This is basically the “were you there?” fallacy, which is nonsense. The fact that the past is not directly observable may mean that we do not know everything, but it most certainly does not mean that we do not know anything.
For starters, I don’t know where you stand on the age of the earth, but if you were trying to argue for an age of 6,000 years, you would have to either propose that God created evidence for a history of events that never happened, or else change the scientific method itself in ways that would kill people if you applied them in any other context. Furthermore, the YEC organisations have themselves admitted that squeezing the evidence into 6,000 years would have raised the Earth’s temperature to 22,000°C. When that is the situation, one can be absolutely 100% certain that the Earth is far, far older than six thousand years, end of story.
As far as evolution is concerned, it’s much the same principle. It’s patent nonsense to claim that we can’t know anything at all about who or what did or didn’t evolve from what; we have to admit that the evidence tells us something in that respect, and make sure that we get our facts straight about exactly what.