The co-evolution of plants and humans

We think we’re so clever, but perhaps we underestimate plants and our co-evolution.

Edible flora have long evolved ways to move seeds away from their parents to survive and thrive – and humans are just another part of their grand plan, argues Robert Spengler from the Max Planck Institute in Germany.

“Note that if the apple does not fall far from the tree, then the apple seedlings will be overshadowed by the parent tree and not survive,” he says.

“Therefore, the apple tree put extensive amounts of energy into producing high-sugar fruits in order to entice animals to spread the seeds.”

This included the earliest hominids, long before humans started consciously domesticating plants through breeding, Spengler writes in the journal Trends in Plant Science.

A largely theoretical paper, it was inspired by early scholars of evolution such as Darwin and Humboldt – and many of his ideas came to fruition while sitting across from the Schiller Garden House in Jena, where Humboldt famously spend his summers debating similar concepts before conceiving of the cosmos.

“I think the domestication of plants and animals is one of the most important factors in the demographic shifts and cultural changes that have led humanity into the modern world,” he says.

“Therefore, a solid understanding of how this process occurred is essential when studying humanity.”

The manuscript draws from paleontological data to highlight parallels between the evolution of seed-dispersal traits in the wild and domestication traits in the fields of early farmers who started intentionally breeding them.

The phenomenon of parallel evolution also appears in the traits of early domestication across different crop species as humans cultivated and harvested them, producing similar selective pressures – known as “domestication syndrome”.

For example, grass crops such as wheat, barley, rice and oats developed a tough rachis (the plant’s stem that holds the cereal grain to the ear) while legumes, such as peas, lentils and kidney beans, evolved a tough pod.

Winding back to the last Ice Age, Spengler extends his vision beyond these popular crops, noting that megafauna – including humans – were pivotal for spreading wild fruits and enabling them to proliferate.

Bright red cherries, for instance, evolved to attract birds with red-green colour vision who then eat the fruit and drop the seed elsewhere. Larger fruits, unrelated to each other, evolved in parallel to recruit larger animals to disperse their seeds.

Megafaunal mammals may also have facilitated the dispersal of small-seeded grains like quinoa, millet and buckwheat, Spengler posits; the small wild seeds adapted to allow animals to graze on them and pass easily through their digestive systems before evolving larger, thinner coats to enable humans to disperse them more efficiently.

“Humans are powerful seed dispersers,” he says, “and plants will readily evolve new traits to spread their seeds and colonise new areas more successfully.”

Spengler suggests therefore that scholars studying plant domestication need to let go of preconceptions around human intentionality and agency to better understand plant evolution.

“Domestication is not a great human innovation; it is an extension of a natural process.

“By modelling domestication as an equivalent process to evolution in the wild and setting aside the idea of conscious human innovation, we can more effectively study the questions of why and how this process occurred.”