6. THE PAST LIFE OF THE EARTH

by Dr Errol I. White

(From a television talk in a series arranged by Granada T.V).

What is the evidence of life of past ages? And how did it come to be preserved? Now you know that much of the earth's land surface is formed by what are called sedimentary rocks - that is to say, rocks formed by the deposition of sands, silts and clays. They are all derived from older rocks gradually worn away by the wind and the rain, and carried down the rivers to be piled up on the beds of ancient seas and lakes, later to be raised up by earth movements to form dry land in their turn. This process is continuous, and rocks have been built up and destroyed in this way since the beginning of time. In them are preserved the remains of the animals and plants that lived at the time of their formation. Usually, it is only the hard parts which are preserved, such as the shells of molluscs, or the bones of the back-boned animals, such as fish. So it comes about that you find the remains of fish miles from the sea, the fossilised bones of animals which seem quite out of place - like the lower jaw of a hyena found in a cave near Torquay - and, of course, fossilised pieces of wood and plant remains. Very rarely indeed are the traces of the soft "innards" preserved, and then usually only as impressions. For instance, a fossilised stomach of a shark complete with its spiral intestine has been found. It looks much the same as in living sharks, although this one lived 350 million years ago. Sometimes one finds even the imprints of still older jelly-fish - and some of them are among the oldest fossils known.

Even with hard-shelled creatures like molluscs, or thick scaled fish, the substance is sometimes eaten away by humic acids percolating through the rocks, and all that remains is a mould of the outside surface in blocks of stone which have cracked open, showing the impression of both sides of the fish. They can be used to make almost lifelike casts of the fishes.

Of other creatures, all that we have found are the footprints that they have left in the wet mud, on some long vanished seashore or lake margin. Occasionally, more than one type of preservation is shown at the same time, as in the examples of the first bird - the reptile-like Archaeopteryx. Not only have the actual bones of the skeleton been preserved but also the impression of the wing feathers, which have long since rotted away; one even shows a cast of the brain. The Archaeopteryx was, as I said, the first bird. He was about the size of a pigeon and his powers of flight were not well developed; he could only glide, and he showed a number of extremely reptilian, un-birdlike features. Instead of having a beak, he had a mouth with teeth; and at the end of the wings there were fingers; while the tail, instead of being as a modern tail, fixed to its pygostyle, or what is more popularly known as the "parson's nose", was long like that of a reptile and to it feathers were attached on either side.

Yet another kind of fossil - and I must use the word "fossil" here - is one in which the form of the organism is lost and all that remains is a compressed, featureless substance which is the most important economic fossil of all - coal. Very rarely indeed, at least to the naked eye, does it show marks of its origin. But a fossil it is. For it is the result of the decay through countless ages, in ancient swamps, of forest trees of kinds long extinct. Some of the outstanding coal-forming plants were giant horsetails and club mosses - things that nowadays are either on a diminutive scale or entirely extinct. The coal seams in Britain come from the Carboniferous period, and are about 300 million years old, although not all coal deposits are as old as this.

It is animals and plants which lived in or near water whose remains are most likely to be preserved, for one of the necessary conditions of preservation is quick burial, and it is only in the seas and rivers, and sometimes lakes, where mud and silt has been continuously deposited, that bodies and the like can be rapidly covered over and preserved.

(from Discovery: published for Granada T.V. by Methuen, London, 1961)