Answer 1:
There are a number of experiments currently
happening to study the origins of life on Earth.
Some of these look to thermal vents at the bottom
of the ocean while others look to the chemistry of
the atmosphere and sources of energy such as
lightning. The important early experiment in this
study was performed by Stanley Miller and Harold
Urey in 1953. They looked at the atmospheres of
major planets such as Jupiter and hypothesized
that Earth's early atmosphere (before the hydrogen
drifted into space) would resemble these other
planets. I think this is the atmosphere you refer
to. More recent research suggests that the Earth's
early atmosphere was more rich in nitrogen, carbon
dioxide, and water vapor than Miller and Urey
assumed. This more oxidizing atmosphere would be
less accommodating for early amino acid
development but some people still think it would
have been suitable for early life.
The exact answer remains unresolved.
Visit the interesting site
here about new theory of earth's early
evolution
The following
link talks about how life adapted to Earth's
changing early atmosphere.
http://www.space.com/scienceastronomy/planetearth/earth_archaean_010716.html |
Answer 2:
Well at the time of the UREY MILLER famous
experiment, the prevailing opinion was that the
Earth's atmosphere was a reducing one. That is,
there was not only no free oxygen (O2)
but that diatomic hydrogen was present. Hence the
primary species in the reducing atmosphere would
have been
H2,CH4,NH3 and
H2S. However, since that time the
pendulum has swung a bit and most geologists
believe that the most likely state of the Earth
early atmosphere was a non oxidizing one (so
called Abelson model). In this model again there
is NO FREE OXYGEN to speak of. However the primary
constituents are CO2, H2O,
SO2/SO3 and N2.
It is more difficult to synthesize by electrical
discharges etc.amino acids in an Abelson
atmosphere than a Urey-Miller (reducing)
atmosphere.There are not many robust constraints
we have on the composition of the early atmosphere
although there are ways to get at the problem.
Free oxygen (O2) first accumulated
about 2 billion years ago...due to photosynthetic
bacteria
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