Previous: Some Ciliates Are Hiding a Secret Weapon
Next: The 2024 Microcosmos Calendar



View count:107,085
Last sync:2024-06-16 20:45
Can life be created spontaneously? Well, a year and a half ago, our master of microscopes, James, was inspired by the idea of spontaneous generation and set up his own little experiment.

Follow Journey to the Microcosmos:

Shop The Microcosmos:

Support the Microcosmos:

More from Jam’s Germs:

Hosted by Hank Green:

Music by Andrew Huang:

Journey to the Microcosmos is a Complexly production.
Find out more at

Stock video from:,_founder_of_experimental_biology.jpg,_foto_av_Paul_Nadar,_Crisco_edit.jpg


This video has been dubbed using an artificial voice via to increase accessibility. You can change the audio track language in the Settings menu.
Are you looking to upgrade your microscope?

Well is the place for you! We’ve got a selection of plan achromatic objectives ranging from 4x all the way up to 100x, and these objectives don’t just work on The Microcosmos Microscope.

They will also work on other DIN-standard compound microscopes. Because of the way lenses are made, regular achromatic objectives display a curved image which results in the edges of the field of view being slightly distorted and out of focus. But these plan objectives produce a flatter image across the entire field of view, resulting in an overall sharper image, which is great if, like us, you enjoy capturing pictures and videos of your samples.

So go to if you want to upgrade your objectives or pick up any other Microcosmos merchandise. In 1864, Louis Pasteur stood up in front of a crowd at the Sorbonne Scientific Soiree to deliver an address. And he started by saying: "A number of imposing problems now have our best minds in thrall." Those imposing questions covered everything from the history of humankind to the concept of species, but Pasteur promised the crowd he would not be answering any of those questions.

What Pasteur wanted to address was a question much more fundamental. Can life be created spontaneously? The concept Pasteur wanted to discuss was called spontaneous generation, which posited that perhaps living things could just like appear from some other kind of matter, though there were various ideas about exactly how that could work.

It was an old theory, and it took several century's worth of experiments to prove that it was wrong. Unfortunately, we of course do not have footage of these experiments. But a year and a half ago, our master of microscopes, James, was inspired by the idea of spontaneous generation and set up his own experiment.

He cut a plastic bottle in half, and flipped the top half so he could place it in the bottom half like a funnel and keep it from spilling. Then he placed his contraption outside to see what would show up. The plan was to return a few weeks later, but… he forgot.

And in his defense, he had a lot of other microbes to watch. When James eventually remembered and went to check on it, he found brownish residue and a bunch of dead bugs on the bottom of the bottle. And of course, because he’s James, the first thing he did was put it under the microscope so we could all see the array of microscopic creatures dwelling within.

The question that spontaneous generation sought to answer is how those creatures got into that bottle. And there were several ideas around how life could spontaneously generate. One theory, called abiogenesis, believed that life could arise from nonliving matter, like scallops forming from the sand they’re found in.

Another theory was called heterogenesis, and it suggested that life could form from another species that was living, or that had once been living— like insects that show up out of nowhere on decaying flesh. Aristotle describes both of these theories of spontaneous generation in his treatise The History of Animals, but he was not the first or only person to believe in spontaneous generation. In Pasteur’s speech, he described an old recipe to make mice.

Simply add a dirty shirt to a vessel filled with wheat grains and then wait 21 days. At the end of the waiting period, you will find the wheat transformed into mice. It’s easy enough to laugh at these ideas now.

Like we know how cells work, and that they exist, and how they reproduce and pass along a life encoded in DNA that in turn begets more life. And as we watch the creatures from James’ bottle floating across our screen, we know enough about microbes to know how they got there. Many of them were in the air, making up an invisible universe that found its way into the bottle through rain or wind or some other element.

But if you lived in a world where you did not know that microbes can travel through the air, James’ experiment would be much harder to interpret. Are there really unseen creatures traveling an imperceptible path all around us in the air and the rain Or are they the product of a chemical reaction we just haven’t found the formula to yet? On its own, this bottle can’t find the explanation.

In fact, no single experiment could resolve the spontaneous generation debate. Every experiment performed was in conversation with the experiments and ideas that came before it. And each experiment set the scene for the work that followed.

For example, in the middle of the 17th century, an Italian scientist named Francesco Redi put meat in sealed jars to demonstrate that maggots did not spontaneously generate from decaying flesh. But while his experiment helped people understand that maggots come from very small eggs, other advances complicated matters, including our favorite invention, the microscope. In Pasteur’s words: But at the end of the seventeenth century, a tremendous discovery, that of the microscope, revealed an entire new world to Man, the world of the infinitesimally small.

So while it had, most assuredly, been vanquished as far as the higher creatures were concerned, the doctrine of spontaneous generation now reappeared, audaciously declaring: Here is my province! Microbes are small and often associated with decaying matter, so they provided a new vessel for the ideas of spontaneous generation and heterogenesis. In the middle of the 18th century, one English biologist named John Needham set out to investigate the spontaneous generation of microbes by boiling broth.

The aim was to kill whatever was living in there so that after the broth cooled down and was then sealed, he could compare it to another flask of broth that had not been heated. When Needham found that both the unheated and heated broths turned cloudy, he concluded that microbes must have spontaneously generated in those flasks. How else could they get there?

Well we know the answer, and we are seeing it in action now: there are microbes in the air that found their way into the experiment. But remember, we did not know this at the time. But Lazzaro Spallanzani, an Italian biologist, had an inkling that this could be the case, and that perhaps there were microbes in the air contaminating Needham’s flasks.

So Spallanzani repeated the experiment, but in a way where he could seal the flask before boiling it. And under Spallanzani’s conditions, no microbes appeared. This seems like a blow for spontaneous generation.

But as scientists better understood the importance of oxygen to cellular respiration, some began to wonder if Spallanzani was wrong. What if microbes could spontaneously generate, just not in the air-deprived environment he had created? Scientists went back and forth over whether this was the case, engaging in a dialogue told through different experimental set-ups.

But by the time Pasteur took the stage, none of the experiments before him on either side could fully seal the holes in their argument. Pasteur disagreed with the theory of spontaneous generation, and he decided to test his ideas by building on the same premise that many of his predecessors had used in their experiments: kill off the organisms in a flask and watch to see if anything grows. But he needed to prevent contamination from any organisms potentially flying through the air, while also ensuring that there was air present to nourish organisms if they did manage to spontaneously generate in the broth.

This was kind of a complicated and contradictory set of parameters for an experiment, and Pasteur found an elegant solution. He bent the neck of a flask into an S, creating a swan-like shape. After Pasteur boiled the nutrient broth in the flask and killed the organisms within it, the s-shaped neck allowed air to flow through.

But Pasteur thought the shape would make it hard for particles traveling in the air to get into the broth, trapping microbes before they could get into the flask. And so it was not a surprise to him that the broth remained clear, uncontaminated by microbes. Pasteur however, was not done.

He wanted to see if the microbes he expected to be trapped in the neck of the flask were really there. So he tipped the flask to the side and allowed the broth to reach into the s-shape. When he did that, the broth eventually became cloudy.

He had shown that his broth did not fill with microbes spontaneously. Pasteur gets much of the credit for disproving spontaneous generation, maybe in part because it fed into his understanding of fermentation and the germ theory of disease. And when we lay out the story like this, the path to his discovery seems kind of linear.

One experiment begets another, which begets another, and so on— flask after flask made murky with microbes like the bottle James set up in his backyard. But there are so many figures missing from our telling of this story, like John Tyndall, the Irish physicist who investigated and validated Pasteur’s theories, or Pasteur’s opposition, like the pathologist Henry Charlton Bastian. And there are so many more characters and conversations of varied forms and tones that shaped the way this science was done and the conclusions that were drawn.

Because ultimately, the uncovering of knowledge is a very human affair— a search for beginnings buried in a hidden world, a treasure hunt with many treasures, but no end in sight. Thank you for coming on this journey with us as we explore the unseen world that surrounds us. These folks here are our Patreon patrons.

If you put them in a flask and boil it, I would be so mad at you. How would we make this show without them? It's not like they spontaneously generate.

They are individual people making individual decisions because they want things like this to exist on the Internet. Thank you so much to all of them. And if you would like to see your name here, you can go to where you can also find out about some of the cool perks you can get!

If you want to see more from our Master of Microscopes, James Weiss, you can check out Jam and Germs on Instagram. And if you want to see more from us, there's always a subscribe button somewhere nearby.