In recent years there has been a surge in interest in gut flora in the wake of research on its substantial effects on personality, so much so that researchers have even taken to describing it as a neutral network.

And much like humans, and even their brains, they are not going to be an exception to recent evolution.

As Chris Kresser writes:

In other words, evolution does not act solely on your 23,000 human genes. Rather, it acts on the 9.02 million genes (both host and microbial) that are present in and on your body, as a single entity.

Moreover, the microbiome can introduce genetic variation and evolve through methods specific to it, such as sharing genes with each other and acquisition of new strains from the environment. And even the borders between bacterial genes and “human” genes are surprisingly porous.

The really interesting observation is yet to come:

Social behavior in primates is also thought to be a critical factor in the evolution of human intelligence (32). Access to microbes may have been a driving force in the evolution of animal sociality, since microbes confer many benefits to the host (33). Social behaviors like grooming, kissing, and sex increased the transfer of microbes from one organism to another. Studies in social mammals have found that development of the forebrain and neocortex in social mammals depends on signals from the microbiota (34), and germ-free mice that lack a microbiota also lack social behavior and show deficits in social cognitive abilities (35).

Depending on the size of these effects there could be some pretty important implications and confounds for psychometrics and genetics of IQ research.

Bacterial composition, for instance, though strongly hereditary, is also going to be affected by the food one eats (a cultural factor), the people with whom one has close contacts with (kissing, certain intimate contacts, and one supposes, effluence in non-hygienic countries), and the local geography, elevation, and climate. Could intelligence be a matter of not just blood and chance, but of soil?

Best not to get too carried away with yet. This paper finds that spousal partners did not have significantly more microbiome similarity than unrelated invididuals (though the sample sizes were small). Still, it might be worth bearing in mind.