Tate Modern is a good place to study the classical nude. There are naked ladies throughout this art gallery among the installations and videos – and not just naked, but nude in an artistic tradition that goes back to ancient Greece.

Paul Delvaux's 1944 painting Sleeping Venus is well aware of its historical echoes. A woman lies asleep on a gilded bed, her body spread out in calm display like that of the famous Venetian Renaissance painting Sleeping Venus. Yet he sets her even further back in time. She lies on a piazza flanked by ancient Greek temples, under a night sky. The moon's silver light makes her flesh pale as she is surrounded by uncanny figures: an Edwardian woman, a skeleton, distraught nudes who hold up their arms in supplication.

Delvaux painted his Sleeping Venus in Brussels during the second world war, and he said its uneasy eroticism was his attempt to capture the terror of night-bombing. Yet the sexual tension in his painting is not a subversion of the classical nude. Sex was always there in older representations of beauty and desire – as Delvaux hints. As the art critic Kenneth Clark wrote soon after the second world war, the nude is the one theme that connects the oldest traditions of western art with the most daring modernism – he cited Picasso's nudes.

There is one of those a couple of rooms away from the Delvaux at Tate Modern. Picasso's glorious 1932 Nude, Green Leaves and Bust is as sumptuously sexy as any Titian. Picasso reinvents everything, yet the lust of his art is primal. Titian would have enjoyed this painting – he too took radical liberties with paint.

The nude is timeless, because human sexual nature evolved before the stone age and has essentially not changed since – apart from a few customs. Francis Picabia concluded that nudes are mere pornography. His painting Otaiti in Tate Modern is savagely sexual. But it is not any more explicit than paintings by the 16th-century German artist Lucas Cranach. It seems the nude in art was born modern.