Asked by: Terry Rees, Manchester
Early trees had wind-borne spores rather than seeds. About 365 million years ago some plants evolved seeds with a food store, to give the developing embryo a head start. These were heavier, so the early conifers began to evolve seed coats with enlarged ribs or vanes to help catch the wind. By 270 million years ago, a tree called Manifera talaris had developed seeds with wings that allowed them to spin as they fell. The seeds were variable; some had two equal wings, some had one wing much bigger than the other and some had just one wing. But aerodynamic reconstructions have shown that a single, asymmetric wing makes seeds fall the slowest. Evolution favoured this design because the seeds were spread further by the wind. Today, all conifers with winged seeds use a single asymmetrical helicopter wing.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.