Proterozoic Era: Stratigraphy

The period of Earth's history that began 2.5 billion years ago and ended 544 million years ago is known as the Proterozoic; it is divided up, rather arbitrarily, into the Paleoproterozoic (2.5 to 1.6 billion years ago), Mesoproterozoic (1.6 billion to 900 million years ago) and Neoproterozoic (900 to 543 million years ago). Near the beginning of the Proterozoic, stable continents first appeared and began to accrete, a long process taking about a billion years.

Ancient Global Pollution

The first "pollution crisis" hit the Earth about 2.2 billion years ago. Several pieces of evidence -- the presence of iron oxides in paleosols (fossil soils), the appearance of "red beds" containing metal oxides, and others -- point to a fairly rapid increase in levels of oxygen in the atmosphere at about this time. Oxygen levels in the Archaean had been less that 1% of present levels in the atmosphere, but by about 1.8 billion years ago, oxygen levels were greater than 15% of present levels and rising. (Holland, 1994) It may seem strange to call this a "pollution crisis," since most of the organisms that we are familiar with not only tolerate but require oxygen to live. However, oxygen is a powerful degrader of organic compounds. Even today, many bacteria and protists are killed by oxygen. Organisms had to evolve biochemical methods for rendering oxygen harmless; one of these methods, oxidative respiration, had the advantage of producing large amounts of energy for the cell, and is now found in most eukaryotes.

Where was this oxygen coming from? Cyanobacteria, photosynthetic organisms that produce oxygen as a byproduct, had first appeared 3.5 billion years ago, but became common and widespread in the Proterozoic. Their photosynthetic activity was primarily responsible for the rise in atmospheric oxygen.

Graham Shields, at the Institute of Geology in Zurich, Switzerland, has written this on-line research paper on the influence of changing oxygen levels on the early evolution of metazoans.

Read about the Llano uplift in central Texas, which is of Proterozoic age.

Source:

Holland, H.D. 1994. Early Proterozoic atmospheric change. Pp. 237-244. In: Bengtson, S. (ed.) Early Life on Earth. Columbia University Press, New York.