One of the weirdest things about being a human being is that we’re actually a composite organism, and most of our genetic information isn’t actually human. The standard line on this is that there are 10 times as many bacteria cells in our body as human cells. The bacteria don’t weigh much – just a few pounds in aggregate – but they constitute the bulk of our collective DNA.

Humans are a composite organism. (Oli Scarff/Getty Images)

From the standpoint of evolutionary biology, human beings are “metagenomic.” Natural selection focuses not only on our fitness as individuals, but on the fitness of the organisms within us, and all around us, and among our close relatives.

There’s been a lot of discussion lately about the possible degeneration of our microbiome due to the overuse of antibiotics, and the resulting effects on human health. Most prominently, Martin J. Blaser’s book “Missing Microbes” says we’re playing with fire:

“For a number of reasons, we are losing our ancient microbes….The loss of microbial diversity on and within our bodies is exacting a terrible price. I predict it will be worse in the future…. An even worse scenario is headed our way if we don’t change our behavior. It is one so bleak, like a blizzard roaring over a frozen landscape, that I call it ‘antibiotic winter’.”

Just in case you were looking for something else to worry about.

This "metagenomic" quality of human existence can be viewed from the macro end of size scale as well as the micro. A new paper, published Monday in the Proceedings of the Natural Academy of Sciences , says that people tend to have a genetic resemblance to their friends, and that these friend cohorts should be part of the discussion when we talk about natural selection and the (ongoing) evolution of human beings. See my story on this, posted this afternoon.

The gist of the paper is that our friends tend to resemble us genetically more you’d expect by random chance. This goes beyond the obvious genetic markers like ethnicity, body type, etc., and includes such things as the olfactory senses. Our friends like and dislike the same sets of odors, or at least they tend to.

“We’re metagenomic with respect to the genes of the friends around us,” co-author Nicholas Christakis told me.

The co-authors conclude in their paper:

“[T]he human evolutionary environment is not limited to the physical environment (sunshine; altitude) or biological environment (predators, pathogens) but also includes the social environment, which may itself be an evolutionary force.”

The authors say in their abstract: "Friends may be a kind of 'functional kin'." This is a highly provocative thesis that will surely incite a lot of discussion and debate. The authors are saying that homophily -- the tendency of people to form connections with people who are look like them -- is not a pure chance event, or culturally driven phenomenon. Evolutionary pressures may be driving this. Their abstract concludes: "[H]omophilic genotypes exhibit significantly higher measures of positive selection, suggesting that, on average, they may yield a synergistic fitness advantage that has been helping to drive recent human evolution."

A bit of a mouthful, that.  And there may be people who say this is sociobiology run amok. Recall that it was here in Washington, D.C., back in the 1970s, that the great biologist E.O. Wilson, the pioneer of sociobiology, had a pitcher of water dumped on his head at a AAAS meeting by an activist who charged the stage. But today it's not particularly controversial to argue that human behavior is to some extent influenced by biology. The new research says that friendship should be part of this discussion. And that there are subtle forces in the mix here that we don't consciously perceive -- any more than we pay close attention to those billions of bacteria in our guts.

Read more: Friends are often genetically like fourth cousins