So a little background: I was raised as very devout Christian and as an adult I sort of loss faith in religion, particularly when thinking critically about both the religion itself and also some of the teachings. Of all the things I thought about critically, the story of Noah's ark was not one of them. However thinking recently about the ardent opposition to evolution provided by many conservative Christians and spurred by something I saw on TV, I took a second to think about the story vs. facts we know now.
Currently scientists estimate that there are about 6.5 million species of animals - based on what we've cataloged and estimate are still undiscovered . This number does not include the millions of species that have gone extinct (said to be incrementally more than what currently exists). So, let's say evolution didn't happen or doesn't happen and let's very conservatively say that there were about 40 million species of animals that ever existed. Per the Noah's ark story, he would have gathered at least 80 million species of animals. Of course for some species, he is said to have gathered 7 and not 2, so we could then conservatively estimate that that he gathered 120 million animals to put on the ark.
Of course it doesn't stop there. You couldn't just gather all the animals together on whim right now, even if you wanted to, because many animals are not found where you are. The Arctic has...worms that live in the ice and polar bears, the desserts have their own animals, the rainforests of the amazon has species of butterfly that live way way up in the trees, giant centipedes (those things are scary) and numerous species of snakes (among many others of course); Australia has Tasmanian devils and platypuses; in Chicago we have cicadas which lay in the ground for 40 years, then come out for a few days after those 40 years. That being said, it would be an INCREDIBLY monumental task for one person to gather all species together especially considering the life span of so many are so very short (some flies live only a few days for example - and how the heck would one get all those flies!)
But wait...there's more. Now the Bible never touched on whether or not God had more of a hand in post-Noah's ark animal life, so we are left to assume that things just progressed naturally? (also if God was gonna create more animals after the flood, then why gather them up in the first place) This doesn't really mesh with what we know right now. Countless species are in jeopardy currently because their numbers are in the hundreds or low thousands (including those held in captivity) that still have pretty bleak chances of survival, despite careful efforts to breed them. Pandas for example can only get pregnant for a few days out of the year, don't particularly like breeding and have a high rate of infant deaths. Did the lions eat grass when they got out? (as well as all the other canivores? Can't imagine the carcasses would be edible or even exist after 40 days in water) What did the spiders eat? Even if the carcasses of the billions of dead animals were edible, spiders don't eat meat.
Then of course, if there were 40 million species of animals, and the MAJORITY of those animals suddenly died, there would be a HUMONGOUS trove of bones left over. Now this is where the bible could intersect with what we know, but there is still some contradictions. Every one of us is of course affected by the presence of fossil fuels, which as the name suggests comes from fossils buried deep in the earth. We of course see oil being present in every continent which suggests huge numbers of animals died and were buried way into the earth. The only contradiction here is that the time required for those fossils to be converted into oil is way way way way longer than the timeline offered by the story of Noah's ark. Even if scientific estimates of how long it takes for fossils to be converted to oil were off by...a couple hundred million years (they estimate 650 million years), it would still be significantly more than the Noah's ark timeline.
That being said...the story of Noah's ark really doesn't stand up to any sort of scrutiny and has way more holes in it than Swiss cheese.