Does kindness come from germs? Are the better angels of our nature really a contagious infection?

Scientific American takes a cold, calculating look at research into the origins of our behavior. Just like parasites can spur suicidal behavior in certain hosts (all the better to reproduce), some microbes might have encouraged the first altruistic behavior in our ancestors:

A recently developed mathematical model and related computer simulations by a trio of researchers at Tel Aviv University appear to validate this theory. The researchers showed that transmissible microbes that promoted altruism in their hosts won the survival battle over microbes that did not — and when this happened, altruism became a stable trait in the host population. The research was published in Nature Communications earlier this year.

[W]hen it comes to altruism, “there are many explanations, but it still sounds like a mystery,” said Ohad Lewin-Epstein, an evolutionary biologist and programmer at Tel Aviv University. As a student in the biology laboratory of Lilach Hadany, he took part in research on how cooperation among members of a population can affect the evolution of new traits. The team came to feel that the classical explanations for the evolution of cooperation weren’t the whole story. In particular, Hadany and Lewin-Epstein, with Ranit Aharonov, a computer scientist visiting the university from IBM Research, wondered if microbes could manipulate their hosts to encourage them to help others.

The researchers in Tel Aviv wanted to lend context and focus to an idea that had been debated for some time: Can transmissible “piggybacking” factors encourage altruism?

To explore this question in depth, the Tel Aviv group created both a mathematical model and a computer simulation that analyzed interactions among members of a population over hundreds (and in some cases, thousands) of generations. The model assumed that altruistic members incurred some fitness cost when they interacted with others, while the recipients of altruistic acts benefited.

The researchers then pitted two types of virtual microbes against each other in the simulation. One microbe promoted altruism in its hosts, while the second did not. In each generation, individuals interacted in ways that allowed both types of microbes to pass from one host to the next, and each individual’s microbes were then transmitted to its offspring. Over the generations, microbes that encouraged altruism in their hosts out-competed their rivals when both passed from one host to another and were subsequently passed from parent to child. This was true even when the population of “pro-altruism” microbes was very small at the outset. Pro-altruism microbe recipients were fitter in that they had benefited from another host’s generosity, meaning they were more likely to produce offspring carrying the same microbe.

By the end of the simulation, the host population consisted mostly of individuals carrying the altruism-promoting microbe — in some scenarios, 100 percent of hosts ended up with the microbe.

If Hadany and Lewin-Epstein’s theory holds up, it could have a profound impact on how we approach medical interventions that affect gut microbes. If microbes influence social behaviors such as altruism, doing things that change our microbial balance — such as taking antibiotics or probiotics — could potentially reshape how we treat one another by weakening or strengthening the manipulations that are part of our normal behavior. What would happen, for instance, if one group of subjects was given heavy doses of antibiotics and another group was left untreated? Would the treated group, now rid of microbial manipulators, act more selfishly than the untreated one?

Early experimental results point to at least some connection between antibiotic use and social behavior.

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