Advantage Aggression

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Evolutionary shifts from hunting to farming and beyond brought about bodily changes that may have led to diseases such as diabetes.

The brain versus brawn debate has taken a curious turn. The collective decision taken by mankind over thousands of years to switch from a muscle-flexing “soldier’s” lifestyle to that of a brain-dominated “diplomat” is an underlying cause of the ticking diabetes time bomb, says a new theory proposed by a team of Indian researchers.

The hypothesis by Milind Watve — professor of biology at the Indian Institute of Science Education and Research (Iiser), Pune — and his colleagues argues that the loss of physical aggression, which our forefathers used abundantly to fend for food and mating opportunities, has resulted in a host of irreversible hormonal and metabolic changes. These evolutionary cues seem to have led to metabolic syndrome, a precursor to diabetes and many other life-threatening diseases, the researchers argue in a paper published in the journal Medical Hypothesis earlier this month.

More than 200 million people worldwide suffer from diabetes and the number is feared to shoot up to 366 million by 2030, according to a study in a journal of the American Diabetes Association. One in four of them will be from India. Some other major diseases associated with metabolic syndrome are hypertension, coronary artery disease and atherosclerosis.

In prehistoric societies, aggression was important for easier access to food and better mating opportunities. However, aggression has an energy cost, in terms of increased risk of getting injured. Therefore, when there is no need for aggression, or aggression is unlikely to be effective, it has to be controlled. A satiated individual does not need to be aggressive and therefore cues of food satiety — such as having a stomach full or energy reserve in the form of fat — should signal aggression control. Sexual satiety is also expected to arrest aggression. There is evidence to show that the underlying hormonal mechanisms for both are similar, say the scientists.

“We are talking about the normal and natural forms of physical aggression that were a routine part of our hunter-gatherer life. We are not referring to pathological aggression (which is caused by hypertension or behavioural disorders),” says Watve.

According to the Iiser scientist, such changes started to become prominent when humans began farming, about 10,000 years ago.

“Agricultural people naturally show more restraint than do hunter-gatherers, as their neighbours are fixed. But they still need to control their oxen with a whip, drive wolves or mad dogs, shoo away birds and rats, catch chicken, and so on. These are moderately aggressive activities and very useful, in terms of maintaining the hormonal balance,” Watve says. “But in urban life, we have given up even this form of physical aggression.”

The new theory is set to replace or encompass the “thrifty” gene hypothesis proposed by American population geneticist James Neel more than 40 years ago. According to this, insulin resistance — the way diabetes manifests in a human body — may have evolved as an adaptive trait that later turned pathological due to the changed lifestyle and diet. He hypothesised a “thrifty” genotype that helped survival in primitive life, which was characterised by intermittent periods of “feast and famine”.

One of the major shortcomings of this theory is that it deals only with energy metabolism, says Watve. “It offers no explanation as to why the immune system changes and why sexual functions alter,” he explains.

In defence of their hypothesis, the Pune researchers have listed 32 biochemical changes that are associated with both insulin resistance and loss of physical aggression. One of them is serotonin, a brain chemical. Studies in the past have shown that aggressive individuals have a low serotonin level as compared to submissive people. Similarly, a high level of serotonin is implicated in reducing sexual motivation. In a complex way, increased serotonin levels also lead to obesity and insulin resistance.

The study throws up another interesting aspect — the biological changes linked to reduced injury-proneness associated with a “diplomat” lifestyle. Aggressive behaviour facilitates the secretion of epidermal growth factor (EGF) — a protein that plays a role in healing wounds — in the saliva and other body fluids. This is in anticipation of wounds during fights. “EGF is also important in pancreatic beta cell regeneration. So chronic lack of injuries will ultimately lead to insulin deficiency as well,” Watve says.

To support their hypothesis, they also gathered evidence of similar changes in many hormones and biomolecules such as dopamine, melatonin, cholesterol, the sex hormones, ghrelin and endorphin.

Watve, however, says the study is more of a synthesis. “Everything in support already exists in literature,” he says. A huge volume of work had accumulated in the last 15 years. There were many unexplained aspects. “Someone had to join the pieces of the big puzzle. Only evolutionary biologists could have done it — and it happened to be us,” says Watve.

But not everybody is convinced. Anoop Misra, an internal medicine specialist at Fortis Hospital in New Delhi, thinks that though an interesting hypothesis, it has several contradictory facets. “The metabolic syndrome is multi-factorial and so is diabetes. Even if eventually proven, it may explain only 5-10 per cent cases of diabetes,” he says.

To Watve, the response is not unexpected. In the last three years, wherever he presented the work, it elicited three types of reactions. One is of curiosity and interest. Some are excited by the novelty of the concept and despite some scepticism, agree that it is worth serious thought. Some others simply ignore it as something from India not worth looking at. The third type of response is from people who have a very religious approach to science. They will oppose anything against their belief system as unscientific, although they may not have any sound argument against it.

The Pune team — which included researchers from the University of Pune and KEM Hospital and Research Centre — also designed an exercise regime based on their findings. “We tested it on a small number of patients. It actually worked,” smiles Watve.

Source: The Telegaph (Kolkata, India)