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
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.