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Scientists believe they have discovered why modern humans develop brain disorders like autism, schizophrenia and Alzheimer's but our near-identical Neanderthal cousins did not.
They say that while present-day humans and our extinct Neanderthal relatives are 99.84 per cent genetically identical, there are important differences.
They discovered the cellular equivalent of ‘on/off’ switches that determine whether DNA is activated or not.
Scientists believe they have discovered why modern humans can develop autism while Neanderthals did not
Hundreds of Neanderthals' genes were turned ‘off’ while the identical genes in today's humans are turned ‘on’.
The international team of researchers, who reported their findings in industry journal Science, also discovered hundreds of other genes were turned ‘on’ in Neanderthals, but are ‘off’ in people living today.Among these genes are those that control the shape of limbs and the function of the brain - traits where modern humans and Neanderthals differ most.
And 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.
The discovery also underlines the power of the ‘on/off’ patterns. Together, they add up to what is called the human epigenome, to distinguish it from the human genome.
The genome is the sequence of three billion molecules that constitute all of a person's DNA, while the epigenome is which parts of that DNA are turned ‘on’ or ‘off’ - even as the molecular sequence remains unchanged.
Some DNA that was switched 'off' in Neanderthals is switched 'on' in modern humans and vice versa
The epigenome exerts such powerful effects that it is often called the ‘second genetic code’.
In an interview Liram Carmel, of the Hebrew University of Jerusalem, who led the study, speculated that any given gene might ‘do many things in the brain’. When dozens of brain-related genes became more active in today's humans, that produced the harmful side effect of neurological illnesses.
But the main effect might have been the astonishing leap in brain development that most distinguishes modern Homo sapiens from our extinct cousins.
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.
One caveat about the research is that one person's epigenome 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.
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