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American paleoneurologist Harry Jerison looks at the fossil record for clues on the selection pressures that led to increases in the size of the primate brain. Mammals evolved about 200 million years ago as the "nocturnal" reptiles. Unlike reptiles (such as dinosaurs), whose cognitive life was based on stimulus-response, mammals were capable of using sound to create a cognitive map of their environment. When the big reptiles disappeared 70 million years ago, vision also became a major source of information for the mammal brain, which evolved accordingly. In particular, the size of the brain increased dramatically. The brain of the mammals was flooded with sensory inputs, and had to develop the ability to recognize an object that could be defined by many (virtually infinitely many) different sets of inputs. The solution was to develop a way to represent the perceptual world and use that representation to recognize objects. Thus the mammalian brain developed the ability to process stimuli by means of a "conscious" perceptual world, as opposed to the reflexes of the reptilian brain. The function of consciousness was thus to create the perception of the object, regardless of what sets of inputs originated the recognition. The reptilian brain was simply "reacting" to stimuli, without any awareness of what those stimuli "meant". The mammalian brain was capable of transforming the stimuli into an "object" existing in time and space, and then "act" accordingly.
Jerison speculated that the human brain is, first and foremost, a marvel of integration. The brain is flooded with sensory data. If the brain had to analyze them one by one in isolation, it would be virtually impossible to cope with the number of sensory data. Jerison believed that the nervous system constructs a model of the world, and then uses that model to "understand" sensory data. The key to constructing the model of the world is to integrate all the sensory data themselves. As the model gets refined, it also gets easier to recognize sensory data for what they are. A sensory datum is not recognized in isolation, but it is recognized as part of a scene. That scene, in turn, represents the integration of all the data that have been perceived. The "world" that we perceive is nothing more than the model that we have created. That model is not necessarily the world as it is: it is a plausible model of the world, given what we have learned so far of it.