Plenty of others have offered excellent commentary; I particularly recommend Biologos' responses, and the conversation that took place on their anotherchoice hashtag. I don't have much to add, except to expound on the very unsettling nature of Ham's refrain:
Were you there?Ham used this phrase several times to question the claims of Big Bang cosmology and the evolutionary development of species. His essential idea is that, if an observer doesn't witness an event or process directly, they cannot claim that it happened. This important rule, he says, marks the difference between "historical science" and "observational science."
There are many concerns I could bring up regarding this argument. I could point out how Moses did not witness any of the events of Genesis, or how the author of Job did not witness any conversations between God and Satan. I could point out that the photons we observe in the Cosmic Microwave Background are reaching us now after journeying for 13.8 billion years, such that what we are measuring is a present event. I could point out that any observation we make is delayed by the amount of time it takes for light from the event to reach our eyes, such that we are always seeing the past, and ask the question of at what time scale observations become invalid. I could point out that Ham's organization was not there to witness the scenes from any of their paintings.
But instead, I'd like to ponder the significance of "being there" as a criterion for "real science."
I am a condensed matter physicist. I study how the behavior of materials is determined by their atomic/molecular/lattice structure. For example, lattice structure is what makes the graphite in your pencil different from a diamond, even though they're both made of carbon atoms. The carbon atoms are arranged differently in each material, so the materials behave differently (in terms of strength, transparency, density, etc.). How do we know what their structures are? We have several ways: We can bombard a sample of each material with x-rays and witness a different diffraction pattern for each; we can probe their surfaces with a tiny metal tip; we can run simulations of different lattice structures until we find one that produces the same material properties we observe experimentally.
But we never actually see these atoms' arrangements. We "aren't there," at the atomic scale; the lattice structure is so many orders of magnitude smaller than we are that we can never hope for a direct observation, just as we "weren't there" for the Big Bang or speciation.
So, if I never directly observe atomic structure, am I forbidden from publishing a lattice structure in a paper, or teaching it to my students? Does that mean that my entire field is invalid, and I should simply stick to reporting materials' properties instead of explaining them? What about chemistry, which faces a similar scale difference? What about a physician who claims that a patient is cured of a microscopic infection?
Whatever the original intention behind the "Were you there?" objection, I think it ultimately undermines all of present-day science, not just the parts that some Christians have trouble accepting.