Page 202 - Introduction to Paleobiology and The Fossil Record

THE ORIGIN OF LIFE 189
then from the carbon isotopes. The carbon
atom has two stable isotopes, carbon-12 and
13
12
carbon-13, usually written as C and C.
13
13
12
The ratio of C to C, usually written δ C,
can indicate the presence or absence of organic
residues of previously living organisms: enrich- is no compelling evidence for this. Perhaps the
12
13
ment in C relative to C is characteristic of
photosynthesizing organisms, and the organ-
isms that eat them. Rosing and Frei (2004)
13
reported values of δ C in organic matter from
the Isua Group rocks that match those of
modern living organic matter, and these might
have come from plankton in the oceans that
were photosynthesizing. This is a dramatic
claim, and it has been disputed, but if
true, this is the first evidence for life on
Earth.
The Archaean world was anoxic: when did
oxygen become a part of the atmosphere, and
why?
The “great oxygenation event”
The Proterozoic Eon, from 2.5 Ga to 542 Ma
(Fig. 8.3), represents a very different world
from the Archaean. Archaean atmospheres
contained volcanic gases, but no oxygen.
Oxygen levels are maintained in the atmo-
sphere today by the photosynthesis of green
plants and cyanobacteria, and the latter were
the source of the initial buildup of oxygen
during the first part of the Precambrian. Then,
2.4 Ga, atmospheric oxygen levels rose to
one-hundredth or one-tenth of modern levels,
not much perhaps, but an indicator of a com-
plete change in the global system that has
been dubbed the “great oxygenation event”
(GOE). What caused this dramatic rise in
oxygen?
The fi rst organisms had anaerobic metabo-
lisms, that is, they operated in the absence of
oxygen. Indeed the fi rst prokaryotes would
have been killed by oxygen. This is a shocking
fact that is confirmed by living microbes:
some can switch from anaerobic to aerobic
respiration depending on oxygen levels.
Others, though, are obligate anaerobes that
have to respire anaerobically and cannot
survive even the smallest amount of oxygen.
Did living things generate suffi cient oxygen to
change the Earth’s atmosphere? Early photo-
synthetic bacteria did not produce oxygen,
and some have argued that modern styles of
photosynthesis that liberate oxygen arose
about 2.4 Ga. However, there is evidence
from biomarkers (see below) that this had
happened by 2.7 Ga. Others have proposed
that the oxygen built up after a dramatic
reduction in volcanic activity; however, there
secret lies in methane.
David Catling and colleagues at the Univer-
sity of Washington in Seattle proposed that
methane was much more abundant in the
Archaean atmosphere than today. Methane
(CH 4 ) is a key product of the activities of
anaerobic microbes that use a form of anaero-
bic respiration called methanogenesis to
breathe. Today, methane is consumed by
oxygen in the atmosphere, but in the absence
of oxygen Archaean methane levels might
have been 100–1500 times as much as today.
Methane is also a potent greenhouse gas,
which would help explain why the early Earth
did not freeze over, given that the 4.0 Ga sun
was about 25–30% less luminous than today.
Methane can diffuse up to the outer fringes
of the atmosphere, where it is decomposed by
ultraviolet light and the liberated hydrogen
atoms are lost into space. In a world without
the escape of hydrogen, Catling and Claire
(2005) have calculated that oxygen would be
mopped up continuously by gases released by
volcanism and metamorphism, as well as by
soluble metals in hot springs and seafl oor
vents, and the world would remain forever
anaerobic. With high Archaean methane
levels, hydrogen atoms were transferred out
of the Earth’s atmosphere, and the oxygen
was not all locked up in water molecules but
eventually fl ooded out as an atmospheric gas.
The collapse of the methane greenhouse
2.4 Ga may have triggered glaciation
worldwide.
The rise of oxygen in the atmosphere had
a profound effect on life and the planet. New
aerobic organisms arose that exploited the
atmospheric oxygen molecules in their chemi-
cal activity. The oxygen also built up a strato-
spheric ozone layer that blocks out solar
ultraviolet radiation. The ozone layer has
been hugely important since this point in the
earliest Proterozoic in blocking solar rays
harmful to life, which allowed diverse life to
colonize the land surface
After the GOE, oxygen levels remained
low, perhaps 1–5% of present levels, for as
much as 1 billion years. In the Archaean,