Why Did Human Lifespans Shorten After the Flood?

The book of Genesis contains a genealogy of men whose lives extended over nine hundred years. Today, however, life expectancies in even the most developed nations only extends into the eighties. How should we understand these ancient numbers? Is it possible for humans to live as long as the early men recorded in Genesis?

What we see in Genesis is that life expectancy remained relatively steady in the nine hundreds until declining exponentially after the great flood. Noah lived nine hundred and fifty years, but his son Shem only lived six hundred and his grandson Arpachshad only made it to four hundred and thirty eight. Eight generations after Noah, Nahor died at a hundred and forty eight, and by David's time the average man lived between seventy and eighty years. How should we interpret these incredible early numbers? Let's explore a few theories.

The first theory is that God only blessed a single family line with exceptionally long lives in order to honor their godly ancestor Seth. The long numbers, then, never represented the average life expectancy of the general human population, they were just miraculous exceptions created by God. If we take the position that the "sons of God" represent the line of Seth, however, then how can we explain God blessing them despite their apparent sin with the "daughters of men?" Furthermore, we have no reason to assume long life was connected with a special blessing on Shem's line because Shem's father Adam lived to nine hundred and thirty.

Another theory suggests a water canopy existed before the flood that blocked out radiation from polluting our atmosphere, this canopy subsequently collapsed after the flood and exposed humans to dangerous elements from outer space. Two PhD thinkers expressed this theory in their 1961 book 'The Genesis Flood' (John Whitcomb and Henry M. Morris).

"in view of what is known about the somatic and genetic effects of radiations to infer that, over the centuries since the Flood, the accumulation of these effects in man in particular has resulted in gradual deterioration and decreasing life-span. Especially marked must have been the effect in the centuries immediately after the Flood… Before the flood, therefore, everything was conducive to physical health and longevity. Equable temperatures, freedom from environmental radiations, and other factors attributable to the vapor canopy all contributed to the effect… After the Flood, the canopy was [gone], its protective effects… removed, and then began a long decline in general health and longevity… Much of this decline… can undoubtedly be attributed to the greatly increased incidence of radiation upon the earth's surface."

The creationist canopy theory has been criticized considerably since it was first proposed. Many creationists have recently abandoned it, but it remains an oft cited explanation for why these early generations had such long lifespans. In my opinion, the theory has never been sufficiently debunked.

A related theory posits general environmental degradation. The flood caused so much destruction to our optimal living environment that post flood humans just couldn't survive for the same lengths to time. Rains, ice ages, animal behaviors, landscape alterations, and other factors contributed to a cumulative negative effect that reduced life expectancy. This cluster of negative variables shrunk our time on earth. However, this raises the question of why humans born immediately after the flood still experienced longer lives than we do today. We'd assume the world was most negatively affected by the flood immediately after it occurred, but we find that people's life spans stabilized at the much lower present limit only centuries after we'd expect the earth's conditions to have recovered somewhat from the trauma. 

Similarly, some have suggested human diets changed. Allegedly, our new post flood consumption of meat shorted our lives. However, almost all our present research and experience casts doubt on this interpretation. Meat provides more protein which allows for better mental and bodily development, and vegetarians probably don't live longer than the rest of us. 

Some see the "hundred and twenty years" in Genesis 6:3 as God establishing a new limit on human lives: "The Lord said, 'My Spirit will not contend with humans forever, for they are mortal; their days will be a hundred and twenty years.'" Was God responsible for reducing our time on earth? This would seem to make sense because no one has lived past a hundred and twenty two, and the oldest man on record died at the age of a hundred and sixteen. However, if God established a new age limit for mankind why did people continue to live well past it for so long after his decree? Abraham was born centuries after the flood and lived a hundred and seventy five years, and his son Isaac lived to a hundred and eighty. It could be argued the limit was slowly implemented over time. However, the more likely interpretation for the "one hundred and twenty years" is that God was going to preserve mankind for that length of time before bringing about the flood.

Another possibility is that the human genome amasses more mutations every generation and the accumulation of them has shortened our lifespans. A blogger named Michael Snyder has discussed this:

"The reason we're heading toward extinction is the increasing number of mutations being passed down from generation to generation. According to Dr John Sanford of Cornell University, every one of us already carries tens of thousands of harmful mutations, and each of us will pass on approximately one hundred new mutations to future generations. Humanity is degenerating at an accelerating pace, and at some point the number of mutations will become so great that we will no longer be able to produce viable offspring."

Humanity is slowing dying. Mutations don't help us evolve or contribute to biodiversity but slowly increase our likelihood of developing illnesses and becoming infertile. However, if this was the reason for humanity's shrinking life expectancy after the flood then why did our lifespans eventually stabilize around the seventy to eighty number mentioned by King David? The flood took place about a thousand and a half years before David was born and it's been about three thousand years since David died, so why haven't our mutations further reduced the lengths of our lives? 

Some have claimed the Tower of Babel incident was the real point at which human life expectancy dropped. The drop was part of God's curse on humanity. The evidence for this comes from Eber outliving six post Babel generations. However, the Bible never mentions this aspect of God's curse on the people of Babel, nor does this theory explain why life expectancy was dropping before God divided humanity.

Finally, some have sought to remove the question of Genesis lifespans altogether by claiming the "years" were really months, or that ten Genesis years represent a single year in our time. This theory creates more problems than it fixes, however, because it results in absurdities like Abraham and Sarah giving birth to Isaac at the age of ten (despite the repeated reminders that they were an old). There are several situations like this throughout Genesis. Multiple men, like Kenan, would of been having kids at exceptionally young ages.  

There aren't many good answers for us to explain why the ancient antediluvian [pre-flood] generations lived into their nine hundreds, but the Sumerian King's List, another ancient pre-flood generational list, records similarly long lifespans for earth's inhabitants. For now, we must simply accept, speculate, and search for more evidence.