We were only "separate" species because for a period of tens of thousands of years we did not interbreed and developed independently due to physical separation.

We were only "separate" species because for a period of tens of thousands of years we did not interbreed and developed independently due to physical separation. But we could and did interbreed when the physical separation was crossed. Despite the separation, we remained similar enough genetically that interbreeding was possible. Were there a higher number of miscarriages than between 2 humans? We will probably never know.