Mankind’s most useful plant – Bottle Gourd’s DNA shows how it migrated across the planet

Mankind’s most useful plant – Bottle Gourd’s DNA shows how it migrated across the planet

The bottle gourd, or calabash, was one of the first plants cultivated by humans – not for food, but for use as a container.  Like us, it came from Africa and has accompanied us around the world for thousands of years.

The bottle gourd’s most obvious use is for carrying water, but they were also used as pontoons for rafts in ancient Egypt, as children’s swimming aids by the Romans, as birdhouses by native north Americans, as gunpowder dispensers in revolutionary France, by pipe smokers in Victorian England and more recently as motorcycle helmets in Nigeria.  Gourd is also possibly mankind’s oldest musical instrument resonator.

Gunpowder gourd from revolutionary France
Gunpowder gourd from revolutionary France

It is the only known plant whose use by humans spanned prehistoric cultures across the entire globe, but one thing that has puzzled scientists was how – given its African origins – it came to be so widely used in the Americas thousands of years before the arrival of Columbus.

A genetic study in 2005, led by botanist David Erickson of the Smithsonian Institution, concluded that American gourds originated in Asia, so were most likely carried across the Beringian land-bridge from Siberia to Alaska by Paleoindian migrants around 16,000 years ago.  However, there is little supporting evidence that the bottle gourd was ever used in Siberia or Alaska in the late Pleistocene era (they tended to use animal hides for storage rather than plants).  Also, the growing season is too short and too cold to successfully cultivate a tropical plant species so close to the Arctic.

A new, more advanced, study of the bottle gourd’s DNA, led by molecular anthropologist Logan Kistler of Pennsylvania State University, has reached a different concluion: the gourd’s seeds drifted across the Atlantic Ocean from Africa to the Americas, then took root and grew wild in the New World.  Much later, Paleoindians arrived and started to domesticate them from around 10,000 years ago.

Kora - instrument made from calabash gourd
Kora – instrument made from calabash gourd

The team used modern domestic gourd samples from across every continent including the Pacific islands, as well as nine ancient American samples cultivated and used by humans around 10,000 years ago in ancient settlements stretching from Florida, Kentucky and Arizona to Mexico and Peru.  The team compared their DNA with that of both domestic and (now rare) wild gourds grown in Africa.

They found that the wild African gourd belongs at the base of the bottle gourd family tree.  The tree then splits into two main branches: African domesticated gourds, and Eurasian ones.  Both ancient and modern American gourd samples belong to the African branch of the tree.  If the gourds had been carried across the Beringian land-bridge, they would have matched the Eurasian branch of the tree.

Furthermore, genetic mutations showed that the American gourds shared a common ancestor with African gourds 60,000 to 100,000 years ago – long before humans arrived in the New World.

According to Kistler, this suggests that the seeds of wild African gourds may have washed out to sea from as long ago as 80,000 years.  Computer models of ancient ocean currents show that the seeds could have made the trans-Atlantic voyage to America in less than a year – a short enough period to successfully germinate after making landfall in the welcoming climates of places like Florida, Mexico, and Brazil.

Calabash gourd pipe
Calabash gourd pipe

Not everyone is convinced by Kistler’s findings.  German botanist and leading gourd expert Hanno Schaefer asked why, if wild gourds made it from Africa to the Americas, they don’t grow wild in the New World today?  He said: “I do now believe bottle gourds didn’t come with Asian colonizers, but I still think that what they have is not enough to tell us what happened.”

Kistler acknowledges that the gourd remains “enigmatic” and there is much more to learn about it, but believes that climate change and the extinction of prehistoric megafauna (which may have spread the gourd’s seeds in their dung) may explain why bottle gourds no longer grow wild in America.

National Geographic science writer Carl Zimmer believes that Kistler’s findings could explain how the bottle gourd spread from Africa to Asia and Europe.

All Eurasian and Pacific island gourds share a common ancestor with a domesticated Ethiopian gourd that lived 60,000 years ago – around the time that modern humans started migrating out of East Africa to colonise the rest of the world.  Archaeologists know that they were using ostrich eggs as water containers at that time, but were they using cultivated bottle gourds as well?  They couldn’t take ostriches with them, but they could have carried bottle gourds and their seeds.

Or is it more likely that a wild ancestor of the Ethiopian gourd spread across the sea to the coastlines of the Near East and South Asia, where later migrating humans found them and started to domesticate them?

Nigerian girls carrying large bottle gourds - image by W Robert Moore, National Geographic
Nigerian girls carrying large bottle gourds – image by W Robert Moore, National Geographic

Sources:  Proceedings of the National Academies of Science, National Geographic and USA Today

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