Where were the first domesticated olives?

Was the Olive tree domesticated once or twice? New Phytologist has been weighing the evidence for both arguments recently, but the answer remains elusive.

If you live it northern climes it can be easy to overlook the importance of the Olive. Ancient Greece was built on the Mediterranean triad of olive oil, wheat and wine. Currently the favoured explanation for the Olea europaea supsp. europaea, but New Phytologist has recently published a paper suggesting at least two domestication events. In theory it should be possible to test whether the Olive had a single domestication or many by looking into its genes, however another paper in New Phytologist that has just gone online highlights the problems with this approach.

Olive tree
Dawn of the Olive. Photo: Feans / Flickr.

Besnard and Rubio de Casas point out that domestication isn’t a simple event. In particular farmers can cross-breed a domesticated tree with wild relatives to breed in a new trait to the stock. So even a single domestication event gets augmented by fresh genes in the pool from time to time.

It might help if we knew how the Olive arrived in the Mediterranean.

The Holocene started around 12,000 years ago, while the Neolithic, with the first farmers, started roughly 6,000 to 5,000 years ago moving from East to West through the Mediterranean. The question is what was it that traveled when the Neolithic spread? Was it a movement of people with new technology into the west? Or was it the idea of farming that spread, with natives taking it up? Or was it a package of technologies that were traded, so that hunter-gatherers bartered for olive trees from neolithic farmers in exchange for other goods?

Looking at human genes, the favoured idea is that groups of settlers slowly spread randomly around landscapes, leading to a drift to the west. If this is the case then it’s reasonable that they would take their plant stock with them. If this is the case then, how could it look like there were multiple origins for olives?

The olives travelling with the settlers would not be the only olives in the Mediterranean. Besnard and Rubio de Casas argue for refugia of olive populations during the last glacial period. Once the Holocene starts conditions change so that these populations can spread out to colonise the Mediterranean as wild olives. These distinct populations then get bred into the domesticated olive as farmers settle in areas and look to improve their trees.

One of the interesting things about the Besnard and Rubio de Casas i that they’re very explicit about what a satisfying explanation for olive domestication would look like. In addition to their own test, they also note that a good explanation should be consistent with the archaeological evidence. It would give a second independent line of inquiry to corroborate any findings.

They jury might still be out on the domestication of olives, but they’re seeing some impressive forensic arguments.

Guillaume Besnard, Rafael Rubio de Casas, 2015, ‘Single vs multiple independent olive domestications: the jury is (still) out’, New Phytologist, pp. n/a-n/a http://dx.doi.org/10.1111/nph.13518

Concepcion M. Diez, Isabel Trujillo, Nieves Martinez-Urdiroz, Diego Barranco, Luis Rallo, Pedro Marfil, Brandon S. Gaut, 2014, ‘Olive domestication and diversification in the Mediterranean Basin’, New Phytologist, vol. 206, no. 1, pp. 436-447 http://dx.doi.org/10.1111/nph.13181

Ornella Semino, Chiara Magri, Giorgia Benuzzi, Alice A. Lin, Nadia Al-Zahery, Vincenza Battaglia, Liliana Maccioni, Costas Triantaphyllidis, Peidong Shen, Peter J. Oefner, Lev A. Zhivotovsky, Roy King, Antonio Torroni, L. Luca Cavalli-Sforza, Peter A. Underhill, A. Silvana Santachiara-Benerecetti, 2004, ‘Origin, Diffusion, and Differentiation of Y-Chromosome Haplogroups E and J: Inferences on the Neolithization of Europe and Later Migratory Events in the Mediterranean Area’, The American Journal of Human Genetics, vol. 74, no. 5, pp. 1023-1034 http://dx.doi.org/10.1086/386295