Jon Kaas, Vanderbilt University, Nashville TN

 

Jon Kaas, Vanderbilt University, Nashville TN

Sage 4101

April 29, 2009 12:00 PM - 1:30 PM

To a large extent, we are what we are because of our large, complex brains. Thus, an understanding of how the human brain evolved is a history of human origins. Concepts of how the human brain evolved over the last 250 million years from those of early mammalian ancestors stem from three types of information. First, although brain tissue does not fossilize very well, we can gain some information about the brains of extinct mammals from the preserved inner surfaces of skulls. Endocasts of skull cavities can indicate the sizes and shapes of brains, and even the locations of some of the brain fissures. From fossils, we know that early mammals were small and had small brains with little neocortex. Brains and especially neocortex enlarged independently in members of several lines of mammalian evolution, especially in primates where the brains of our ancestors enlarged greatly over the last two million years. Second, we can gain an understanding of what brain features are primitive and how features become specialized in detailed, cladistic, comparative studies of the brains of present-day mammals. Specialization of the human brain, besides size, become most evident in comparison with what is known about the brains of our nearest living relatives, the great apes. Finally, these are conclusions that are supported by brain scaling rules. As brains get bigger, as they did in our hominin ancestors, we did not just get bigger neurons and bigger brain parts, but more neurons and parts. This is partly because neurons do not scale upward in size very well. Larger brain sizes and numbers of neurons resulted in problems of connectivity that placed restrictions on brain evolution. Surprisingly, larger brains in rodents evolved by increasing the sizes of neurons and decreasing the ratio of neurons to glia, and probably by maintaining the same number of cortical areas, while larger brains in primates maintained high densities of neurons by both increasing the size of some neurons while increasing the number of very small neurons, and they also increased the number of cortical areas. In hominins, especially humans, the neuron connection problem was reduced by specializing each of the cerebral hemispheres in different ways.

 

From mice to men:the evolution of the large, complex human brain

 

Behavioral and Cognitive Neuroscience

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