Evolutionary origins of the nervous system

THE human brain is a true marvel of nature. This jelly-like 1.5kg mass inside our skulls, containing hundreds of billions of cells which between them form something like a quadrillion connections, is responsible for our every action, emotion and thought.

How did this remarkable and extraordinarily complex structure evolve? This question poses a huge challenge to researchers; brain evolution surely involved thousands of discrete, incremental steps, which occurred in the mists of deep time across hundreds of millions of years, and which we are unlikely to ever fully understand.

Nevertheless, a number of studies published in recent years have begun to shed some light on the evolutionary origins of the nervous system, and provide clues to some of the earliest stages in the evolution of the human brain. These clues come from the most unexpected of places – from sea sponges, which lack nervous systems altogether, and from the extant descendents of a primitive worm which lived some 600 million years ago.

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The evolution of manual dexterity

The unique capabilities of the human hand enable us to perform extremely fine movements, such as those needed to write or to thread a needle. The emergence of these capabilities was undoubtedly essential in human evolution: a combination of individually movable fingers, opposable thumbs and the ability to move the smallest finger and ring finger into the middle of the palm to meet the thumb gives us dexterity that is unparalleled in the animal kingdom.

Last year, geneticists identified a stretch of DNA which has undergone rapid change in humans but not in chimps, our closest relatives, or in other organisms. This short DNA sequence, named HACNS1, regulates the activity of genes involved in limb development. In chimps, it is active only in the upper arm, but in humans, it is active in the part of the hand which is destined to become the thumb, and so it was proposed to have been involved in the evolution of thumb opposability.

Neurobiologists from the University of Pittsburgh have now discovered a neuroanatomical specialization which also seems to have been important in the emergence of manual dexterity. In Proceedings of the National Academy of Sciences, they report that the area of the brain which controls voluntary movement in the higher primates is subdivided into two distinct regions, one of which is evolutionarily more recent and is essential for highly skilled movements.

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