When animals first evolved, they did so in a microbial world. They did so on a planet that had been home to all these organisms for billions of years. This is Ed Yong, he’s written a book about the microorganisms that live inside us and all around us.
. and that pre-date us, by a lot. If we condense all of earth’s history into a single calendar year, then life emerged around about March, and multicellular life, all the organisms that we are familiar with only really came up in October or so.
And humans emerged very, very recently indeed. So we're just the icing on life's cake. From the time humans evolved, bacteria and other microbes would have colonized every nook and cranny of our bodies, just like they had other animals.
And they lived with us like that for some 200,000 years until eventually someone looked close enough to notice. That person was Antony van Leeuwenhoek. So this is the late 17th century in the Netherlands and he is not a trained scientist.
He’s not a scholar. But he has two things that are really important: One is an insatiable curiosity and the other are really really good lenses, which he grinds himself. At that point, microscopes had been around for decades.
In England, Robert Hooke had published a book of observations that was a popular success. It introduced the term “cell” for the pores that he saw in a slice of cork, because they reminded him of the cells of a monastery. But bacterial cells are much smaller than plant cells.
While Hooke was using a compound microscope that could magnify 20-30 times, Leeuwenhoek used single-lens microscopes that could magnify up to 260 times. Though you might not expect that from looking at them. It’s really like two brass rectangles that sandwich this tiny tiny glass lens, this spherical lens between them.
But it basically looks like a door-hinge. The whole thing was just a few inches long and the lens was as small as a grain of rice. And he would have held it really close to his eye.
He would have had to basically press his face up against this thing and look through the lens. It would have been deeply uncomfortable and very straining. To understand why his lens was so small, you have to know how magnifying glasses work.
Like any other lens, they make use of the fact that light bends when it enters glass at an angle. That’s because light moves more slowly through glass than air, and so the portion of the wavefront that enters the glass first will slow down while the other part continues at the faster speed. That’s what makes it change direction, both when it enters and exits the lens.
Now if you put an object close enough to the lens, the bending light creates a virtual image that’s not the real size of the object, but the size it would be if the light from the lens had traveled in a straight line all along. And if the lens is more curved, those lines are even steeper, meaning greater magnification. So Leeuwenhoek’s lenses weren’t just biconvex, they were spherical.
And he knew that he could make that curve even more drastic by shrinking the whole lens down as small as possible, even if it meant he’d have to get his eye uncomfortably close to it. And with that tiny bead of glass, Leeuwenhoek opened a peephole into a whole new world. He looked at his blood and discovered blood cells.
He looked at his semen and discovered sperm. He looked at samples of water and saw algae cells, protozoa, and bacteria. He called them little animals.
He went on to peer into his own dental plaque and found bacteria there too. And he described what he saw in letters to the Royal Society. He's the first person to see bacteria.
He’s the only creature, let alone human, he’s the only organism in the entire history of life on earth to actually see the things that have been the dominant players in life’s history. And I think the great thing is that he wasn’t disgusted by it. He saw tiny living things living in his own mouth and, you know, he thought they were cool.
It would be nearly two more centuries before microbes were linked to disease. And even longer before we understood that our microbes help us digest food, train our immune system, and crowd out harmful bacteria. Now microscopes are much more powerful than Leeuwenhoek’s were, but these days it’s a different tool altogether that’s enabling researchers to explore how microbes affect human health.
Just like Leeuwenhoek found microbes because he had the right tools, now we’re seeing that the microbiome is important because we have the right tools, because we have the ability to sequence the DNA of microbes, to identify them by just scooping up a sample from the environment and looking at their genes. If you were to take all the water out of your poop, 25-50% of what's left is bacteria, both dead and alive. So if you were feeling sad that you can't see the microbes that make a home on your body, Well, it turns out you kinda can!
