End of eternal life as shown by the Bible and birth of Man

At first Adam and Eve weren't limited to any specific lifespan, not until they were kicked out of Eden. It is possible that their numerous children weren't aware of the concept either until Kain murdered Abel. When the young man laid there dead, there must have been quite a turmoil going through the minds of everyone. This was the first human to die. The fact that the immortality could be ended so easily and foolishly must have terrified everyone to their very bones. This could be the turning point - the realization that the humans could die. After that nothing was ever the same. The fear of death must have caused a subconscious desire that life wasn't really eternal, because if it were, it was much too horrible. But if it was only temporary, people found it easier to accept death - a little more time, a little less - it didn't seem so bad. That is probably why a long list of people in the bible keeps getting shorter and shorter life spans; starting at as much as 900 years and down to a hundred, until it settled to some average 60 years old or so. This is a very strange fable indeed, if you think about it. If humans evolved from the apes, whose life spans are 35-50 years, then how was it possible that our ancestors were written to start out at 900 years and to come down? Of course, the easiest thing would be just to dismiss the Bible as nonsense and a bunch of fairy tales. Is there any way at all a person with a scientific mind could somehow look at the story from a perspective of a possibility? Well, not if we, indeed, are direct descendants from the neanderthals. My theory on this is alien intervention, a bit of genetic engineering, to change us from the cavemen to the modern men. Perhaps more than modern, perhaps even superior to us. Perhaps instead of evolving we devolved. That would explain huge pinnacles of science such as the pyramids, which still remain a mystery to us. There is enough proof, to be sure, that aliens from the outer space or some other intellectually and technologically superior beings from an unknown source have been visiting Earth continuously and still do. Are they trying to 'fix' us or have they given up?