It's not just Jehovah's Witnesses, it's a problem that Christianity must grapple with in general.
In one apocryphal scripture: "Wherefore, all things which have been revealed unto the children of men shall at that day [of the Millennium] be revealed; and Satan shall have power over the hearts of the children of men no more."
We have no writings of men that date back to 5000 B.C. or earlier, though many anthropologists say people have been around for much longer. Christians pretty much stand on the notion that when Jesus returns, He will make all things known back to the Beginning. "And not only this, but those things which never have been revealed from the foundation of the world, but have been kept hid from the wise and prudent, shall be revealed unto babes and sucklings in this, the dispensation of the fulness of times."
I had a professor who was a rabid atheist, and he made everyone read Holy Blood, Holy Grail and many of the works of French philosopher Albert Camus, including The Myth of Sisyphus. He quoted someone (can't recall who) who said that the three most common forms of suicide were: 1) marriage, 2) a 40-hour work week, and 3) a revolver. It was an entertaining course, but I didn't buy into atheism. In fact, I worked back in the early 70s with a very sharp guy who was a grounds keeper. He also was an alcoholic and he happened to know Camus and worked with him during the War. He talked about how he was almost poisoned with a piece of chocolate and he knew quite a bit about Camus' personal philosophies. In fact, he said that Camus, shortly before his death, not only completely reversed himself on atheism, he was considering entering the priesthood in the Catholic church! (Camus died in an automobile accident when a friend offered him a ride in lieu of a railroad trip.) Anyway, the guy was fascinating when he wasn't trashed! He had a lot of war stories.
Christianity depends on there being an Adam and Eve. Adam fell and Christ redeemed man from the Fall. This is Christian dogma, and alas, the JWs eschatology, while ridiculous in many ways, is still built on this Christian doctrine. The history of science also is laced by many errors that make it far from reliable. I believe Christ will clear all this up during His Millennial reign. Meanwhile it's a leap of faith for most Christians.