We would expect natural selection to remove such genes from a population. Yet, many populations include individuals carrying such genes, due to which diseases persist.
For example, diabetes is clearly a very harmful medical condition. Yet, it is quite common.
According to WHO, an estimated 1.6 million deaths were caused by diabetes in 2016. Therefore, it can be assumed that harmful genetic variants associated with this common disease have not yet been eliminated by natural selection.
Two questions come to one’s mind: How did this burden of unfavourable or deleterious gene variants arise in the first place? What keeps natural selection from getting rid of them?
Origin of genes causing diabetes
While investigating ancient human DNA, researchers found the gene variants responsible for diabetes were apparently inherited from Neanderthals-the closest extinct relative of contemporary humans.
The legion has it that Neanderthals lived before Deluge. At that time, social evils were rife in the society so much that God felt that He had made a mistake by creating human beings. So in an act of divine retribution, He wiped out the whole mankind, including Neanderthals. In view of their piety, only Noah and his family escaped that punishment.
The genetic make-up of Neanderthals has now been sequenced from ancient DNA obtained from their fossils and compared with those of modern humans. Subsequently, they were assigned a new species, different from Homo sapiens. Their ancestors left Africa before modern humans, venturing into ice age Europe and Asia as far back as 500,000 years ago, and were still there when ancestors of modern humans embarked on the same journey about 70,000 years ago. Compared to modern humans, Neanderthals were stockier, with shorter legs and bigger bodies. They are known for their large cranial cavities and huge noses for humidifying and warming cold dry air. As hunter-gatherers, they consumed meat, plants, fungi and, when available, shellfish. They made complex tools with stones and buried their dead. In a hunter-gatherer society, food was gotten largely through physical activity.
Natural selection works by weeding unfit variants out of a population, including genes that cause disease
Extinction of Neanderthals
Neanderthals and modern humans lived alongside each other in Europe and Asia for several thousand years before Neanderthals vanished some 30,000 years ago. Their disappearance is one of the most enduring mysteries in all of human evolution. One popular theory, Inter Alia, is that modern humans contributed to the demise of their close cousins, either by outcompeting them for resources or through open conflict. As the migrants were technologically advanced, they dominated the scene. Recent research suggests that modern humans did not cause Neanderthals to rapidly go extinct. Other researchers believe modern humans outnumbered Neanderthals on the account of their reduced fecundity. Yet another theory suggests the environmental catastrophe played a major role in their disappearance.
Interbreeding
Genetic studies revealed the ancestors of modern humans mated with Neanderthals after leaving Africa. This means Neanderthal genes were introduced into the genetic makeup of all non-Africans living today. It is estimated that round two per cent of genes of European and Asian heritage originated from Neanderthals. It is suggested that gene variants that seem to increase the risk of diabetes were introduced into the human lineage-but not all humans. Human lineages that had remained in sub-Saharan Africa never encountered Neanderthals and so did not wind up carrying any Neanderthal DNA. This scenario would help explain how the diabetes-contributing gene could be so old and not be found in Africans.
The rise in the incidence of diabetes and obesity
In 1962, Geneticist James Neel proposed “The Thrifty Gene Hypothesis” to partially explain the rise in diabetes and obesity in the modern world. He stated that Neanderthals went through cycles of feasts and famines. Through natural selection, and over many generations, they evolved to be better at food storage and utilisation and were more likely to survive during the famine portion of the cycle. The ability to store extra calories as fat during times of plenty could help someone stay healthy and fertile when food was scarce. The gene variants did not predispose Neanderthals to diabetes or obesity. Rather, they conferred a selective advantage to them in that harsh environment where food was scarce and a great deal of physical activity was necessary to survive.
However, the advantageous DNA inherited from Neanderthals became detrimental to the health of modern humans in view of their sedentary lifestyles, unhealthy foods and plenty of alimentation; increasing their susceptibility to diabetes and obesity. Problems arise when there is a mismatch between what we were designed to do and what we currently do, thus, complications such as obesity or diabetes might develop.
In modern times, famine never comes and those “thrifty genes” become a liability. Diabetes afflicts humans in late life, something less likely to affect Neanderthals, who used to die younger.
There are, however, many serious objections to “The Thrifty Gene Hypothesis.”
Still, it has had an enduring appeal; potentially helping to explain the epidemic nature of diabetes in newly affluent populations. Work by Southam and colleagues found little evidence in support of the thrifty gene hypothesis.
What keeps natural selection from getting rid of Neanderthal genes?
How could harmful Neanderthal gene mutations persist among modern-day humans?
“It’s just really difficult sometimes to get rid of bad mutations, especially if they’re only a little bad, or if the negative effects appear late in life,” said Rebecca Rogers, an assistant professor of bioinformatics at the University of North Carolina, Charlotte.
Diabetes usually develops later in life when the person already has had children so there is not a great selection pressure to prevent those genes from being passed on. It is possible that the gene variants causing diabetes are on their way out but selection may not have had time to completely remove them.
In addition, there have been some compelling arguments made that the improved ability to treat diabetes would lead to an increased incidence of the disease in future. This is because medical science is allowing traits that otherwise would have been selected against by evolution, to be passed to future generations with greater regularity.
Other effects of Neanderthal ancestry
Neanderthal ancestry is blamed for an increased risk of Crohn’s disease, lupus, depression and biliary cirrhosis. On the other hand, it has also conferred some advantages to modern humans. Researchers found that Neanderthal DNA is commonly found in regions that affect skin and hair. This suggests some gene variants provided a rapid way for modern humans to adapt to the new cooler environments they encountered as they moved into Eurasia. When the populations met, Neanderthals had already been adapting to these conditions for several hundred thousand years. The DNA inherited from Neanderthals seem to be involved in boosting immunity; perhaps providing a quick answer to local infection.
The writer is Chest specialist in San Francisco