The oldest known drawing of a snake eating its own tail is inside the tomb of the pharaoh Tutankhamun, painted on a golden shrine in the fourteenth century BC.

The Greeks named the image Ouroboros, meaning 'tail-devourer'.

It became the great symbol of alchemy: the endless cycle of destruction and rebirth, of the substance that is its own beginning.

The Norse had their own version — Jormungandr, the world-serpent that circles the earth and bites its tail.

In the nineteenth century a German chemist named August Kekule said he worked out the ring structure of benzene after dreaming of a snake curled biting its tail.

Whether he really did or not, the story stuck.

An idea drawn on a tomb wall in Egypt, three thousand years later, unlocked modern organic chemistry.