Hank: There's something that's been puzzling scientists for a while. We knew that modern humans originally came from Africa about 200,000 years ago, but we didn't really know when we left.

According to a paper published this week in Nature, teeth found in Fuyan Cave in the Hunan province of China puts prehistoric modern humans, or homo sapiens in southern China more than 80,000 years ago.

That's at least 30,000 years earlier than most scientists thought. Now there were models of human migration that put homo sapiens in China that long ago, but they never really had substantial evidence until now. It's also 30 to 70 thousand years before modern humans reached Europe.

The 47 teeth found in Fuyan Cave came from at least 13 different people, and they're shaped almost exactly like the teeth found in the mouths of modern Chinese people, so these are definitely homo sapiens. And they were found beneath a layer of flowstone that was at least 80,000 years old.

Flowstone is the smooth stone you see inside of old limestone caves that forms into the stalactites and stalagmites that you got sticking out of the floor and ceiling. Since we can measure how long it takes for those things to form by studying the geology of the cave they're found in, we can use them to date whatever's underneath them, like teeth.

A stalagmite in Fuyan Cave that was on the layer above the new fossils was dated as being between 80,000 and 120,000 years old, so the people who left those remains must have died before that. In the same layer of sandy clay under the flowstone where we found the teeth, there were also fossils from a number of different animals that lived during the Upper Pleistocene period 125,000 to 10,000 years ago.

So if these people were contemporaries of those animals (which they were), if they died around the same time and left remains in the same layers of clay, that means that they can't be more than 125,000 years old.

So thanks to these teeth we know that modern humans probably first left Africa around 80,000 years ago and spread to the eastern Mediterranean and southern China. Then, around 40,000 years ago, they went from the eastern Mediterranean to northern China and Europe.

When you think about it, it makes sense. Modern humans originated in a tropical African climate. It would've been a lot easier for them to adapt to the conditions they found in southern Asia than in the northern Mediterranean or Europe, where temperatures are regularly below zero. Expanding north also would have meant competing with the Neanderthals, who were well entrenched in Europe and much hardier in the cold.

But enough about people; let's talk about eels. According to a study in Nature Communications, electric eels use their zap both as a weapon and a tracking device.

We've talked about how that zap works before. Electric eels have long chains of special cells called electrocytes that are all linked to a common nerve. When the eel sends a signal down that nerve, all of the electrocytes open at once, creating a current capable of discharging at around 700 volts. That's enough to stun or kill whatever the eel wants to eat.

We understand how they shock things. What we didn't understand until this study is how they found those things after they shocked them.

Electric eels live in thick, dark, muddy water in the Orinoco River in the southern Amazon Basin in South America. There's very little visibility down there and because of that, electric eels have evolved to have very weak eyesight. They navigate and hunt by sensing the electric fields around them.

But once they've zapped their prey, it kind of just floats where ever the water wants to take it. So they can't see it and they can't sense it; how are they gonna get it into their mouths? What we know now is that electric eels are much more sophisticated hunters than we thought. They don't just zap everything in the water around them and then scoop up the tasty debris.

Instead, they send a small electric pulse through the water, just enough to make a tiny fish twitch. That twitch is their electric field responding to the current and that's something that the eels can detect. Then, as the eels strike, they emit a high-voltage, highly accurate volley of electrical discharges aimed directly at their prey.

So the electricity functions both as a stun and as a homing signal. It's kinda like how echo-locating bats will use high frequency sound to zero in on their prey.

The researchers tested this by taking a spinning disk and putting one conductive rod and lots of non-conductive plastic rods inside it. When the eels sent out an electric pulse, the conductive rod would twitch so that's what it tried to eat. The team found that if an electric eel can't see or smell its prey at all, it will only strike towards prey that responds to that electric signal, and they'll always miss the prey that doesn't.

Turns out "electric eels" are more like "electro-locating homing eel bats with stun guns."

Thanks for watching this episode of SciShow News, and thanks especially to all of our Patreon patrons who make this show possible. Thanks to you, if you're one of those people, we are now working on a new sound-proof studio. If you wanna listen to me complain about the noise outside, we'll just put a little cut of that in right now.

Hank: Oh, you had to stop, huh. Wow.
 
Woman: That's gonna explode or...
 
Hank: I'm a little worried.

Hank: That happens every single episode that we film! Just me complaining about... that. Thanks for watching (laughs).