You seem to be assuming that in reptiles red blood cells are produced by the division of other red blood cells. But a few seconds' thought would show you that since a zygote is not a red blood cell, they must in fact have a mechanism for producing red blood cells from other types of cell. And then a quick google round the internet would have shown you that, like mammals, they make red blood cells in their bone marrow. So if you want a "just-so story" as to how mammals acquired this trait, here's one: we inherited it from the reptiles, from which we are descended.
However, as we know, you did
not want an answer to your question, so you didn't bother to spend the few seconds thought and effort it would have taken you to find it yourself. What you wanted was ignorance, which can be had with no thought or effort at all.
Your remaining confusion about what evolution is and how it works could best be corrected by reading a frickin' biology text book.
Edited by Dr Adequate, : No reason given.