9. Geological and Glacial Time
A remark should be made here as to the age of these ancestral forms. The record of life on earth, as known from the fossils in stratified rocks, is divided into four great periods. The earliest, the Primary or Palæozoic, comprises about two-thirds of the total lapse of geologic time. During the Palæozoic all the principal divisions of invertebrate animals came into existence, but of the vertebrates only the fishes. In the Secondary or Mesozoic period, evolution progressed to the point where reptiles were the highest and dominant type, and the first feeble bird and mammal forms appeared. The Mesozoic embraces most of the remaining third or so of the duration of life on the earth, leaving only something like five million years for the last two periods combined, as against thirty, fifty, ninety, or four hundred million years that the Palæozoic and Mesozoic are variously estimated to have lasted.
Fig. 3. The descent of man in detail, according to Gregory (somewhat simplified). Extinct forms: 1, Parapithecus; 2, Propliopithecus; 3, Palæosimia; 4, Sivapithecus; 5, Dryopithecus; 6, Palæopithecus; 7, Pliopithecus; P, Pithecanthropus erectus; H, Homo Heidelbergensis; N, Homo Neandertalensis.
Fig. 4. The descent of man in detail, according to Keith (somewhat simplified). Extinct forms: 2, 5, 6, 7 as in [Figure 3]; Pith(ecanthropus), Pilt(down), Neand(ertal). Living forms: Gb, Or, Ch, Go, the anthropoid apes as in [Figure 3].
These last five million years or so of the earth’s history are divided unequally between the Tertiary or Age of Mammals, and the Quaternary or Age of Man. About four million years are usually assigned to the Tertiary with its subdivisions, the Eocene, Oligocene, Miocene, and Pliocene. The Quaternary was formerly reckoned by geologists to have lasted only about a hundred thousand years. Later this estimate was raised to four or five hundred thousand, and at present the prevailing opinion tends to put it at about a million years. There are to be recognized, then, a four million year Age of Mammals before man, or even any definitely pre-human form, had appeared; and a final period of about a million years during which man gradually assumed his present bodily and mental type. In this Quaternary period fall all the forms which are treated in the following pages.