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Why Pumpkins And Squashes Aren’t Extinct

The pumpkin shouldn’t exist. Nor should squashes, gourds, and courgettes. These common dinner-table vegetables are all part of a group of plants called Cucurbita, whose wild ancestors were deeply bitter and encased in tough rinds. They depended on large animals like mammoths to break them apart and disperse their seeds. And when these megabeasts went extinct, the squashes should have followed them.

Logan Kistler from Pennsylvania State University thinks that they almost did. Their populations started to decline and fragment. They avoided extinction only by forming a new partnership with the only remaining animal with the right qualities to spread their seeds—us.

When humans domesticated squashes, we inadvertently bred them to be bigger, easier to infiltrate, and much tastier. The wild counterparts are very different, Kistler says. “They’re about a fifth of the size, and with very tough, hard rinds. Cutting them open is quite a task. And they’re extremely bitter. Unpalatably nasty. If you’ve ever grown a garden and had a cucumber that’s a little off and unbelievably bitter, that’s because of the compounds found in the wild counterparts.”

The taste comes from defensive chemicals called cucurbitacins, which are lethal to small mammals that might be tempted to eat the fruits. They’re even strong enough to sicken or kill any sheep or cow desperate enough to have a go. With few exceptions, only the largest of mammals can eat these plants, because their bulky bodies are better able to tolerate or metabolise the poisons. Elephants do so today. And we know their counterparts did so in the past because scientists have found squash seeds in fossilised, 30,000-year-old mastodon dung.

This is all tied into a popular idea, proposed in 1981 by Daniel Janzen and Paul Martin, that the seeds of many New World fruits used to be dispersed by now-extinct megabeasts. These plants, including avocados and chocolate, are “evolutionary anachronisms“, clinging to existence in a world that’s bereft of their ancient partners.

When Kistler heard about the squash seeds in mastodon dung, he wondered if these plants are also ghosts of evolution. To find out, he collected samples of squashes from archaeological sites and compared their DNA to that of modern plants, both wild and domestic.

He found that people domesticated different squash lineages on at least three separate occasions throughout the Americas, starting around 10,000 years ago. Some groups, including the pepo subspecies that includes courgettes and pumpkins, seemingly went extinct in the wild. Others, like the fraterna subspecies, still exist in the wild but only in restricted regions. And yet others, including three wild species, are so similar that they probably represent a single population that only recently split apart.

All of these patterns suggest that the ranges of wild squashes have shrunk, fragmented, and sometimes disappeared over the last ten thousand years, ever since the last giant mammals kicked the dust. Partly, that’s because these animals grazed, browsed, trampled, uprooted, and reshaped the surrounding vegetation, created a diverse mosaic of niches where weedy plants like squashes could thrive. When the megabeasts disappeared, the Americas became more uniform, and the  squashes were squashed out.

They also had a tougher time spreading their seeds. Their old dispersers were gone and the most likely substitutes were small rodents with diverse diets, who could have chiselled through the fruits and carried the seeds to pastures new. But Kistler found that these same animals are the most likely to be put off by the squashes’ bitter tastes. Compared to larger animals like elephants or rhinos, he found that smaller ones like mice and shrews have far more TAS2R genes, which allow them to taste bitter compounds.

That makes sense: little, unfussy eaters would encounter a wide range of toxic chemicals in their food, and be more susceptible to them. It pays for them to detect as many such substances as possible. But it’s bad news for the beleaguered squashes.

Humans can’t stomach cucurbitacins either. People who’ve been unfortunate enough to swallow high levels of these chemicals have come down with a severe diarrhoeal illness called Toxic Squash Syndrome. But perhaps some ancient hunter-gatherers became skilled at finding individual squashes that produced low or tolerable levels. After eating such plants, they would have pooped the seeds out, inadvertently sowing the land with more palatable strains.

The details of this process are unknown, but the results are clear: Squashes found salvation through domestication. “They  formed a symbiotic relationship with humans, who acted as substitute dispersers and provided a growth habitat,” says Kistler. “They also became more palatable, and their rinds became softer.” According to this view, domestication wasn’t something that happened to the squashes, but a process that they were active parts of. It’s not a case of humans exerting  our dominion over nature, but of nature exploiting us in return.

9 thoughts on “Why Pumpkins And Squashes Aren’t Extinct

  1. “But perhaps some ancient hunter-gatherers became skilled at finding individual squashes that produced low or tolerable levels.”

    Could there have been preparation to remove or neutralize the cucurbitacins, kind of like leeching acorns to remove the tanins? That still leaves open the door for selective breeding, but provides a plausible scenario where humans would have started to eat the squashes.

  2. Really interesting article 🙂
    I wonder to what extent this is true for all domesticated plant species? Even those crop species which did not depend on now-extinct-animals for survival had to adapt/be selected for according to certain traits before being used for farming by humans. And even though wild ancestors of most modern crops may still be extant, none of them are anywhere near as successful (ecologically speaking) as their agricultural derivates. In a way, agriculture might simply have been nature’s way of latching onto a highly efficient disperser of seeds.

  3. Is it cucurbitacins that are the “active ingredient” in karela/bitter gourd, because that’s widely eaten by millions of people who don’t seem to suffer any ill effects? Or is the bitterness in those from some other compound, and purely coincidental?

    1. Chris Y asked,
      “Is it cucurbitacins that are the “active ingredient” in karela/bitter gourd, because that’s widely eaten by millions of people who don’t seem to suffer any ill effects?”

      Many people have been seriously ill after eating karela/bitter gourd (or drinking juice made from bitter gourd). At least three people have died from eating it. You can read about it in the India Journal of Medical Research.

      “Three deaths were reported, one from Delhi and two from Uttar Pradesh after consumption of extremely bitter bottle gourd juice. Three persons who died after consumption of freshly prepared bottle gourd juice or juice mixed with bitter gourd (karela) juice were over 59 years of age and had diabetes since last 20 years. This juice was reported to be extremely bitter by all three. Twenty six persons were admitted to various hospitals of the country on complaint of abdominal pain and vomiting following consumption of freshly prepared bottle gourd juice. Diarrhoea and vomiting of blood (haematemesis) was reported in 18 (69.2%) and 19 (73.1%) patients, respectively. Biochemical investigations revealed elevated levels of liver enzymes. More than 50 per cent patients had hypotension. Endoscopic findings showed profusely bleeding stomach with excessive ulceration seen in distal oesophagus, stomach and duodenum in most of the cases. All these patients recovered fully and no sequeale was recorded for any of the cases.”

      More details at:
      http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3307184/

      1. Thank you. I wasn’t aware that such serious reactions were known. Nevertheless, it’s a trivial number compared to the millions of people who eat the things regularly – surely fewer proportionately than the instances of serious illness from recognised allergies. Karela appear quite frequently on restaurant menus in the west, and I can’t help feeling that if they were regarded as seriously toxic, health and safety authorities would have intervened long ago.

        1. The aforementioned article in the Indian Journal of Medical Research mentioned a commonality among the fatalities after consuming keral/bitter gourd:

          “The juice was extremely bitter as compared to other days.”

          “The Committee noted that the common finding was that the juice consumed on the day of the incident was unusually bitter. “

          The implication is that certain varieties (or rare cultivars) of kerala/bitter gourd contain much more cucurbitacins than the typical bitter gourds. The fatalities and sicknesses may be attributable to the consumption of those rare variants of bitter gourd which contain unusually high concentrations of cucurbitacins. These rare variants may occur randomly among populations of bitter gourd. Or it may be a reliably heritable trait that has not been traced to a particular field. More research is needed to determine why certain bitter gourds contain clinically harmful concentrations of cucurbitacins.

  4. Maybe maybe not .As every farmer knows white tail deer love pumpkins and curcurbits of all kinds and have been around for milleniums .They are perfectly capable of spreading the seeds probably better than humans

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