Evolution

I

Introduction

Evolution, in biology, descent with modification, the process whereby all living things on Earth have diverged, by direct descent, from a single origin of life that occurred more than 3,000 million years ago.

II

History of Evolutionary Ideas

To most people through history it has always seemed obvious that the teeming diversity of life, the uncanny perfection with which living organisms are equipped to survive and multiply, and the bewildering complexity of living machinery, can only have come about through divine creation. Yet repeatedly it has occurred to isolated thinkers that there might be an alternative to supernatural creation. The notion of species changing into other species was contemplated, like so many other good ideas, in ancient Greece. It went into eclipse until the 18th century, when it resurfaced in the minds of such advanced thinkers as Pierre de Maupertuis, Erasmus Darwin, and the man who styled himself the Chevalier de Lamarck. In the first half of the 19th century the idea became not uncommon in intellectual circles, especially geological ones, but always in a rather vague form and without any clear picture of the mechanism by which change might come about. It was Charles Darwin (Erasmus's grandson), spurred into print by the independent discovery by Alfred Russel Wallace of his principle of natural selection, who finally established the theory of evolution. In 1859 he published his On the Origin of Species by Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life, usually abbreviated to The Origin of Species. After 1859 it was difficult for reasonable people to doubt that all living species, ourselves included, had evolved from other species. Modern molecular biology makes it hard to doubt that all species can be traced back ultimately to a single common ancestor, for all known life forms share the same genetic code and it is wildly improbable that they could have tumbled upon it independently.

III

Evolutionary History

Between 5,000 million and 4,000 million years ago the Earth was formed. By 3,000 million years ago life had arisen and we have fossils of microscopic bacteria-like creatures to prove it. Some time between these two dates—independent molecular evidence suggests about 4,000 million years ago—that mysterious event, the origin of life, must have occurred. Nobody knows what happened, but theorists agree that the key was the spontaneous arising of self-replicating entities, i.e. something equivalent to “genes” in the general sense. There is less agreement over how this happened.

The atmosphere of the early Earth probably contained methane, ammonia, carbon dioxide, and other gases still abundant today on other planets in the solar system. Chemists have experimentally reconstructed these primeval conditions in the laboratory. If plausible gases are mixed in a flask with water, and energy is added by an electric discharge (simulated primordial lightning), organic substances are spontaneously synthesized. These include, most significantly, amino acids (the building blocks of proteins, including the all-important enzymes that control the chemical processes of life), and purines and pyrimidines (the building blocks of RNA and DNA). It seems probable that something like this happened on the early Earth. Consequently, the sea would have become a “soup” of prebiological organic compounds.

It is not enough, of course, that organic molecules appeared in the primeval soup. The crucial step, as noted above, was the origin of self-replicating molecules, molecules capable of assembling copies of themselves. Today the most famous self-replicating molecule is DNA (deoxyribonucleic acid, see Genetics and Nucleic Acids), but it is widely thought that DNA itself could not have been present at the origin of life because its replication is too dependent on support from specialized machinery, which could not have been available before evolution itself began. DNA has been described as a “high-tech” molecule which probably arose some time after the origin of life itself. Perhaps the related molecule RNA, which still plays various vital roles in living cells, was the original self-replicating molecule. Or perhaps the primordial replicator was a different kind of molecule altogether. Once self-replicating molecules had been formed by chance, something like Darwinian natural selection could have begun: variation would have come into the population because of random errors in copying. Variants that were particularly good at replication would automatically have come to predominate in the primeval soup. Varieties that did not replicate, or that did so inaccurately, would have become relatively less numerous. A kind of molecular natural selection led to ever-increasing efficiency among replicating molecules.

As the competition between replicating molecules warmed up, success must have gone to the ones that happened to hit upon special tricks or devices for their own self-preservation and their own rapid replication. Such devices probably were constructed by the manipulation of other molecules, proteins perhaps. Other manipulated devices were the forerunners of membranes which provided circumscribed volumes for the enclosure of chemical reactions. It may have been soon after this stage that simple bacteria-like creatures gave rise to the first fossils, between 3,000 million and 4,000 million years ago. The rest of evolution may be regarded as a continuation of the natural selection of replicator molecules, now called genes, by virtue of their capacity to build for themselves efficient devices (cells and multicellular bodies) for their own preservation and reproduction. Three thousand million years is a long time, and it seems to have been long enough to have produced such astonishingly complex contrivances as the vertebrate body and the insect body. Genes are often referred to as the means by which bodies reproduce themselves. This is superficially undeniable, but it is a more profound truth that bodies are the means by which genes reproduce themselves.

Fossils were not laid down on more than a small scale until the Cambrian era, nearly 600 million years ago. By then most of the major animal phyla (the large groups into which the animal kingdom is classified) had appeared. Obviously creatures with hard skeletal parts, including teeth, are more likely to fossilize, and they dominate the fossil record. The first vertebrates may date back 530 million years, according to fossil evidence of agnathans—primitive, jawless fishes with fins, gills, and fish-like muscle patterns—found in China in 1999. Vertebrates appear abundantly in fossil beds between 300 and 400 million years ago: fish-like creatures, completely covered with heavy armour-plating, perhaps adapted to escape from Eurypterids, giant undersea scorpion-like predators which infested the seas at that time. Among vertebrates, the land was first colonized by lobe-finned and lung-bearing fish about 250 million years ago, then by amphibians and, in more thoroughgoing fashion, by various kinds of animals that we loosely lump together as “reptiles”. Mammals and, later, birds, arose from two different branches of reptiles. The rapid divergence of mammals into the rich variety of types that we see today, from opossums to elephants, from anteaters to monkeys, seems to have been unleashed into the vacuum left by the catastrophic extinction of the dinosaurs, 65 million years ago.

Although we naturally emphasize the evolution of our own kind—the vertebrates, the mammals, and the primates—these constitute only a small branch of the great tree of life. Some dozens of animal phyla are recognized, of one of which the vertebrates are only a subphylum. In addition to the animal kingdom, other evolved groupings that are conventionally granted kingdom status are the plants, the fungi, and the single-celled protoctista, all within the single, major grouping, the Eukaryotes. Creatures that are not Eukaryotes are called Prokaryotes and they include various kinds of bacteria (the status of viruses as living things is a matter for argument: many of them are probably relatively recently “escaped” fragments of parasitic genetic material). It is now widely accepted that the eukaryotic cell originated as a symbiotic union of several prokaryotic cells. Organelles such as mitochondria and chloroplasts, within eukaryotic cells, contain their own DNA and are almost certainly the lineal descendants of ancestral prokaryotes.

IV

Human Evolution

Our own species evolved within the group of African apes by a rapid evolutionary spurt during the last few million years. Molecular evidence suggests that our last common ancestor with chimpanzees and gorillas lived not much more than five million years ago. The fossil record of our immediate ancestors is now richer than it is often said to be in older textbooks. It shows various archaic forms of Homo sapiens with heavy brow ridges (including the famous Neanderthalers of Europe), preceded by Homo erectus which extends back to nearly two million years ago. Homo erectus lived in Asia as well as Africa but it is controversial whether the Asian members of this species leave any surviving descendants. Several anthropologists favour the view that there was a second “out of Africa” migration of Homo sapiens within the past couple of hundred thousand years, and they trace all modern humans back to a Homo sapiens ancestor who lived in Africa less than a quarter of a million years ago (the quaintly named “African Eve”). Homo erectus had a smaller brain than Homo sapiens and our still earlier ancestors had even smaller brains. With the possible interpolation of species of Homo such as Homo habilis, our immediately previous ancestors seem to have been members of the genus Australopithecus. These have been described as bipedal apes and certainly their brains were not noticeably larger than those of modern chimpanzees. Before them, our ancestry merges with that of the other African apes, the chimpanzees and gorillas, and for some tens of millions of years back becomes dominated by adaptations for living in trees, for example forward-pointing eyes and grasping hands and feet. Before that, our ancestors seem to have been small, shrew-like, insectivorous creatures who lived nocturnally in a world dominated by dinosaurs. These small mammals were descended from the large group of “mammal-like reptiles” which enjoyed their great flowering before the rise of the dinosaurs.