Many have been excited this week about headlines claiming marijuana (Cannabis) was domestication in China 12,000 years (making it the first crop in East Asia). As the reputable journal Nature put it "pot farming first blossomed" in China 12,000 years ago. But was it so? How clear or otherwise in the evidence? Is it really farming? The study by Ren et al in Science Advances paper is important-- it represents the largest collection of Cannabis genomes sequences, it provides some important information on subpopulations and genes that have been selected for differently in fibre hemp from drug strains. However, I finds is dicussion and conclusions riddles with both imprecision (about chronology, geography and cultures) and inaccuracies. So what should be questioned?.
Given how few wild/feral samples that they have can they really rule out multiple domestications. Genetic analyses often err on the side of single origins. Simulation work (Allaby et al 2008 PNAS) have shown that this will be true even for crops with multiple origins, because gene flow among crops of different origins and pruning of lost branches (not sampled or not surviving to present). Their analytical methods are more modern and more sophisticated but I am not sure they can rule out multiple origins, and they certainly can’t rule out origins from regions not sampled or where wild populations are extirpated (e.g. Japan). Also wild populations (lets assume there are some in central Asia) can be heavily inundated with gene flow from crops over time make their original dinstictiveness hard to find in modern genomes.
A major issue is imprecision in dating. Genomic dervied estimates will never rival radiocarbon dating on archaeobotanical remains. Their proposed date of origins (is this domeatication?) has 6000 year error margins. And it not clear what they date! Was a divergence between two wild population separated due to climatic vicariance at the end of the Pleistiocene or start of the Holocene, or is it meant to be the domestication bottleneck? The equivalent genomic dates for Asian rice domestication are ca. 18,000 BP and something close to 10,000 for African rice. Both of which are way off. African rice is domestication at more like 3000 BP and Asian rice at more like 8000 BP (of course it matters whether one is talking about the beginning or end of a process, as domestication takes 3000-4000 years in terms of morphological evolution/ genetic fixation.
By contrast archaeological dates are much more
precise, at worst with 100-200 year error margins, but Ren at all quote these
very imprecisely. It is as if they wish archaeology was less precise, but even then it would not approach the dating imprecission attached the the genomic dates. The say "~3000 BP" for the appearance of Cannabis in India, but if we are rounding off it is closer to 4000, as it occurs in the Late Harappan horizen (3900-3500
BC). Although Indian epics are not well dated some parts of them are from oral traditions
that probably also date around then and make reference to Cannabis- I think Ren
et al refer to this as ~2000 BP.
They are also quite imprecise about geography: are
they suggesting a NW China (Xinjiang) or NE (Chifeng) source? And if either of
these then discussion of early cord-marked pottery in South China (mostly south the Yangtze) is really not
relevant, and yet they discuss this as though it indicates the use of hemp cords-- for which there is simply no evidence.
Cannabis is undoubtedly an important crop brought into cultivation early in east Asia, esepcially for its medicinal and/or social uses, but becoming important for larger oily seeds and fibres over time. But in my view its development as a crop either parallels or is even inspired by the increasing importance of cultivation of other taxa, like the China millets. And this process could easily have played out multiple times-- perhaps in very different contexts in parts of central Asia or Jomon Japan, even amongst non-farming cultures. While genomic data will contribute to this, modelling such data really requires some calibration points in time and space, which will ultimately come from archaeobotany.