Michael: Almost half a million of the people alive today were alive 100 years ago. And they’ve seen a lot: the roaring 20s, the swinging 60s, the Lost Decades, the wars, the art, and the viral cat videos. Our understanding of the world has also changed a ton, just in their lifetimes. Around 100 years ago, humans didn’t know much about genetics and quantum mechanics, or why certain foods and fungi can keep us healthy. But 20th century scientists have made some important discoveries about humans, the Earth, and our entire universe. Here are just 10 of the things we didn’t know a century ago.

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Number One: We didn’t know that there are uncharged particles in atoms, which we call neutrons.

By 1920, we knew that every atom had both negatively-charged electrons, and positively-charged protons, which sit in the nucleus and make up most of its mass. But there was a problem: for example, it didn’t make sense that a helium atom only had two protons because it was as heavy as four. And there wasn’t any proof that some kind of neutral particle existed, one that added mass but not charge. At least, not until the 1930s.

Researchers were experimenting with alpha particles, basically helium nuclei, and firing them at the element beryllium, which released a bunch of different particles. The physicist James Chadwick noticed that these mystery particles had no charge, and they were massive enough to bump out protons from other atoms. By 1932, he was convinced that these were different from electrons and protons, and were the particles the scientific world had been looking for: neutrons.

Number Two: We didn’t know that DNA is in the shape of a double helix.

By the early 1950s, researchers working on DNA were pretty sure it held a lot of genetic information, and was passed down from parents to their children, but they couldn’t really figure out how it worked without knowing its structure. Two scientists named Rosalind Franklin and Maurice Wilkins were experimenting with a process called x-ray diffraction, where they basically shined x-rays onto DNA. These x-rays bounced off the atoms and showed up as patterns on a sheet of film, which revealed a helical structure.

In 1953, these images, along with lots of other research about the chemistry of DNA, ended up in the hands of James Watson and Francis Crick. They pieced it all together to make the double helix model. Knowing this structure inspired lots of new biology research, to figure out how DNA’s chemical code can make something as complicated as us.

Number Three: We didn’t know that there’s a vitamin C... and others.

For a long time, we’ve known that eating, or not eating, certain foods can affect our health. Like, doctors in the mid-1700s realized that eating citrus fruits could prevent scurvy, a disease where people would develop ulcers, anemia, and exhaustion. But they didn’t know what these fruits had in common: vitamin C.

Around a century ago, scientists began studying, isolating, and naming all the vitamins we know today. For example, the biochemist Albert Szent-Györgyi spent the 1920s researching how our cells use food to make energy. And he came across a molecule in the adrenal gland, the hormone-releasers right on top of our kidneys, that sparked his interest: It had 6 carbons, and acted like a sugar and an acid, so he called it hexuronic acid. In the 1930s, he and other researchers ran some experiments on guinea pigs. Without this compound in their diet, these furry friends developed scurvy... sound familiar? So they realized that hexuronic acid was, in fact, the vitamin C that scientists had been trying to isolate for years.

Number Four: We didn’t know how to kill germs and fight off infections.

The idea of killing harmful microbes has been around for a long time. Some ancient cultures treated infected wounds with chunks of moldy bread. But they didn’t know why those treatments worked. The first medicine that could fight off microbial infections, called an antibiotic, was discovered by a scientist named Alexander Fleming in 1928. After coming back from vacation, Fleming was looking at petri dishes with bacteria that could cause nasty infections and sore throats in humans.

And he noticed something strange: there was a little bit of mold at the edge of one dish, with no bacterial colonies growing nearby. Fleming figured out that that this Penicillium notatum mold produced a compound, which he called penicillin, that could kill different kinds of harmful bacteria. For years, many researchers worked to purify penicillin from different molds, test it on humans, and eventually mass-produce the first antibiotic. This so-called “wonder drug” has saved lots of people from life-threatening infections.

Number Five: We didn’t know that the universe was bigger than the Milky Way.

At the start of the 20th century, astronomers knew the universe was filled with things like planets, stars, gases, and dust, but they didn’t fully understand how big it was, o why certain clouds of gas and dust, or nebulae, were spiral-shaped, and appeared to be so far away. This spiraled into the so-called “Great Debate” of astronomy in the 1920s: Some astronomers thought everything in the universe was contained in the Milky Way galaxy, including these spiral nebulae. While others thought the universe was much bigger, and that these nebulae could be entire galaxies on their own.

To try and resolve this argument, Edwin Hubble used the largest telescope in the world at the time, the Hooker telescope, to take pictures of the spiral-shaped Andromeda nebula in 1924. He found stars upon stars, including some Cepheid variable stars, which vary in brightness and can be used to figure out how far away something is. By his calculations, Andromeda was hundreds of thousands of light years away, and almost certainly a galaxy. So our Milky Way is actually just one of many, many galaxies in the universe.

Number Six: We didn’t know that hydrogen is the most abundant element in the universe.

Before 1925, we assumed that the rest of the universe was made of the same kinds of elements in the same proportions as the Earth. However, the astronomer Cecilia Payne-Gaposchkin turned to the sun and observed something very different. Her PhD research was focused on understanding the light emitted and absorbed from stars, which can be used to figure out what elements they’re made of.

She realized that the Sun was mostly made up of hydrogen and helium, and it was different forms of these elements that caused the variations in the light we observed. Her 1925 thesis concluded that hydrogen was about a million times more abundant in stars than we thought, and thus is the most abundant element in the universe.

Number Seven: We didn’t know that the Earth’s crust was made of tectonic plates.

At the start of the 20th century, most geologists believed that the Earth’s crust was pretty much... set in stone. They figured the Earth was once a hot molten blob, and the surface shrank and wrinkled as it cooled to form things like mountain ranges and the oceans. Like, imagine a grape turning into a raisin.

But, in 1915, a scientist named Alfred Wegener came up with the theory of continental drift: that a long time ago the continents all made up a single landmass called Pangaea and slowly shifted apart. Evidence started piling up, like we found similar fossils from the same time periods on completely different continents. But we still couldn’t really explain how the continents moved.

That came in the late 1960s, when geologists connected this theory to ideas about large underwater ridges, which oozed molten rock and gradually pushed the existing seafloor away. With some more research, they concluded that the Earth’s crust was broken up into giant, slow-moving chunks that they called tectonic plates.

Number Eight: We didn’t know that modern humans probably came from ancestors in Africa that migrated across the world.

Throughout the 18th and 19th centuries, nobody really agreed where modern humans came from. And it’s even still debated today. Some scientists believe that different races of modern humans evolved separately, but from a common ancestor that migrated across the world.

While others think a population of modern humans started on one continent and eventually migrated around the world. Starting in the late 1980s, Charles Darwin’s Out of Africa idea became one of the most widely-accepted theories, because researchers started looking for genetic evidence to trace human ancestries. A team of scientists compared human mitochondrial DNA, which is passed on from mothers to their children, and found it was very similar in humans all across the world.

And more researchers keep finding evidence that modern humans are probably descended from one original group in Africa, that grew different as they multiplied and migrated across the Earth over thousands of years.

Number Nine: We didn’t know that CFCs destroy the Earth’s ozone layer.

The ideas of air pollution, burning too many fossil fuels, and global warming have been around for a while, but we didn’t realize how much some man-made compounds could affect our atmosphere. Chlorofluorocarbons, or CFCs, are organic chemicals made of some combination of carbon, hydrogen, chlorine, or fluorine. Throughout the 20th century, CFCs were being produced as coolants in refrigerators and air conditioners, and in aerosol sprays.

But some chemists realized that CFCs were floating up into the stratosphere and being broken apart by the strong UV light from the sun. The CFCs released chlorine that reacted with ozone molecules, and were slowly depleting the ozone layer around the Earth that normally protects us from harmful UV radiation. At first, the researchers’ ideas received a lot of criticism, but in 1975, they won the Nobel Prize in Chemistry for their work. And by 1989, countries around the world began to sign an environmental treaty called the Montreal Protocol, and promised to stop using CFCs in order to protect our planet.

Number 10: We didn’t know just how right Einstein was.

Now, Einstein wasn’t right about everything, but his General Theory of Relativity has really stood the test of time. The basic idea is that the universe is like a huge fabric of space and time, and heavy masses can bend and warp it. When it was first published 101 years ago, there wasn’t much evidence to support his ideas. But, over time, more and more tests kept proving Einstein right.

For example, astronomers noticed that Mercury’s orbit wasn’t really acting the way they had predicted, but Einstein used his own equations to re-calculate the orbit and was way more accurate. Then, there’s gravitational lensing, his prediction that light should be bent by the presence of heavy objects. And Einstein was right again. When we look into the night sky, we see light getting bent around galaxies, making circular streaks in our telescopes. The cherry on top was the discovery of gravitational waves.

Over a hundred years ago, Einstein predicted that there should be small ripples in the fabric of space and time, like when a boat leaves ripples in a lake. And just a few months ago, the LIGO Scientific Collaboration observed them for the first time. These gravitational waves could allow scientists to peer farther out, and further back in the universe, and uncover even more of its secrets.

We’ve made so many discoveries in the past century that have changed how we understand the world, the universe, and our place in it. And scientists are far from done exploring. So who knows what we might learn next?

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