Humans may have saved pumpkins, squash, and gourds from extinction
When you take a bite of pumpkin pie or caramelized butternut squash this Thanksgiving, you might want to give thanks for those living in the Americas some 10,000 years ago.听
If it weren鈥檛 for ancient American agriculturalists, the precursors to today鈥檚 squash, gourds, and pumpkins probably would have gone extinct, say scientists. And Thanksgiving dinner would look a little less orange.
A new study has found that human domestication saved those plants, Cucurbita, from following large herbivores into extinction across the Americas.
Cucurbita relied on humongous plant eaters, like mastodons, to disperse their seeds across the landscape. Without such assistance, the plants likely , according to new research published Monday in the journal Proceedings of the National Academy of Sciences.听
鈥淧umpkins and squashes are evolved from a plant that is really meant to have megafauna聽on the landscape,鈥� study lead author Logan Kistler tells 海角大神 in an interview. Mastodons and other large herbivores would eat these wild fruits and then deposit the seeds across the landscape in their dung. Then new squash and gourd plants would grow, the megafauna would eat the fruit, and the whole cycle would continue.
鈥淎s megafauna disappeared,鈥� he says, 鈥渢hat had serious effects on the wild populations. But humans, in a way, filled the dispersal role of those disappearing large mammals.鈥�
Humans, 鈥渁 new feature on the landscape,鈥� Dr. Kistler says, picked up where mastodons left off.听
But people probably didn鈥檛 like the fruit very much right away. In the wild, 鈥渢hey are about the size of a baseball, they鈥檙e incredibly tough, and they鈥檙e not edible by any standard that humans would use,鈥� says Kistler. 鈥淭hey鈥檙e unbelievably bitter.鈥澛�
So domestication likely began with choosing the sweeter varieties. 鈥淥bviously the toxic bitterness is one of the first things that would have had to have gone聽away when it became an edible domestic species,鈥� George Perry, another study author, tells the Monitor.听
The giant herbivores likely weren鈥檛 bothered by the bitter fruits.
鈥淲e have mastodon dung deposits that go back 30,000 years that have squash seeds in them,鈥� says Kistler. 鈥淭he fact that we do have the squash seeds in the mastodon dung suggests that they must not have been bothered by the bitterness.鈥�
The researchers investigated DNA sequences of various mammals, looking for a relationship between the ability to taste bitterness and body size. 鈥淭he megafauna, the larger-bodied species, tend not to have as many bitter taste receptor genes,鈥� says Dr. Perry.
Bitter taste serves a purpose, the scientists explain. "As far as we know, we evolved the capability to taste bitter substances to detect and avoid toxicity,鈥� says Kistler.听
The researchers posit that the size of the animal鈥檚 meal can determine its tolerance for bitter toxins. For large animals, like the present-day elephants, a single gourd would be just a tiny portion of its food. 鈥淚t鈥檚 part of several kilograms of food in a particular feeding session,鈥� says Kistler. So, 鈥渕aybe that toxin just isn鈥檛 nearly enough of the biomass to cause a problem.鈥�
But for a smaller animal, a squash might constitute its entire meal.听
Before the scientists compared bitter taste receptors in animals, they dove into the DNA of the squashes themselves. Extracting DNA from modern and ancient, wild and domestic squashes across the Americas, they examined markers indicating where different species lived and evolved.听
The researchers found that domestication happened across multiple locations independently.听
They also found evidence that the range of some wild squashes extended over large expanses. 鈥淭his lineage that appears to have been around the Gulf of Mexico before, now is only in a few spots,鈥� explains Perry. But in those spots, 鈥渢hey鈥檙e highly similar sequences, suggesting there used to be one genus range.鈥�
Between the large herbivores acting as dispersal mutualists and the big animals鈥� disruption of the landscape, the region must have been conducive to the 颁耻肠耻谤产颈迟补鈥檚 vines, the researchers say.听
Although humans made the fruits edible and helped them survive this mass extinction, ultimately it was their relationship with the ancient megafauna聽that gave the gourds, squashes, and pumpkins their start.听
鈥淭he pumpkins in the pie are the product of this long coevolutionary process with things like mastodons,鈥� Kistler says. 鈥淯nderstanding the ecological context of the species that feed us on a daily basis, I think, is worthwhile.鈥�
