Using logic and reasoning all I can offer as a layman is that a greenhouse canopy type atmosphere would have had different properties than ours which would likely affect the whole ecosystem of the planet which in turn would likely render modern dating methodoly inaccurate. This would likely be the reason that humans lived multiple century lives as per the Biblical record.
Not to put words into your mouth but what does a greenhouse canopy atmosphere look like? Are you talking a variance in the composition of the atmosphere like say higher concentrations of carbon dioxide and changes in the concentrations of other gases in the atmosphere?
And at this point you go one step further and you start asking questions like how do gases interact with the human body? And at a physiological level what do they do? How would variance in atmospheric gas concentrations affect the human body? How does this translate into contributions of life expectancy?
Some helping info:
Oxygen ToxcicityArterial Blood GasHemoglobinCarbon Dioxide Poisoning
Edited by Discreet Label, : Added Links
Edited by Discreet Label, : Cleaned up link typos
™ Version 4.2
