Is it not rather ridiculous to suggest that the first men lived between 900 and 1000 years (Gen 5:5)? Is this not obviously inconsistent with known science?

There are several ways to interpret these claims; the question assumes we are to take the claims “literally.” So let us examine this approach. First, I refuse to ignore science insofar as doing so involves ignoring observations and obvious inferences from them. Are there any observations or inferences about organisms that make extreme longevity in early man impossible or unlikely? So it seems; there are no instances of any animals living so long, and while we do not understand the biological cause of aging, we know of no genetic or environmental variations that reliably cause extreme old age. Thus the theory that the Flood changed the environment in some way such as to prevent extreme longevity appears to be ad hoc from a scientific point of view. But, of course, divine intervention allows for anything. So it is entirely possible that early man could live literally 900 years, and God simply changed the rules after the Flood. This is, it seems to me, the best way to support the text rationally. One might instead claim that there is a mythical element in Genesis 1-11, rather than any scientific explanation. This does not prevent much in the way of unreasonable theological commitments, because all the rest of the Bible requires miracles.