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Geneticist Liram Carmel and a team of Israeli scientists at the Hebrew University of Jerusalem have discovered the cellular equivalent of functional “on/off” switches in DNA, which may help to explain fundamental differences between modern humans and extinct Neanderthals.
The Israeli researchers have just published what may be break through research online, in the web edition of the prestigious journal “Science”, which may help explain how modern man, Homo sapiens, evolved from Neanderthals.
Present-day humans and our extinct Neanderthal cousins are 99.84 percent the same genetically but yet are completely different.
Scientists discovered just four years ago that the genomes of the two species differ by only a fraction of a percent. Now geneticists say they understand why: there are embedded biological
cellular equivalents of “on/off” switches that may determine whether parts of the DNA is activated or not and what effects this may have.
It also turns out from the research that such on/off patterns are very important. Cumulatively these patterns add up to what is called the human “epigenome”, in order to distinguish it from the entire human genome. While the whole genome is that sequence of 3 billion molecules that constitute all of a person’s DNA, the epigenome refers only to those switchable bits of DNA that are turned on or off, even as the total molecular sequence remains unchanged.
A number of scientific researchers have focused on the epigenome in the last few years, shedding new light on how gene “silencing” can lead to cancer, for example, or why even identical twins with identical DNA sequences can sometimes be very different people The epigenome element of our DNA exerts such powerful effects that it is now frequently referred to as our “second genetic code.”
Now the Israeli researchers have discovered that the epigenome also offers clues as to what makes modern humans distinct from their immediate ancestors.
Geneticists led by Liram Carmel of the Hebrew University of Jerusalem studied DNA from three different limb bones: those from somebody alive today, from a Neanderthal and from a Denisovan. the Denisovian is another extinct form of early human that lived in Eurasia during the Stone Age period. His/her remains, a little finger bone and a tooth, were not discovered until 2010 when they were found in a cave in Siberia.
Geneticist David Gokhman and others on the Israeli team then looked at the on/off patterns of the DNA of each. They identified about 2, 200 areas that are activated in today’s humans, but switched off in either or both of the two extinct species, or vice versa. When a gene is switched off, it does not produce the trait it otherwise normally would.
Amongst all the epigenetic differences discovered, there was one particular cluster of five genes called HOXD, which influences the shape and size of limbs, including arms and hands. It was largely silenced in both of the two extinct ancient species, the scientists discovered.
These could very possibly explain anatomical differences between ancient and modern humans, including the shorter legs and arms of the Neanderthal, their bowleggedness, large hands and fingers, and even their curved arm bones.
Paleoanthropologist Chris Stringer of the Natural History Museum in London called the work “pioneering, ” and “a remarkable breakthrough, ” in an interview and he added that the HOXD gene finding “may help to explain how these ancient humans were able to build stronger bodies, better adapted to the physical rigours of Stone Age life.”
Of course any one person’s epigenome can vary markedly from another’s just due to diet, environment or other factors. It is therefore not yet possible to determine whether such on/off patterns found in Neanderthal genes are typical of the species overall, or simply particular to the individual studied. Further research will be required to look at many such individuals therefore.
Other major differences in their on/off patterns between the extinct, and present-day, humans may be associated with neurological and psychiatric disorders including autism, schizophrenia and Alzheimer’s disease. More of the Neanderthal versions that relate to these traits were silenced than in modern man.
Carmel speculated out loud in an interview 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 somehow may have also produces the harmful side effects of neurological disorders.
But the main effect might also have been the astonishing leap in brain development itself, that most distinguishes modern Homo sapiens from our extinct ancestors. So it may be we have to take the good with the bad, in genetics as in other areas of life.