ANALYSIS: Irish science has confirmed that humans are indeed unique – we have genes exclusive to us and not, apparently, evolved from genes shared with other species
WHAT DOES it take to become a human? Alarmingly little, it now seems.
Scientists working at Trinity College Dublin have achieved a remarkable first, discovering three genes that are unique to humans. These genes do not exist in the genetic blueprint of our nearest relatives, the apes and monkeys.
Yet Dr Aoife McLysaght, who conducted the research with Dr David Gonzales Knowles, now believes there may be no more than 18 completely human genes to be found in our genome. The identification of these specifically human genes is a profound discovery, however, no matter how many are eventually found. The small collection of genes may well be what differentiates us specifically from other less complex primates.
This could be the genetic stuff that allows us to exist as humans, the very genes that impart what we consider as human-specific traits such as language, reason and consciousness of self.
We seem to have startlingly few uniquely human genes given we parted ancestral company with our closest relatives – the chimpanzees – six million years ago. Genome studies of humans and chimps have shown that 99 per cent of our DNA is identical, so anything that seems to set us apart genetically may help us finally and fully to understand exactly what it is that makes us human.
Creationists chaff at the reality that we are descended from apes, but whatever one chooses to believe in terms of a creator, there is no question about our ancestry.
Until McLysaght’s and Knowles’s discoveries, all of our “human” DNA could be traced back to earlier ancestors. Our “modern” genes can be shown to be no more than an inheritance, either fully duplicated or partially rearranged versions of pre-existing genes passed on by older organisms over millions of years.
The power of genetic analysis has put this beyond doubt. We share genes not just with apes or monkeys but also with mice, dogs,
rats and even fruit flies. This is what natural selection is all about, the driver that allows evolution to deliver new genes and unique new species.
As natural selection threw up new versions of genes, those that helped species better survive their existing environment were retained and passed on to offspring. Once a useful gene emerged there was usually no need for natural selection to deliver a new version of the same gene and so some have persisted for an extremely long time including ones we share with the likes of fungi and yeasts.
But the Trinity work has now provided us with three genes that were never shared with ancestors and that only exist in humans. These were discovered after a painstaking comparison of human, ape and monkey genomes by the team.
In keeping with natural selection, the apes and monkeys including chimps, gorillas, orang-utans, gibbons and macaques, all had within their genomes sections of DNA that perfectly matched up with these three human genes.
In these species, however, the DNA was “non-coding”, it was not capable of becoming active and producing a protein, the substances produced by genes to power the inner workings of our cells.
All genomes including our own contain huge sections of non-functioning DNA. Most of the effort in genetic research that followed publication of the human genome focused on the race to identify active genes, and the long, repetitive sections were dismissed as so much “junk DNA”.
Scientists were taken aback, however, when early assumptions that there might be up to 100,000 genes arranged along our two billion-step human genome were wildly out of line, with only about 20,000 or so genes there to be discovered.
Then geneticists started to reassess whether all that junk DNA might have hidden value. Scientists began to find examples, although very few, where evolutionary processes could change a non-coding length of DNA into a working, functioning gene. Eleven examples were discovered in flies and one in yeasts, with suggestions that perhaps there might be examples in primates.
None were found in humans until McLysaght began her search, looking for active gene sites in humans that were inactive in other primates.
The three found so far are reported this morning in the important journal Genome Research. McLysaght and Knowles’s painstaking study identified three active, coding genes in humans that in other primates were non-coding junk DNA.
This means that at some point less than six million years ago after the human lineage separated from chimps, these three inactive DNA sites began producing proteins that were distinctly human.
The search is now on to understand what these genes do. They already know what building blocks are required to make the three proteins produced by the genes and these “are unlike any other human genes”, says McLysaght.
If first discovered in a mouse it would be a simple matter for researchers to track the protein and learn where in the animal it is active. They can also produce “knock-out” mice incapable of producing the proteins to understand how this might affect the animal.
Such experiments obviously are impossible to conduct in humans, and so other methods are used, for example searching cells to see where the three proteins collect or where they might interact with other proteins or tissues.
The two scientists have searched only a portion of the human genome and now believe that there will probably be just 18 strictly human genes found across our entire genetic blueprint.
McLysaght admits it is “tempting” to speculate these genes might be included among those that confer humanity upon us as a species. No evidence is available yet to prove that this is so.
Perhaps these should be considered the “God genes” – the ones that allow us to think and reason, learn and invent. Believers will want these to have been brought into play by a power beyond natural selection, but whatever the source of the change, these truly are genes that belong to each of us as humans and ones that we do not share with any other living thing.
Dick Ahlstrom is Science Editor