Weedy seeds gathered in once-green Sahara

by Mary Caperton Morton
Tuesday, June 5, 2018

Today, the vast and arid Sahara Desert seems an unlikely place to find early signs of seed gathering and plant cultivation in Africa, but new evidence shows that, 10,000 years ago, people were collecting, sorting and saving seeds near a rock shelter known as Takarkori.

Located in what is now southwest Libya in the central Sahara, the Takarkori rock shelter was occupied first by hunter-gatherers, and then pastoralists, between 11,500 and 8,500 years ago, when this region was more verdant. The archaeological site has been excavated for decades. “Takarkori has a unique archaeological record describing the last 10 millennia in a nearly continuous deposit, with fireplaces, penning structures, burials of human skeletons and decorated pottery,” says Anna Maria Mercuri, a botanist at the University of Modena and Reggio Emilia in Italy and lead author of the new study, published in Nature Plants.

In a four-year-long study at Takarkori spearheaded by archaeologist Savino di Lernia from the Sapienza University of Rome, researchers uncovered millions of artifacts, including more than 200,000 seeds gathered in small circular concentrations. The team ruled out the possibility that the seeds were gathered by ants or other insects, leaving them as “the first unquestionable evidence of early Holocene seed collection in the central Sahara,” Mercuri says. The carefully arranged seed assemblages also represent the earliest documented examples of seed selection, transport, planting, cultivation and harvesting — all behaviors associated with the shift from hunting and gathering to the more pastoral lifestyle that preceded the dawn of agriculture.

Analysis of the seeds showed that they were grouped by type and sorted by size, most likely by sieving through baskets, fragments of which were also found at the site. The researchers also found dried plant remains and microscopic pollen grains; some seeds and fruits were still colorful. “The state of preservation of this rock shelter is outstanding,” Mercuri says. The team extracted ancient DNA from the grains of millets and wild sorghum. The grains “were so well-preserved that they look shiny and seem modern under the microscope,” she says.

Most of the seeds came from wild cereals — a category of grass — that are now extinct in this region. But during the early Holocene, the area sprouted more savannah-like vegetation, including widespread grasses. The wild cereals identified by the team likely grew in clumps near waterways or ponds. Several of the species, including Echinochloa colona, Panicum laetum and Dactyloctenium aegyptium, are still grown elsewhere in Africa today for human consumption.

“In general, grasses are quite edible,” says Regina Baucom, a botanist at the University of Michigan in Ann Arbor, who was not involved in the new study. “The bulk of the grain, or endosperm, consists of carbohydrates for the developing embryo, along with proteins, oils and some vitamins, making it a nutritious food source for humans.”

Today, wild cereals are often dismissed as weeds — in competition with cultivated plants — but weedy traits would have been advantageous to the first farmers, Baucom says. “Most of the species this team found are highly prolific. They grow fast even in adverse soil conditions and produce a lot of seeds.” Wild cereal seeds are also known to have high survival rates over periods of dormancy, so seeds could be saved from one rainy season to the next without germinating or dying.

Previously published paleoclimate studies have shown that the Sahara went through periods of rain and drought. “The wild cereals we identified in Takarkori can adapt very quickly to environmental stress, including climate change and also human interactions, which can also be considered an environmental stress,” Mercuri says. “In times of change, these plants were chosen [for cultivation] because they were weeds.”

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