while the provenience of some of the relevant fossils is imperfect. Neverthe- 

 less, the general sequence of taxa shown corresponds approximately with the 

 time sequence. 



Cranial capacity and the time dimension 



If we classify endocranial capacity according to the time dimension, we 

 see that no Lower Pleistocene hominid specimen thus far recovered ex- 

 ceeded 685 c.c. 



Lower Pleistocene hominids: * 435 (428), 480 (435), 480 (485), 500 (428), 



540 (440), 500 (530), 530 (436), 530, 633, 684 

 Middle Pleistocene hominids (earlier and middle): 652, 750, 775, 780, 850, 



890, 915, 915, 975, 1000, 1015, 1029, 1030, 1030, 1225 



At the same time, no Middle Pleistocene hominid had a capacity 

 lower than 650 c.c. In the above list I have included the Swartkrans speci- 

 men with the Lower Pleistocene hominids: Cooke (1963, p. 103) has stated 

 that it is very likely that the Swartkrans and Kromdraai faunas "are close 

 to the Villafranchian-Cromerian boundary, but it is still not certain on 

 which side they lie." That is to say, it is not certain whether they are from 

 the Lower or Middle Pleistocene. Since Swartkrans seems to be older than 

 Kromdraai, it would seem reasonable to put the Swartkrans specimen 

 among the Lower Pleistocene group. 



It is not suggested that these sample limits tally with the population 

 limits at any one time. Our estimates of standard deviations and ranges 

 for the population have already shown that this is not so. Nonetheless, the 

 samples may provide us with some evidence for a trend. 



In brief, the proposition is suggested that during the latter part of 

 the Lower Pleistocene, conditions of life became such that strong selec- 

 tive pressures for an increase in brain size became evident. The effect of 

 such selection became manifest with the emergence of the larger-brained 

 group known as H. habilis, but more emphatically with the arising of the 

 next major systematic category, H. erectus. 



We shall consider below what aspects of the life of Homo habilis and, 

 especially, of Homo erectus could have been relevant for these selective pres- 

 sures toward increasing brain size— factors such as the rise of systematic stone 

 tool-making, organized and systematic hunting, and symbolic behavior in- 

 cluding symbolic speech. 



* Holloway's (1970b) reestimates are cited in parentheses. 



99 & 



