Biology doesn't know in advance what the end product will be; there's no Stuffit Compressor to convert a human being into a genome. But the genome it… - Gary F. Marcus

What propels an embryo from one stage to the next-and makes one species different from another-is not a blueprint but rather an enormous autonomous library of the instructions contained within its genome. Each gene does double duty, specifying both a recipe for a protein and a set of regulatory conditions for when and where it should be built. Taken together suites of these IF-THEN genes give cells the power to act as parts of complicated improvisational orchestras. Like real musicians, what they play depends on both their own artistic impulses and what the other members of the orchestra are playing. As we will see in the next chapter, every bit of this process-from the Cellular Big 4 to the combination of regulatory cues-holds as much for development of the brain as it does for the body.

"To take one example, even a brief exposure to light in a newborn kitten, rat, or monkey can launch a complex cascade of gene expression. The light activates photoreceptors-which send signals-which trigger a pathway-which leads to the expression of neural growth factors and a set of genes known as "immediate early genes" or "early response genes"-each of which, in turn, triggers the expression of many more genes. One study of cichlid fish suggests that a change in social status (from submissive to dominant) is tied to changes in the expression levels of at least fifty-nine different genes-a phenomenon not entirely unrelated to the testosterone rush that Joe-six-pack gets when the home team wins."

Memory, perception, coordination, strength: guitar is just plain hard.