Scientists have identified three genes that may have played an important part in a major development in human evolution. The genes are being linked to the increase in brain size that took place in ancestors of human beings. This increase led to the development of mental abilities that define what it means to be human. Researchers published two reports on their observations of the genes in the publication Cell in late May. The researchers believe the genes first appeared between three and four million years ago. This was just before a period in the fossil record that shows a major brain enlargement in the species from which humans evolved. The three genes share almost the exact same qualities. Along with a fourth one that seems to serve no purpose, they are called NOTCH2NL genes. They came from a gene family believed to have developed hundreds of millions of years ago. They are also important in the development of embryos.