Ancient societies deliberately grew weeds

Plant domestication, the theory runs, comprises a long history during which humans select traits advantageous to farming practice. Qualities such as seed size, nutritional content, climatic resilience and reproductive reliability ideally become concentrated, resulting in a comparatively small number of widely cultivated plants.

Research by a team of archaeologists and archaeo-botanists, however, has found that these cultivated qualities have not always been viewed as priorities. Indeed, in uncovering the practices of early societies that occupied “the middle ground between farming and foraging”, the scientists discovered that the plant species most encouraged to grow exhibited “traits that overlap a considerable degree with traits that are characteristic of plants now considered as weeds”.

To make their findings, the scientists, led by Anna Maria Mercuri of the Università degli Studi di Modena e Reggio Emilia in Modena, Italy, examined plant remnants found near the Takarkori rock shelter in southwest Libya, all dating to between 7500 and 3500 BCE. At the time the land around the rock shelter was rich and fecund, part of a region today referred to as the “green Sahara”.

During the first part of the period, the rock shelter was inhabited by a culture known as Late Acacus hunter-gatherers. After about 6400 BCE, their place was taken by Saharan Pastoral Neolithic groups, which kept domesticated animals, notably Barbary sheep.

Mercuri and her colleagues uncovered 30 collections of dried seeds, accumulated in successive layers of sand around the shelter and around what used to be a nearby river plain. Their presence in layers allowed accurate dating of when the seeds had been dropped, or stored, and provided, the scientists note, “evidence for the systematic gathering, processing and cultivation of wild cereals”.

Based on their shape, aided in some case by DNA extraction, the recovered seeds fell into 12 genera. Among the species represented were blue signal grass (Brachiaria leersioides), African foxtail grass (Cenchrus ciliaris), crowfoot grass (Dactyloctenium aegyptium), and jungle rice (Echinochloa colona).

The seeds were found arranged in either “spot” or “mix” configurations. Spots comprised primarily a single species, without other bits of plant – such as stems and husks – present. Mercuri and her colleagues suggest these accumulations were the result of threshing and winnowing.

The mixed groups comprised mainly non-seed fragments and their purpose, the scientists note, was “more ambiguous”. They suggest they may represent food processing waste, or detritus deposited by feeding livestock.

And while more research is needed to resolve open questions about how the hunter-gatherers and early pastoralists used plant products, one particular quality about the species they encouraged became startlingly clear. In today’s terms, they would all be called weeds.

The cereals were not bred to be fat and well-behaved, as today’s cultivars are, but quite the opposite.

“The primary quality that attracted hunter-gatherers in Africa seeking the benefits of cultivation — implying a shift of labour investment to just a few food plant species — must have been the invasive and opportunistic behaviour of some wild grasses,” the researchers write.

For these pre-agricultural societies, the name of the game in securing cereals was cultivation without domestication. Preferred plants were encouraged to grow in areas near settlement points. To achieve this, invasive behaviour was a boon. In today’s language, plants that spread like weeds were very good things indeed.

Neither was this type of opportunistic exploitation of wild plants a short-lived fad between the end of nomadic foraging and the start of settled farming. Mercuri and her colleagues present evidence that some species, especially grasses belonging to the genus Urochloa, “were continuously managed for four millennia, with systematic gathering and processing, possibly shifting towards some forms of cultivation more than once”.

The research, published in the journal Nature Plants, provides the first evidence of sophisticated plant management in the Saharan region during this period.

It also provides strong evidence that the domestication of plants was not always geared to isolating and promoting the kind of qualities viewed as optimal by modern farmers. The researchers conclude: “These wild plants were selected for features that were precious in the past but pernicious for agriculture today.”