Watermelon was bitter, perhaps even difficult to swallow 6,000 years ago

Watermelons come from the ancient continent of Africa. As early as about 4300 years ago, on the murals of ancient Egyptian tombs, an oval green object with striped patterns appeared, much like the familiar fruit, watermelon. Moreover, besides this suspected watermelon, there are also paintings of grapes and other fruits. Such a scene makes it easy to believe that people at that time ate watermelon to enjoy its sweet taste.

But scientists know that watermelons haven't always been sweet. Rather than saying that sweet watermelon is a gift from nature, it is better to say that it is the beautiful result of years of artificial domestication. Before domestication, the pulp of wild species of the watermelon genus was often bitter, even unpalatable. Many researchers are exploring what kind of unpalatable melon is and how it evolved into today's delicious watermelon.

On this road, there is still a very important question that needs to be answered: If watermelons are originally bitter, why did humans start to grow watermelons and domesticate watermelons? Recently, a group of scientists found an answer when they looked at 6,000-year-old watermelon seeds.

Can you tell what it smells like just by looking at the seeds?

The watermelon (Citrullus lanatus) we eat today is just one species in the genus Citrullus. It also has a few relatives with bitter flesh, from a more distant time and space. In the 1950s, archaeologists began excavating (next to Egypt) a site in Libya called Uan Muhuggiag. In the decades since scientists have found the seeds of many plants at this site, and the oldest watermelon seeds ever found also come from here-according to the results of carbon 14 dating, they are more than 6000 years old in history.

These Neolithic seeds are important clues to deciphering the ancestry of watermelons. In addition, the researchers also obtained another set of watermelon seeds from Sudan (a country in northeastern Africa), which were about 3,300 years ago. There is little difference in appearance between the different species of the watermelon genus, scientists say. To confirm the seed-to-seed relationship, they needed to use genome sequencing to find their similarities and differences.

Of course, melon seeds that are thousands of years old are not enough. To know where the ancestors of modern domesticated watermelons are, it is natural to compare them with younger samples. Therefore, the research team found 47 watermelons (Citrullus lanatus) samples made between 1824 and 2019 from the herbarium of the Royal Botanic Gardens (Kew), and collected watermelons that had been sequenced by previous studies from various countries. Genomes of various species of the genus in order to find the relationship between old melons to new ones.

The results of genome sequencing told scientists that the melons corresponding to the group of Libyan seeds 6,000 years ago may have green and white flesh and taste bitter. The reason why it is speculated to be green and white flesh is because of a gene called LYCB, which has a V226F mutation in the red flesh watermelon we eat, but there is no such mutation in ancient Libyan seeds. The reason why it is speculated that there is a bitter taste is that a bitter taste regulator gene ClBT, the sweet watermelon we eat now, has the non-bitter allele of this gene, while Libyan seeds carry the bitter allele.

In the Sudanese seeds 3,300 years ago, the researchers did not find the DNA fragments corresponding to these two genes, and there was no way to judge whether the flesh of the melon was sweet or red. If scientists want to understand how the two groups of ancient melon seeds in Libya and Sudan are related to modern watermelon species and find out why humans domesticated watermelons, they have to dig for more clues from the genome.

It is so bitter, why do humans plant it?

When comparing the genomes, scientists found that the group of Libyan melon seeds 6,000 years ago was closer to the sticky-seeded watermelon (Citrullus mucosospermus) that grows in West Africa today. Sticky-seeded watermelon also has bitter pulp, which is inedible, and people today grow it, not to eat it as fruit, but more often to use the seeds as a snack or stew them in soup.

So, would the ancients also eat the seeds of bitter watermelon? These Libyan melon seeds were found in a Neolithic human settlement. The researchers found typical traces on some of the seeds, which are very similar to the traces of modern watermelon seeds bitten by human teeth.

UMB-6 represents the 6,000-year-old group of seeds in Libya (Image source: original paper)

This makes scientists believe that humans 6,000 years ago also had the habit of eating melon seeds. People at that time lived a life of hunting and gathering. After a tiring day, they might return to the cave to rest, sit around and chat and crack sunflower seeds. If it is not for eating melons, but for eating melon seeds, then it is not difficult to understand that the bitter watermelons in ancient times would be collected and cultivated by humans.

The domestication of watermelons may have been driven by eating melon seeds in the first place, a finding that surprised scientists. The researchers also found that many living species can be found in the genome of Libyan seeds. The genes of the sticky-seeded watermelon just mentioned, there are also some sub-species of the watermelon (Citrullus lanatus) we often eat. species, and the Amaru watermelon (Citrullus amarus) from South Africa, among others.

Scientists say there is a clear genetic introgression between Libyan seeds and modern domesticated watermelons (Citrullus lanatus). The so-called gene introgression refers to the flow of genes between two gene pools, usually resulting from interspecific hybridization. Then, the Libyan seeds may not be the wild ancestors of modern domesticated watermelons, and their flesh may not taste good, but they should have been domesticated by humans.