We have science or deductive reasoning to help us contemplate what those numbers may have been at the time as exampled here:
The Great Dying - NASA Science
"Becker's team had previously found such gas-bearing buckyballs in rock layers associated with two known impact events: the 65 million-year-old Cretaceous-Tertiary impact and the 1.8 billion-year-old Sudbury impact crater in Ontario, Canada. They also found fullerenes containing similar gases in some meteorites. Taken together, these clues make a compelling case that a space rock struck the Earth at the time of the Great Dying.
But was an asteroid the killer, or merely an accomplice?
Many scientists believe that life was already struggling when the putative space rock arrived. Our planet was in the throes of severe volcanism. In a region that is now called Siberia, 1.5 million cubic kilometers of lava flowed from an awesome fissure in the crust. (For comparison, Mt. St. Helens unleashed about one cubic kilometer of lava in 1980.) Such an eruption would have scorched vast expanses of land, clouded the atmosphere with dust, and released climate-altering greenhouse gases."
As to the rest of your statement, its kind of like saying hypothetically that even though we don't know we need heat shielding and even though we don't know the distance so as to carry the sufficient amount of fuel and even though we haven't even figured out we need oxygen yet let alone how to store or produce enough, that "It's very unlikely that humans will have" a problem getting to the moon and back.
The only reason any moon mission was successful was because we had the above described info. With genetic engineering we simply don't yet have that info and the potential consequences are far greater reaching than a rocket failing to get to the moon as such unintended or intended effects could be as great as the 'great dying' in a worst case scenario.