As a result of further intelligence-driven technology, humans then 

 went on to become highly effective social hunters or predators in 

 their own right. The selective pressures driving this process were 

 presumably similar to those that had allowed the human emergence 

 from a former subservient role. 



Among the variety of selective pressures that drove the evolution 

 of the large human brain, it can be argued that the demands of 

 predation — first in surviving its dangers and later in the successful 

 practice — were ever present and powerful in their effects. 



Some Constraints on a Technologically Competent Animal 



By any standards, the human animal is a very successful one. 

 Homo sapiens is the most dominant species that the world has seen 

 thus far, even if it is also the most destructively invasive one. This 

 dominance is the result of its intelligence, most often translated 

 into tangible technology, so it is perhaps worthwhile to enquire 

 why no other animals have not followed the same route of brain 

 expansion that has characterised the human lineage. It seems to me 

 that there are at least five obvious constraints that would apply to 

 any candidate for the role of a "technologically-competent ani- 

 mal." Although some would apply only if the candidate were a 

 mammal, they are: 



1. A critical minimum brain size. As was pointed out by Phillip 

 Tobias (1971) in a previous James Arthur Lecture, the human brain 

 is characterized by a very large number of "extra neurons," over 

 and above those needed to handle the basic functions of the body. 

 These are the equivalent of the computer hardware needed to process 

 incoming information and convert it into intelligent responses. For 

 human-style intelligence, it seems that a volume of at least 500 cc 

 of "extra" neural tissue hardware is required, so a constraint is 

 inevitably imposed on the minimum size of an animal that could 

 afford and accommodate such additional brain tissue. 



2. Appropriate birth-canal adjustments to allow the passage of 

 the offspring's large head in such technologically-competent mam- 

 mals. 



25 



