Why did human beings evolve to be nice to one another? From a scientific standpoint, it doesn't make much sense for us to go out of our way to help others, especially when we don't receive any direct benefit. But new research suggests there may be an evolutionary reason that kindness exists, and it may have more to do with microbes than genetics.

Most theories that attempt to explain the evolution of altruism focus on the individual; some people see the benefit of helping the community to help their own species. These theories assume that altruism is genetically encoded — that some people just have bigger hearts than others, and that quality is determined by the genes passed down to them. But a new study has found that altruism may have less to do with the kindness in someone's heart and more to do with the number of microbes in their gut.

Researchers at Tel-Aviv University in Israel recently took a look at the role that microbes play in human behavior to determine the evolutionary benefit of altruistic behavior. We already know that viruses and bacteria can change a host's behavior. Rabies, for example, can make an individual more aggressive. There are certain parasites that can cause their insect hosts to commit suicide, and there are types of plasma that can manipulate their bacterial hosts into cooperating with one another.

The new study, which was published in a recent issue of Nature, proposed that microbes could make humans act altruistically, meaning it's microbes that explain and determine the evolution of human kindness.

How the researchers reached this conclusion

Using a series of computer models, researchers tested a number of scenarios involving interactions between humans, some with altruism-inducing microbes and others without. They found that humans could not only be influenced by microbes to act altruistically, but that doing so would help promote the transfer of these microbes from one individual to another. In other words, microbes may make their human hosts act altruistically to give the microbe a better chance of spreading to the new host. That's evolution.

Researchers also compared the altruism-inducing microbe theory with the possibility that niceness is simply encoded in our genes. In these models, they found that genetically-encoded altruism would not evolve over time as it would with a microbial influence. They also noted that while genetic kindness could persist from generation to generation, microbe-induced niceness is much more likely to spread to the next generation.

"I believe the most important aspect of the work is that it changes the way we think about altruism from centering on the animals (or humans) performing the altruistic acts to their microbes," Dr. Lilach Hadany, a researcher of population genetics and evolution theory at Tel Aviv University and a lead researcher for the study, told Phys.org.

The microbial theory explains why altruism tends to "spread" within a community. One act of kindness often causes a snowballing of such acts within a population. That wouldn't be caused by genetics, but it does make sense when you consider the possibility that altruism is caused by microbes.

Can microbes make us better people? It's certainly possible. And if we have to "catch" something while interacting with another human being, wouldn't it be nice if that something was a dose of kindness?