Although the antiquity of Homo sapiens will be the subject of detailed study in later chapters, we may here profit from a consideration of the life spans of our fellow mammals during the Pleistocene and Recent (or post-Pleistocene) periods, the only periods in which man or any of his close kin are known to have lived. By international agreement the beginning of the Pleistocene has been established at the point when modern genera of elephants, horses, oxen, deer, and some other large mammals were first seen on the continents of the Old World, excluding Australia. The movement that brought them in, mostly from the New World, took place about one million years ago.
Before that stretched the vast temporal expanse of the Tertiary —Paleocene, Eocene, Oligocene, Miocene, and Pliocene—comprising some 77 million years, the Pliocene alone taking up some 12 million. During this long span individual species were born, flowered, and died at what seems to us a leisurely pace. The life expectancy of a mammalian species was then anywhere from one to eight million years.
During the first 300,000 or 400,000 years of the Pleistocene this pace continued, but it was suddenly quickened in various parts of the Old World, particularly its northern portions, by geological events. The planet's crust wrinkled more rapidly than before, raising the toothed edges of mountain ranges and creating great contrasts of climate, both regional and seasonal. First mountain glaciers, then continental icecaps crawled forth and melted away, blowing, like pairs of bellows, alternately cold and warm. In large tropical land masses, as in much of Africa, the bellows blew wet and dry.
s This section is based on many sources, including books by G. G. Simpson. However, specific facts and figures come principally from two works of Bjorn Kurten: "Rates of Evolution in Fossil Mammals," CSHS, Vol. 24 (1959), pp-205-15, and "Chronology and Faunal Evolution of the Earlier European Glacia-tions," SSF-CB, Vol. 31, No. 5 (i960) pp. 1-62.
In response to these changes, new species evolved rapidly. Many became extinct, but others survived. The life expectancy of a species now dropped to a mere 360,000 years. At a point in time pegged at 300,000 years ago, all or nearly all the living mammals of the European and neighboring fauna, which were fox-sized or larger, had come into existence. The species which have since appeared are bats, insectivores, and rodents, all small animals. During the last 75,000 years, no new mammalian species seem to have evolved at all. Three hundred thousand years ago the evolution of new species of medium-sized and large mammals came to a halt. The heyday of speciation was over.
The oldest known Homo erectus is believed to be 700,000 years old. He appeared during the period of frenzied mammalian speciation mentioned above, and seems to have lasted until less than 100,000 years ago in remote parts of the Old World. His known life span as a species, about 600,000 years, was within the normal range for a mammal of his size and vintage. As I shall show in Chapter 11, Homo sapiens appeared about 250,000 years ago in an archaic form. Completely modern forms of our species appeared at least 35,000 years ago. Unless our species is a curious exception to the rules by which the game of speciation is played, Homo sapiens should go back to 360,000 or 300,000 years ago. This figure would place Homo sapiens in the fauna to which he belonged, and would give Homo erectus, who appeared exactly when he should have, ample time for speciation by succession.
So much for the actuarial statistics of Pleistocene and Recent species. With subspecies the reckoning is more difficult because subspecies are not easy to sort out when found among fossils. We nave no satisfactory information except that subspecies of the ibex nave been traced back at least 230,000 years.9 In the case of man, the subspecies of Homo sapiens are probably of different ages, depending on the times at which regional populations of Homo erectus, in one way or another, crossed the sapiens threshold. But all of them did this before the end of the Pleistocene.
in modern times we have seen whole tribes and peoples disappear after their lands had been invaded by Europeans and other culturally dominant strangers. The native Tasmanians are gone, and so are the Indians of Lower California. The Andamanese of the main islands, the Fuegians, and many others are on their way out. These sad cases of ethnic oblivion give us a feeling that human history is a long record of utter extinctions, but this is not true.
All species are destined to become extinct, but, except as they are parts of species, subspecies need not follow this rule. By definition, species do not ordinarily interbreed, but subspecies do. The Tasmanians were absorbed by the Caucasoids who replaced them on their island. A mixed Tasmanian-European population survives today. If the Indians of Lower California left no mixed descendants—which is unlikely—other Indians very much like them are still alive. When subspecies disappear, they usually, if not always, do so by absorption. Their genes linger on polymor-phously with those of their conquerors, to re-emerge, now and then, when needed. The principle is that when a population has been invaded by members of another race the genes that give it its special adaptation to its local environment retain their selective advantage and eventually come to characterize the mixed population through the process of natural selection. For example, central Europe was invaded from the East many times from the Neolithic through the Iron Age, but central Europeans still look more like the hunters of the Mesolithic than like the invaders.1 Without the concepts of absorption and re-emergence it would be difficult for us to explain the physical diversity and geographical distribution of the living human races.
Part of this diversity may be relatively new. I refer here especially to the reduction in body size that has affected many species of mammals since the end of the Pleistocene, some 10,000 years ago. As will be explained in Chapter 3, extreme cases of size reduction in plants and animals take the form of dwarfing, which means that an irreversible genetic change has taken place. Our species includes a dozen or more populations of dwarfs, living in Africa, southern Asia, and Indonesia. As far as we know, all human dwarf populations are geologically recent.