Article 19: Many of the most exciting events during the history of the Earth and of life occur during the Proterozoic. Stable continents first appear as well as the first living organisms.
The period of the Earth’s history that begins 2,5 billion years ago and ends 542 million years ago is known as the Proterozoic.
The name Proterozoic comes from Greek and means earlier life.
Cyanobacterial stromatolites in the Early Proterozoic (Source: K.M. Towe, NASA)
About 2,5 billion years ago, enough shield rock has formed to start recognizable geologic processes such as plate tectonics. Geology is about to be joined by biology to continue Earth’s progress, from a molten hell to a living planet. Evidence for the first known glaciations occurs during the Proterozoic; one starts shortly after the beginning of the eon.
During the Precambrian, the most important events in biological history take place. Consider that the Earth forms, life arises, the first tectonic plates arise and begin to move, eukaryotic cells evolve, the atmosphere becomes enriched in oxygen and, just before the end of the Precambrian, complex multicellular organisms, including the first animals, evolve.
Many of the most exciting events during this history of the Earth and of life occur during the Proterozoic. Stable continents first appear and begin to accrete, a long process taking about a billion years. Also coming from this time are the first abundant fossils of living organisms, mostly bacteria and archaeans – still alive and kicking in our intestines, producing methane, in dental plaque and in the vagina – but by about 1,8 billion years ago eukaryotic cells appear as fossils too. About 1,6 billion years ago comes the first evidence of oxygen build-up in the atmosphere. This global catastrophe spells doom for many bacterial groups, but makes the explosion of eukaryotic forms possible. These include multicellular algae and, toward the end of the Proterozoic, the first animals.
The first traces of life appear nearly 3,5 billion years ago, in the early Archean. However, clearly identifiable fossils remain rare until the late Archean, when stromatolites, layered mounds produced by the growth of microbial mats, become common in the rock record. Stromatolite diversity increases through most of the Proterozoic. Until about one billion years ago, they flourish in shallow waters throughout the world.
Their importance for understanding Proterozoic life is tremendous; stromatolites that have been silicified often preserve exquisite microfossils of the microbes that made them. Stromatolites begin to decline in abundance and diversity about 700 million years ago. A popular theory for their decline (though certainly not the only possible explanation) is that herbivorous eukaryotes, perhaps including the first animals, evolve at about this time and begin feeding extensively on growing stromatolites. Stromatolites are rare fossils after about 450 million years ago. Today, as mentioned earlier, they are found only in restricted habitats with low levels of grazing, such as the shallow, saline waters of Shark Bay, Australia.
The oldest fossil that may represent a macroscopic organism is about 2,1 billion years old. There are tantalizing hints from trace fossils and molecular biology that animals may have appeared as much as one billion years ago. However, the oldest, relatively non-controversial, well-studied animal fossils appear in the last hundred million years of the Proterozoic, just before the Cambrian explosion.
So after the Earth forms, it takes nearly 4000 million years before the first animals leave their traces on the planet. This span of time makes up roughly seven–eighths of the Earth’s history.
The time from 635 million years ago to 542 million years ago, known as the Ediacarian Period (sometimes still called the Vendian), sees the origin and first diversification of soft-bodied organisms. This period and the fauna as well are named after the Ediacara Hills of Southern Australia, where the first abundant and diverse fossils of this kind are found.
Earth’s first animals have soft bodies. This illustration shows a community of soft-bodied Ediacaran animals. Some species resemble living ocean creatures. Others are unlike any known organisms and cannot be classified. Scientists have found fossils of these fauna in sedimentary rocks worldwide. Credit: Smithsonian Institution
The first pollution crisis hits the Earth about 2,2 billion years ago. Several pieces of evidence — the presence of iron oxides in 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. Atmospheric oxygen levels in the Archean have been less than 1% of present levels.
But by about 1,8 billion years ago, oxygen levels are greater than 15% of the present levels and rising. 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 uni-cellulars are killed by oxygen. Organisms have to evolve biochemical methods for rendering oxygen harmless; one of these methods, oxidative respiration, has the advantage of producing large amounts of energy for the cell and is now found in most eukaryotes.
Where is the oxygen coming from? Cyanobacteria, photosynthetic organisms that produce oxygen as a byproduct, have first appeared 3,5 billion years ago, but become common and widespread in the Proterozoic. Their photosynthetic activity is primarily responsible for the rise in atmospheric oxygen and for our existence as well.
The Proterozoic is a tectonically very active period in the Earth’s history. It is believed that 39% of modern continental crust formed in the Archean, 43% in the Proterozoic and only 18% in the periods since then.
This map illustrates the break-up of the supercontinent Rodinia, which formed 1100 million years ago. The Late Precambrian was an ‘Ice House’ World, much like the present-day. Credit to Christopher R. Scotese.
This Ice House was ‘Snowball Earth’, a deep freeze that began around 715 million years ago and held Earth in its icy grip for about 120 million years.
Credit to Stocktrek Images
BBC’s Kate Ravilious reporting: “The ice brought Earth to a standstill. Where there were once waves lapping onto a tropical shore and warm waters teeming with life, there was just the whistling of the wind and a cold barren landscape, covered in ice as far as the eye could see. Even at the equator – the warmest place on Earth – the average temperature was a frigid -20°C, equivalent to modern-day Antarctica. Most life was wiped out, and the creatures that did survive huddled in small pockets of open water, where hot springs continued to bubble up.
“However, some scientists now believe that this crushing catastrophe drove one of the most incredible steps in evolution: the development of the first animals and a dramatic flourishing of life known as the Cambrian explosion. So why on Earth would this period of extreme cold cause life to switch gear so rapidly? Maybe because it pumped lots of life-giving oxygen into the air. The idea is that the ice gave a boost to microscopic plants, which released oxygen as a waste product. Finally, around 660 million years ago, Earth’s volcanoes topped up the atmospheric carbon dioxide enough to haul the climate out of its frozen state.”
Thanks to the University of California, Berkeley, Museum of Paleontology, to LiveScience, and to BBC’s Kate Ravilious