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 iron-containing bacteria.

The oxygen released by the algae would be used by the bacteria, which would not then be dependent on significant amounts of atmospheric oxygen.

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 level.

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.