Does rebirth conflict with evolution? I'm trying to understand how modern Buddhism deals with our current scientific understanding. 

Science indicates that life began on this planet with a small set of life forms. So there was a point in time where only a few life forms would be capable of rebirth. Now, however, there are over 7 billion humans and immensely more other creatures which can all be reborn. Where did all the souls/consciousnesses that exist now come from? How does the idea of rebirth fit with the evolution of life on Earth? 

I am very curious as to how modern Buddhism deals with this question. Please forgive me if it is a dumb question, my understanding of Buddhism is very minimal. Thanks in advance!
There are an infinite number of beings in all the places of existance and also an infinite number of universes. The earth being one of the humanoid worlds.  [Kosala Sutta 1] In some of the higher planes the number of beings are finite. In lower planes this might not be the case. As life span decreases the merit required to be born in the human plane decreases hence the population increase. Sometimes conflicts and natural disasters do reduce the population.
Aggañña Sutta does mention about evolution from being in higher plane and how the deteriorate incidentally occupying the lower planes. (At the start of the cycle there only a higher plane)

Their body was still coarse and roughly shaped. Thus, after a very long time, the mud-like substance began to be exhausted. Then, mushroom-like plants began to grow so fast that they replaced the mud-like ocean. The creatures began to devour them as well, and they also found it delicious, like sweet honey and milk. Their body hardened more and details began to turn finer.
After another very long time, the mushrooms also began to be exhausted, replaced by cassava or turnip-like plants. They also began to devour them night and day, and thus they began to notice differences amongst them. As the changes of their bodies varied between each other, the concept of difference arose. The concepts of the beautiful and the ugly were born. The beautiful scorns the ugly and they became arrogant because of their appearance.

Sourced from: Aggañña Sutta
Also see:

Buddhism and evolution
Buddhist cosmology of the Theravada school
Buddhist cosmology