While I don't believe in reincarnation myself, I had asked that same question to someone who did believe in it. I didn't think they'd be able to answer and would just shrug their shoulders, but instead they said something that really surprised me.

What they said was that it was folly to assume all souls would start out as human. Instead, they suggested that all life (including microscopic) has a soul that would be reincarnated, and that eventually they would basically 'evolve' into a human soul. About the world population increasing, they suggested that with fewer areas available for animal and plantlife, more and more souls would be forced to evolve towards a human soul before they were ready to do so. As a result of not being ready for this stage, fewer humans would be moving on to the next one, leading to overpopulation and an increase in problems.

I still don't believe in reincarnation, but I thought it was a really interesting view.