Evolution tries to explain the development of life on Earth - It fails. One could easily see life in its simplest form developing from an accidental combination of elements and evolving from one cell to many cells. But what could not have happened by accident is sex as not only a means of reproduction but also as a way of 'experimenting' with life and genetics to produce a variety of organisms and the development of species that advance and become more adaptable to the environment. Some other beings not of this Earth had to have deliberately and consciously created the sexual paradigm to life on the planet Earth and had to have done this with a deliberate intent and purpose. Those who are prone to giving existence a religious explanation could say God did it. But we do not know who or what God is or his intentions and so even in that case we are still talking about an alien phenomenon - An alien {not of human origin} being or beings was and still is responsible for the continuous evolution of life on this planet.