sitchin says (and i think this is possibly credible) that the nefilim came from a planet (twelth planet of our solar system he says called nibiru) but
this planet (or in my opinion, whatever planet they came from) had a very long orbit round the sun (3600 years) so, whereas our earth year is 365
days, their nibiru year was 3600 earth years. So assuming that biological life forms on a planet develop in correspondence to the planets orbit;
gravity and other attributes; you might have a nibiruan turning 1 when they are actually 3600 earth years old. So living on earth, a nibiruan would
seem to be almost immortal - hence the gods being called immortals... 'a thousand ages in thy sight are like an evening gone' etc.
and maybe if their planet had different gravity etc, they could be giants, or they could run really fast, or be really strong (like superman) on our
planet, in different gravity conditions,e tc.
there are many tales of ancient gods having superhuman abilities: achilles could run/fight really fast; ajax was a giant; chinese gods came from the
land of snow and could run as fast as a train; see the american indian stories of skinwalkers (and modern sightings) who are beings who can supposedly
keep up with a car driving fast on a freeway...
(just to freak you out..)
alternatively, maybe our planets magnetic field was different at one time in the earth's past, and maybe this explains why animals and trees and
insects were a lot bigger in ancient times - dinosaurs; giant sharks; giant crocs; giant mammals; giant insects; huge trees... (giant humans???
'there were giants on the earth in those days.." the bible; david and goliath; hercules and tiryns; etc) and perhaps this also explains why before
the flood (a catastrophic global event that maybe affected our planets magnetic field???), humans lived a long time; but after the flood, acc to the
bible, the ages of humans starts to decrease from 900 years to around 120 years.
?
But note that the 'gods' are still 'immortals' - poss cuz they come from another planet?
to an ant or a butterfly, we probably seem immortal, as they only live a few days. Maybe to a lifeform on a fast rotating, small planet close to the
sun, we would seem to live forever and be called immortals?