The problem is that whenever it happened, there were only a very few at first. And it is difficult to find fossil remains of small primates that are that old. But by comparing the DNA of Chimps and Humans and noting the number of
"variations", they think they can "back calculate" when the two species separated. That nuber seems to be about 4-5 million years ago.

They have also found parts of a skeleton that appears to be more than 7 million years old. However, that seem to be from an earlier "split" which died out and is not really related to Humans.

One of the problems is that we can't actually say how many times a new species may have split off from the apes. But about 5 million years ago there was a sharp change in climate - with things getting warmer and drier in sub-Saharan Africa. This is believed to have been the "trigger" which allowed the new species to find a favorable environment for survival.

d;-)