Inbreeding played an attribute to the supply of life in the beginning, and that attributed many flaws into the genetic system.
But I think the most affecting factor ni our life span is this: the atmospheric and world change that the Flood brought with it. Check out even the Sumerian King List which basically gives the records of so many kings, from the beginning to the writer's time. ALL the kings show insane amounts of lifespan, into the thousands, which, you know, goes along with the Biblical accounts of the ages of say, Adam and his descendants, at least six of them over nine hundred years. Then after a recorded flood that their deities made, the kings' lifespans were drastically shortened, even to 126 for our Uruk warrior king Gilgamesh.
To my next point, we must begin with the Sumerian deities: they freaked over the noise the humans were making below them, and thus decided to wipe them out with, gasp, a flood. So Ea, being the deity of wisdom and all, decided to warn Atrahasis about it and to save himself by making a boat and to save animals and the boat's craftsman, so he tears his house down and makes a boat out of it, in which he saves animals and the craftsmen of the boat. So after a week of rain, the gods subside their wrath, because they are fed by the people's sacrifices.
Wowza! God wiped us out for wickedness, and saved those who were righteous. The Sumerian gods wiped out the humans for their noise, but saved some for service. The parallels are eery.
And then, according to both texts, our lives were shortened...related? I think so.
I think our lifespans were significantly longer pre-Flood. Obviously, so do the Aztecs, and the Aztecs also think that a man came on a boat during a great flood that wiped out the world, and he gave life back to the world. Hrmm. The Aztecs, the Babylonians, AND the Hebrews all agree, along with about 2000+ other cultures. This is getting scary.
Most of these cultures all agree that after these great devastating waters, the years of man went down. Maybe due to atmosphere? That cloud canopy, protecting from harmful sun rays, a misty atmospheric irrigator, near-direct genetic descendence from the first man, and a life span of 900 years, are all connected. Something catastrophic completely changed what man was used to. Even God said prior the Flood, "My Spirit will not contend with man forever." Something was going to happen to alter this near-millennial age limit set to his people. Something happened.