The temperature range for life is very 
very very narrow. Here are 2 goldilocks
zones:

1.
"Chemical reactions could have bound a greenhouse
gas from the atmosphere in minerals (carbon
dioxide to carbonate for instance). This could
have led to a drop in the average temp by just a
few degrees, which could have increased the size 
of the polar ice caps and thus the fraction of
sunlight reflected into space unused 
(termed the albedo, i.e., whiteness of a planet)
Postivive feedback among a reduced energy intake, 
further drop in temps, and further growth of the
ice caps could have led to a planet in the deep
freeze, like our neighbor Mars.

Conversely, we could have ended up in a
planetary oven, considering that the luminosity
of the Sun has increased by some 40 % over the
past 3 billion years. This would have been
more than enough to send us on an overheating
feedback loop, ending in a state comparable to that
of our other neighbor, Venus?  
(Life on the Edge, Gross)

2.
But it just isn't the temperature that matters. Lovelock
has pointed out that it is also the type of sunlight.

"If the Sun were just dull red hot instead of blazing
white, and if the Earth were in a closer orbit and just
as warm as now, no life would be possible. the energy
of light coming from a dull red Sun is insufficient to 
break the chemical bond between carbon and oxygen."

I'm beginning to think that life is so rare that ...
the odds are strongly against us being here now.

Comment?
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