FOR AT LEAST a million years and hundreds of thousands of generations, the ape-people survived essentially unchanged.
The average ape-woman weighed about 65 pounds, and the ape-man about 100, and they stood three to four feet tall. Each had a brain about the size of an orange--roughly as big as that of the species they most resembled, the chimpanzees. Like the chimps, they spent much of their time in the treetops of the thick, lush African forest. The fruits they mostly lived on grew there, and the forest canopy offered safety from ground-dwelling saber-toothed cats and other killers for whom the ape-people were easy prey. Three million years in the future, the inheritors of the ape-people's genes would deduce that the ape-humans' daily routine, like the chimpanzees', was to move through the forest, picking fruit as they went, scavenging an occasional piece of carrion, and killing and eating an occasional small animal. After combing perhaps two or three miles of landscape in a day they would retire to the treetops, there to sleep in relative safety from the predators below.
In most other ways, there was a wide difference between the ape-people and the chimps. The former walked upright, on two legs, the first species of their evolutionary family to do so routinely. But not all the time. The result, as often happens in evolution, was a compromise: The ape-human could neither climb as well as an ape nor walk as well as a modern human. The ape-people, it would seem, lived a two-part existence--in the trees and on the ground. In a departure from the ape lifestyle, their walking ability enabled them to venture farther into open spaces within the forest to look for food; they were not tied so closely to what was available in the trees.
Their world was hardly an Eden. The ape-human probably could not run fast, and so would have been unable to escape from a big cat or hyena without a substantial head start. Constant vigilance was necessary while on the ground, and numerous ancient skulls crushed by carnivores testify that the ape-human was more prey than predator.
Nevertheless, the ape-people have to be judged an evolutionary success: Their kind persisted perhaps ten times as long as modern humans have so far existed.
A little less than 3 million years ago, that long and successful run was coming to an end. Great forces were about to transform not only the world of the ape-people but also the ape-people themselves. No mysterious black monolith, planted on the savanna by some extraterrestrial force to strike the spark of humanity, was responsible. Rather, great ice sheets were beginning to grow far to the north. The Inheritors, with bigger brains amplified by computers, would later deduce that the ice sheets were accompanied by a drop in the temperature of the North Atlantic Ocean of as much as 25 degrees Fahrenheit, causing cooler, drier air to blow from Europe over Africa. They would also discover that as the northern ice sheets grew higher, they diverted colder, drier air toward the homeland of the ape-people.
The effect of these far-reaching climatic changes was to make Africa drier. This in turn shrank the forest refuges of the ape-people and expanded the open grasslands. All of this forced a survival crisis on the ape-people and the other animals of their world. They would have to adapt or die.
Out of the crisis would emerge the genus Homo, our genus.
Behind that development lay more than four and a half billion years of an unimaginably complex and violent coevolution of the earth's climate system and the life it shaped, and by which it was reshaped in return.
THE EARTH AND ITS ATMOSPHERE were born of fire and fury, setting in motion vast processes that are still playing out today and that will continue, scientists believe, for eons to come.
As far as modern scientists can tell, a slowly rotating cloud of dust and gases like hydrogen and helium, perhaps 15 billion miles across, a nebula floating in interstellar space, began to contract. Gravity pulled the smaller dust particles toward larger ones and the cloud began to spin faster and faster, flattening into a disk. Gradually, as the particles compressed into a single mass, the nascent sun materialized. Ever more intense compression boosted the temperature of the mass to nearly 2 million degrees Fahrenheit, setting off nuclear fusion. In fusion, hydrogen atoms combine to form helium, and energy is released as sunshine.
As the sun formed, not all the material in the rotating disk was drawn into it; some continued to orbit the new star. From this rocky material, the planets formed. No earthlings were around to see this, of course, but astronomers have observed many such disks around young stars, and they are confident that our solar system formed in the same way.
In the currently favored scenario of the earth's birth and the formation of its atmosphere, everything happened quickly, relative to geological time. Myriad chunks of rock orbiting the sun coalesced into the building blocks of planets. These planetesimals, as scientists call them, then began to merge with each other, and soon a planet took shape. As it grew bigger, its gravity force increased, attracting more and more planetesimals. The shock of the colliding planetesimals created such heat--several thousand degrees--that all of the naturally occurring water, nitrogen, and carbon in the planetesimals and on the new earth was instantly vaporized.
When the proto-planet reached only three-tenths of its eventual diameter of almost 8,000 miles, it began to hold the vaporized chemicals in its gravitational field, and an atmosphere thus started to accumulate. Scientists cannot be exactly sure about what the early atmosphere was like. "We don't have any direct data," says James F. Kasting of Pennsylvania State University, an expert on the early earth and ancient climates. "We're sort of relying on theoretical models we build up partly on computers and in our heads."
But according to the dominant view, a key constituent of the early atmosphere was the one that provokes such contention today: carbon. Reacting with sunlight, it took the form of carbon dioxide, a powerful heat-trapping greenhouse gas. Like other greenhouse gases, carbon dioxide lets visible radiation from the sun penetrate to the planet's surface, where it is absorbed and reemitted as infrared radiation. Then the greenhouse gas reabsorbs this heat-producing radiation and bounces part of it back to the surface, warming the planet. Water vapor is an even more potent greenhouse gas, and most of the early atmosphere consisted of water in the form of steam. This combination of factors, along with heat from the interior, might have made the earth some 2,000 degrees Fahrenheit warmer than it otherwise would have been.
The young earth is thus believed to have been a largely molten ball hidden behind a cloud of steam. For perhaps 100 million years, or maybe in as few as 10 million, it continued to absorb hits by planetesimals big and small. Some were probably the size of full-fledged planets. The single most violent and jarring event in the earth's history took place in this early period, it is widely believed by scientists, when a planet-size body struck the earth a glancing blow, chipping off massive chunks that reaccreted to form the moon.
The formation of the earth and its atmosphere were largely completed between about 4.6 billion and 4.5 billion years ago. If all the time between 4.6 billion years ago and today were compressed into a century, the main period of the earth's formation would have taken between one and a half and three years. For about the next fifteen years (some 700 million years in actual time), heavy bombardment by smaller planetesimals, asteroids, and comets continued. Eventually, as the raw materials of the accreting earth were used up, bombardment slowed and the new planet cooled enough to allow the steam in the atmosphere to condense and fall as rain, forming the first ocean. Asteroids and comets still crashed into the earth from time to time, and if they were big enough, as they often were, they would revaporize the ocean. Eventually it would condense again. This may have happened repeatedly.