For much of the twentieth century, domestication was imagined as relatively rapid.
Perhaps early farmers noticed useful mutations, selected them, replanted their seeds and transformed wild cereals into crops within a few centuries.
Some models suggested even shorter timescales.
Archaeobotany has dramatically changed that picture.
For several major crops, domestication appears to have unfolded across thousands of years.
Archaeological seeds are evolutionary time capsules
Plant remains survive surprisingly well at archaeological sites.
Seeds can become charred during cooking or fires. Pieces of cereal chaff may survive. Plant impressions can remain embedded inside ancient pottery.
These fragments allow archaeobotanists to reconstruct plant populations from different periods.
The paper's Figure 1, on page 4, is especially important. It plots changes in traits such as non-shattering and seed enlargement through time across crops including wheat, barley, rice, pearl millet and sorghum.
Instead of an abrupt leap from wild to domesticated forms, the graphs show gradual evolutionary trajectories.
Barley tells the story clearly
Wild barley disperses its grain by breaking apart when mature.
Domesticated barley retains its grain.
Archaeological barley from early Holocene sites in Syria, Jordan, Israel and western Iran shows changing proportions of these forms.
Before roughly 9000 BC, domesticated-type non-shattering forms were essentially absent.
Over subsequent millennia their proportion increased.
After roughly 7000 BC, some populations contained predominantly or almost entirely domesticated-type forms.
That transition took around two thousand years or more.
Similar patterns occur in wheat.
Rice followed its own long trajectory
Rice spikelet bases from archaeological sites in the Lower Yangtze provide another sequence.
Non-shattering forms gradually increased.
The process had begun before around 6000 BC and appears to have been largely completed before 4000 BC.
Again, we are dealing with millennia rather than a few generations.
Why slow domestication matters
The timescale radically changes explanations for why agriculture began.
If wheat domestication took several thousand years, it cannot easily be explained as a direct response to one short climatic event.
Neither can it be reduced to one cultural innovation or a sudden burst of population pressure.
During three thousand years, climates changed repeatedly.
Human settlements changed.
Harvesting tools changed.
Population density changed.
Cultivation practices changed.
Trade networks changed.
Domestication therefore unfolded while both human societies and environments were moving targets.
Thousands of plant generations
For an annual cereal, three thousand years can mean roughly three thousand generations.
For people it may correspond to around 100 to 150 generations.
Imagine an experiment started by your ancestors more than a hundred human generations ago, modified continuously by changing climates and technologies, and completed by people who had no memory of why it started.
That resembles early domestication more closely than a deliberate breeding programme.
Domestication without a domestication plan
This creates one of the most fascinating ideas in the paper.
The people whose activities generated domesticated crops did not necessarily have “domestication” as their objective.
They harvested.
They sowed.
They cleared.
They moved plants.
They altered soils.
Plants possessing characteristics better suited to those environments gradually left more descendants.
Domestication could therefore emerge from repeated economic behaviour without anyone intending to redesign the species.
Evolution was doing the bookkeeping.
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Further reading: This post is inspired by and draws on Dorian Q. Fuller, Tim Denham, and Robin Allaby, “Plant domestication and agricultural ecologies,” Current Biology 33, no. 11 (2023): R636–R649, doi:10.1016/j.cub.2023.04.038. Read the original article in Current Biology
For a classic and highly readable perspective on why plant domestication arose in some regions but not others, see Jared Diamond, Chapter 8, “Apples or Indians: Why Did Peoples of Some Regions Fail to Domesticate Plants?”, in Guns, Germs, and Steel: The Fates of Human Societies (W. W. Norton, 1997), pp. 131–156. Read “Apples or Indians” View Guns, Germs, and Steel on Google Books
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