sized, trends such as this are most plausibly due to the effects of 
interspecies competition, a process presumably enhaneed in our own 
case by the complex ecogeographic effects of the “Ice Ages,”’ al- 
most exactly the period within which we have seen hominid brain 
size mushroom. 
There is another factor, too, one having to do with systematic 
analysis rather than with notions of evolution per se. Drawing a 
straight line between two points is easy; indeed, two points are the 
definition of a straight line. And the fewer points you choose to deal 
with, the more likely all are to fall close to the same straight line. 
The tendency in paleoanthropology (for reasons that there is no time 
to discuss here, but see Tattersall, 1995a), has been toward the min- 
imization of the number of points, i.e. species, recognized over the 
course of human evolution; and this peculiar paleoanthropological 
perception has reinforced the notion of linearity in our evolution. 
Indeed, some paleoanthropologists (see Wolpoff et al., 1993), while 
having had to abandon the “‘single-species hypothesis” for Hom- 
inidae as a whole, still recognize no more than two successive spe- 
cies in the entire history of the genus Homo. However, an alternative 
(and, to me, both theoretically and empirically much more persua- 
sive) interpretation of the human fossil record reveals a much greater 
systematic complexity (fig. 1). 
Putting together the notions of the episodic nature of innovation 
in human evolution, and of greater taxic diversity than usually rec- 
ognized in the fossil record, I had planned to plot brain sizes vs. 
time in the context of a realistic number of taxa. I was thwarted, 
however, when I realized two things. First, I was totally unsure how 
to sort the human fossil record into what I would regard as a rea- 
sonable number of taxa (my colleague Jeffrey Schwartz and I are 
currently engaged in an attempt to do this, but it will take some 
time). For, despite the huge morphological diversity that the human 
fossil record evidently contains, littlke has been done either to de- 
scribe it properly at the requisite level of detail, or to evaluate it in 
the appropriate terms. The result of this legacy of linear thought is 
that many of the fossil human crania for which brain sizes have 
been accurately estimated actually exist in a taxonomic limbo—in 
which they will remain at least until the human fossil record is 
