once placed a considerable premium on big brains have somewhat relaxed 

 since the late Pleistocene. Whatever advantages a larger cranial capacity must 

 earlier have imported were no longer evident. It was no longer a key to 

 survival to have a large, well-filled braincase. 



The relaxation of the selective pressure, the reversal of the age-old trend 

 toward increased brain size, casts into strong relief the other side of the coin. 

 What was the advantage in having an increased brain size? Why did selection 

 in favor of larger brains continue for as long as it did, and why did it seem 

 to operate more energetically at some stages than at others? What is the 

 neurological basis of increasing brain size? And what are its microscopic and 

 physiological counterparts? The ensuing part of this book will be devoted to 

 exploring some of these questions, although it will not presume to offer 

 adequate and conclusive answers. 



SEVEN 



U THE STRUCTURAL MEANING OF 



Q VARIATIONS IN BRAIN SIZE 

 K 



On the perils of palaeoneurohistology 



One is least qualified to speak on the paleohistology of the hominid 

 brain. On theoretical grounds brains may become larger from any one or 

 any combination of the following factors: (a) more nerve cells; (b) bigger 

 nerve cells; (c) more neuroglia; (d) more nerve processes; (e) longer nerve 

 processes; (f) thicker nerve processes (thicker myelin sheaths); (g) more 

 highly-branched nerve processes. These are 7 variables that may affect the 

 total quantity of brain tissue alone. At any one moment in time, within a 

 single species, are larger brains larger predominantly because they possess 

 more neurons? Or are larger brains larger through a combination of any 

 2 or more of these 7 possible variables? 



We simply do not know the answers to these questions for modern 

 human brains within the range of normality. We shall see in a moment that 

 we have a little more information when it comes to comparisons between 

 different animals of different average brain sizes. But within our species we 

 do not know what the microscopic or cellular basis is of varying brain size. 



103 & 



