Thursday, 5 November 2015

How Humans And Rice Co-Evolved

Three geographically separate domestications of Asian rice : Nature Plants

Here is a good example of how a change in a species' environment can cause evolution and how that evolutionary change can feed back into the environment to cause changes in yet another species. In this case it's about how the different varieties of rice evolved and how these new varieties then facilitated changes in human populations and human culture.

It has generally been assumed that rice cultivation began in China along the Yangtze River Valley some 9,000-10,000 years ago when humans first domesticated wild rice Oryza sativa of the japonica variety. This cultivar then spread throughout Asia and into the Indian subcontinent where is hybridized with other wild varieties to give, for example, Basmati aromatic rice.

Now however, a team of researchers led by Peter Civáň of the University of Manchester, UK, believe they have shown that, although japonica rice was cultivated in China, so were two other varieties independently in the Indus Valley in modern Pakistan where the indica variety was cultivated, and in what is now India and Bangladesh along the Brahmaputra River system where the aus variety was cultivated. Both of these river valleys gave rise to flourishing early civilisations as humans learned agriculture and changed from hunter-gatherers to settle agriculturalists.

Our knowledge is never definitive. It will be interesting to see if other types of genetic data confirm our conclusions or provide even deeper insights into rice’s domestication history.

Peter Civáň, University of Mancherster, UK.
It was in these urbanised communities, along with those in similar river valleys such as the Tigris-Euphrates and the Nile that most of the foundations of our modern cultures were laid, including the world's major religions, the foundations of writing, mathematics, accounting, trade, political organization, art, music and story-telling.

The researchers arrived at this conclusion having examined the genomes of 1083 varieties of modern rice and 446 samples of wild rice from all over southern Asia. It contrasts with a 2011 conclusion by a New York University team who concluded that rice had been domesticated just once, in China. Typically of scientific debate however, both 'sides' acknowledge that these different conclusions could be due to the regions of DNA chosen for the study and that nether answer is definitive.

Domesticated rice (Oryza sativa L.) accompanied the dawn of Asian civilization and has become one of world's staple crops. From archaeological and genetic evidence various contradictory scenarios for the origin of different varieties of cultivated rice have been proposed, the most recent based on a single domestication. By examining the footprints of selection in the genomes of different cultivated rice types, we show that there were three independent domestications in different parts of Asia. We identify wild populations in southern China and the Yangtze valley as the source of the japonica gene pool, and populations in Indochina and the Brahmaputra valley as the source of the indica gene pool. We reveal a hitherto unrecognized origin for the aus variety in central India or Bangladesh. We also conclude that aromatic rice is a result of a hybridization between japonica and aus, and that the tropical and temperate versions of japonica are later adaptations of one crop. Our conclusions are in accord with archaeological evidence that suggests widespread origins of rice cultivation. We therefore anticipate that our results will stimulate a more productive collaboration between genetic and archaeological studies of rice domestication, and guide utilization of genetic resources in breeding programmes aimed at crop improvement.*

Three geographically separate domestications of Asian rice
Peter Civáň, Hayley Craig, Cymon J. Cox & Terence A. Brown
Nature Plants 1, Article number: 15164 (2015) doi:10.1038/nplants.2015.164

*Copyright © 2015, Rights Managed by Nature Publishing Group. Reprinted with permission under licence #3742490331503

Biologically, humans created the conditions in which the genes of the different varieties of rice could form an alliance with those of humans and so both flourished in this alliance. Both humans and rice are now vastly more numerous and biologically successful than they would have been otherwise.

It shows that the question of the domestication history of rice remains open, with different approaches reaching different conclusions.

Michael Purugganan, New York University
Exactly the same point can be made for other domesticated plants and animals, of course. It's recognition of the power of 'selfish' genes to act in mutually self-interested alliances that gives the lie to claims that 'selfish gene' evolution can never give rise to operation and altruism. In fact, nothing could be further from the truth.

By being spread geographically by humans, these different varieties were brought into close proximity, having diverged into geographical varieties earlier in their evolutionary history. When this gave rise to cultivars which humans preferred, such as the wonderfully savoury aromatic Basmati rice - a hybrid between indica and aus - or the conveniently 'sticky' japonica rice where chopsticks are used, or the round, absorbent varieties used for puddings and paellas, these were selected for and so the rice plant evolved into today's multiple varieties.

If people would cease the religiously inspired, nonsensically arrogant assumption that humans are somehow apart from nature, selective breeding like this would be seen as merely a form of natural selection and just as much an evolutionary process as any other adaptive process. From the rice plant's 'point of view', this evolutionary change can be seen as rice genes co-opting humans to aid in their propagation and persistence over time.

Just as we saw recently with the emerging new species of wasp brought about by the emerging new species of fruit fly brought about by humans cultivation of apples in America, so the change in the rice plant's environment brought about evolutionary changes in rice and in human culture and had profound effects throughout the larger environment of Earth's ecosystem.

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