After analyzing a lot of science researches and articles about who invented sex we can say that the first sex was done about 450 million years ago by already extinct placodermi class fish.
Placoderms were living from the late Ordovician period (480 million years ago) to the late Devonian period (360 million years ago). It is believed that our sex inventors were living 50-80 million years before extinction. The oldest fossil is from Llandovery epoch and is about 445 million years old. It is believed that placoderms extinct in the end of Devonian period (360 million years ago) because of Hangenberg event which happened about 360 million years ago. Hangenberg event was the event then oceans depleted in oxygen and covers with algae.
Who invented sex