I often think about the long and winding road from organic compounds floating in the so-called primordial soup to humans. Lately I’ve been wondering if microbes helped drive the bus.
Even just a few years ago, that would have been a truly ludicrous idea. But thanks to our growing understanding of the human microbiome, it could represent a thrilling example of evolutionary symbiosis that has mutually benefitted humans and their microbial passengers.
Our bodies are made up of many more microbial cells than human cells. Thousands of species of bacteria, fungi, viruses, and other microbes live almost everywhere in and on our bodies, including the digestive system, nose, and skin, to name just a few. Some of the earliest research showed that the microbes that live in our digestive systems help us digest food, make some of the vitamins we need, and balance the immune system.
Since then, we’ve learned that these microbes, collectively called the microbiome, can affect body weight, susceptibility to cancer, and even behavior. The gut microbiome interacts with its host using signaling networks that employ the immune system, hormones, and the nervous system. In short, it has a profound effect on our overall health.
I’ve been studying the microbiome for more than 20 years. My research team at Massachusetts Institute of Technology explores how microbes help keep us healthy. We’ve learned that our daily diet and habits dramatically influence our microbiomes. We’ve specifically studied aspects of wellness in mice (which often make good stand-ins for humans) that are influenced by diet and microbes, including healthy skin, a slender physique, and breeding success across generations. Several findings from our work make me think that microbes helping steer the evolution of humans isn’t such a far-fetched idea.
For starters, in our “glow of health” study, we fed to mice bacteria extracted from human breast milk. This dietary addition gave them thicker skin, more lustrous fur, and, in females, more acidic vaginal mucus. That change in mucus is correlated with increased fertility in mice — and in humans.
Or take the case of oxytocin, sometimes called the love hormone. In humans, oxytocin not only stimulates reproductive behaviors, but also induces childbirth, releases breast milk, bonds babies with their moms, and joins couples in monogamy to share child rearing. Oxytocin promotes nerve growth, fosters creativity in the brain, and serves as glue for complex mammalian social networks that have been integral in evolving social organizations. When fed to mice, certain kinds of bacteria found in human breast milk elicit production of oxytocin in the brain and bloodstream.
Likewise, testosterone levels in mice soar after eating these bacteria. Such microbe-treated mice display larger testicles with higher sperm counts and also build extra muscle. The resulting “mouse swagger” would give these mice a competitive edge in combat and romance, letting them spread their genes and microbes more widely and for a longer time. During bad times, these microbiome-related changes could provide a huge survival advantage for both the host and its microbial allies.
Even thyroid hormone, sometimes called the gas pedal that controls the body’s metabolism and thus body temperature, is influenced by our resident bacteria. It makes sense that heat-loving (thermophilic) bacteria originally dwelling in decaying swamp plants would try to set the body temperature of their new hosts so they could live year-round in total comfort with a competitive edge over other microbial interlopers. This stabilized host environment could then have chaperoned the evolution from external egg laying to internal placental pregnancy. As a bonus for microbes, by increasing mother-infant intimacy, internal pregnancy abets the transfer of microbes from mother to child, and thus the creation of future suitable dwellings for the mother’s microbial descendants.
It turns out that our minuscule microbial manipulators also boost levels of a transcription factor (a protein that helps turn the instructions of DNA into body-building proteins) called Forkhead Box N1. It helps build tissue in the thymus gland that produces specialized immune cells that sustain pregnancy in mammals. Thanks to the exquisitely synchronized immune interactions choreographed by this tissue, the immune system doesn’t swarm and kill sperm cells or the developing fetus. Instead, it opens the door to internal fertilization and lengthy pregnancy while still combating invading bacteria and other pathogens.
Microbe-stimulated Forkhead Box N1 is also involved in the growth of body hair which, along with the production of thyroid hormone, supports the stable body temperature (called endothermy) needed for an extended pregnancy. Forkhead Box N1 is also implicated in the development of mammary glands. It’s just a small stretch to imagine that microbes helped modify sweat glands into lactating breasts in order to create a yummy and nutritious food for human infants and at the same time spread their own microbial sprouts to future generations.
Interestingly, mouse moms consuming probiotic bacteria from human breast milk actually take better care of their infants — and are less likely to eat them — compared with untreated mice or those eating other types of diets. Following this line of reasoning, the bacteria help make more mice and thus more future microbe hosts.
The idea that humans are a kind of deluxe love bus for microbes sounds preposterous, even diabolical. But maybe it’s actually a winner for everyone.
Susan E. Erdman, DVM, is a principal research scientist and assistant director of MIT’s Division of Comparative Medicine.