146 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”front end of a flatworm, its function is no more than a hub for sensory
and motor activities, not far from a reflex center. As the head ganglion
expands in size, as in insects, instinctive motivational behaviors become
evident. Some sort of primitive “urge” seems to be in place when a fly is
attracted to a sugar solution.^13 Lower vertebrates express strong emotions
in feeding and mating. Mammals develop a much more elaborate brain
structure, in particular the neocortex, enabling them not only to better
coordinate their behavior but also to store information for associative
and retrieval purposes. The development of the neocortex culminates
in the appearance of the prefrontal lobe, which in humans occupies one
third of the entire cerebral cortex. The advent of the prefrontal cortex
enhances rational thinking, enabling an animal to “foresee” the conse-
quences of alternative behaviors in order to take proper action. Rational
thinking also helps to put a brake on the urge from the primitive emotional
Fig. 7.12. Diagram showing evolution of behavior. The mode of behavior goes from
the simplest to the most complex from bottom to top. Time course of evolution goes
from left to right. Values are not strictly quantitative. Note that instincts, defined as
genetically determined behaviors, never disappear from higher vertebrates, including
humans.