Iron Age plant remains tell new agricultural story in east Africa

Archeologists can tell quite a bit about ancient human life from plant fossils–dietary preferences, farming techniques, and even which sports were popular. Now, an international team of researchers found 2,300-year-old plant remains that are the earliest known evidence of plant farming in east Africa. These old flora are 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 area over time.

The findings also fill in some gaps in the history of a region that was critical to the development of agriculture. Despite its importance in human history, little is known about where and how plant farming started in and around present day Kenya, Tanzania, and Uganda.

“This is where human evolution occurred,” Natalie Mueller, a study co-author and an archaeologist and paleoethnobotanist at Washington University in St. Louis, said in a statement. “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.”

9,000 years of human history

In the study, the team worked at the Kakapel Rockshelter. This rock art site sits north of Lake Victoria, in the foothills of Mount Elgon near the Kenya-Uganda border. It contains various archaeological artifacts that reflect over 9,000 years of human life in the area. It 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,” study co-author and University of Pittsburgh anthropological archaeologist Steven T. Goldstein said in a statement.

Mueller used a flotation technique to separate the remnants of both wild and domesticated plant species from ashes and other debris in a hearth excavated at Kakapel. While Mueller has used this method in other parts of the world, it is sometimes difficult to do in water-scarce locations and has not been as widely used in east Africa.

After separating these plant remnants, the team used direct radiocarbon dating to determine when the cowpea, or black-eyed pea, arrived in the region. They found that the cowpea arrived in east Africa about 2,300 years ago, at roughly the same time that people in the area began to domesticate cattle. These remains indicate that there was a pattern of gradual introductions of different crops that originated from other parts of the continent.

“We found a huge assemblage of plants, including a lot of crop remains,” said Mueller. “The past shows a rich history of diverse and flexible farming systems in the region, in opposition to modern stereotypes about Africa.”

The cowpea at Kakapel rock shelter represents the earliest documented arrival of a domesticated crop–and potentially farming itself–to eastern Africa. Cowpea is believed to have originated in west Africa. According to the team, the plant may have arrived in the Lake Victoria basin at 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 herders and incoming Bantu-speaking farmers,” Emmanuel Ndiema, a study co-author and a senior research scientist from National Museums of Kenya.

A pea mystery

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

One unusual crop that the team uncovered was a burnt, but intact field pea. Previously, peas were not thought to be part of early agriculture in this region. It represents the only known evidence of peas in Iron Age eastern Africa, but also represents its own mystery.

“The standard peas that we eat in North America were domesticated in the near east. 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,” Mueller said. “But there is another kind of pea that was domesticated independently in Ethiopia called the Abyssinian pea, and our sample could be either one!”

Several of the plant remnants couldn’t be positively identified, partly due to a lack of reference collections in the area. The team is also working on a separate project to build a comparative collection of Tanzania’s plants. A more complete database and the findings 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 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.”