Wheat is eaten by more of the world's people than any other cultivated plant. Behind this near-ubiquity lies a complicated genetic history that only became clear in the twentieth century, and whose understanding has guided the breeding of the wheats that now dominate modern fields.

Almost all wheat grown commercially today is one of two species: bread wheat (Triticum aestivum), used for leavened loaves and for most baked goods, and durum wheat (Triticum durum), used for pasta. Both have unusually large genomes built up from three sets of chromosomes, a condition known as hexaploidy. Most plants and animals are diploid, carrying two sets of chromosomes; bread wheat carries six, inherited from three ancestral species. The history of wheat is therefore a history of three hybridisations, spaced across thousands of years.

The first of these hybridisations occurred naturally, long before cultivation, when two wild grasses crossed in the Fertile Crescent to produce an early tetraploid wheat. Early farmers, working with this tetraploid as a harvested grass around 10,000 years ago, selected strains whose seeds remained on the ear at harvest rather than dropping to the ground. This was the first step in domestication. The second hybridisation, probably around 8,000 years ago, added a third genome from another wild grass, Aegilops tauschii, a plant native to the region south of the Caspian Sea. The result was hexaploid wheat - larger-seeded, more adaptable to cooler and wetter climates, and, importantly for later bakers, richer in the gluten proteins that allow dough to rise.

Plant geneticist Dr. Mariana Orsini has described this history as 'a stack of accidents, edited for a while by farmers'. The accidental nature of the original crosses meant that much of the genome of modern bread wheat is a patchwork, with many redundant genes and considerable variation in which copies are active. This complexity made wheat genetics particularly difficult until the full genome sequence was finally published in 2018, decades after those of many simpler crops.

Modern breeding has worked with, and sometimes against, the consequences of these ancient crosses. The dramatic increase in yields during the green revolution of the 1960s came from introducing dwarfing genes - from a Japanese wheat called Norin 10 - that shortened the plant's stem. A shorter stem supported heavier grain without collapsing. The American agronomist Norman Borlaug, working in Mexico, combined the dwarfing trait with disease resistance and wide climatic adaptation to produce semi-dwarf varieties that were taken up across South Asia and the Middle East. The result was an often-cited doubling of wheat yields in countries such as India and Pakistan.

The green revolution was not universally welcomed. Critics argued that the new varieties required more water, more fertiliser and more pesticide than older ones, and that they pushed traditional landraces out of cultivation. Landraces are genetically diverse local varieties that have evolved in specific regions over centuries. Their loss reduces the pool of traits that future breeding can draw on, and several seed banks, including those of the International Maize and Wheat Improvement Centre in Mexico, now hold tens of thousands of landrace samples precisely to protect against this erosion.

Current research focuses on problems that the green revolution did not solve. Climate change requires wheat varieties that tolerate heat and drought; rising concerns about greenhouse gas emissions from fertiliser production push breeders towards plants that use nitrogen more efficiently; and older problems such as rust diseases, which periodically threaten large areas of wheat, keep resistance breeding central. Dr. Orsini's work at the University of Adelaide has identified genes from landrace wheats that confer tolerance of high night-time temperatures, an increasingly important trait in parts of Australia.

Bread wheat remains, in a sense, a young crop. Its genome has been stable for perhaps six thousand years, a short time on the evolutionary scale. The fact that it feeds so much of the world depends on continuing work in breeding rather than on any intrinsic perfection of the plant. Dr. Orsini argues that the history of wheat offers a lesson for food security: the most important crops are best treated as projects, requiring ongoing investment and protection of their genetic diversity, rather than as settled resources that can be taken for granted.