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Ancient origins of the cerebral cortex

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Just how special is the human brain? Compared to other mammals, the thing that stands out most is the size of the cerebral cortex – the thick sheet of cells on the outside of the brain, which is so expanded in humans that it has to be folded in on itself in order to fit inside the skull. The cortex is the seat of higher brain functions, the bit of the brain we see with, hear with, think with. In particular, one of its main functions is association – bringing sensory information together with information on internal states and motivation to enable flexible and context-dependent decisions to be taken, rather than simple reflexive actions in response to isolated stimuli. While undoubtedly vastly more developed in humans, a new study suggests the cerebral cortex may have much more ancient origins than previously suspected.

All mammals have a cortex and it generally increases in size over evolution. Mice and rats have a smooth cortex, while that of cats is somewhat expanded and fold…

Wild-type humans

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Wild-type is the term geneticists use to refer to non-mutants. It literally means organisms that are the same, genetically, as those in the wild, compared to ones that have been grown under coddled conditions in the lab for generations, going soft in the absence of natural selection, or that are specifically mutant at some gene or other. There are no wild-type humans.

Well, maybe there are a few, somewhere, but even they are not really non-mutants. We all carry millions of mutations in our genome – positions where the sequence in our genome differs from the typical sequence. Where everyone else has a “T”, you might have an “A”, for example. Most of these mutations have no consequence – they are simply neutral variation in DNA that has no discernible function. It turns out that most of the genome is not made of genes – the bits of DNA that code for proteins actually comprise only about 2-3% of the total sequence. Mutations that change the code for proteins are by far the most l…