For unambiguous evidence of photosynthesis one must return to
no more than 1.6 billion years ago, to limestone deposits identical
to those produced today in hot springs by blue-green algae.
These deposits, called stromatolites, are scattered widely over
the world. Some in Rhodesia are as much as 2.7 billion years old.
The 1.6-billion-year-old stromatolites in the western Sahara are
unusual in that they contain alternating layers of CaCO3
and Fe(OH)3 as if they were laid
down by colonies of photosynthetic blue-green algae and O2-respiring
The oxygen released by the algae would be used by the bacteria,
which would not then be dependent on significant amounts of atmospheric
It is likely that such mutual aid, or symbiosis, was common in
this era, with respirers living next to and using the oxygen from
photosynthesizers, just as bacteria live in mixed colonies in sewage
and swamps today, with one species being dependent on the waste
products of another species for its food or raw materials.
It is not necessary to assume that oxygen respiration had to wait
for the complete conversion of the atmosphere to oxidizing conditions
before it could develop.
It is clear that organisms resembling bacteria and blue-green
algae were in existence 3 billion years ago, and is probably true
that some of these organisms were photosynthetic and oxygen-liberating.
Well over a billion years may have been required for photosynthetic
life to pour so much O2 into the
atmosphere that its character was changed. By 1.6 billion years
ago, oxygen-emitting photosynthesis and oxygen-using respiration
were in full swing.
It is encouraging that the date for the Sahara stromatolites falls
right in the middle of the atmospheric transition period predicted
from the oxidation states of iron deposits.
What is remarkable is that the South African rocks from the Transvaal
and Swaziland tell us that less than 1.5 billion years elapsed from
the condesation of the Earth to the evolution of life at the bacterial
As an indication of how difficult the next step - the development
of eucaryotes - was, this second step required fully as much time
as the creation of the planet and evolution of bacteria.