7. The “Missing Link”
No modern zoölogist has the least doubt as to the general fact of organic evolution. Consequently anthropologists take as their starting point the belief in the derivation of man from some other animal form. There is also no question as to where in a general way man’s ancestry is to be sought. He is a mammal closely allied to the other mammals, and therefore has sprung from some mammalian type. His origin can be specified even more accurately. The mammals fall into a number of fairly distinct groups, such as the Carnivores or flesh-eating animals, the Ungulates or hoofed animals, the Rodents or gnawing animals, the Cetaceans or whales, and several others. The highest of these mammalian groups, as usually reckoned, is the Primate or “first” order of the animal kingdom. This Primate group includes the various monkeys and apes and man. The ancestors of the human race are therefore to be sought somewhere in the order of Primates, past or present.
The popular but inaccurate expression of this scientific conviction is that “man is descended from the monkeys,” but that a link has been lost in the chain of descent: the famous “missing link.” In a loose way this statement reflects modern scientific opinion; but it certainly is partly erroneous. Probably not a single authority maintains to-day that man is descended from any species of monkey now living. What students during the past sixty years have more and more come to be convinced of, was already foreshadowed by Darwin: namely that man and the apes are both descended from a common ancestor. This common ancestor may be described as a primitive Primate, who differed in a good many details both from the monkeys and from man, and who has probably long since become extinct.
Fig. 1. Erroneous (left) and more valid (right) representation of the descent of man.
The situation may be clarified by two diagrams ([Fig. 1]). The first diagram represents the inaccurate view which puts the monkey at the bottom of the line of descent, man at the top, and the missing link in the middle of the straight line. The illogicality of believing that our origin occurred in this manner is apparent as soon as one reflects that according to this scheme the monkey at the beginning and man at the end of the line still survive, whereas the “missing link,” which is supposed to have connected them, has become extinct.
Clearly the relation must be different. Whatever the missing link may have been, the mere fact that he is not now alive on earth means that we must construct our diagram so that it will indicate his past existence as compared with the survival of man and the apes. This means that the missing link must be put lower in the figure than man and the apes, and our illustration therefore takes on the form shown in the right half of [figure 1], which may be described as Y-shaped. The stem of the Y denotes the pre-ancestral forms leading back into other mammalian groups and through them—if carried far enough down—to the amphibians and invertebrates. The missing link comes at the fork of the Y. He represents the last point at which man and the monkeys were still one, and beyond which they separated and became different. It is just because the missing link represented the last common form that he was the link between man and the monkeys. From him onwards, the monkeys followed their own course, as indicated by the left-hand branch of the Y, and man went his separate way along the right-hand branch.