Plausibility of Thousand-Year Life Spans
People up until the Flood of Noah's days lived on average about 900 years. Is it possible that people could really live that long? Or, is the Bible mistranslated or misinterpreted? If you believe evolution happened randomly and was not a designed mechanism, then you probably can't deal with the long life spans of the pre-Flood world. However, if humans were designed by God then there is no reason to doubt the long life spans of Genesis.
Scientifically, there is no reason to believe that very well-engineered organisms could not live indefinitely. All it would take is some masterful genetic coding. Biologists and geneticists, if they're honest, will openly admit that there is no good reason (other than diseases) why humans couldn't live much, much longer if genetically modified. I have seen some evolutionary biologists who have candidly admitted as much [6,7]. There is still a lot to learn about aging and what controls the life span of an organism, but all the more reason to believe that it is really possible. If God, the perfect Geneticist, wanted, He certainly could have made Man able to live many hundreds of years initially.
Valter Longo is one of the small but influential group of specialists in this area [the study of ageing] who believes that an 800-year life isn't just possible, it is inevitable. ... "We're very, very far from making a person live to 800 years of age. I don't think it's going to be very complicated to get to 120 and remain healthy, but at a certain point I think it will be possible to get people to live to 800. I don't think there is an upper limit to the life of any organism." Consider the wide range of average life spans in nature. Fruit flies, some species of which are as genetically similar to humans as primates , live typically only 2 weeks . Some animals can live past four hundred years, like a type of mollusk. Some bowhead whales (mammals like us) can live in excess of 200 years, also . Then you have the middle of the road mammals like mice that live only roughly 3 years, which is still about 78 times as long as fruit flies. On the high end of things, you have plants and simple organisms like sponges that can live thousands of years. One pine tree was about 4,900 years old when it was cut down . One sponge was estimated to be in excess of 2300 years old.
Fruit flies. As I have said, and you may want to check my reference  to verify this, some fruit fly species are genetically very similar to humans. The genetic differences in such a case are minimal and yet the life spans of humans are on the order of 1500 times longer. Small genetic difference can make a huge difference in life spans, clearly. Is it improbable that relatively minor divinely orchestrated changes in Homo sapiens resulted in a huge difference in life spans? Not at all. Man only lived about 13 times longer in Adam's time.
The Drastic Changes in Life Spans
Young earth creationists believe that the environment was much better before the Flood and mutation rates were very low. After the Flood, mutations proliferated and caused people to become less healthy and live shorter lives, the hypothesis goes. The problem is that that is pure speculation and not stated in the Bible at all. All we know is that people lived long, long lives before the Flood, and then afterwards people lived shorter and shorter lives until about the time of David (c. 1000 BC). Even during Moses' time people were still living longer. Amram lived 137 (Ex 6:20). Moses lived 120 years, which is nearly miraculous by modern standards. Aaron also lived a long time by today's standard (123 years). A major problem with this hypothesis is that Noah's son Shem, who was born before the Flood, only lived 600 years. Mutations could not account for him living so much shorter than his father, since mutations generally occur at conception. Even if the post-Flood environment caused health problems for Shem, that does not fit with Noah who lived more than a third of his life after the Flood and seems to have suffered no ill effects. So, this young-earth hypothesis is not stated in the Bible and it is not scientifically likely to be true.
But, the old earth view seems to present even more of a problem in explaining the sudden, rapid decline in life spans after the Flood. The answer, though, is hinted at in the Bible, I believe. First, we learn that there was intermarriage between the "sons of God" and the "daughters of men" (Gen. 6:1-2) both before and after the Flood, which by itself could mean many different things. From genetic research, though, we learn that humans interbred with Neanderthals in Mesopotamia (probably before 60 Ka and the Flood) and perhaps Denisovans (probably after 50 Ka and the Flood), both of which may have been non-human hominids. It is reasonable to believe that the Bible is talking about this interbreeding. Second, we learn that Noah was spotless in his generations (Gen. 6:9), which could easily be a reference to the purity of his bloodline -- meaning he personally was not contaminated by Neanderthal DNA. There is the good possibility that Noah's wife and the son's wives were not pure humans but had been contaminated by Neanderthals. Third, we learn in Genesis 47:8-9 that Jacob was rather unusual in living 130 years [he died at 147], but he was not unusual in his own family; the Egyptians apparently did not live that long, according to the Bible. Egyptian records also indicate that this is true. (By the way, the young earth hypothesis discussed above does not as easily account for this disparity of life spans.) So, apparently Abraham's line was somehow different.
What do these three hints lead to? Well, here is one answer. Adam and Eve were originally genetically superior to previous hominids. Some genetic change took place with Adam, maybe as a result of eating of the Tree of Life or maybe just because a bundle of new genetic information was activated by evolution or by a direct miracle. That genetic change allowed him to live to be 900 years or indefinitely if partaking of the Tree of Life. Neanderthals and other hominids were not able to live any more than 30 to 60 years. Noah's line remained uncontaminated allowing them to all live hundreds of years. But, because there was intermarriage with Neanderthals among other people groups before the Flood, many other people were not living that long anymore. Some of these contaminated women married Noah and/or his sons. This resulted in Shem living much shorter than his father. The other wives of the sons of Noah were likely even more contaminated and resulted in a sharp decline in life spans immediately after the Flood. Due to ongoing, occasional but consistent contamination with Neanderthals and other hominids after the Flood, life spans consistently declined for thousands of years. At the point of the Tower of Babel (c. 50 Ka), when people spread out rapidly across the world, the average life spans had probably decreased to almost modern, short life spans. However, the godly line of Noah, a faithful remnant, understood that marrying the "sons of God" was wrong, and so Abraham's line remained more pure than most other people groups. We see from the account of Abraham getting a wife for Isaac that they were careful not to marry into foreign families (Gen. 24:2-4). As a nice side-effect, Jacob and Isaac and others in that line lived longer than most others living in other parts. And so, all of the Bible pieces of the puzzle fit nicely in this alternative, old-earth hypothesis.
By the way, the people of South and Central Africa live the shortest lives on average in the world (~48 years). Studies have shown that these people groups are genetically closest to the Homo sapiens who lived in Africa before Adam. This implies that the humans who lived in Africa, who migrated from Mesopotamia after the Flood, intermarried more with the ancient Homo sapiens (evidence of this is now coming to light ).
It should go without saying—but I'll say it anyway—that once a human intermarried with a pre-human, the children were all fully human. People either have a spirit or they don't. There's no such thing as a partial spirit. It is the spirit that makes people aware of God and the spirit world and arguably makes us have a great intellectual edge over the animals; it clear that all people groups today are spiritually aware and have the image of God that defines us as 'human,' biblically. The exception to that is the possible case where demons were possessing non-humans, in a minority of cases, especially before the Flood. Such hybrids resulted in 'mighty men,' apparently. Some of the ancient people of Canaan appear to have been this kind of hybrid (cf. Num. 13:33).