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Weeding Out Weeds

Dawn of farming sparked speed-evolution in weeds

 

IT DIDN’T take long. Just a few thousand years after humans began to domesticate crops, a wide variety of weeds had adapted to exploit the new farmlands – with some species seeming to have evolved, like crops, to be completely dependent on cultivated land.

 

19 November 2014 by Colin Barras

Given the chance, weeds will take root in most agricultural settings.

 

Today their presence lowers crop yields by 10 per cent globally, causing losses of tens of billions of dollars in profits each year.

 

But weed woes are nothing new. Ehud Weiss at Bar-Ilan University in Ramat-Gan, Israel, and his colleagues studied ancient seeds, fruits and other plant remains recovered from Atlit-Yam, a 9000-year-old coastal settlement now submerged a few metres below the Mediterranean Sea off the coast of Israel.

 

The material was waterlogged by seawater, meaning it was extremely well preserved.

 

The remains, which date to a time just 2000 years after farmers sowed their first seeds, include durum wheat, figs, chickpeas (garbanzos) and herbs.

 

Alongside these important crops there is evidence of at least 35 weed species – suggesting that it didn’t take long for opportunistic herbaceous plants to adapt to our agricultural revolution.

 

What’s more, five of those species – including darnel and Egyptian hartwort – appear to have been obligatory weeds, meaning they can usually only grow in cultivated fields.

 

They provide the earliest evidence yet that some weeds – as well as crops – adapted to agricultural settings within just a few millennia of the agricultural revolution (Vegetation History and Archaeobotany, doi.org/w5n).

 

George Willcox of the Lumière University Lyon 2 in France describes the findings as a useful and important contribution to the study of the origin of agriculture, and the origin of weeds.

 

Most of what we know about early agriculture comes from the charred remains of cooked plants.

 

So finding plants from the dawn of agriculture that were exquisitely preserved in water is exciting, allowing researchers to accurately identify all the species.

 

“Species-level identification allows the authors to assert with confidence that obligatory weeds were present,” says Willcox.

 

What is a weed today may have had some uses yesterday, warns Simone Riehl at the University of Tübingen in Germany.

 

“Unfortunately, I can’t interview the settlers of Atlit-Yam,” says Weiss. But he adds that if a modern weed species’ nutritional benefits hasn’t improved much through the ages, it suggests it was probably a weed in the past too.

 

Is it possible that such useless, unwanted plants could have adapted so completely in just a few thousand years?

 

Absolutely, says Kenneth Olsen at Washington University in St Louis, Missouri, who studies weeds as examples of fast adaptive evolution.

 

“The development of agriculture would have opened up a new niche, and species that were already adapted to naturally disturbed habitats would be well positioned to rapidly spread to agricultural habitats,” he says.

 

After this initial period of colonisation, weeds would have continued to adapt to the peculiar conditions found in agricultural settings, says Ana Caicedo at the University of Massachusetts Amherst.

 

For instance, down the millennia some obligatory weeds have evolved to mimic the appearance of crop plants, allowing them to evade detection and removal by farmers.

 

Remarkably, even this stage of weed adaptation might have been under way 9000 years ago at Atlit-Yam: darnel is sometimes referred to as “false wheat” because of its resemblance to the crop.

 

Whether weeds would have been as problematic for early farmers as they are today is less clear.

 

Bruce McDonald at the Swiss Federal Institute of Technology in Zurich thinks that the earliest farmers probably planted fields of mixed crops rather than monocultures, so even if a weed like darnel brought down wheat yields, there should still have been a good harvest from other crops.

 

Monocultures might have come later, but there is evidence that they are relatively ancient. References to monocultures in the Old Testament suggest that farmers living a few millennia after Atlit-Yam was abandoned may have planted them.

 

“I’ll bet that weeds were the big problem in early monocultures – and picking weeds may have consumed a lot of time for those early farmers,” says McDonald.

 

This article appeared in print under the headline “Weeds evolved fast on first farms”

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