There is a little magic embodied in every bit of bread or cheese and every sip of beer and wine. That magic is microbial and, at least in the case of the bread, beer and wine, the microbe doing the magic is yeast, of a single species, Saccharomyces cerevisiae, what one might reasonably call civilization’s yeast, or at least the yeast at the heart of civilization-its slightly sour lollipop center.

We don’t think about yeast. We buy it in packets at the store. We add water and wait. It is a creature akin to the sea monkeys you can order in the back of magazines, a sort of cheap novelty. Yet, yeast has long been, if not the bread and butter, at least the bread, beer and wine of agricultural peoples (which is to say, nearly all of us). Yeast, in many ways, saved lives (though it also takes them) and may even have been part of the reason we began to farm in the first place.

In a series of six articles published on the Guest Blog, I considered the story of yeast.

On day 1, I began by considering the possibility that alcohol (and within it yeast) may have been one of the reason crops were first planted, in part because in times of contagious disease and polluted water supplies alcohol might have helped keep us healthy.

On day 2, I next considered new research on fruit flies suggesting fruit flies do use alcohol as medicine, just as our ancestors might have.

On day 3, I found that unknown to those studying the fruit flies, it turns out a separate literature has considered whether alcohol kills human pathogens and, in turn, influences our fate, whether historically or even today.

On day 4, I began to consider the wicked side of fungi such as yeast. In thinking about the story of alcohol and yeast, we tend to think about having tamed yeast, in order to meet our needs. But in other stories in nature, for example that of termite balls, animal societies seem more likely to be tricked by fungi than in control of them. Fungi, like humans, can have ill intent.

On day 5, I wrote about the other societies we tend to think of as farming, all of which we tend to say “farm fungi.” But in each of these cases, including our own, it is just as reasonable to say the fungus is farming the animals.

On day 6, I concluded by considering what remains to be understood in the story of fungi, alcohol and humans. In the end, if we are to carve the story of our interactions on the cave wall, the fungi need to be there. They are among our most important partners. They may even be in control.

These articles form part of the bigger story I’ve lost myself in over the last year or so, a story of our interactions with other species. These interactions not only influence us, they make us who we are. Yeast has shaped humans and societies, just as worms, bacteria, lions and many other species have too. Sister Sledge said it best when she sang, “we are family, all my microbes, worms, predators and me.” Well, she did not sing it just like that, but she should have.

In working hard on a story like this one, a story that has required hundreds of hours of work, lost sleep and dreams filled with writhing worms and dividing cells, one begins to wonder what the end is (or when it ends…). I can’t precisely explain why I find these species so fascinating or why I feel I need to write about them. I can’t help myself. But I think there is precedent to my wanderings.

Here, I’ll return to the caves I was able to visit this week, deep caves in which, more than ten thousand years ago our ancestors made art. A hundred years of research has concluded resolutely that it is unclear why the early peoples of Europe and elsewhere painted caves, carved bison sculptures or made small sculptures of voluptuous women. This art was not easy. The artists crawled into caves as deep as they could and painted what was meaningful in one way or another to them. They did so using fire for light. They did so when they could have been running around playing pin the spear-head on the mammoth. Why? Maybe it was magic. Maybe it was religion. Maybe they did not know why they were doing it either. They painted similar subjects again and again, in different forms and from different directions. They painted them because for whatever reason they “needed to." I can’t explain why they painted the caves and yet I get it. The species with which we coexist make us human. They are who we are and so maybe that is what the cave artists were after, images of who they were, a kind of self-portrait a confrontation in those dark corridors with their existence. They were bisons, mammoths, reindeer, lions, and magic. We are those same things and also, bacteria, yeast, wheat and worms and thousands of other species, species I want to record, if not on the cave wall, at least on paper, so we will know who they, and we, are.o they were.

[Image 1: An animal scene from the caves in Lascaux in the Dordogne Valley of France as redrawn in Lascaux inconnu, Leroi-Gourhan Arl. and Allain J.]

Table of evolutionary contents: Here you can skip ahead or backward to the other chapters in the story of the other species in our daily lives, whether they bethe cow, the chicken, the hamster, bacteria (on Lady Gaga, on feet, in bathrooms,as influenced by antimicrobial wipes, as probiotics, in the appendix), pigeons and urban gardens, house sparrows (to be published next week, stay tuned), predators,diseases, dust mites, basement dwellers, lice, field mice, viruses, yeast, the fungus that produces penicillin, bedbugs, houseflies, or something more.

Or for the big picture of how Rob thinks these stories come together to make us who and who we are, check out The Wild Life of Our Bodies. Rob Dunn is a writer and evolutionary biologist in the Department of Biology at North Carolina State University. Find him on twitter at robrdunn. Find him in person somewhere in Europe with his family while they are all on sabbatical.