THE EVIDENCE OF FOSSIL REMAINS Pithecanthropus Erectus.-The discovery which throws most light on the evolutionary progress of man was made in Java dur ing 1891-92 by Prof. Eugene Dubois, then a surgeon in the col onial military service, and later professor of geology in the Uni versity of Amsterdam. In a stratum which contained the fossil bones of many extinct species of animals he obtained five frag ments of a strange kind of being, one of which he regarded as a transitional form between man and ape—a real missing link. He named it Pithecanthropus erectus, and assigned it to a separate family of primates—one lying on the borderline between anthro poids and man. (Pithecanthropus erectus, eine menschenaehnliche Uebergangsform aus Java, 1894.) The five fossil fragments found were : a skull cap which outwardly had the form which might be expected in a giant form of gibbon, a left thigh bone and three teeth. The most distant of the fragments were 20 paces apart. Later he added a sixth fragment—part of a lower jaw found in another part of the island but in a stratum of the same geological age. The skull cap is flat, low and has great eyebrow ridges ; its characters are more simian than human, yet when Prof. Dubois succeeded in obtaining a cast from the interior of the skull cap, that cast bore on it the convolutionary pattern of the brain of Pithecanthropus, and that pattern proved to be altogether human. Pithecanthropus, the fossil man of Java, had a brain which was smaller, simpler and infinitely more primitive than that of the lowest living men.
By this discovery Prof. Dubois caught the human brain in the act of evolving. Certain cortical or convolutionary areas in man's brain are known to be concerned with sight, hearing and touch, and the reception of messages from other sense organs; a "motor" area is concerned in the initiation and control of voluntary move ments. Between these primary areas of the cortex lie association areas which have to do with the memory and the interpretation of what is seen, heard or felt. The cortex of part of the frontal
lobe—the prefrontal cortex—is concerned in the acquisition of skilled movements. These secondary or association areas of cor tex, which lie between and separate the primary areas, are the basis of man's educability—his capacity to learn from experience. In the brain of Pithecanthropus the association areas are much less developed than in the brains of the lowest of living human races. Yet all the essentially human parts are represented. It is even possible that the.owner of this brain was capable of speech.
A further study of the brain-cast has convinced Prof. Dubois that Pithecanthropus must be placed in the human family (Proc. Roy. Acad. Sc. Amsterdam, vol. xxvii., nos. 5, 6, 1924). The brain of this "fossil" man is now estimated to have had a volume of at least goo c.c., the largest-brained gorillas rarely rise above 600 c.c.; the lowest-brained of human beings occasionally falls below I,000 cubic centimetres. Pithecanthropus in size of brain lies on the verge of humanity. His teeth, if large, are essentially human in form of crown and root; the socket for the canine, in the fragment of lower jaw, shows that this tooth was not massive and pointed as in anthropoid apes. The thigh bone is human altogether, and gives proof that Pithecanthropus walked as men do.
Pithecanthropus was assigned by Prof. Dubois, on reliable evi dence, to a date late in the Pliocene period ; others on weighing the evidence suppose that he lived early in the Pleistocene period. If we accept the duration of the Pleistocene as 250,000 years, and regard Pithecanthropus as representing the evolutionary stage reached by mankind at the beginning of this period, then we have to conclude that man's body had become adapted to its peculiar posture and gait before the end of the Pliocene period, and that the higher development of the brain took place in the ensuing Pleistocene period.