Your Microbiome Might Play A Role In Altruism

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They might be microscopic, but single-celled bacteria are surprisingly powerful. Research has linked certain bacteria in the gut to symptoms of anxiety, depression, and even autism. Now, scientists are beginning to think that bacteria may not just be out to destroy our mental health. They could be responsible for acts of altruism, too.

Good Bacteria, Bad Bacteria

When it comes to evolution, altruism has always been a conundrum. If the main goal for a species is to do things that help it survive and procreate, how could selflessness (literally doing something without the expectation of reward) have evolved? Even Darwin puzzled over it. "It is extremely doubtful whether the offspring of the more sympathetic and benevolent parents...would be reared in greater number than the children of selfish and treacherous parents of the same tribe," he wrote in "The Descent of Man". "He who was ready to sacrifice his life...would often leave no offspring to inherit his noble nature."

Scientists over the years have put forth a number of theories to explain why animals persist in being so gosh-darn selfless, but it's still mostly an open question. Tel Aviv University researchers Lilach Hadany and Ohad Lewin-Epstein wondered if there was something else at play. They knew that microbes and other parasites could change their hosts' behavior for the worse—rabies, for instance, encourages animals to become aggressive and bite, thereby spreading the virus; and the liver fluke Dicrocoelium dendriticum makes ants climb to high places where they can be eaten by the animals the fluke wants to colonize. Why, when, wouldn't some microbes change their host's behavior for the better?

Escherichia coli, one of the many species of bacteria present in the human gut.

Kill 'Em With Kindness

To find out, the Tel Aviv researchers teamed up with IBM computer scientist Ranit Aharonov to create a mathematical model and a computer simulation to analyze the effects of altruistic microbes on their hosts. The simulation looked at one population in the presence of two types of microbes: one that promoted altruism, and one that didn't. In each generation of the population, the individuals interacted in ways that were sure to spread both types of microbes from one individual to the next. The simulation also assumed that microbes were automatically passed to a host's offspring.

The result? Over hundreds of generations, the altruism-loving microbes stomped the competition, even when the researchers started with a tiny number of them. That makes some sense, since individuals who received the altruism-loving microbe also benefited in some way from a host's altruism, so they were better off evolutionarily speaking. That is, the host's altruism made those new hosts more likely to produce offspring, and those offspring also had the altruism-loving microbe.

It might sound surprising that a single-celled organism could trigger a behavior as complex as altruism, but it's not beyond the realm of possibility. Research has shown that gut microbes encourage the production of serotonin, the neurotransmitter that some antidepressants are designed to help produce. And while your microbiome could be linked with symptoms of anxiety and depression, studies have shown that certain bacteria found in yogurt can reduce those symptoms in mice.

So when Ebenezer Scrooge changed his ways on Christmas morning, it could have been the result of those three ghosts or, more likely, the bacteria in his gut finally got their way.

Your Microbiome and Your Brain

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