"The origin and spread of olive cultivation"

Fossil pollen dataset reveals the origin and spread of olive cultivation in the Mediterranean Basin

Olive (Olea europaea L.) was one of the most important fruit trees in the ancient Mediterranean region and one of the founder species of horticulture in the Mediterranean Basin. Produced and traded across the Mediterranean over thousands of years, olive oil was both a staple food ingredient and a precious commodity used for cosmetic, medicinal and ritual purposes as well as for illumination. Cured olives are common in many Mediterranean cuisines. Today, olive orchards still form a significant component in the agricultural economy of many Mediterranean countries, with Spain, Italy and Greece accounting for about two-thirds of the total world production of olive oil. But when and where did olive cultivation begin, how did it spread, and who is to be thanked for the invention of this vital component of Mediterranean diet, touted by the World Health Organization as healthy and sustainable, and inscribed by UNESCO as intangible world cultural heritage?

In the wild, olive trees grow in habitats characterized by a typical Mediterranean climate (Figure 1), usually in hilly areas, in evergreen woody vegetational associations. These habitats can be found mainly near the coastal Mediterranean areas of Morocco, Libya, the Levant, Anatolia and at the southern coasts of Greece, Italy, France and the Iberian Peninsula. Olive is also native to Mediterranean islands such as Palma, Sardinia, Corsica, Sicily, Crete, Cyprus and the Aegean Islands.

Conflicting evidence has been offered regarding the geographical origins and timing of olive cultivation. Genetic studies differ among themselves regarding the exact area of origin of domestication and whether there was one primary domestication event or several independent ones. The archaeobotanical evidence also allows for varying interpretations: The first modern proposal concerning the date and geographic origin of large-scale olive management, based on archaeobotanical remains and natural distribution, was that of Zohary and Spiegel-Roy (1975), who suggested that the olive tree was already cultivated (and consequently domesticated) at Chalcolithic Ghassul in the southern Levant, ca. 6,000 years BP. The northern Levant and some north-western Mediterranean areas were also suggested in the literature as areas where olive cultivation began. The macro-botanical evidence also offers several options regarding the exact timing of the beginning of olive horticulture ranging between late 7th millennium to the 5th millennium BP.

The genetic evidence and the archaeobotanical data cannot be easily reconciled, probably because of complex secondary domestication processes, with hybridization between local wild, feral and domesticated genotypes and introduced domesticated olive trees, followed by repeated local selection events. While DNA data can indicate areas of potential genetic contributions to the domesticated gene pool, it lacks information on the timing of such events. We therefore focused on another proxy – the palynological evidence.

Our study (Langgut et al. 2019) used circum-Mediterranean fossil pollen records, correlated with archaeological proxies for olive cultivation and oil-production (such as concentrations of crushed olive pits or the presence of extraction technologies), to shed new light on the history of olive cultivation around the Mediterranean. We employed a fossil pollen dataset composed of high-resolution pollen records obtained across the Mediterranean Basin, covering most of the Holocene. Pollen records which met the criteria of our study were available from Portugal, Spain, Italy, Greece, Turkey, Lebanon and Israel. The pollen data were provided by the EPD (European Pollen Database) and by palynologists who work across the Mediterranean. These palynologists are co-authors of our new study published in the journal "The Holocene" (29: 902-922).

Within the new fossil pollen dataset, human activity was indicated when olive pollen percentages rose rapidly, unaccompanied by an increase of other Mediterranean trees with similar environmental requirements, and when the rise occurred in combination with consistent archaeological and archaeobotanical evidence. Based on these criteria, our results showed that primary olive horticulture occurred in the southern Levant, not later than ~6,500 years BP. The palynological evidence indicated that olive cultivation occurred in the Aegean (Crete) several centuries later, during the early/mid 6th millennium BP. It is not yet clear whether olive cultivation in the Aegean can be considered an independent cultivation event or as having resulted from knowledge (and possibly plant) transmission from the southern Levant. In any event, this early olive horticulture corresponds to the establishment of the Mediterranean village economy and the completion of the ‘secondary products revolution’ during the Chalcolithic period, rather than to Bronze Age urbanization or state formation. It was primarily a rural staple economic strategy that was only secondarily (and much later) co-opted by Early Bronze Age elites as an instrument of political-economic leverage.

From the two areas of origin, the southern Levant and the Aegean, olive horticulture spread across the Mediterranean. Based on the pollen dataset used in this study, the beginning of olive cultivation is dated to ~4,800 years BP in the northern Levant. In Anatolia, large-scale olive horticulture is dated to ~3,200 years BP and in mainland Italy to ~3,400 years BP. In the southern sectors of the Iberian Peninsula olive horticulture is evident palynologically only during the last two millennia. The archaeological record for the Iberian Peninsula supports a slightly earlier date, during the mid/late 3rd millennium BP.

Our study gives us a better understanding the cultivation history of the olive tree across the Mediterranean, underlining its emergence in Levantine rural communities well before the advent of urbanism. It also illustrates the positive potential of basin-wide scientific collaborations, combining regional datasets to provide a broad geographical and temporal perspective on this economically important species and on the conditions of its long-term survival.

Scolars who contributed to our database were Josѐ Sebastián Carrión, Rachid Cheddadi, Daniele Colombaroli, Warren John Eastwood, Raphael Greenberg, Thomas Litt, Anna Maria Mercuri, Andrea Miebach, Neil Roberts, Henk Woldring and Jessie Woodbridge.