Hank Green: So, who has saved the most lives in history?

I could make the case that it was the French chemist Louis Pasteur. It's thanks in part to his work that we understand where germs come from. A lone scientific genius whose experiment saved millions of lives. 

Or at least that's the story we are so often told. But the more complex truth can teach us a lot about how science really works. 

Hi. I'm Hank Green, and this is Crash Course: Scientific Thinking. 

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Now please do not get me wrong, Pasteur was a smart dude and his work really was revolutionary.

There have been plenty of brilliant individual scientists throughout history: Ibn Sina, Marie Curie, George Washington Carver, all of them did some very cool stuff, including making lost of delicious snacks out of peanuts. 

But here's the thing: none of them could have done what they did without the support of the scientific community around them. 

As Isaac Newton himself famously said, "If I have seen further it is by standing on the shoulders of giants."

It is very rare for just one person or one experiment to change our understanding of the world. Behind each breakthrough, there's a community of scientists connected across decades through many experiments, all in the name of scientific progress. 

Like today, germ theory is common knowledge. We know that certain microbes can invade our bodies and make us sick. 

It is hard to overstate how game-changing this knowledge has been. Because of it, we have vaccines, antiseptics, and doctors who wash their hands before surgery. 

And Pasteur often gets credit for developing germ theory. But there's a lot more to it than that. 

By the early 19th century, knew that microbes existed thanks to the microscope. But at the time, many people thought microbes arose spontaneously from rotting food and flesh. Delicious. 

And though that idea of spontaneous generation had a long line of doubters, nobody had convincingly presented evidence that put it to bed. 

So picture this:

It's the 1860s. Just a couple years earlier, Charles Darwin had published a new book about evolution that was reshaping how scientists thought about where life comes from. 

It was a time of shifting world-views. Other scientists had hypothesised that microbes already existed all around us.

They had lived and travelled on microscopic particles in the air, occasionally landing in an environment like spoiled food or an open wound, where they could cause disease. 

Enter Louis Pasteur. 

He waa sceptical of spontaneous generation, and wanted to test this claim that microbes floated in the air. 

He knew that he would have to start with something completely free of microbes. 

So, he heated up a brother that was crawling with them until all the microbes were dead. An early example of sterilisation.

And then, as the broth cooled, he exposed it to the air. Before long, it was a micro pool party once again. 

In his eyes, this was evidence that microbes didn't just arrive spontaneously, that this new idea was right, and that microbes hitch-hiked into the broth through the air. 

But not everyone was convinced. The idea that microbes float in the air just sounded really bizarre. 

One biologist of the time, Félix Pouchet, reasoned that if that were true, "The air in which we live would almost have the density of iron."

And besides, the sceptics said, "The experiment hadn't totally ruled out spontaneous generation. What if fresh air was the thing that makes life burst forth in liquid?"

But listen, Pouchet and the sceptics weren't just being haters. Exactly the opposite. They were being scientists.

Nobody, not even Pasteur, could design and refine such great experiments without challenged and inspired by their peers. This back and forth scepticism and critique is all part of what makes science such an effective tool for building knowledge. 

So Pasteur was like, "Hold my flask."

And he set out to gather more evidence. 

This time, he followed the same steps as before, but he used a swan neck flask, a glass container with a long curvy neck.

After sterilising his broth, he opened it to the air again, only this time, he observed a lack of new microbial growth in the flask.

Air still got in, but since nothing formed, he concluded that the microbes got trapped in the curving neck of the flask, which meant the germs had to be travelling from the air, not arising from the sterile environment of the liquid. 

And then, just to be sure, he tipped the flask sideways so the microbes that had gathered in the neck slid down into the broth. And sure enough, it waa micro city again. 

"But what if in that test you somehow destroyed the life force in the liquid and that's why microbes didn't burst forth?" the sceptics might say.

Well, Pasteur thought of that, too. He iterated with several more types of flasks with the same conclusion. 

Which just tells me that so much of science history has relied on excellent glass blowers. 

Pasteur's work ultimately led people to move away from the idea of spontaneous generation, which paved the way for germ theory as we know it today. And he went down in history for the experiment he designed, cuz it was a great experiment. 

He treated the shape of the flask as an independent variable, a factor that can be changed so its effects can be observed. 

The presence of microbes was a dependant variable, an outcome that's measured to understand its relationship to the independent variable. 

Those are some of the hallmarks of any well-designed science experiment.

But more than that, his experiment presented an elegant, creative approach to controlling each of those variables. The shape of the flask prevented anything heavier than air from touching the sterile broth until Pasteur wanted it to.

Pasteur's work was an example of experimental science, where scientists formulate a question about the natural world and then devise experiments and gather evidence to answer the question. 

A lot of times, these experiments happen in controlled situations within laboratories, but not always, which can make things more challenging for scientists. 

To understand what that means, I think it's time for some sage advice. 

[Sage advice title card]

Sage: Hey, Hank. 

Did you know that Pasteur himself once said, "Everything gets complicated away from the laboratory"?

Hank: He must have been fun at parties, but not wrong. 

Sage: Right.

And sure, Pasteur was able to create a super controlled environment by narrowing his experiment to this tiny little bit of sterilised liquid.

In that way, he was able to isolate so many other factors that could have been affecting the outcome. 

But we can't always do that with all the things we want to study. 

Hank: Right? Like something big. Like the climate of the Earth, for example, hard to put into a laboratory. 

Sage: Totally. We can't generate a mock Earth with all of its complicated overlapping systems and just put it into the lab. 

So instead, much of what we know about climate change is from measuring the stuff that's out there in the real world, like surface temperature, sea level rise, and how much carbon dioxide is in the atmosphere now vs in the past. 

We call observational science, when scientists gather evidence by observing events that have already happened, or would have happened anyway, and then use that evidence to form, test, and refine hypotheses.

As a self-styled naturalist, observational science is my favourite. It's the art of, "Woah, what's going on over there? I got to write that down." 

Hank: Yeah, science isn't always about designing the perfect experiment, because sometimes an experiment isn't even the right method for doing science. 

Sage: But of course, there are other areas where experimental science is used to understand the climate. 

Like, we measured the ways carbon dioxide traps heat in the labs all the time. Something we've been doing ever since Eunice Newton Foote placed glass cylinders of it in direct sunlight and observed how it heated much more quickly than a cylinder with ordinary air. 

And that was all the way back in 1856.

Ultimately, what kind of study works best can depend on a lot of factors. And that's been your Sage advice. 

[Title card]

Hank: Thanks, Sage. 

Another thing scientists have to consider when performing any kind of study is, can I do this ethically? 

Like, when I got cancer, there were people with my same cancer actively enrolled in a study testing out a new treatment.

It would have been unethical for the researchers to give half of those people the new treatment and the other half nothing. They couldn't just let those people die of cancer. 

Instead, they gave the other half the treatment I got, called the standard of care. 

Essentially, they ran an experiment testing the new treatment against the old treatment. 

Observational studies, like the ones Sage and I talked about, are another ethical way of learning more about a person's health.

In these, scientists collect information about each participant's behaviour and their health outcomes. 

And this gets really tricky. The world isn’t set up to be interrogated like a research sample in a lab. 

Say, for example, you'd like to know how drinking alcohol relates to heart disease.

Sounds simple. It is not. 

The relationship between those two things could be influenced by other things. Like, if someone drinks a lot of alcohol, they might also eat way more high calories foods.

These potential side influences are called confounding variables, factors that can distort the true relationship between the things you want to understand. 

Ideally, observational studies will measure and account for any confounding variables like this, but we don't always know what the confounding variables are. 

And that's a big part of why science builds on evidence from many different types of studies. 

Like, when Pasteur performed his first experiments, the scientific community didn't just say, "Oh, well, case closed. Well done, Louis " They pushed for more and more evidence. 

In the end, Pasteur's experiment was a major turning point in science history, but his work was built on a foundation of previous research and refined in its day by the healthy scepticism of his peers. 

Not only that, but scientists continued to build on Pasteur's findings even after he published them. 

People like Robert Koch, who furthered the study of microbes and eventually discovered the bacteria responsible for tuberculosis. I swear. 

And the other Green brother, that was just a coincidence. 

And our understanding disease continues to develop today, not only as a result of well-designed experiments, but also as a result of really smart observational studies and collaboration among the scientific community. 

Next time, we're going to talk about peer review, and why it is a key part of how science helps us know more over time. I'll see you then. 

This episode of Crash Course Scientific Thinking was produced in partnership with HHMI BioInteractive, bringing real science stories to thousands of high school and undergrad life science classrooms. If you're a teacher, visit their website for resources that explore the topics we discussed today in this video. 

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