The Three Sisters fundamental to the traditional diet of many American Indian tribes may have been just two without the ingenuity of ancient Indigenous Peoples living in what is now the continental U.S.
An international team of scientists has discovered that squashes and pumpkins—members of the Cucurbita genus—were domesticated here 3,000 to 5,000 years ago. This achievement took place too far back for archaeologists to identify exactly whose ancestors were responsible. “This is before we have really good ideas of the linkages between modern people and specific prehistorical groups on the landscape,” says Logan Kistler, NERC Independent Research Fellow at the University of Warwick in the U.K., and lead investigator.
But investigators are certain that Cucurbita husbandry evolved independently in at least six different regions, including the eastern U.S.
About 10,000 years ago, they report, the megafauna that once dominated the landscape—mammoths and mastodons, for example—died out. The extinctions took place at the end of the last ice age and were probably caused by the combined effects of climate change and human predation. The warming climate ushered in the Holocene Epoch in which we are now living.
The extinction of the Pleistocene Epoch giants should have meant that the squashes also died out. The megafauna had been essential to their survival in two ways—they trampled the ground and other vegetation, creating the disturbed habitat favored by the Cucurbita, and they transported the seeds to new habitats in their dung.
Courtesy Lee Newsom
Cucurbita seeds retrieved in Florida from the dung of a 12,000-year-old Pleistocene-Epoch mastodon.
Smaller animals could not do this. They did not sufficiently disrupt the landscape and the Cucurbita fruit tasted bitter and was toxic to them. By analyzing the genomes of 46 living mammal species for bitter taste receptors, the scientists found that they have fewer receptors for bitter than do smaller mammals, so the huge animals of the Pleistocene would presumably have been less sensitive to the unpleasant taste of squashes and pumpkins.
The Cucurbita had evolved a symbiotic relationship with the Pleistocene megafauna and were dependent on the large mammals for their survival. But all was not lost when the mammoths and mastodons became extinct. Through genomic analysis of extant species of Curcurbita and archaeological evidence (such as seeds preserved in dung) of ancient wild Cucurbita species, the researchers found that the plants developed a symbiotic relationship with humans that allowed them to survive.
Before the wild squashes disappeared altogether, humans began to domesticate them for use as a food source. Through careful experimentation and observation, they were able to create squashes that did not taste bitter or contain toxins.
The disappearance of the megafauna was crucial to this step. It meant there were fewer squash in the wild, greatly reducing the possibility of cross-pollination between wild and domesticated varieties.
“The squashes needed to be a little bit isolated in order to become domesticated. They are very bitter and if there are wild plants pollinating your garden crops, your garden crops are going to produce very bitter fruits that are going to be inedible,” says Kistler.
“So as the population of the wild plants [lessened] when megafaunal mammals disappeared, it seems that was the point at which this sort of reproductive isolation could begin to take effect and the palatable phenotypes that we know and love today could start to evolve,” says Kistler. In the end, many of the wild species disappeared.
In the eastern United States, says Kistler, the Three Sisters was not the first crop complex to be domesticated.
“Maize wasn’t introduced to the area until about 2,000 years ago,” says Kistler. “In this region, the maize-beans-squash [triumvirate] was sort of a late prehistoric phenomenon. Going back quite a bit further, before the introduction of maize and beans from Central America, there was an entirely native crop complex. It was based on chenopodium, which is a relative of quinoa and spinach, sunflowers and a couple other crops including one of these squashes.”
Kistler continues, “And of course maize comes to be a really important player in agriculture all over the Americas, but certainly in the eastern U.S. Before that happened there was already this agricultural complex that was very much shaped by the people who lived in eastern North America. Maize sort of came and latched onto that system rather than the whole idea of agriculture being introduced with maize. That’s not really how it went.”
This research shows how in one instance plants and humans adapted to climate warming, to the benefit of both. But whether such fortunate adaptations will be the rule in the future is an open question, says Kistler.
George Perry, assistant professor of anthropology and biology at Penn State and a co-author of the study, says it is a “good example of how beneficial it is to bring a broader ecological perspective to studying the history of human plant domestication because the plants in the wild environment were changing due to changes in the habitat and to their seed dispersal at the same time.”
The research was published in the Proceedings of the National Academy of Sciences.