Scientists have identified a gene that controls the size of the brain, a finding that will give an insight into what it means to be human. Humans have extraordinarily large and complex brains, even when compared with our close relatives.
Andrew Jackson of the Medical Research Council, MRC, Human Genetics Unit, Edinburgh and colleagues at the MRC Genome Damage and Stablity Centre studied families who have members with Seckel syndrome, which retards growth in the womb, leading to short stature and a markedly reduced brain size (microcephaly). They report that the small brain size is linked to faults in a gene called PCNT and find that this gene works with another gene linked to the condition, called ATR, which is involved in DNA repair.
The PCNT gene is responsible for the manufacture in the body of a protein called pericentrin, a component of the centrosome, an organ in cells (organelle) that is essential for the process of cell division that underpins growth.
"These results help us understand how microcephaly genes work, bringing together centrosomal proteins and DNA damage signalling" says Dr Jackson.
Other genes involved in the working of the centrosome have previously been found to be mutated in different forms of microcephaly, suggesting that it is essential for determining brain size.
When comparing different species, brain size does present a correlation with intelligence. For example the ratio of brain weight to body weight for fish is 1:5000; for reptiles it is about 1:1500; for birds, 1:220; for most mammals, 1:180, and for humans, 1:50. Among people, modern studies using brain scans have shown that brain size shows a rough correlation with IQ among adults of the same sex. The brain is a metabolically expensive organ, and consumes about 25% of the body’s energy. Another evolutionary constraint is fitting the head through the birth canal.
Thus, although larger brains are associated with higher intelligence, smaller brains might be advantageous from an evolutionary point of view if they are equal in intelligence to larger brains.