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Blog científico

Publicado el 12 de septiembre de 2012 por José María Bermúdez de Castro

The brain of Homo sapiens: lights and shadows

The growth, development and functioning of our brain is very expensive in terms of energy. To start with, when we are at rest, we dedicate 20 percent of the biological energy we consume to maintaining brain activity. Clearly this extravagance must have adaptive advantages. Otherwise, our brain would still be the same size it was in our Pliocene ancestors, for four million years. But our big brain development also happens to be a cause for concern. Let's see why. Our species's brain reaches a volume of about 1,350 cubic centimetres, on average, compared to 400 in chimpanzees or Australopithecus. For a couple of decades we've known that the exponential growth of the brain of the genus Homo, which occurred in the last two million years, was a result of the extension of childhood into a new stage of development, which is generally known as the childhood stage. Our brain grew not only faster than in Pliocene Australopithecus, but had more time to triple its size. Chimpanzees' brains complete their growth by the fourth year of life, while the human brain reaches its final size around the age of eight. Before sexual maturity, chimpanzees still have seven years to develop their brain. In that time, neuronal connectivity progresses through the necessary learning to lead fulfilling lives as adults. We humans have even more time to get to that point. Moreover, adolescence (a totally new phase in our species) allows not only a restructuring of brain connectivity, but the formation of the myelin sheath that protects neuronal extensions, and the ability to multiply the transmission speed of information a hundred times. This is true `broadband` through which the huge amount of processed information circulates at each moment of our lives. The brain of the genus Homo achieved a considerable increase in size due to the increase of the rate of brain growth both during gestation and in the postnatal period. At the end of our first year after birth, the brain of Homo sapiens reaches the respectable size of 800 cubic centimetres. According to research by Jianzhi Zhang in the early 21st century, the key to this achievement may be in changes in the ASPM gene, which regulates the number of stem cells involved in the formation of the brain. As I said in the previous paragraph, we also have four additional years to achieve our brain size compared to Pliocene ancestors. On the other hand, we have slowed down the achievement of brain maturity thanks to changes in genes such as Cux1 and Cux2. Thus, we have more time to get the connectivity of billions of neurons that make up our brain neocortex. Thanks to these developmental processes we have enhanced cognitive abilities compared to anthropoid apes. Important developments include our development of a very complex language and the capacity for symbolic thought. However, our brain structure keeps the same functional scheme as that of other apes. Yes, we are smarter than chimpanzees thanks to a highly developed neocortex, in which the prefrontal `executive area` stands out as the centre of decision-making, planning or anticipation. However, the `emotional brain` continues to dominate our actions. We would be unable to survive without the activity of regions such as the amygdala, the hippocampus, the hypothalamus and the anterior cingulate cortex in perfect connection with the `rational brain`. The great paradox of our species lies in our enormous capacity to generate a culture that progresses exponentially thanks to the accumulation of information and interaction among millions of human beings, and our inability to manage it effectively. Issues such as global warming, unsustainable population growth or notable social inequalities are the result of the huge gap between technological and biological evolution. Our brain has evolved significantly, but its structure is still that of an ape.