Humans are one of the most successful species on the planet: We live on frozen continents and arid deserts, create tools that help us survive and even push the boundaries of our biology. Our ingenuity and adaptability serve as our species' superpowers, but the origins of that power may be found in unexpected places — like the armpit sweat of our closest living relatives.
When we hear the term "human microbiome," we tend to think of the thriving community of bacteria that live in our gut. Scientists know that these bacterial communities can influence things from our mood to our mental health. But the microbiome extends far beyond our gut — it also includes the bacteria that live in the stomach, on our skin, and in our armpit sweat glands.
In a new theory, scientists argue that this vast community of microbes tells the story of human evolution in a way we have never thought about it before,.
"Human sociality is a hallmark of human evolution, and for far too long, we’ve been disregarding the impact that the microbiome could have on these particular questions," Aylssa Crittenden, an anthropologist at the University of Nevada, Las Vegas and a study co-author, tells Inverse.
The theory highlights just how deeply the microbiome is tied to human social behavior — one of the defining characteristics of our species, she says.
The theory is detailed in a paper published Wednesday in Frontiers in Ecology and Evolution.
More work is needed to confirm the hypothesis, but the paper provides an intriguing overview of three aspects of humanity's origin story in which the microbiome is a central character.
1. The microbiome advantage
For better or worse, the human gut microbiome — home to about 100 trillion microorganisms — is flexible. Large changes in the gut microbiome, and especially changes that lead to a less diverse cast of characters, have been linked to health problems like mental health issues and obesity. But historically, this flexibility has been a boon for humans, the paper argues.
Where we live and how we eat influence what species are in our microbiomes. Take a study of immigrants from isolated communities in East Asia who immigrated to the United States, for example. Immediately after relocation, their microbiomes started to "westernize," and resemble those of people born in the United States.
This ability to adapt your microbiome may have helped humans get more nutritional value out of diverse food sources at crucial times in our evolution, the new paper posits.
A 2010 study of Japanese individuals highlights this adaptive power: Their microbiomes were better able to extract nutrients from seaweed than North Americans were — as a result, a seaweed-rich diet benefited the Japanese more than it might someone who does not live in a place where eating seaweed is common.
This kind of flexibility would have allowed groups of humans, moving on foot into new areas, to tap into new nutritional sources, and sustain themselves, Crittenden and her colleagues argue.
Those niche adaptations, they say, may have been "critical to the human evolutionary trajectory," and may have helped us to make the most of new surroundings.
2. Armpit ecology
The human armpit is loaded with lymph nodes, hair follicles, and sweat glands that all produce bacteria. We mainly encounter these bacteria via scent — aka, body odor. Turns out that body odor is not just stinky — it is also a powerful social force, the researchers say.
Certain glands in your armpits, called apocrine glands, contain tidbits of information that can be conveyed to a sensitive smeller. Humans are surprisingly capable at distinguishing between individual gorillas based on their individual odors. And within our own species, dating services have been created (and since disappeared, we cannot fathom why) entirely based on smelling a stranger's T-shirt.
"Human sociality is a hallmark of human evolution, and for far too long, we’ve been disregarding the impact that the microbiome could have."
The information coded into these odors may have shaped the way that humans first interacted and formed social groups, Crittenden says.
"Those apocrine glands produce bacteria that signal information to the smeller, perhaps signaling health or even infection," she says.
"We don't know exactly what is being signaled, [but] we know that information is indeed being signaled."
It is unclear how, exactly, this information may have played a role in our past, but it is an example of a biological force that we often underestimate, Crittenden and her colleagues say.
3. Extending the microbiome
At a key moment in humanity's history, we decided to take our relationship with microbes to the next level. We extended the microbiome outside of the body, by fermenting foods and using microbes to our advantage.
Fermenting foods comes with a few obvious advantages, like the ability to store food for later use, stay in one place, or help us cultivate the microbes most useful to us, the authors argue. Over the course of our evolution, humans have cultivated foods with acidic qualities, certain smells, or traits — like alcohol — that add a certain degree of zest to a life dedicated to survival, they say.
Fermentation also comes with a few less obvious advantages that are nonetheless critical to human evolution — like the ability to pass certain kinds of bacteria from one person to another, the authors say.
The use of chicha, a Peruvian fermented drink, is a case in point Crittenden says. The ancestral way of brewing chicha consists of chewing corn and then spitting it out — salivary bacteria begin the fermentation process.
If the subsequent drink is then shared around, it could "inoculate consumers," and help certain types of microbes spread throughout a community, she says. If those microbes helped humans occupy an ecological niche, this could have added an additional advantage, the researchers say.
Chicha is just one example, but ultimately, fermentation forms an under-appreciated process in human evolution in which microbes, social rituals, and survival are deeply intertwined, Crittenden says.
"The use of these technologies to make nutrients more available, associated with a place in our evolutionary history where microbes began to play a central role not only in our biology, but also in terms of our social interactions," she says.
Abstract: The social structure of primates has recently been shown to influence the composition of their microbiomes. What is less clear is how primate microbiomes might in turn influence their social behavior, either in general or with particular reference to hominins. Here we use a comparative approach to understand how microbiomes of hominins have, or might have, changed since the last common ancestor (LCA) of chimpanzees and humans, roughly six million years ago. We focus on microbiomes associated with social evolution, namely those hosted or influenced by stomachs, intestines, armpits, and food fermentation. In doing so, we highlight the potential influence of microbiomes in hominin evolution while also offering a series of hypotheses and questions with regard to evolution of human stomach acidity, the factors structuring gut microbiomes, the functional consequences of changes in armpit ecology, and whether Homo erectus was engaged in fermentation. We conclude by briefly considering the possibility that hominin social behavior was influenced by prosocial microbes whose fitness was favored by social interactions among individual hominins.