The God Species_ How the Planet Can Survive the Age of Humans - Mark Lynas [12]
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-armored fish first ventured onto land—already colonized, 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 trade-off. 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 specialized 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 “gray 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 plowed into the sea off modern-day Mexico.