Evolution of a human brain. Dissemination 96th
In 10 previous articles of VD disclosure (No. 3, 13, 17, 22, 28, 46, 48, 71, 76 and 86) we have exposed the origin and evolution of the nervous system. From the "rope ladder" of nerve of invertebrates which can be considered as the most primitive form of the brain, through the brain of apes and hominids to the brain of H. sapiens each pair of nodes, the primitive chain, has been developing and assuming increasingly complex structures both anatomically and functionally. These changes contain the process of cephalization and are closely related to the appearance of the new skills in the new species. With each step of cephalization varies the number of neurons, the area, volume, weight and density of each of the pairs of nodes that can be called lobes of the most developed brains.
H.sapiens’ brain is composed of 100 000 billion neurons, has an area of about 2000 cm2 (1800/2300 cm2), a volume of 1,300 ml (1,100 / 1,500 ml), a weight of 1450 g (1300/1600g) equivalent to 0.8-2% of body weight and its density is 66.6 billion neurons per ml and 1.1 g / ml.
In the last decade there have been several hypotheses about the connection between weight, volume and brain surface, with the degree of human intelligence without reaching a final agreement. However if we apply to the conclusions of studies in other species, we tend to defend the importance of the percentage of weight of a part of the brain related to its total weight.
However the difficulty of finding fossilized hominid brains, being an organ of the difficult mineralization, significantly reduces the knowledge of how the brain of Australopithecus has evolved to become the brain of H.sapiens. At this point it becomes important the studies conducted by Dr. Ralph Holloway of Columbia University, in the endo plaster molds inside the skulls of various species of hominids, which can be considered as predecessors of the H.sapiens. These studies have been able to determine the volume and distribution of some features of the different brains.
The first and most notable character is the continued increase of brain volume in Australopithecus afarensis (-3.9 million years ago) 500 ml, Homo habilis (- 2.5 million years ago) 750 ml, Homo erectus (-1.9 million years ago) and Homo sapiens 1000 ml (-0.2 million years) 1,500 ml.
The second feature of the internal structure is the constant increase, the step by step development of temporal lobes (increase visual functions, hearing and memory), parietal lobes (increase balance, coordination and symbolism especially reading and math) and of the bulge of Broca's area (third convolution of the left hemisphere involved in language and understanding).
From this information can be deduced that the internal structure of the brain of H.afarensis has features (lower volume but similar cast) of actual brain and confirms that in other living beings, the expression previously "sleeping " genes contributes to the appearance of new bodies, their subsequent evolution, and therefore new opportunities that biological nature deals with the selection, promotion or elimination.