• Choose a category
• All
• Classic-Fiction

By Root 807 0

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 color, 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 while 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. Orangutans 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 color, 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 centimeters in volume. Early Australopithecus had expanded its brain to about 500 cm3, while Homo erectus measured up with a brain size of