A progressive enlargement of the hominid brain started about 2 million years ago, probably from a bipedal, australopithecine form with a brain size comparable to that of a modern chimpanzee. Since then, a threefold increase in endocranial volume has taken place, leading to one of the most complex and efficient structures in the animated universe, the human brain. In view of the central importance placed on brain evolution in explaining the success of our species, one may wonder whether there are physical limits that constrain its processing power and evolutionary potential.

In this paper I will explore some of the design principles and operational modes that underlie the information processing capacity of the cerebral cortex in primates, and I will argue that with the evolution of the human brain we have nearly reached the limits of biological intelligence.

Biological limits to brain size

The human brain contains about 100 billion neurons, more than 100,000 km of interconnections, and has an estimated storage capacity of 1.25 × 1012 bytes (Cherniak, 1990; Hofman, 2000). These impressive numbers have led to the idea that our cognitive capabilities are virtually without limit. The human brain, however, has evolved from a set of underlying structures that constrain its size, and the amount of information it can store and process. In fact, there are a number of related factors that interact to limit brain size, factors that can be divided into two categories: (1) energetic constraints, and (2) neural processing constraints (Fig. 6.1).