The Sunflower Wars
by Amy McDermott
The domesticated sunflower, Helianthus annuus, is an enduring symbol of summer. Its big-as-a-dinner-plate flowers tower on their long, thin stems, exulting the season as we do.
As autumn begins and the last of these golden beauties disappear, we ask where our cheery summer sunflowers came from. It's a more complicated question than you might expect.
Today, the domesticated sunflower is grown worldwide for its oil-rich seeds. With the increasing homogeneity of the global diet, oil crops including sunflowers are rapidly expanding. Endless acres of sunflower stretch across Russia and Ukraine– the largest sunflower producers. Argentina, the European Union, the United States, Turkey, India, and China are all major growers as well.
But for all its ubiquity, the domesticated sunflower's evolutionary origins have not always been clear.
The story begins in 1951, when the leading sunflower expert of the 20th century, Charles Heiser, published evidence of early sunflower domestication in North America. Combined with fossils of several other domesticated plants in the region, Heiser's findings provided strong evidence for an independent origin of agriculture. This was big news in anthropology, and for the rest of the 20th century academic careers were built on the origins of agriculture in the Eastern US.
Then, 15 years ago, a new set of fossilized sunflower seeds was unearthed in San Andrés, Tabasco, Mexico.
At 4000 years old, they predated the oldest US specimens by 1000 years. Loren Rieseberg, a leading sunflower expert in the Department of Botany at the University of British Columbia, was one of the few to examine the Mexican fossils before they were destroyed for carbon-14 dating. Initially, both he and Heiser thought that the specimens were sunflower.
Rieseberg recalls, "4000 years ago, all of a sudden you just, BOOM, have two seeds that are fully domesticated [in the fossil record]. When I first saw them I thought that there had been a separate origin [of sunflower domestication] in Mexico..."
The sunflower world was launched into controversy.
Researchers broke into camps. Some saw the Mexican sunflower fossils as evidence of a single origin of agriculture in Mesoamerica, followed by the northward spread of domesticated crops. But for anthropologists who had built careers studying the independent origin of agriculture in North America, emerging Mesoamerican theories threatened to gut them academically.
To resolve the debate, Rieseberg and a team of researchers from the US, UK, and Mexico turned to genetic analysis of the historical relationships between wild and domesticated sunflower strains.
They found that H. annuus is genetically more similar to indigenous populations in the Eastern US than to native Mexican strains. Based on these findings, the consensus is once again for a single origin of sunflower domestication in the Eastern-Central US, roughly 3000 years ago.
Today, most scholars feel that the controversy was based on a false premise. Sunflower is not the only evidence of agricultural origins in the Eastern US, and even if it hadn't been domesticated in North America, other plants, like squash, goosefoot, and sumpweed certainly were.
What's more, the identity of the fossils found in Southern Mexico has been called into question. When oil-rich plants like sunflower are domesticated from wild stock, they are selectively bred for larger and larger seeds, which produce more and more oil. The relatively small seeds of wild plants evolve into the large seeds of domesticates over generations. So after the first Mexican fossils were reported, researchers expected to find a time series of smaller seeds in the region, but none were ever found.
Although the Mesoamerican specimens looked like sunflower, they may actually have been another well-known Mexican species says Rieseberg, "We now think that they were probably a misidentification... that looked a lot like sunflower but actually were probably bottle gourds. There’s a lot of diversity in bottle gourd seeds, and out of context we thought that they were sunflowers. "
Genetic evidence has laid the sunflower wars to rest. The scientific community has largely moved on, and sunflower remains strong evidence for an independent origin of agriculture in Eastern North America today.
"I think [the controversy] was a worthwhile thing for us," Rieseberg reminisces. "When something’s controversial you tend to sample in much more detail and do more comprehensive studies than if you knew the answer and it was a byproduct of something else you were doing. So I think it was a helpful spur for us to do the job well."
Amy McDermott is a graduate student at Columbia University, where she studies marine conservation biology. She can be reached at firstname.lastname@example.org