A change came about when we learned how, and where, to look for fossil microorganisms. Elso Barghoorn and his associates have studied polished thin sections of silica-rich cherts from the Gunflint region of northern Minnesota and southern Canada.

With the aid of optical and electron microscopes they have found a rich collection of fossil bacteria, blue-green algae, fungi, and other microorganisms with unknown present-day relatives. Two examples are shown on the previous page.

Their association with banded iron ore formations means that these cherts probably were laid down under reducing conditions, and radioisotopic methods date them at 1.8 billion to 2.1 billion years old.

The Gunflint cherts also were found to contain pristane and phytane, diagrammed on the right. These are organic compounds that can occur as breakdown products of chlorophyll, and have been regarded as possible evidence for photosynthesis.

Other microfossil deposits around 2.7 billion years old, from Australia, Rhodesia, and South Africa, contain what appear to be fossil remains of bacteria and blue-green algae.

The oldest sediments with true microfossils are the Fig Tree cherts from the Transvaal, and the Onverwacht sediments from Swaziland, both in South Africa. The Fig Tree cherts, which are 3.1 billion years old, contain fossil bacteria that are spheroids resembling blue-green algae, filamentous organic structures, and complex hydrocarbons including pristane and phytane.

The Onverwacht sediments are more than 3.2 billion years old, and are carbon-rich cherts containing spheroids and filaments that possibly are of biological origin.