A trove of ancient plant remains excavated in Kenya helps explain the history of plant farming in equatorial eastern Africa, a region long thought to be important for early farming but where scant evidence from actual physical crops has been previously uncovered.
In a new study published July 10 in the Proceedings of the Royal Society B, archaeologists from Washington University in St. Louis, the University of Pittsburgh and their colleagues report the largest and most extensively dated archaeobotanical record from interior east Africa.
Up until now, scientists have had virtually no success in gathering ancient plant remains from east Africa and, as a result, have had little idea where and how early plant farming got its start in the large and diverse area comprising Kenya, Tanzania and Uganda.
"There are many narratives about how agriculture began in east Africa, but there's not a lot of direct evidence of the plants themselves," said WashU's Natalie Mueller, an assistant professor of archaeology in Arts & Sciences and co-first author of the new study. The work was conducted at the Kakapel Rockshelter in the Lake Victoria region of Kenya.
"We found a huge assemblage of plants, including a lot of crop remains," Mueller said. "The past shows a rich history of diverse and flexible farming systems in the region, in opposition to modern stereotypes about Africa."
The new research reveals a pattern of gradual introductions of different crops that originated from different parts of Africa.
In particular, the remnants of cowpea discovered at Kakapel rock shelter and directly dated to 2,300 years ago constitute the earliest documented arrival of a domesticated crop — and presumably of farming lifeways — to eastern Africa. Cowpea is assumed to have originated in west Africa and to have arrived in the Lake Victoria basin concurrent with the spread of Bantu-speaking peoples migrating from central Africa, the study authors said.
"Our findings at Kakapel reveal the earliest evidence of domesticated crops in east Africa, reflecting the dynamic interactions between local herders and incoming Bantu-speaking farmers," said Emmanuel Ndiema from the National Museums of Kenya, a project partner. "This study exemplifies National Museums of Kenya's commitment to uncovering the deep historical roots of Kenya's agricultural heritage and fostering an appreciation of how past human adaptations can inform future food security and environmental sustainability."
Constantly changing landscape
Situated north of Lake Victoria, in the foothills of Mount Elgon near the Kenya-Uganda border, Kakapel is a recognized rock art site that contains archaeological artifacts that reflect more than 9,000 years of human occupation in the region. The site has been recognized as a Kenyan national monument since 2004.
"Kakapel Rockshelter is one of the only sites in the region where we can see such a long sequence of occupation by so many diverse communities," said Steven T. Goldstein, an anthropological archaeologist at the University of Pittsburgh (WashU PhD '17), the other first author of this study. "Using our innovative approaches to excavation, we have been uniquely able to detect the arrival of domesticated plants and animals into Kenya and study the impacts of these introductions on local environments, human technology and sociocultural systems."
Mueller first joined Goldstein and National Museums of Kenya to conduct excavations at the Kakapel Rockshelter site in 2018. Their work is ongoing. Mueller is the lead scientist for plant investigations at Kakapel; the Max Planck Institute of Geoanthropology (in Jena, Germany) is another partner on the project.
Mueller used a flotation technique to separate remnants of wild and domesticated plant species from ashes and other debris in a hearth excavated at Kakapel. Although she has used this technique in her research in many other parts of the world, it is sometimes difficult to use this approach in water-scarce locations — so it has not been widely used in east Africa.
The scientists used direct radiocarbon dating on carbonized seeds to document the arrival of cowpea (also known as the black-eyed pea, today an important legume around the world) about 2,300 years ago, at about the same time that people in this area began to use domesticated cattle. Researchers also found evidence that sorghum arrived from the northeast at least 1,000 years ago. They also recovered hundreds of finger millet seeds, dating back to at least 1,000 years ago. This crop is indigenous to eastern Africa and is an important heritage crop for the communities that live near Kakapel today.
One unusual crop that Mueller uncovered was field pea (Pisum), burnt but perfectly intact. Peas were not previously considered to be part of early agriculture in this region. "To our knowledge, this is the only evidence of peas in Iron Age eastern Africa," Mueller said.
The exceptional pea is pictured in the paper, and it represents its own little mystery. "The standard peas that we eat in North America were domesticated in the near east," Mueller said. "They were grown in Egypt and probably ended up in east Africa by traveling down the Nile through Sudan, which is also likely how sorghum ended up in east Africa. But there is another kind of pea that was domesticated independently in Ethiopia called the Abyssinian pea, and our sample could be either one!"
Many of the plant remnants that Mueller and her team found at Kakapel could not be positively identified, Mueller said, because even modern scientists working in Kenya, Tanzania and Uganda today don't have access to a good reference collection of samples of plants from east Africa. (As a separate project, Mueller is currently working on building such a comparative collection of Tanzania's plants.)
"Our work shows that African farming was constantly changing as people migrated, adopted new crops and abandoned others at a local level," Mueller said. "Prior to European colonialism, community-scale flexibility and decision-making was critical for food security — and it still is in many places."
Findings from this study may have implications for many other fields, Mueller said, including historical linguistics, plant science and genetics, African history and domestication studies.
Mueller is continuing to work on identifying the wild plants in the assemblage, especially those from the oldest parts of the site, before the beginning of agriculture. "This is where human evolution occurred," Mueller said. "This is where hunting and gathering was invented by people at the dawn of time. But there has been no archaeological evidence about which plants hunter-gatherers were eating from this region. If we can get that kind of information from this assemblage, then that is a great contribution."
