The Kaapvaal Craton in South Africa holds some of the secrets about early life on the earth. Entombed in the rocks are the remains of single-cell prokaryote organisms and other simple algae that gave life its first foothold on the planet.
Craton is the terminology used to designate the geological core or nucleus of a continent. It constitutes a large, stable segment of the earth’s crust around which a continent forms. The North American Craton, for example, encompasses much of the eastern, midcontinent, and western portions of the USA. It then extends northward to underly most of Canada, and even farther north to take in Greenland. But Oregon, Washington, and California are not part of the North American Craton. They formed as accreted terranes that attached to the craton at various points in the past.
The Kaapvaal Craton underlies the northeastern third of South Africa. It also extends into Botswana, the southern edge of Zimbabwe, and the southwesternmost fringe of Mozambique. This craton formed during the Archean Eon between 3.6 and 2.7 billion years ago. But the oldest portions of the Kaapvaal Craton are exposed in the Barberton Greenstone Belt on the eastern edge of the craton. This area is also called the Makhonjwa Mountains.
Greenstone belts contain a variety of rock types. However, they are primarily composed of metamorphosed mafic and ultramafic volcanic rocks mixed in with sedimentary formations. Both mafic and ultramafic are terms indicating that these igneous rocks have low silica content. Greenstone belts are widely interpreted to represent the remains of ancient oceans. Therefore, the presence of sedimentary rock formations in greenstone belts is important in the search for early life.
Creatures existing in the oceans 3.5 billion years ago lived and died in their aqueous home. When they died, some of their remains sank to the ocean floor. Thus, their organic remains, along with silts and clays, accumulated as layers of sediment in these ancient oceans. Eventually, the sedimentary and volcanic rocks underwent burial, limited metamorphism, and then incorporation into the craton.
So, cratons and their associated greenstone belts contain some of the oldest sedimentary rocks on the planet. Therefore, the search for first life on earth inevitably drifts towards these rock formations. The Kaapvaal Craton is no exception. Research into the sedimentary record preserved in Barberton greenstones yields multiple lines of evidence for an ancient world populated by microbial life.
Single-cell prokaryote organisms probably dominated this ancient world. But this first influx of life on earth occurred prior to the existence of significant free oxygen, so early life was probably chemosynthetic. Chemosynthetic bacteria derive their energy from converting inorganic molecules into organic substances. So, they don’t need sunlight, and they don’t require existing organic materials. Technically their energy is derived from the chemical oxidation of available inorganic molecules.
The exact time that life appeared on our planet is impossible to know. We can only search the geological record for the earliest preserved remains of life. But the requirements for preservation are steep. The first requirement is that other creatures don’t completely consume the organic remains. Secondly, the remains must be preserved in the sediments such that their traces are detectable. Once buried, the rocks containing the fossils cannot undergo too much burial, since metamorphism from deep burial destroys the organic remains.
Undoubtedly life is older than the 3.6 billion years registered by the Barberton greenstones. The Isua Greenstone Belt of Greenland dates to 3.8 billion years, and maybe definitive discoveries there will eventually push back the detection of early life another 200 million years.
Chemosynthetic Life: The First Mass Extinction – https://archeanweb.com/2020/04/02/chemosynthetic-life-the-first-mass-extinction/ Also:
Feature Image: Verdite (microcrystalline fuchsite metamorphite) South Africa (Modified)- By James St. John – Verdite (microcrystalline fuchsite metamorphite) (Archean; South Africa), CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=35317571 – (verdite lens or pod in ultramafic host rocks in the Onverwacht Group, Barberton Greenstone Belt, Paleoarchean, 3.30-3.55 Ga; southwest of the town of Barberton, South Africa)