NEONATAL BRAIN DEVELOPMENT
When mammals are born, they have to pass through a narrow birth canal, which places a practical limit on neonatal head size. But this does not necessarily limit ultimate brain size, so long as further brain development can take place after birth. This is not too much of a problem for human infants, whose particularly helpless state at birth is made feasible by parental care. Small animals like rats can hide their young safely away in holes. So, like humans, the young can be born immature without incurring excess risk. In contrast, herbivores that inhabit open grassland may need to be able to run with the herd within minutes, or at most a few hours, of their birth. Such creatures could not afford a long postnatal period of general brain development. This problem has been solved in a different way by the kangaroo, which is born very early in its development but remains protected in the safety of its mother’s pouch, where it continues to develop until it is capable of independent movement. Our own protracted postnatal development not only allows us to grow a bigger brain (the adult brain is around four times the size of a new-born baby’s brain). It also ensures that our brain continues to develop while we are interacting with our environment. So each person’s brain will, to a certain extent, be adapted to the circumstances of their lives. Survival of the most useful The first sign of what will become the brain appears very early in human gestation. By the end of the second week, a neural plate made up of precursor neurons can be identified. By the end of the first month, a primitive brain has already formed. Like other parts of the body, the brain develops when cells migrate to the appropriate place. Those cells have to know how to differentiate into the right kinds of eventual cell types, and when to stop differentiating. But brain development requires more than cells simply knowing how to get to the right place and what to do when they are there. For this particular organ, a high level of competitiveness is involved. During development, connections in the brain respond to what is going on. We start off with many more potential neurons than will eventually survive. Neurons compete to make connections with their targets, and it is the connections that are actually used which seem to have a better prospect of survival. Unsuccessful neurons die, through a process of programmed cell death, called apoptosis. If one set of neurons fails to make its normal connections, then another set of opportunist neurons may colonize the vacant space.
When mammals are born, they have to pass through a narrow birth canal, which places a practical limit on neonatal head size. But this does not necessarily limit ultimate brain size, so long as further brain development can take place after birth. This is not too much of a problem for human infants, whose particularly helpless state at birth is made feasible by parental care. Small animals like rats can hide their young safely away in holes. So, like humans, the young can be born immature without incurring excess risk. In contrast, herbivores that inhabit open grassland may need to be able to run with the herd within minutes, or at most a few hours, of their birth. Such creatures could not afford a long postnatal period of general brain development. This problem has been solved in a different way by the kangaroo, which is born very early in its development but remains protected in the safety of its mother’s pouch, where it continues to develop until it is capable of independent movement. Our own protracted postnatal development not only allows us to grow a bigger brain (the adult brain is around four times the size of a new-born baby’s brain). It also ensures that our brain continues to develop while we are interacting with our environment. So each person’s brain will, to a certain extent, be adapted to the circumstances of their lives. Survival of the most useful The first sign of what will become the brain appears very early in human gestation. By the end of the second week, a neural plate made up of precursor neurons can be identified. By the end of the first month, a primitive brain has already formed. Like other parts of the body, the brain develops when cells migrate to the appropriate place. Those cells have to know how to differentiate into the right kinds of eventual cell types, and when to stop differentiating. But brain development requires more than cells simply knowing how to get to the right place and what to do when they are there. For this particular organ, a high level of competitiveness is involved. During development, connections in the brain respond to what is going on. We start off with many more potential neurons than will eventually survive. Neurons compete to make connections with their targets, and it is the connections that are actually used which seem to have a better prospect of survival. Unsuccessful neurons die, through a process of programmed cell death, called apoptosis. If one set of neurons fails to make its normal connections, then another set of opportunist neurons may colonize the vacant space.
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