The human immune system may limit future evolution

Scientists from Imperial College London have suggested why the human genome may possess far fewer genes than previously estimated before the human genome project was begun.

Research published in the July issue of Trends in Immunology, shows how a more advanced immune system in humans could explain why the human genome may have only a slightly greater number of genes than the plant Arabidopsis thaliana, and probably less than rice, Oryza sativa.

Dr Andrew George, from Imperial College London and based at the Hammersmith Hospital comments: “Although humans are normally thought to be considerably more complex than organisms, such as plants, rice, yeast and earthworms, this is not reflected in their number of genes, with humans having less genes than other supposedly less complex organisms.”

Dr George suggests that the limited number of functional genes in the human genome may be a result of the presence of a more advanced immune system. The immune system is designed to protect us from disease, but it is important that the cells of the immune system do not recognise our own tissues or cells, as this would lead autoimmune disease.

Autoimmune disease is avoided by killing off any immune cells that recognise molecules produced by the body (self-molecules). This means that the larger the genome, the more self-molecules the immune system needs to tolerate.

As a result, the immune system has to kill more immune cells. If there are too many genes then this results in the vast majority of immune cells dying, paralysing the immune system, and leaving the body unable to fight off disease or infection.

Dr George adds: “The limited size of the human genome could make further evolution for humans difficult. Fortunately, the human genome has been able to create genes which have multiple uses, thus making the best use of a limited number of genes.”