Were You There?
If this response were a valid challenge to evolution, it would equally invalidate creationism and Christianity, since they are based on events that nobody alive today has witnessed.
As a flagrantly anti-intellectual dismissal of the validity of any evidence pertaining to events we don't personally observe, it can be quite amusing - since the person making this argument was certainly not alive to witness the Bible being revealed or written down, he/she has no way of knowing whether what we call the Bible is the result of actual divine inspiration, or of Satanic interference or of a succession of stoners down through the ages scribbling down whatever came to mind and rationalizing it later as revealed truth. The claim that the Bible is uniquely exempt from this problem is a textbook example of special pleading. In such usage, the argument contradicts itself, which is the point. Not so incidentally, the Bible does not make the claim that the narrative of Genesis is eyewitness testimony.
The obvious contradiction here is that anyone using this argument also wasn't there to see the event and wasn't there to verify whether the anyone else was there. If the accusation is that no one was around to see evolution in action, the accusation that no one was around to see God magic the universe into existence 6,000 years ago is equally true. Someone can claim that the Bible is such first-hand evidence, but unless the person using the "were you there?" argument was around when the Bible was written (unlikely) or has personally supervised every copy of it made between creation in 4,004 BC and today, the same accusation applies.