Washington, April 20 (IANS): In a significant discovery, scientists have found why modern humans develop brain disorders like autism or schizophrenia but our ancestor Neanderthals did not.

They have discovered the cellular equivalent of "on/off" switches that determine whether DNA is activated or not.

Hundreds of Neanderthals' genes were turned "off" while the identical genes in today's humans are turned "on".

Alternatively, hundreds of other genes were turned "on" in Neanderthals but are "off" in people living today.

“The genes related to autism, as well as to schizophrenia and Alzheimer's disease, were more likely to be "off" in Neanderthals than in modern humans,” said lead author Liram Carmel from Hebrew University of Jerusalem.

The discovery underlines the power of the "on/off" patterns.

“The 'on/off' switches could also explain the anatomical differences between archaic and present-day humans, including Neanderthals' shorter legs and arms, bow-leggedness, large hands and fingers, and curved arm bones,” Carmel explained.

When dozens of brain-related genes became more active in today's humans, that produced the harmful side effect of neurological illnesses, the study noted.

One person's genome can vary markedly from another's due to diet, environment and other factors.

“It is, therefore, impossible to know whether the 'on/off' patterns found in Neanderthal genes are typical of the species overall or peculiar to the individual studied, the study, published in the journal Science, concluded.