The domestication of plant species was a major step along the way to our current culture. It was once thought that the practice had two origins (the Fertile Crescent and Central/South America) and spread culturally from there, picking up further crops as it went. Evidence has poured in, however, that crop domestication developed separately in a number of locations around the globe. The latest issue of Science has two research papers that look at different aspects of the process, along with a perspective that presents the big picture.
The first paper looks at material isolated from sites on the west slope of the Peruvian Andes, focusing on material that appears to come from domesticated plants. The big surprises are the dates obtained: squash appeared around 8-9,000 years ago, peanuts just shy of 8,000 years ago, and quinoa at roughly the same time. With agriculture in place, production moved to cotton by about 5,500 years ago. None of these items are likely to have originally been domesticated in this region, suggesting that seeds and techniques arose earlier and elsewhere.
The authors suggest a date of about 10,000 years ago might be a reasonable guess, which is a striking figure: the most firm date we have for human occupancy of the Americas is only about 13,000 years. The authors suggest that limited local cultivation acted as a supplement to a hunting-gathering society for a few thousand years and only gradually fostered the transition to an agricultural society.
The issue also contains a review of what's gone on in the genome of wheat since we domesticated it at about the same time as squash. Compared to its wild relatives, domesticated wheat is tetraploid: it has four copies of every chromosome, instead of two. At least some cultivars are hexaploid, with a further set of chromosomes derived from a different species of grain by hybridization.
The researchers suggest that all this extra genetic material has proven useful to the species in two ways. The domestication process normally creates a genetic bottleneck, in that relatively few ancestral plants wind up being expanded into a large population with little genetic diversity. Normally, low genetic diversity is harmful, but the extra chromosomes (especially those from a different species) have helped wheat remain diverse. The second is that it allows for an extremely flexible genome, one that has produced many variations for humans to select traits from. The extra copies of genes allow deletions, mutations, and even larger rearrangements to tweak gene expression and function in many ways, some of which would probably be fatal in an an organism with only two copies of a chromosome.
The news focus nicely pulls these ideas together. It notes that the South American model—one in which domesticated plants spread rapidly, but only gradually changed people to farmers—appears to apply in most other cases of domestication. It also discusses how slowly the genetic diversity that helped produce modern wheat allowed selection of a domestic species. It turns out that seeds from wild and domestic species are intermixed at agricultural sites for centuries after the tetraploid wheat appeared, and that the percentage of domestic wheat rose only slowly during that period.
It ends up with the big mystery: why did people suddenly think domesticating plants was a good idea? The 10,000 year figure is suspiciously close to the rapid climactic changes that accompanied the end of the last ice age, suggesting that changing temperatures might have disrupted food supplies enough to make domestic plants essential. The end of the ice age also pushed atmospheric carbon levels up by nearly 50 percent, which may have triggered favorable changes in the plants themselves. But it ends on an enigmatic note: maybe there was something cultural going on that we may never be able to reconstruct.