The God of Summer: The Wild Truth About Watermelon

Hi everyone, welcome to Cold-Knowledge Blind Box. This summer has been brutally hot, so this time we're skipping the miracle medicines and talking about the most common summer fruit: the watermelon. It's a giant fruit everyone loves in summer, and everyone knows it. When summer arrives and you bite into a slice, nothing cools you down and quenches your thirst quite like it, right?

So we should really get to know it properly first. Watermelon's origins and history are full of interesting stories. And as a fruit, it's actually quite unusual. Once you've learned about it, you might feel differently the next time you eat one. Give it a try.

In biological classification, the watermelon is a eudicot in the order Cucurbitales, family Cucurbitaceae, genus Citrullus. That makes it a relative of the gourd.

A lot of the melons you know belong to this family: winter melon, watermelon, pumpkin, cucumber, luffa, and muskmelon are all Cucurbitaceae.

At this point some listeners might be wondering how these melons got their names. Why is the watermelon called "xigua" (西瓜, "western melon") in Chinese? The reasoning is pretty casual, and it works the same way as the name for pumpkin, "nangua" (南瓜, "southern melon"). Neither was originally native to China. Nangua came in from the south, so it's called "southern melon." Xigua came in from the west, from Central Asia, so it's called "western melon." That's really all there is to it.

The English name for watermelon is "watermelon," which translates literally as "water melon," and that's a very fitting name, right? It refers to its extremely high water content. About 92% of a watermelon is water. Compared with other fruits, it's much juicier, and there's no way to eat a watermelon without dripping. Everyone has experienced that. That's also why it's so refreshing, and why it's called watermelon.

The Japanese name is the most fun of all: "スイカ" (suika). The word actually came from Chinese. It's a phonetic borrowing of "xigua," imitating how "xigua" was pronounced in certain southern Chinese dialects. And here's the coincidence: in Japanese, that sound can also be understood as "water melon" (水瓜), which happens to match the English name, watermelon. So it's a double meaning, a pun that just happened to work out.

Now, the watermelon's love of water is also directly connected to where it came from. Let's keep going.

The watermelon's birthplace is Africa. Africa has many wild watermelon species in the genus Citrullus. They mostly grow in arid regions, in sandy soil. Some look very much like modern watermelons, though they're usually much smaller, the kind you could hold in one hand. The color and stripes? One look and you'd know it's a watermelon.

But once you cut one of these wild watermelons open, it's a completely different story. The flesh inside isn't red at all. It's mostly white or pale green. Some are very hard, others are soft like a sponge, and they taste bitter and astringent. Everything about them says: this is not something you can eat directly.

So this is puzzling, right? Why would anyone ever think of growing this thing? It's hard to imagine.

A similar question came up in our corn episode. Corn's ancestor also looked inedible, as hard as tiny pebbles. Scientists guess that it was burned, popped like popcorn, and that's when the Indigenous peoples of the Americas noticed it and discovered its value.

So what was the potential value of the wild watermelon? Its water.

In the Kalahari Desert in southwestern Africa, there's a wild watermelon called "tsamma." During the dry season, local Indigenous people treated these melons as a water source, like little water bottles, very convenient to carry. Later, besides collecting the wild ones, they also began growing some themselves as a water reserve, which was vital for surviving the dry season.

Of course, storing water is also the plant's own survival strategy. In the rainy season, it soaks up as much water as it can and stores it in its flesh. In the dry season, it uses that water to attract all kinds of animals. Once the animals eat it, they help spread its seeds. That's its purpose.

So this trait is especially useful in dry regions. Humans and animals alike love watermelon. The likely reason humans first started cultivating and breeding it is precisely this water. That's the hypothesis, at least.

Now, to be more specific: the genus Citrullus has seven living species. So which of them is the wild ancestor of the modern watermelon? To understand the origin of the watermelon, we have to start with ancient Egyptian tombs more than 4,000 years old.

When did humans first start growing watermelons? The earliest clear record comes from ancient Egypt. The Egyptians had domesticated the watermelon at least 4,000 years ago. Watermelons in their tomb paintings already look a lot like today's: elongated, unlike the round wild ones. That suggests they were cultivated by people.

Then the next question: which wild watermelon did the ancient Egyptians domesticate? That puzzled scientists for a long time. Several wild species could have been candidates.

Here's another example. In West Africa, people also grew a wild watermelon called the "egusi melon." Locals didn't use it for water, and they didn't eat its flesh as fruit. They grew it for its seeds, which are very numerous and very nutritious, rich in fat and protein. West Africans make a traditional dish from them called egusi soup. People used to wonder whether the egusi melon might be the ancestor of the modern watermelon, but that was only speculation.

Then, finally, in 2019, scientists found a clue.

In ancient Egyptian belief, death was not the end. The dead went on to the heavens. So their tombs were stocked with the essentials for the journey: bread, honey, meat, and also clothes, money, even pets, as grave goods.

And what about drinking water along the way? That's right: watermelon.

The ancient Egyptians placed watermelons in tombs so the dead would have something to drink on the road. This applied to pharaohs too. In Tutankhamun's tomb, for example, watermelon seeds and paintings of watermelons were found, which shows that watermelons were already very common in ancient Egypt.

So the clue scientists found comes from an ancient Egyptian tomb about 3,500 years old. When that tomb was excavated, fragments of watermelon leaves were found on the mummies. Archaeologists preserved them.

In 2019, a team in Germany studied these fragments and found they were so well preserved that high-quality DNA could still be extracted. Genetic analysis showed that this watermelon was closely related to a watermelon subspecies from Sudan called the "Kordofan melon."

But because Sudan has been unstable and at war for a long time, finding the wild ancestor of the watermelon would have to wait.

Beyond the watermelon fragments, scientists also found two very important gene mutations.

One is a mutation in the gene that produces cucurbitacin. Why are wild watermelons bitter? Because they contain cucurbitacin, the source of the watermelon's bitterness. This mutation shows that by that time, watermelons had already stopped being bitter.

The other is a mutation in the gene that produces lycopene. Lycopene, also known as tomato red, is what makes tomatoes red, and it's also the source of the watermelon's red color. The mutation in the lycopene gene shows that by that time, watermelon flesh was already red.

Some studies have also found that the gene that makes watermelon sweet is linked to the lycopene gene. That means the watermelons ancient Egyptians ate 3,500 years ago may already have been quite delicious, not just a source of water.

Making watermelon tasty also set the stage for it to spread around the world. If it had only been good for storing water, other places might not have needed it at all. So the delicious watermelons we can eat today owe a great deal to the foundation the ancient Egyptians laid.

Since ancient Egypt, watermelons have been bred around the world for thousands of years, and today there are over a thousand varieties. The flesh isn't always red, and the shape isn't always round. You can find all kinds.

And since the watermelon is the representative of giant fruits, naturally a lot of people are passionate about one thing: growing them as big as possible.

The current Guinness World Record for the largest watermelon was grown in 2013 by a farmer in Tennessee, USA. It weighed an astonishing 159 kg. That's ridiculous, and who knows how they ate it.

Some are big, and some are expensive. Certain watermelon varieties are considered premium. For example, in Japan there's a watermelon called Densuke, a black-skinned variety said to be extraordinarily delicious. Before it goes on sale, it has to be polished and buffed. That's how particular they are.

And how expensive is it? In 2008, a Densuke watermelon sold at auction for 650,000 yen. Pretty eye-opening.

Some people find the seeds annoying when they eat watermelon and wish it had none. So seedless watermelons appeared on the market.

Seedless watermelons were first invented by the Japanese around 1939. The method was to treat the seeds or seedlings of an ordinary diploid watermelon with an alkaloid called colchicine, turning them into tetraploid plants. Then they crossed these tetraploids with diploids to produce triploids. Triploids can't form seeds, so they became seedless watermelons. That's the principle.

Although the watermelon is just an ordinary fruit, some people even turned it into a tool for discrimination.

The cause goes back to the American Civil War we talked about in the last episode, on cotton. After slavery was abolished, many Black Americans no longer worked as enslaved laborers on cotton plantations. They started growing and selling watermelons to make a living.

The plantation owners, who could no longer enslave them, were resentful. So they spread propaganda to mock and ridicule Black people, claiming they were lazy and satisfied with just a slice of watermelon. In short, it was an extremely arrogant display.

As a result, some people began using watermelon eating to discriminate against Black people and to show off their own sense of superiority. Even now, some still bring it up, which is really absurd.

Old jokes more than a hundred years old really should have been retired long ago. Alright, that's it for this episode of Cold-Knowledge Blind Box. Thanks for listening. See you next time.

举