Dr William Reville Column: The human brain had achieved its present size, and, presumably, its present complexity, by at least 100,000 years ago. At that time humans were in a primitive phase of existence and the question naturally arises as to how such a complex brain evolved with a capacity far in excess of that needed to solve the relatively simple problems of the time. Nobody has come up with a satisfactory answer to this question.
When we look at the very big and the very small we encounter equally huge numbers. Compare the large-scale structure of the universe with the small-scale structure of the brain. Our brains have 10 to 15 billion nerve cells while our Milky Way galaxy contains about 15 billion stars. Nerve cells are associated with glial cells that nourish and sustain them and glial cells occur in about the same abundance as nerve cells.
Interconnections between nerve cells occur at structures called synapses and there are 15 to 20 thousand billion synapses in the human brain. This is the same number as the number of stars in a thousand galaxies.
The basic unit of the brain is the nerve cell. These cells can be connected to each other through the synapses in a virtually infinite number of ways. The number of possible connections is greater than the number of atoms reckoned to be present in the universe.
Brain volume within the human ancestor lineage has increased from less than 400 millilitres (ml) to roughly 1,400 ml in the last three to four million years. The famous Lucy, whose fossil skeleton was unearthed in central Ethiopia, lived between three and four million years ago. She walked on two legs, was less than four feet tall and had long arms, massive jaws and a small brain (450 ml).
About three million years ago Lucy's kind diverged into two lines of descent, one of which, a million years later, gave rise to the genus Homo. The first hominid, Homo habilis, lived between two and 1.5 million years ago, was still of small physique, but had a brain size of 700 ml. About 1.5 million years ago Homo habilis evolved into the larger-brained Homo erectus (850 ml), who was succeeded by Homo sapiens with a brain size of about 1,400 ml.
As a lump of stuff, the adult human brain is an organ of considerable size, weighing about 1.4kg in man and 1.25 kg in women. This does not mean that men are a bit smarter than women, because the female brain represents about 2.5 per cent of body weight compared to a figure of two per cent for men. The brain is certainly not the biggest organ in the body. It is very much smaller than muscle, which contributes 42 per cent of male body weight, and much smaller than the weight of the bony skeleton.
The brain is an organ of vast computational and other capacities. However, most people don't exploit their brainpower to any significant extent and spend their days doing repetitive and simple tasks. I know that not everybody, despite their effort and application, can switch on brain-power to operate at genius-level, but any brain that is not being asked to perform will operate at a far inferior level to its optimum capacity.
But look at how some brains can perform. Wolfgang Amadeus Mozart (1756-1791) heard a piece of music in the Vatican that was played only once a year. This music was carefully guarded and Mozart had never seen the score or heard the piece being practiced. He left the Vatican and wrote down the entire score, note by note, from memory.
The works of William Shakespeare (1564-1616) are the single greatest achievement in literature. In a relatively short working life he wrote 37 plays and a cycle of 154 sonnets of exquisite quality. His works cover the full range of human activities and emotions. He acquired a great knowledge of literature, law, Latin, French, politics, science, sociology, psychology, history, music, mathematics and the arts, and used all of this in his plays. In his writings, he used a vocabulary of over 25,000 words, greater than any other English writer.
Some people can use their brains to perform amazing computations. Professor AC Aitken of Edinburgh University was once asked to express four divided by 47 as a decimal. He started his answer after four seconds and gave another digit every three quarters of a second, saying, "Point08510638297872340425531914".
He stopped after 24 seconds, discussed the problem for one minute and repeated the last four digits before adding two more: "191489". Then after a five second pause he continued: "361702127659574468".
Aitkin stopped at the repeating point, the point at which the entire sequence of numbers begins again with 085. (Quoted by Anthony Smith in The Mind, Hodden and Stoughten, 1984).
As far as can be judged from the fossil evidence, there is no difference between the modern human brain and that of our ancestors who lived 100,000 years ago. How did a brain capable of remembering whole symphonies note by note evolve in a world where the demands to be made on it were surely simpler than that? And why has it not grown any bigger since? Nobody knows the answers to these questions.
William Reville is associate professor of biochemistry and director of microscopy at UCC