Domestication History of the Squash Plant (Cucurbita spp)

Squash (genus Cucurbita), including squashes, pumpkins, and gourds, is one of the earliest and most important of plants domesticated in the Americas, along with maize and common bean. The genus includes 12–14 species, at least six of which were domesticated independently in South America, Mesoamerica, and Eastern North America, long before European contact.

Six Main Species

There are six cultivated species of squash, which in part reflect different adaptations to local environments. For example, the figleaf gourd is adapted to cool temperatures and short days; butternut squash is found in the humid tropics, and pumpkins grow in the widest range of environments.

In the table below, the designation cal BP means, roughly, calendar years ago before the present. Data in this table has been assembled from a variety of published scholarly research.

Why Would Anybody Domesticate Gourds?

Wild forms of squashes are harshly bitter to humans and other extant mammals, so bitter that the wild plant is inedible. Interestingly, there is evidence that they were harmless to mastodons, the extinct form of American elephants. Wild squashes carry cucurbitacins, which can be toxic when eaten by smaller-bodied mammals, including humans. Large-bodied mammals would need to ingest a huge amount to have an equivalent dose (75–230 whole fruits at once). When the megafauna died off at the end of the last Ice Age, wild Cucurbita declined. The last mammoths in the Americas died off about 10,000 years ago, around the same time squashes were being domesticated.

Archaeological understanding of the squash domestication process has undergone a considerable rethinking: most domestication processes have been found to have taken centuries if not millennia to complete. In contrast, squash domestication was fairly abrupt. Domestication was likely in part the result of human selection for different traits related to edibility, as well as seed size and rind thickness. It has also been suggested that domestication may have been directed by the practicality of dried gourds as containers or fishing weights.

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Bottle Gourd Domestication and History

By K. Kris Hirst

Bees and Gourds

Evidence suggests that cucurbit ecology is tightly bound up with one of its pollinators, several varieties of an American stingless bee known as Peponapis or the gourd bee. Ecologist Tereza Cristina Giannini and colleagues identified a co-occurrence of specific types of cucurbit with specific types of Peponapis in three distinct geographic clusters. Cluster A is in the Mojave, Sonoran and Chihuahuan deserts (including P. pruinosa); B in the moist forests of the Yucatan peninsula and C in the Sinaloa dry forests.

Peponapis bees may well be crucial to understanding the spread of domesticated squash in the Americas because bees apparently followed the human movement of cultivated squashes into new territories. Entomologist Margarita Lopez-Uribe and colleagues (2016) studied and identified molecular markers of the bee P. pruinosa in bee populations throughout North America. P. pruinosa today prefers the wild host C. foetidissima, but when that is not available, it relies on domesticated host plants, C. pepo, C. moschata and C. maxima, for pollen.

The distribution of these markers suggests that modern squash bee populations are the result of massive range expansion from out of Mesoamerica into the temperate regions of North America. Their findings suggest that the bee colonized eastern NA after C. pepo was domesticated there, the first and only known case of a pollinator's range expanding with the spread of a domesticated plant.

South America

Microbotanical remains from squash plants such as starch grains and phytoliths, as well as macro-botanical remains such as seeds, pedicles, and rinds, have been found representing C. moschata squash and bottle gourd in numerous sites throughout northern South American and Panama by 10,200–7600 cal BP, underlining their probable South American origins earlier than that.

Phytoliths large enough to represent domesticated squash have been found at sites in Ecuador 10,000–7,000 years BP and the Colombian Amazon (9300–8000 BP). Squash seeds of Cucurbita moschata have been recovered from sites in the Nanchoc valley on the lower western slopes of Peru, as were early cotton, peanut, and quinoa. Two squash seeds from the floors of houses were direct-dated, one 10,403–10,163 cal BP and one 8535-8342 cal BP. In the Zaña valley of Peru, C. moschata rinds dated to 10,402-10,253 cal BP, alongside early evidence of cotton, manioc, and coca.

C. ficifolia was discovered in southern coastal Peru at Paloma, dated between 5900-5740 cal BP; other squash evidence that has not been identified to species include Chilca 1, in southern coastal Peru (5400 cal BP and Los Ajos in southeastern Uruguay, 4800–4540 cal BP.

Mesoamerican Squashes

The earliest archaeological evidence for C. pepo squash in Mesoamerica comes from excavations carried out during the 1950s and 1960s in five caves in Mexico: Guilá Naquitz in Oaxaca state, Coxcatlán and San Marco caves in Puebla and Romero’s and Valenzuela’s caves in Tamaulipas.

Pepo squash seeds, fruit rind fragments, and stems have been radiocarbon dated to 10,000 years BP, including both direct-dating of the seeds and indirect dating of the site levels in which they were found. This analysis allowed also to trace the dispersion of the plant between 10,000 and 8,000 years ago from south to north, specifically, from Oaxaca and southwestern Mexico toward Northern Mexico and the southwestern United States.

Xihuatoxtla rock shelter, in tropical Guerrero state, contained phytoliths of what may be C. argyrosperma, in association with radiocarbon-dated levels of 7920+/- 40 RCYBP, indicating that domesticated squash was available between 8990–8610 cal BP.

Eastern North America

In the United States, early evidence of the initial domestication of Pepo squash comes from different sites from the central midwest and the east from Florida to Maine. This was a subspecies of Cucurbita pepo called Cucurbita pepo ovifera and its wild ancestor, the inedible Ozark gourd, is still present in the area. This plant formed part of the dietary complex known as the Eastern North American Neolithic, which also included chenopodium and sunflower.

The earliest use of squash is from the Koster site in Illinois, ca. 8000 years BP; the earliest domesticated squash in the midwest comes from Phillips Spring, Missouri, about 5,000 years ago. 

Selected Sources

• Brown, Cecil H., et al. "The Paleobiolinguistics of the Common Bean (Phaseolus Vulgaris L.)." Ethnobiology Letters 5.12 (2014): 104–15. 
• Giannini, T. C., et al. "Ecological Niche Similarities of Peponapis Bees and Non-Domesticated Cucurbita Species." Ecological Modelling 222.12 (2011): 2011–18. 
• Kates, Heather R., Pamela S. Soltis, and Douglas E. Soltis. "Evolutionary and Domestication History of Cucurbita (Pumpkin and Squash) Species Inferred from 44 Nuclear Loci." Molecular Phylogenetics and Evolution 111 (2017): 98–109. 
• Kistler, Logan, et al. "Gourds and Squashes (Cucurbita Spp.) Adapted to Megafaunal Extinction and Ecological Anachronism through Domestication." Proceedings of the National Academy of Sciences 112.49 (2015): 15107–12. 
• López-Uribe, Margarita M., et al. "Crop Domestication Facilitated Rapid Geographical Expansion of a Specialist Pollinator, the Squash Bee Peponapis Pruinosa." Proceedings of the Royal Society of London B: Biological Sciences 283.1833 (2016). 
• Zheng, Yi-Hong, et al. "Chloroplast Phylogeny of Cucurbita: Evolution of the Domesticated and Wild Species." Journal of Systematics and Evolution 51.3 (2013): 326–34.