HUMAN KIND, AGE OF. — The problem of the age of humankind remains one of the most difficult and controversial, its solution being tied to that of other geological, geophysical, and astronomical problems that are still the subject of lively debate. Among these may be cited: the boundary between the Pliocene and Pleistocene; the synchronization of Neogene deposits across various continents; and the interpretation of the famous "eoliths" found at various levels of the Cenozoic. These issues will be briefly reviewed.
The appearance and spread of humankind are certainly among the most important phenomena characterizing the Neogene or Quaternary era, to the extent that it is also called the Anthropogene. Yet it remains unclear whether the emergence of humankind coincided with the beginning of this era, preceded it, or followed II.
To provide a definitive clarification of this matter, it would first be necessary to establish the precise date of the beginning of the Pleistocene—that is, the first period of the Neogene era. However, scholars are far from unanimous on this point. Some hold that the Pleistocene began with the migration into Europe of certain important groups of Asian mammals, such as elephants and bovids. Yet it is objected that the European fauna of that time (the Villafranchian) still displays markedly Pliocene characteristics and should therefore still be considered Cenozoic. Others, more appropriately, equate the beginning of the Quaternary with the onset of the great glacial expansions, which draped vast regions of the Northern Hemisphere and all the principal mountain chains in thick ice sheets. This grand phenomenon is attested not only by morainic and fluvioglacial formations but also by the migration of Arctic faunas—both continental and marine—into European and Mediterranean regions. The latter view is favored here. Even so, it remains to determine precisely when, relative to the glacial period, the first appearance of humankind occurred—whether it preceded the glaciations, coincided with one of the glacial expansions, or occurred during one of the intervening warm periods known as interglacials, and if so, which one. Here the difficulties multiply, for the number of glaciations is still uncertain, with estimates among scholars ranging from two to as many as twelve.
At present, most specialists favor the view of three or at most four glaciations, designated respectively as Günz, Mindel, Riss, and Würm, after four Alpine rivers. Yet since not all accept the first glacial period (Günz), it happens that the same deposits are variously attributed by different scholars either to the preglacial (Villafranchian) or to the first interglacial (Günz-Mindel).
The problem is further complicated by the difficulty of synchronizing deposits from different regions, especially those from areas that did or did not experience glacial incursions—for example, European deposits with those of the Far East or tropical Africa. Even within Europe, it is evident that evidence of climatic change appears earlier in the deposits of northern regions than in coeval formations of the south, particularly in the Mediterranean.
Bearing in mind the foregoing, we may now consider the earliest known evidence of human existence and the periods to which it may be assigned. In Europe, the oldest known human remains are represented by the Mauer mandible (Palaeanthropus heidelbergensis), discovered in western Germany, which, according to the most reliable authorities, dates to the first interglacial. Roughly contemporary would be the Sinanthropus pekinensis of China and, according to some scholars, certain individuals from the famous Java sites (Pithecanthropus cretus, P. robustus). All these most ancient representatives of humankind exhibit markedly primitive traits. Until recently, most scholars considered the Piltdown find (Eoanthropus dawsoni) of England to be roughly contemporaneous with these, but recent fluorine tests have seriously cast doubt on its antiquity.
To the same interglacial period are attributed the first secure evidence of human industry, consisting of very crude but unmistakably intentional stone artifacts belonging to the Abbevillian (= Chellean) and Clactonian cultures, as well as hearth pits. Some scholars, however, claim to have found traces of human industry even in older, preglacial formations. Few still give credence to the much-debated "eoliths" recovered from Miocene and Eocene deposits, but the presence of genuine artifacts cannot be ruled out in certain preglacial "crags" of England and in some Villafranchian deposits of Europe or Africa. A very recent report (Arambourg, 1949) describes curious polyhedral flints from the Villafranchian of Algeria, which are difficult to dismiss as anything other than artifacts. Thus, the possibility cannot be excluded that humankind may have appeared as early as the close of the Pliocene, before the onset of the great glacial expansions.
In sum, it may reasonably be concluded that the emergence of humankind corresponds roughly to the transitional period between the Pliocene and the Pleistocene. This conclusion accords well with current (albeit still limited) knowledge of the primates from which the human body may have evolved. Indeed, it is precisely at the close of the Pliocene and the beginning of the Pleistocene that we find both the most human-like anthropoid apes (the australopithecines) and the earliest hominids mentioned above (Mauer, Sinanthropus, the Pithecanthropi).
Given this, can we estimate how many years ago humankind first appeared? Here too, no secure foundation exists, for absolute chronology is still in its infancy. Several methods are currently used to date the principal events of the Neogene. The first and most reliable is based on the study of so-called "varves"—that is, particular stratified clays deposited in large lakes near the sources of Neogene glaciers during the last retreat period. Since each pair of layers in these clays corresponds to one year, it is possible to calculate the duration of the retreat of the last glaciation, which De Geer and his pupils estimated at about 8,000 years for Scandinavia. This method, however, applies only to postglacial times, corresponding, as is well known, to the most recent periods of prehistory (from the Mesolithic onward).
For earlier periods, recourse is had to a second method, which is less secure, being based on theoretical principles whose application to Quaternary climatic phenomena is not yet fully demonstrated, though it is now accepted, albeit with some reservations, by many Quaternary specialists. This is the method based on the so-called "Milankovitch curve," that is, the curve of variations in solar radiation calculated by the author cited on the basis of periodic changes in certain elements of the Earth's orbit—specifically, the eccentricity of the orbit, the obliquity of the ecliptic, and the heliocentric longitude of perihelion. The variation in the amount of solar radiation, dependent on these elements, and from which, according to Milankovitch, the climatic alternations of the glacial period derive, has been calculated precisely for the last 600,000 years and with some approximation for a further 400,000 years before that.
From these calculations it would appear that the first glaciation (that of Günz, since Milankovitch admits four glaciations) would have begun approximately 600,000 years ago, and that if Man appeared—as the most reliable authors believe—at least during the first interglacial period (Günz-Mindel), his existence would date back to approximately 300,000 years ago.
It is well, however, to bear in mind the hypothetical character that still attaches to these estimates, which in any case can only give the order of magnitude of the periods under consideration.
At present, the possibility is being studied of applying to the chronology of the Neozoic area methods based on radioactivity, that is, on the computation of time corresponding to the disintegration of certain isotopes of radioactive minerals, among which thorium 230 for research on deep-sea sediments and carbon 14 for substances containing this isotope, which sometimes accompany prehistoric human remains, may be cited. The latter research relating to carbon, however, is valid only for limited durations, and, in general, methods based on radioactivity are still in an initial phase and for the present afford only relative certainty.
Nevertheless, it can now be considered established, on the basis of a whole complex of factual data and well-founded considerations, that human prehistory corresponds to a very long period of time, and that the appearance of man dates back to a much more remote epoch than was hitherto believed.