Iron Age plant remains tell new agricultural story in East Africa

Archaeologists can tell a lot about ancient human life from plant fossils: dietary preferences, farming techniques, and even the sports that were popular. Now, an international team of researchers has discovered 2,300-year-old plant remains that represent the earliest known evidence of plant agriculture in East Africa. This ancient flora is described in a study published July 10 in the journal Proceedings of the Royal Society B and indicate that various crops were likely introduced to the region over time.

(Related: East Africa’s oldest human fossils are older than we thought.)

These discoveries also help fill in some gaps in the history of a region that played a crucial role in the development of agriculture. Despite its importance in human history, little is known about where and how agriculture began in and around present-day Kenya, Tanzania and Uganda.

“This is where human evolution happened,” study co-author Natalie Mueller, an archaeologist and paleoethnobotanist at Washington University in St. Louis, said in a statement. “This is where hunting and gathering were invented by humans at the dawn of time. But there is no archaeological evidence to show what plants hunter-gatherers were eating in this area. If we can get that kind of information from this dataset, then it will be a great contribution.”

9,000 years of human history

As part of this study, the team worked at Kakapel Rockshelter. This rock art site is located north of Lake Victoria, in the foothills of Mount Elgon, near the Kenya-Uganda border. It contains various archaeological objects that provide evidence of more than 9,000 years of human life in the area. It has been recognized as a national monument of Kenya since 2004.

“Kakapel Rock Shelter is one of the only sites in the region where we can observe such a long sequence of occupation by so many diverse communities,” study co-author Steven T. Goldstein, an anthropological archaeologist at the University of Pittsburgh, said in a statement.

Mueller used a flotation technique to separate the remains of wild and domesticated plant species from ashes and other debris in a fire pit dug in Kakapel. Although Mueller has used this method in other parts of the world, it is sometimes difficult to implement in places where water is scarce and has not been as widely used in East Africa.

After separating these plant remains, the team used direct radiocarbon dating to determine when cowpea, or black-eyed cowpea, arrived in the region. They found that cowpea arrived in East Africa about 2,300 years ago, around the same time that people in the region began domesticating livestock. These remains indicate that there was a pattern of gradual introduction of different crops from other parts of the continent.

(Related: Bronze Age nomads used cauldrons for black pudding and yak milk.)

“We discovered a vast array of plants, including many crop remains,” Mueller said. “The past shows a rich history of diverse and flexible agricultural systems in the region, in contrast to modern stereotypes about Africa.”

The Kakapel rock shelter cowpea represents the first documented arrival of a domesticated crop – and potentially of agriculture itself – in East Africa. Cowpea is thought to have originated in West Africa. The team suggests that the plant may have arrived in the Lake Victoria Basin around the same time that Bantu-speaking peoples began migrating from Central Africa.

“Our findings at Kakapel reveal the earliest evidence of domesticated crops in East Africa, reflecting the dynamic interactions between local pastoralists and incoming Bantu farmers,” said Emmanuel Ndiema, study co-author and senior researcher at the National Museums of Kenya.

A pea mystery

The team also detected evidence that grain sorghum arrived from the northeast at least 1,000 years ago and recovered several 1,000-year-old pearl millet seeds. Millet is a cereal indigenous to East Africa and remains an important heritage crop for communities near Kakapel today.

The team discovered an unusual crop: a burnt but intact pea. Peas were not previously thought to be among the earliest agricultural crops in this region. This is the only known evidence of peas in Iron Age East Africa, but it is also a mystery in its own right.

Mueller discovered an unusual crop: the wild pea, burned but perfectly intact. Peas were not previously considered to be among the earliest agricultural crops in this region. CREDIT: Courtesy of Proc. Royal Soc. B

“The classic peas that we eat in North America were domesticated in the Near East. They were cultivated in Egypt and probably ended up in East Africa down the Nile through Sudan, which is probably also how sorghum ended up in East Africa,” Mueller said. “But there is another kind of pea that was domesticated independently in Ethiopia, the Ethiopian pea, and our sample could be either!”

(Related: Ancient agricultural practice could help sustain future humans on Mars.)

Several plant remains could not be identified with certainty, partly due to the lack of reference collections in the region. The team is also working on a separate project to build a comparative collection of Tanzanian plants. A more comprehensive database and the results of this study can be applied to several fields, including plant science and genetics, historical linguistics, African history and domestication studies.

“Our work shows that African agriculture has constantly evolved through migration, the adoption of new crops and the abandonment of others at the local level,” Mueller said. “Prior to European colonialism, flexibility and community-based decision-making were essential for food security – and they still are in many places.”

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