the sieve of selection. Organisms are selected to engage in sex—the ones that find it uninteresting quickly become extinct. And this is true not only of the microbes of two billion years ago. We humans also have a palpable devotion to exchanging segments of DNA today.
By one billion years ago, plants, working cooperatively, had made a stunning change in the environment of the Earth. Green plants generate molecular oxygen. Since the oceans were by now filled with simple green plants, oxygen was becoming a major constituent of the Earth’s atmosphere, altering it irreversibly from its original hydrogen-rich character and ending the epoch of Earth history when the stuff of life was made by nonbiological processes. But oxygen tends to make organic molecules fall to pieces. Despite our fondness for it, it is fundamentally a poison for unprotected organic matter. The transition to an oxidizing atmosphere posed a supreme crisis in the history of life, and a great many organisms, unable to cope with oxygen, perished. A few primitive forms, such as the botulism and tetanus bacilli, manage to survive even today only in oxygen-free environments. The nitrogen in the Earth’s atmosphere is much more chemically inert and therefore much more benign than oxygen. But it, too, is biologically sustained. Thus, 99 percent of the Earth’s atmosphere is of biological origin. The sky is made by life.
For most of the four billion years since the origin of life, the dominant organisms were microscopic blue-green algae, which covered and filled the oceans. Then some 600 million years ago, the monopolizing grip of the algae was broken and an enormous proliferation of new lifeforms emerged, an event called the Cambrian explosion. Life had arisen almost immediately after the origin of the Earth, which suggests that life may be an inevitable chemical process on an Earth-like planet. But life did not evolve much beyond blue-green algae for three billion years, which suggests that large lifeforms with specialized organs are hard to evolve, harder even than the origin of life. Perhaps there are many other planets that today have abundant microbes but no big beasts and vegetables.
Soon after the Cambrian explosion, the oceans teemed with many different forms of life. By 500 million years ago there were vast herds of trilobites, beautifully constructed animals, a little like large insects; some hunted in packs on the ocean floor. They stored crystals in their eyes to detect polarized light. But there are no trilobites alive today; there have been none for 200 million years. The Earth used to be inhabited by plants and animals of which there is today no living trace. And of course every species now on the planet once did not exist. There is no hint in the old rocks of animals like us. Species appear, abide more or less briefly and then flicker out.
Before the Cambrian explosion species seem to have succeeded one another rather slowly. In part this may be because the richness of our information declines rapidly the farther into the past we peer; in the early history of our planet, few organisms had hard parts and soft beings leave few fossil remains. But in part the sluggish rate of appearance of dramatically new forms before the Cambrian explosion is real; the painstaking evolution of cell structure and biochemistry is not immediately reflected in the external forms revealed by the fossil record. After the Cambrian explosion, exquisite new adaptations followed one another with comparatively breathtaking speed. In rapid succession, the first fish and the first vertebrates appeared; plants, previously restricted to the oceans, began the colonization of the land; the first insect evolved, and its descendants became the pioneers in the colonization of the land by animals; winged insects arose together with the amphibians, creatures something like the lungfish, able to survive both on land and in the water; the first trees and the first reptiles appeared;
M. S. Parker, Cassie Wild
Robert Silverberg, Damien Broderick