284 ! The Process of Evolution 



The details of the human fossil record are not particularly per- 

 tinent to our theme. A brief review of the salient features of the 

 record (as we interpret it) is given here for the convenience of 

 those not familiar with them. The earliest fossil organisms generally 

 conceded to be "men" are the Australopithecines ( members of the 

 genus Australopithecus). These men, who may have made their 

 appearance in the upper Pliocene (1.75 million years ago) and who 

 disappeared some 500,000 to 600,000 years ago, were fully erect and 

 bipedal, and made and used stone tools. The time of emergence 

 of Australopithecus is doubtful because of uncertainties in the dating 

 of recent finds, especially the so-called "Zinjanthropus." The pri- 

 mate fossil record prior to Australopithecus sheds little light on the 

 line leading to man, although one fossil from the lower Pliocene 

 (some 10 million years ago) known as Oreopithecus seems to be 

 more related to our distant ancestors than to those of monkeys or 

 apes. 



Direct descendants of one group of the Australopithecines prob- 

 ably are the Java and Peking men and their relatives. Homo erectus. 

 The first fossil H. erectus has an estimated age of 600,000 years, 

 indicating a possible overlapping in time with some of the Australo- 

 pithecines. Most Homo erectus probably disappeared around the 

 time of the Riss glaciation, some 200,000 to 250,000 years ago. The 

 last remnants of this species may well have persisted in geographic 

 isolation after selection had transformed other groups of H. erectus 

 into what we now call Homo sapiens. Other less well-documented 

 remains indicate that H. erectus was widespread and quite variable 

 and had achieved a culture involving the use of fire and tools. 



It seems likely that much of the confusion regarding the emergence 

 of Homo sapiens has been caused by reticulate evolution. About the 

 time of the last interglacial ( 100,000 to 200,000 years ago ) , various 

 populations of the geographically variable H. erectus probably 

 gave rise to numerous populations of H. sapiens. These populations 

 had varying fates; some died out, others met and fused through 

 interbreeding, and some may have persisted and evolved in relative 

 isolation until after the last glaciation. The famous Neanderthal 

 man seems to have been a geographic variant of H. sapiens, one 

 which disappeared, in all probability, from different combinations 

 of causes in different areas (interbreeding with more modern H. 

 sapiens, competition from more modern H. sapiens, starvation due to 

 disappearance of game, etc. ) . 



Once our predecessors became upright, there remained only one 

 major physical change to convert them into modern men. This 

 change was a great increase in brain size and skull volume. Austra- 



