they so evidently have done and continue to do—in the emergence of the first human brains?

From these questions, it is easy to see that any scientific account of the development of the human brain has to meet a formidable challenge. For such an account must not only explain a sequence of development of great orderliness and efficacy but also allow room for the creative effects of chance—in the form of random mutations and the ensuing natural selection—that have led to the propagation of this particular form of brain in the first place. The majority of developmental neuroscientists today respond to this challenge by proposing a series of stages in which built-in instructions and the effects of arbitrary external events are mingled to an intriguing degree.

FIGURE 6.1. The development of the human brain during gestation is a highly complex project on a tight schedule. In this 12- to 14-week-old embryo, nerve cells are proliferating at the rate of about 15 million per hour. The physical bases for perception are beginning to emerge: one can make out an eye (the black dot) and the future site of the ear (the white area just above). Source: National Institute of Child Health and Human Development.