Theory: In the beginning

Ok. Here I have another musing of mine, just something to pass the time.

What if, when we were a young species, Male and Female were actually completely different? What if, one was a dog-like creature, with fearsome fangs, and the other, a plant eater, or some combination of different species. Then, one day, somehow, these two entirely different species "came together", and, by survival of the fittest, the early human outlived both these species?

Ok, here's an idea: what if those two species were actually intelligent, explaining our intelligence?