system
Given the following post, respond as if you're commenting on Reddit:
human
How far back in human history would we have to go before modern humans could not reproduce with their ancestors?

i.e. how far back would we have to go to be technically considered a different species?
gpt
Other great apes have 24 chromosome pairs while humans have 23. There is speculation that this happened when human population was very small and was a mutation that spread through the entire population.

If you were to try to breed with a human from before this mutation you would probably be unsuccessful. 

I don’t know when this happened, it must have been during Homo Erectus or earlier since we know that Sapiens bred with at least two other descendants of Erectus the Neanderthals and Denosovians.