It was Prophets, not Elites that were around in the past. Otherwise, the Geas in humans are what allows us to use Forerunner tech. When John activates the lightbridge on 04, he did by instinct, just pressing a random button. His Geas told him what button to press. In addition, both humans and forerunners were both created by the precursors. Once our species had more similarities, but the forerunners embraced mutations. Forerunners who have undergone no mutations look pretty similar to humans. As they go through more mutations, they look less and less human like. Before a mutation, Didact looked like Voldemort wearing a combat skin, after a mutation he looked like a cross between Voldemort and a vampire, for example.