Malus sieversii and the origin of the apple

The wild forests of the Tian Shan carry a wild apple, Malus sieversii, that is unusual among wild fruit in that some of it is already large, sweet and palatable. Most wild relatives of cultivated fruit are small and unrewarding; the Tian Shan apples include trees whose fruit would not look out of place in a modern orchard. That variability is the raw material domestication worked on.

The species was described in the eighteenth century and its significance for the domestic apple was argued through the twentieth, notably by Russian and Central Asian botanists working in the region. The argument then rested on morphology, distribution and the sheer diversity of the wild populations.

Genomic sequencing settled it. Analysis of the apple genome and of wild and cultivated populations shows M. sieversii as the principal progenitor, with the domesticated apple retaining a large share of its wild diversity rather than passing through the severe genetic bottleneck typical of domesticated cereals.

Apples travelled west from Central Asia along the trade corridors that later became known collectively as the Silk Road, carried as fruit, as seed, and eventually as grafted material. Grafting matters here more than in most crop histories: an apple grown from seed does not resemble its parent, so a valued tree can only be moved as a tree or as scion wood.

Along the route the moving populations met other wild Malus species and hybridised with them, including Malus orientalis in the Caucasus and Anatolia and, in Europe, the crab apple Malus sylvestris. Genomic work has shown the M. sylvestris contribution to be substantial — in some analyses larger, in the European cultivated apple, than that of M. sieversii itself in particular genomic regions.

This makes the domestic apple a hybrid crop with a principal ancestor rather than a straightforward descendant of one species. The single-origin story is not wrong so much as incomplete, and the part it leaves out is the part that happened in Europe.

Cider apples are distinguished from dessert and culinary apples largely by phenolic content: the tannins that give bitterness, astringency and structure. High-phenolic fruit is not what domestication selects for when the goal is eating, and the traits that make a good cider apple are closer to the wild condition than the traits that make a good eater.

The European crab apple contribution is therefore of practical, not just academic, interest. Small, sharp, astringent fruit is what M. sylvestris offers, and a European orchard population carrying substantial crab apple ancestry is a population in which such fruit can reappear from seedlings.

It also bears on how cider cultivars arose. Traditional West Country and Norman cider varieties were overwhelmingly chance seedlings — found in a hedge, at a field edge, on a farm — and selected because someone judged the fruit useful. That process draws on whatever diversity is present in the local population, which is why regional cider fruit differs so markedly between traditions.

The genomic picture is strong on ancestry and much weaker on chronology. When apples were first deliberately cultivated rather than gathered, when grafting was first applied to them, and how quickly cultivated forms replaced wild ones in any given region are questions the genome does not answer directly.

Nor does it identify a moment of domestication. The apple appears to have been domesticated gradually and repeatedly, with cultivated and wild populations remaining in genetic contact for a long period. That contact is why the crop retained so much diversity.

For cider history the practical implication is that the arrival of apples in a region and the arrival of cider-making in that region are separate questions with separate evidence, and answering one does not answer the other.