8. Family Tree of the Primates
While this second diagram illustrates the most essential elements in modern belief as to man’s origin, it does not of course pretend to give the details. To make the diagram at all precise, the left fork of the Y, which here stands for the monkeys as a group—in other words, represents all the living Primates other than man—would have to be denoted by a number of branching and subdividing lines. Each of the main branches would represent one of the four or five subdivisions or “families” of the Primates, such as the Anthropoid or manlike apes, and the Cebidæ or South American monkeys. The finer branches would stand for the several genera and species in each of these families. For instance, the Anthropoid line would split into four, standing respectively for the Gibbon, Orang-utan, Chimpanzee, and Gorilla.
The fork of the Y representing man would not branch and rebranch so intricately as the fork representing the monkeys. Many zoölogists regard all the living varieties of man as constituting a single species, while even those who are inclined to recognize several species limit the number of these species to three or four. Then too the known extinct varieties of man are comparatively few. There is some doubt whether these human fossil types are to be reckoned as direct ancestors of modern man, and therefore as mere points in the main human line of our diagram; or whether they are to be considered as having been ancient collateral relatives who split off from the main line of human development. In the latter event, their designation in the diagram would have to be by shorter lines branching out of the human fork of the Y.
Fig. 2. The descent of man, elaborated over [Figure 1]. For further ramifications, see Figures [3], [4], [9].
This subject quickly becomes a technical problem requiring rather refined evidence to answer. In general, prevailing opinion looks upon the later fossil ancestors of man as probably direct or true ancestors, but tends to regard the earlier of these extinct forms as more likely to have been collateral ones. This verdict applies with particular force to the earliest of all, the very one which comes nearest to fulfilling the popular idea of the missing link: the so-called Pithecanthropus erectus. If the Pithecanthropus were truly the missing link, he would have to be put at the exact crotch of the Y. Since he is recognized, however, as a form more or less ancestral to man, and somewhat less ancestral to the apes, he should probably be placed a short distance up on the human stem of the Y, or close alongside it. On the other hand, inasmuch as most palæontologists and comparative anatomists believe that Pithecanthropus was not directly ancestral to us, in the sense that no living men have Pithecanthropus blood flowing in their veins, he would therefore be an ancient collateral relative of humanity—a sort of great-great-granduncle—and would be best represented by a short stub coming out of the human line a little above its beginning ([Fig. 2]).
Even this figure is not complete, since it is possible that some of the fossil types which succeeded Pithecanthropus in point of time, such as the Heidelberg and Piltdown men, were also collateral rather than direct ancestors. Some place even the later Neandertal man in the collateral class. It is only when the last of the fossil types, the Cro-Magnon race, is reached, that opinion becomes comparatively unanimous that this is a form directly ancestral to us. For accuracy, therefore, [figure 2] might be revised by the addition of other short lines to represent the several earlier fossil types: these would successively spring from the main human line at higher and higher levels.
In order not to complicate unnecessarily the fundamental facts of the case—especially since many data are still interpreted somewhat variously—no attempt will be made here to construct such a complete diagram as authoritative. Instead, there are added reproductions of the family tree of man and the apes as the lineages have been worked out independently by two authorities (Figs. [3], [4]). It is clear that these two family trees are in substantial accord as regards their main conclusions, but that they show some variability in details. This condition reflects the present state of knowledge. All experts are in accord as to certain basic principles; but it is impossible to find two authors who agree exactly in their understanding of the less important data.