Pumpkins, gourds, and other Thanksgiving motifs very nearly didn’t survive to become part of the modern American autumn. The extinction of ice age megafauna several millennia ago killed off mastodons and other herbivores that helped disperse the seeds of these fall staples and other members of the genus Cucurbita. They might have gone extinct were it not for the help of human cultivation, according to new findings published in Proceedings of the National Academy of Sciences this week. Humans domesticated Cucurbita multiple times beginning around 10,000 years ago.
The wild, unpalatably bitter precursors of the domestic squashes we enjoy today were well adapted for landscapes inhabited by large mammals who were undeterred by their bitterness. Mastodon dung deposits, for example, have been known to contain intact Cucurbita seeds. But these days, nearly all herbivores weighing more than 1,000 kilograms (2,205 pounds) have disappeared from the Americas, and wild Cucurbita are rare. Meanwhile, pumpkins, gourds, and squashes flourish in our patches, gardens, and farms.
A team led by University of Warwick’s Logan Kistler conducted a genetic analysis of 91 ancient, modern wild, and modern domestic Cucurbita samples. They discovered that humans domesticated the plants on several independent occasions throughout the Americas. Zucchinis, pumpkins, and summer and winter squashes were domesticated in the Oaxaca Valley region around 10,000 years ago, for example, while the domestication of scallop and acorn squashes took place in eastern North America. The team also found a previously unknown pathway to Cucurbita domestication in northeastern Mexico.
Many ancient wild varieties that aren't found nowadays were widespread before the extinction of large herbivores, and the decline of these wild Cucurbita coincided with the rise of cultivated ones.
So, to test whether the disappearance of large herbivores struck a critical blow against wild Cucurbita, the team screened 46 mammal genomes for genes that code for bitter-taste receptors. Smaller mammals – the ones who survived the environmental upheaval that accompanied the beginning of the Holocene – have more bitter-taste genes than the now-extinct megafauna. While small mammals with diverse diets were able to detect and avoid those bitter fruits, mastodons and other massive plant-eaters chewed them up, tolerated their moderate toxins, and then spread their seeds out through poop.
Without their seed-dispersing partners, the weedy plants would have been crowded out. They also preferred disturbed habitats, such as those created by heavy stomping and trampling. Many went extinct, and the team thinks that without their new human partners, some Cucurbita may not have survived.
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