"First, modern maize pollen grains are larger and turn red when stain is applied, whereas ancient grains do not, " they said.

BBC: Maize was key in early Andean civilisation, study shows

In the specific case of maize, to ensure that seed stocks achieve 99.9% purity, a separation distance of 200 metres is required from any source of "contaminating" maize pollen.

BBC: GM trial 'no threat to organic centre'

The team commented that the scarcity of macroscopic remains was in marked contrast to an abundance of microscopic evidence of maize in the guise of maize pollen samples collected from soil at the sites.

BBC: Maize was key in early Andean civilisation, study shows

In traditional plant breeding, some inbred maize plants destined to be crossed with others have their male, pollen-producing tassels cut off to prevent self-fertilisation (maize is a horticultural hermaphrodite, carrying both male and female sex organs).

ECONOMIST: Terminator genes

Zeneca does this by genetically engineering its maize to produce an enzyme, called ribonuclease, which interferes with pollen production.

ECONOMIST: Terminator genes