According to the nebular hypothesis, when did nuclear fusion begin?

There is evidence that Emanuel Swedenborg first proposed parts of the nebular hypothesis in 1734.[3][4] Immanuel Kant, familiar with Swedenborg's work, developed the theory further in 1755, publishing his own Universal Natural History and Theory of the Heavens, wherein he argued that gaseous clouds (nebulae) slowly rotate, gradually collapse and flatten due to gravity, eventually forming stars and planets.[2]