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A lot of what we know about ancient human civilizations comes from the things they leave behind, things like pottery, tools, and bones. Except, sometimes, humans don’t live in places long enough to leave these obvious clues.

And even when they do, time can make those remnants unrecognizable. To get around this, some researchers have turned to techniques outside traditional archaeology, to find evidence of ancient humans in less obvious places. For example, a study published in 2020 found that clues might be hidden in stone.

Or more specifically, in stalagmites. Stalagmites are cave formations that form when mineral-rich water drips down from the ceiling. As the water dries, the minerals remain, and over time, these deposits build up into spiky pillars on the cave floor.

In climate science, you’ll hear stalagmites referred to as speleothems. This is a category that includes both stalagmites and stalactites, the pillars that hang from ceilings. But climate scientists focus on stalagmites because they grow in more predictable ways from the ground up.

And they’re pretty good records of a cave’s history. See, the water and minerals it takes to make speleothems are both chemically affected by the local environment. For instance, the water might have different kinds of oxygen atoms in it depending on the temperature and humidity.

So by looking at a pillar’s chemistry and age, scientists can figure out what the area must have been like. Also, because speleothems grow very slowly, they can be millions of years old, allowing researchers to look at changes from way back in time. But these pillars can also hold information from fairly recent events, like early human activity.

A group of researchers found that out when they collected speleothem samples from caves in southwest Turkey. Originally, they intended to use the samples to look at regional climate changes, but when they cut them open, they found something unexpected:. They were threaded through with black, sooty layers, up to a millimeter thick.

And after looking at them under a microscope, they realized that these layers were the result of fire. See, when wood burns, it gives off particles of black charcoal. In the open, the wind might carry them away, but in a cave, these particles would stick to growing stalagmites, getting embedded into the stone for millennia.

Now, natural fires do happen, but these caves also had human remains in them, leading the researchers to believe the soot could have been from human-made fire. And that changed the course of their investigation. Ultimately, the team discovered three notable stalagmites from two caves: Tabak Cave and Kocain Cave.

These structures had particularly well-defined black layers, and both had some other evidence of human activity. The big question was when those people were around. Unfortunately, one popular technique in archeology was of basically no use here: carbon dating.

In that process, you track how a kind of carbon breaks down, and use that to get a sense of how old something is. Except, that requires organic material with carbon in it. So, rocks and minerals can’t be dated this way.

And while soot is technically organic material, the researchers would have needed to completely separate it from the minerals to use carbon dating on it, which would have been impractical. So, the team decided to figure out the age of the minerals themselves, borrowing a technique more often used in geology than archeology: uranium-thorium dating. In the geosciences, this is a common technique for calculating the age of structures like speleothems that contain the mineral calcium carbonate.

And like the name says, it relies on measuring the ratio of two elements: uranium and thorium. Just about all water on Earth has a tiny amount of uranium in it, not enough to be dangerous, but enough to measure. As a result, minerals deposited from water, including calcium carbonate, also have a trace amount of uranium.

But the thing is, uranium is radioactive, and over time it decays:. It breaks down into another element. More specifically, one type of uranium, called uranium-234, decays to a kind of thorium called thorium-230.

This is actually really convenient, though, because thorium is very rarely present in water, so, the only way it usually ends up in calcium carbonates is through uranium decay. The tricky thing is that over time, thorium also breaks down. So, you can’t just directly measure the amount of thorium in a sample to figure out how old it is.

But luckily, both uranium-234 and thorium-230 do decay at a predictable rate, so by looking at the ratio of the two elements, geologists can ultimately figure out when a mineral formed. Using uranium-thorium dating, the researchers discovered that the speleothems f found in Tabak Cave are about 7000 years old. Because of its narrow, steep entrance, uncomfortable temperature, and poor ventilation, they don’t think people lived here, but they do think the cave was likely used as a burial ground or a ritual site.

That’s because they found ceramics and animal bones that might have been left over from funerary feasts, as well human bones in the cave nearby. Meanwhile, the speleothems in Kocain Cave are more recent, dating back to about 500 to 3000 years old. Now, this cave has a wide entrance, so it’s possible that the soot could have been blown in from elsewhere.

But there is also evidence of human activity here, like, it has a cistern that dates back to the Roman era. Either way, the big thing is that researchers found a way to track human history that doesn’t require any other evidence. And in the future, that might really come in handy at Stone Age sites, which are often millions of years old.

In science, there are a lot of techniques that are associated with certain disciplines that aren’t really used outside their field. Speleothem analysis used to be one of those techniques:. It was used in geology and climatology, but not much else.

But every now and then, methods like this get a chance to spread beyond their borders, and as a result, we get to learn more about the world we live in, and the people who came before us. Thanks for watching this episode of SciShow. If you really like getting smarter with us, we’d love for you to consider supporting us on Patreon.

You can get started at patreon.com/scishow. [♪ OUTRO].