Look at your hand. Look deep down into all those wrinkles and crevices. There, smaller than your eye can see, are millions of creatures living out their lives and playing a role in everything you do.

They may not be from outer space, but they are more different from us than anything in the animal kingdom. If we want to know what aliens might look like, all we gotta do is look between the cracks in our skin, the walls of our guts or the spaces between our teeth to get a sense of how completely different forms of life might look and act, and how different their worlds might be.

You started as a single cell, and for the first 30 hours, that’s all you were. Then you divided, becoming 2 cells, then 15 hours later 4 cells, and so on. At some point you absorbed your first foreign cell while floating around as an invisible spec inside your mother’s womb. As you grew, more of these tiny creatures found their way inside you.

This first wave of microbiota played a vital role in keeping you alive, allowing you to absorb the nutrients your mom gave you. Over time, their numbers swelled, and new forms of them moved in, settling in and building communities in all the different crevices and organs inside you.

As soon as you left your mom’s womb, you immediately started to encounter ones your mom never had. Waves of these immigrants flooded in, further adding to the biodiversity now inside you, quietly preparing you for life on earth.

There are now more of them in you than there are you in you, and each of us is home to a universe of them unlike anyone else’s. Some of them even live inside your cells, making it hard at times to distinguish ‘them’ from ‘you’.

You could not survive without them. Without a functioning gut microbiome we would starve rather quickly. And without us, many of them would never find the homes they need to live. They are part of you, they influence your likes and dislikes, even the things you think about.

Every piece of food and drop of water you’ve ever had contribute to their biodiversity. Many of them were never able to make a home, discharged through your waste or kicked out by your bodies immune system. Those that survived did so by finding a way to adapt to life inside you, learning how to feast on the food you eat and the waste you create.

But some cause problems. Like those neighbors you never invite for dinner, they can make you sick, tired, or sometimes they just smell funny. Others create bigger problem, unable to get along with the communities inside you, yet too unruly and stubborn to just leave. These caused most of the illnesses and infections you have had over your life.

You can see billions of them every day, half to two-thirds of the waste you produce consists of dead gut bacteria. They are short-lived fellows and masses of them die every minute and clump together in your intestines. Maybe next time when you look down at your waste, give a salute to your fallen comrades. They spent their lives eating the parts of your food you could not digest yourself and turning that food into nutrients you could absorb. Once they have played their part, they are unceremoniously flushed away, never having even been acknowledged by their host.

But perhaps a more fitting salute would be to help their offspring on their way through life by giving them more of the kinds of food they thrive on. Most of the food we know to be good for us is not actually good for us, it’s good for them. A healthy assortment of fats, proteins and carbs, as well all the essential vitamins and nutrients from quality fresh food, gives the ones that help you what they need to live longer happier lives.

(Actually it is a little more complicated than that as what is good for each of us differs because no two people have the same microbiome. But science is making progress and one day your toilet will be able to tell you precisely what you should be eating. In the meantime just ask your doctor.)

We are only beginning to understand these tiny creatures and their role in our health. Yet problems with them are already known to contribute to neurodegeneration, heart disease, cancer, and more. Learning more about them will lead the way to better diagnostics and better treatments for a wide assortment of disorders.

For the longest time we thought we were it, that life was just us and the animals we see. Only relatively recently, when our tools for examining the microscopic world improved, have we understood how wrong we were.

If we do encounter aliens, they are far more likely to be microbial than anything we can see with the naked eye. Or, like the microbes that have been all around us since our beginning, they could be on a scale so radically different from what we know that we don’t even have the tools to detect them.

Written with the help of Prof. Gerold Riempp.