Category: Domestication

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Crops made, and remade

Two new studies of very different crops – banana and chrysanthemum, of all things – end up telling surprisingly similar, deliciously complicated stories. For these crops, domestication was not once and done, so to speak. They were both repeatedly remade as people moved them through landscapes containing new wild diversity.

In the banana study, the authors suggest that a partly domesticated Musa acuminata lineage from New Guinea was carried westwards through Southeast Asia, encountering and hybridizing again and again with different local wild bananas along the way. Each encounter added new genetic material to an already changing crop, helping produce the genomic mosaics found in mainland Southeast Asian bananas today.

The chrysanthemum study reveals a strikingly parallel history: cultivated plants originating in China were introduced to Japan, where they encountered local wild populations and acquired new genetic diversity, before later movements to Europe and further breeding reshaped the crop again.

The papers also show that an essentially similar process played out somewhat differently in the two crops. In banana, repeated hybridization appears to have been important in the building of the crop itself, as domesticated or partly domesticated plants became the starting material for successive encounters with wild Musa. In chrysanthemum, genomic analysis reveals a more complex network of relationships among multiple wild and cultivated groups, with C. indicum among the important ancestral contributors. In this genetic cauldron, hybridization and introgression repeatedly diversified an established cultivated genepool, contributing to traits such as flower form, colour and plant architecture.

Taken together, these papers challenge the familiar “funnel,” or bottleneck, image of domestication: a one-way downward slide from diverse wild relative to genetically narrow crop. Instead, they point to a more stop-start, two-way, non-linear, geographically contingent process, in which (semi-)cultivated plants continue to encounter, absorb and be reshaped by wild diversity.

In both banana and chrysanthemum — and probably many other cases — wild relatives have been more active, continuous participants in creating the diversity of the crops we know today than we sometimes give them credit for. Or I have given them credit for at any rate.

That argues for treating the wild genepool not simply as a reservoir from which to fish out useful genes one at a time. The historical evidence suggests that crops have benefited in the past from repeatedly absorbing larger chunks of wild genetic diversity, allowing selection to reconstruct useful combinations. Might it be worth trying to make that happen again? Is anyone out there doing pre-breeding explicitly with an eye to the past?

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The fruits of migration

As chance would have it, following my recent speculative foray into the possible connection between language and crop diversity, an essay by the great Andrea Wulf 1 has just appeared that tells the story of how a remarkable German naturalist used breadfruit and linguistics to work out Polynesian migration patterns long before DNA or archaeology could confirm them.

George Forster sailed with his father aboard the HMS Resolution on Cook’s second voyage (1772-1775) and encountered breadfruit across the South Pacific. He noticed that the trees were seedless and could only be propagated by root cuttings. And also that there didn’t seem to be wild breadfruit trees anywhere in the islands. That meant every tree he saw, and there were many of them, had been deliberately planted by people.

Digging through the university library back in Göttingen, Forster later found that wild, seeded breadfruit with little flesh do exist, but in Southeast Asia. If the Pacific islands only had the improved, seedless types, he reasoned, they must have been carried there by people. That was proof both of sophisticated Polynesian plant management and of a migration route running from Southeast Asia eastward across the Pacific.

Forster combined this with careful attention to language. He deduced that most Pacific languages he encountered were dialects of a single root language, with small, systematic sound shifts. He and his father also charted how the presence of Malay loanwords decreased the farther east an island was located, which suggested that islands closer to Southeast Asia — the home of Malay — were settled earlier.

Where his fellow naturalist Joseph Banks saw the breadfruit purely as an economic resource (later lobbying to transplant it to the West Indies to feed enslaved plantation workers, hence HMS Bounty and all that), Forster saw it as historical and ethnographic evidence. That’s a strikingly different, more humanistic and — to me at least — more appealing way of reading a plant.

Wulf then jumps forward to modern archaeology and genetics, which broadly confirm Forster’s reconstruction. There was an initial human population in the region of Papua New Guinea dating back 45,000 years. This was followed by a second, Austronesian-speaking wave migrating out of Taiwan and thereabouts around 3,500 years ago. They travelled by the stars in “outrigger or double-hulled canoes loaded with hogs, chickens, root vegetables, coconuts, bananas, and breadfruit seedlings.” And they mixed with the descendants of the earlier wave through Melanesia. Their own descendants eventually reached Fiji, Samoa, Tonga, and, much later, Tahiti, Hawaii, Easter Island and New Zealand.

The essay closes by tying this to Tongan historian Epeli Hau’ofa’s idea of “Oceania” — the Pacific not as scattered, isolated islands, lost in a vast ocean, but as one interconnected “sea of islands.” Exactly what Forster intuited two centuries before it became conventional wisdom.

We’ve blogged a lot about breadfruit over the years. Check out the archive. And in particular listen to the recently-retired Dr Diane Ragone describe her 40-year journey to understand this remarkable tree.

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A Babel of agrobiodiversity

“The rise and fall of language diversity through the Holocene” by Blasi, Hamilton, Gray & Bowern, published in Science last month, does something I really love. It complicates a standard narrative.

The received story of language diversity is that it has been declining steadily for centuries, largely driven by colonialism and globalization. But direct evidence is scarce and recent: writing only reliably takes us back about 6,000 years. So the team combined ethnographic data from 171 hunter-gatherer and fisher societies, paleodemographic estimates of global population, and statistical and social-computational modelling to reconstruct plausible trajectories of global language diversity over the 12,000 years of the Holocene, which began after the last major ice age, triggering the birth of agriculture.

At the beginning of this period, they estimate there were roughly 4,500–6,200 languages. That’s actually fewer than the roughly 7,500 languages spoken today. Language diversity then increased through most of the Holocene, driven by the population growth engendered by agriculture, reaching a peak between about 1,000 and 3,000 years ago, when the models suggest that tens of thousands of languages may have existed. That’s an order of magnitude more than today.

Then came a major collapse. The authors argue that the decline in linguistic diversity began with the expansion of large states and empires, long before European colonialism. Expanding populations and their languages, cultures and institutions displaced smaller ones. Today’s languages are therefore “survivors of a massive and highly selective historical bottleneck.” They persist because the populations speaking them expanded. Extinction, the authors argue, has shaped linguistic and cultural diversity far more — and for longer — than we have appreciated.

Now, there are obvious caveats. The authors are modelling linguistic diversity, not measuring it. Their estimates depend on heroic assumptions about early ethnolinguistic group sizes, global population and the relationship between population and language number. Regional histories could have been very different, and the model can’t reconstruct individual episodes of extinction caused by war, disease or environmental change.

But the paper provides a provocative template for thinking about diversity more generally. It suggests that population growth can initially generate diversity; but expanding societies can subsequently erase it through homogenization.

And that makes me wonder about agricultural biodiversity. You saw that coming, I’m sure.

I’m not suggesting that crop diversity followed the same pattern as language diversity through the Holocene. We simply don’t have the evidence to say that, though it’s an intriguing thought. But could we borrow the methodology? Could we reconstruct plausible trajectories of crop or agricultural-system diversity from fragmentary archaeological, ethnobotanical, genetic and demographic evidence?

Rather than assuming that agricultural diversity was vast and stable in the distant past, happy in its Vavilovian centres, and has been declining recently, perhaps we might end up complicating the familiar genetic erosion narrative too. That would be worthwhile. And fun.

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