The science of life : fully illustrated in tone and line and including many diagrams

THE EVOLUTION OF MAN

present time. And even for the remoter periods we are already approaching a sufficiency of knowledge whereby the margin of error can be narrowed down to ten million years or so, which on the geologist’s scale of time is not a large figure, for it only means being out by about five or ten per cent.

Such are the data upon which we base the figures we have inserted upon our diagram of geological formations. If the reader will turn back to that he will find one or two points of very great interest in relation to our curiosity about the future course and duration of Evolution (Fig. 122).

In the first place he will remark that there is a huge disproportion between the lengths of the great eras of geological time. The Age of Mammals seems hardly to merit the name of Era at all: it has only endured for about 50 million years. The Age of Reptiles lasted about 125 million years, while the Paleozoic droned on, without even a bird or mammal or flowering plant, for over 300 million. That puts the date of the Early Cambrian back to 500 million. Below these lowest Cambrian rocks there is still a huge thickness of earlier rock-deposits —at least 180,000 feet of them—in which, however, save in the uppermost 10,000 feet, no fossils have been found. The time taken for the formation of these Pre-Cambrian rocks was longer than for all the later eras together ; for one of them is radium-dated back to over 1,200 million years.

The earth came into being when the ancestral sun was disrupted by the too-near approach of some other star. Of that stupendous birth you can read in Jeans’ Astronomy and Cosmogony. The date of that remote event cannot yet be estimated with such accuracy as can the ages of rocks, though the limits are rapidly being narrowed down. As Holmes shows, it cannot be less than 1,600 nor more than 3,000 million years ago; probably the lower estimate is nearer the truth.

Let us provisionally take 2,000 million years as the age of Earth. Life, as revealed by actual fossils, has been in existence for nearly a third of this time. Without any doubt the real age of life is greater, since the first living things would be soft and squashy and very seldom fossilized, and even if they were preserved would, in most cases, have been later baked and crushed out of recognition.

It is only in the last fifth of the earth’s history that we know of vertebrates, and in less than a sixth of it that we know of land-

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vertebrates. Mammals have been on the scene for only a thirteenth of the time, and modern placental mammals for only a thirtieth. And as for man, he is a mere upstart. The earliest creatures that could be called men, or at least not apes, cannot possibly have been on earth’s stage for more than a paltry ten million years, or one twohundredth of the total, and it may well prove necessary to halve even this estimate while our own species of man boasts perhaps a million years of history—one-twentieth of one per cent. of the earth’s full record.

Time is telescoped, and the centuries of history shrivel. Fifteen thousand years ago man was still in the Old Stone Age. Civilization, in the sense of a stable social life based upon agriculture and metal-working, dates back to less than ten thousand years. Ten thousand years—when it took at the very least ten million to generate man from the first tailless ape, and a hundred million for that ape to be brought into being from the first mammal !

With this revelation of the huge spaces of earth’s past the doors of the future, too, seem to open. We know a good deal now about the rate of cooling of the earth and of the sun. That sort of knowledge also grows more and more exact. The discoveries that heat may be generated by radio-activity, by shrinkage, and by the actual transformation of matter, have enormously enlarged the possible future of life upon earth. There is every reason to suppose that conditions on our planet will continue to allow life to flourish in the future for as long as they have allowed it to flourish in the past ; indeed, this trifle of a thousand million years or so for the future of terrestrial life is almost certainly an under-estimate, granted of course that no unforeseen catastrophe breaks in upon it.

Man is part of an unbroken stream of life. That same stream in the dawn of life on earth manifested itself in the form of single microscopic cells; hundreds of millions of years later, after transformation through forms we dimly guess at—forms of polyps, of worm-like creatures, of headless things like lancelets, it flowed through thousands of generations in the form of fish ; it emerged on land, it learnt to be a reptile, it covered itself with hair and warmed its blood, and fed its young with milk. Still without break of continuity, it transformed itself to become fully mammalian, its young to Be eed parasites upon its life. Four-footed, race and hairy, it took to the Eocene forests ; it grew into lemur, into monkey, into ape ;

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