hands’ splaying out to form wings, becoming bats. Still more returned to herbivory, and headed out into the grasslands now spreading through the continents, their bodies growing rapidly in size: these became bison, elephants, horses and other grazing and herding animals.
But our story follows a different group of mammals who struck out in a new direction. They headed off not into the land or out to sea but up the trees. Perhaps to escape predators on the forest floor, or to take advantage of succulent arboreal fruits, the lives of these ‘prosimians’, who appear in the fossil record about 55 million years ago, demanded a whole new set of skills. The paws of their ratlike ancestors evolved into gripping hands, more suited to a life spent grasping branches. Their requirement for smell declined. But their need for vision increased enormously, and not just any vision: their eyesight had to reveal excellent colour, and, most important, had to be front-of-head and stereoscopic to give depth perception.
The pressure was on for bigger brains. Mental calculations performed whilst speeding through the treetops had to be fast and accurate. Memory was once again useful, aiding decisions as to what types of trees could support what weight, how to grasp certain branches, or when to visit different fruiting bits of the forest. These were still small animals, but as they evolved better agility in the forest, their bodies grew larger. By 35 million years ago, true monkeys had appeared. By 22 million years ago, gibbons had split away from the evolutionary line. Orang-utans followed, at about 16 million years ago, and chimpanzees 6 million years ago. That left the hominids, and we are their only surviving descendants – all other hominid species, of which there have been a dozen at least, were destined to perish.
BIRTH OF THE FIRE-APE
Our lineage may be ancient, but modern Homo sapiens has been a very short-lived phenomenon, perhaps illustrating the biological anomaly that we are. Although bipedal hominids were stalking the African plains as long as 3 million years ago, true Homo sapiens – the evolutionary descendant of Australopithecus, Homo habilis and later Homo erectus – appeared less than 500,000 years ago, and perhaps as recently as 200,000 years ago.
Mitochondrial DNA passed through the maternal line suggests in fact that we are all descended from a single individual – the so-called Mitochondrial Eve – who lived in Africa 200,000 years ago. Further evidence comes from the remarkable homogeneity of human DNA: despite superficial differences in hair straightness, noses and skin colour, we are far more closely related than might be expected. (A single breeding group of chimpanzees will show more genetic variation than do all humans.7) This is strong evidence that modern humans did all descend from the same original group, and our dominance may have begun with a characteristic act of genocide, as the last Homo neanderthalensis survivors were ethnically cleansed from Europe and Asia by the new migrants. Since then, no other animal, whether on two legs or four, has challenged the dominance of Homo sapiens.
The most striking biological characteristic of the human ancestral line over the last few million years is the extraordinary progress of its brain development. Chimpanzee brains measure about 360 cubic centimetres in volume. Early Australopithecus had expanded its brain to about 500 cm3, whilst Homo erectus measured up with a brain size of about 800 to 900 cm3. Half a million years ago, the brain was expanding at an extraordinary rate of 150 cm3 every hundred thousand years.8 Modern humans typically have a brain size of 1,350 cm3, nearly four times the size of those of our nearest relatives, the chimpanzees.
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