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Humans evolved to have an instinct for deadly violence, researchers find

Humans are six times more likely to kill each other than the average mammal, but researchers stress that's no excuse for violence: 'Humans are moral animals and we cannot escape from that'

Ian Johnston
Science Correspondent
Wednesday 28 September 2016 18:00 BST
An Iraqi man inspects the remains of Yazidi people killed by Isis and buried in a mass grave near the village of Sinuni, in the northwestern Sinjar area
An Iraqi man inspects the remains of Yazidi people killed by Isis and buried in a mass grave near the village of Sinuni, in the northwestern Sinjar area (Getty)

Humans have evolved with a propensity to kill one another that is six times higher than the average mammal, according to new research.

Scientists calculated that when we first developed into modern humans about two per cent of deaths were caused by fellow Homo sapiens, according to an article about the research in the journal Nature.

While this rate is well below the highest figure – found among meerkats where nearly 20 per cent of deaths are caused by other meerkats – many mammals kill each other only rarely or not at all.

For all their ferocious reputation, tigers are much less likely to fight each other to the death – with a rate of 0.88 per cent.

And we are also prone to periods of extreme violence that can put even meerkats in the shade. Between about 1200 and 1500 in the Americas more than 25 per cent of the people there were killed by other humans.

The researchers compiled information about more than four million deaths among more than 1,000 mammals from 80 per cent of the mammalian families, including some 600 human populations from the Palaeolithic era to the present day.

They then used this information to create an evolutionary tree of different mammals’ propensity towards violence.

Humans, they found, were closely related to mammals who were more likely to kill each other than most.

Writing in Nature, the researchers said: “Lethal violence is considered by some to be mostly a cultural trait.

“However, aggression in mammals, including humans, also has a genetic component with high heritability. Consequently, it is widely acknowledged that evolution has also shaped human violence.

“From this perspective, violence can be seen as an adaptive strategy, favouring the perpetrator’s reproductive success in terms of mates, status or resources.”

The researchers found that lethal violence was used by nearly 40 per cent of mammals, but suggested this was probably an under-estimate.

The average percentage of deaths caused by members of the same species was about 0.3 per cent.

But, about 160,000 to 200,000 years ago, the same figure for humans was estimated to be about two per cent, more than six times higher than the average.

The Nature paper said there analysis “suggests that a certain level of lethal violence in humans arises from the occupation of a position within a particularly violent mammalian clade, in which violence seems to have been ancestrally present”.

“This means that humans have inherited their propensity for violence,” it added.

“We believe that this effect entails more than a mere genetic inclination to violence. In fact, social behaviour and territoriality, two behavioural traits shared with relatives of Homo sapiens, seem to have also contributed to the level of lethal violence.”

The researchers stressed this inherited tendency towards violence did not mean humans were unable to control themselves.

“This prehistoric level of lethal violence has not remained invariant but has changed as our history has progressed, mostly associated with changes in the socio-political organization of human populations,” they wrote.

“This suggests that culture can modulate the inherited lethal violence in humans.”

And, in an email to The Independent, the lead author of the paper, Dr José María Gómez, of Granada University in Spain, said: “Do not fall into the trap of ... making over-simplifications.

“Humans are moral animals and we cannot escape from that.”

The researchers compared their estimate for the ‘murder’ rate among the earliest Home sapiens – made using comparative methods developed by evolutionary biologists – with studies on observed levels of human-on-human killings.

The Paleolithic era was quite peaceful in human terms and close to the researcher’s estimate.

In the ‘Old World’, there were spikes during the Mesolithic, about 10,000 to 5,000 years ago, and Mediaeval age, about 500 to 1500AD, when about 10 per cent of humans died at the hands of their own species.

But things were significantly worse at times in the New World.

During a period from 3,000 to 1,500 years ago, more than 15 per cent of deaths were caused by humans, the study found.

Then came the astonishing surge to more than 25 per cent in the run-up to the arrival of Christopher Columbus in 1492, when millions more people died as they were exposed to European diseases for the first time.

In the modern world, there are some remarkably low rates of death from violence.

In the UK, which has one of the lowest levels in the world, 0.9 out of every 100,000 people (0.0009 per cent) is intentionally killed by someone else, according to the United Nations Office on Drugs and Crime.

Professor Mark Pagel, commenting on the research in an article in Nature, wrote that the question of whether humans were naturally violent had been long debated with philosopher Thomas Hobbes suggesting humans lived in “continual fear and danger of violent death” in the 17th century.

Others, such as French thinker Jean-Jacques Rousseau, suggested we were more the product of our environment.

“The work by Gómez and colleagues opens up a new approach to uncovering the origins of human violence, giving good grounds for believing that we are intrinsically more violent than the average mammal,” Professor Pagel wrote.

“Their findings fit well with anthropological accounts that describe hunter–gatherer societies as being engaged in ‘constant battles’.

“But societies can also modify our innate tendencies. Rates of homicide in modern societies that have police forces, legal systems, prisons and strong cultural attitudes that reject violence are, at less than 1 in 10,000 deaths (or 0.01%), about 200 times lower than the authors’ predictions for our state of nature.”

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