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Ancient Oceans and Fossils

Updated: Jan 28, 2022

By Rosalyn Brady
December 19, 2021

[Photo Credit: Smithsonian Ocean - A Collection of Cambrian Fossils]

Among many theories speculating the origins of life, the most popular of these is the hydrothermal vent theory, hypothesizing that life began in hydrothermal vents in the depths of Earth’s ancient oceans. Evidence for this theory includes fossilised microbes found in sedimentary rock in the Nuvvuagittuq greenstone belt, a belt of sedimentary rock near the eastern shore of Hudson’s Bay. These rocks have been interpreted as sediment from the emissions of hydrothermal vents on the ocean floor, and they date back between 3.7 and 4.2 billion years - to around the time when life began. Similar fossilised microbes have also been found in the Barberton greenstone belt near the coasts of South Africa and Swaziland dating back between 3.5 and 3.2 billion years, also within sedimentary rock formed by the emissions of hydrothermal vents. Additionally, some of the most primitive life forms, such as Archaea microorganisms and prokaryotic organisms - single-celled organisms without any organelles - are found near hydrothermal vents.

Ancient ocean fossils reveal a plethora of information regarding how life evolved beyond hydrothermal vents. For example, fossils called stromatolites, the oldest of which dates back 3.5 billion years, tell researchers that photosynthesis was already present in bacteria by this time. This is because stromatolites are formed by cyanobacteria, prokaryotic bacteria that perform photosynthesis, trapping sediment, which then forms sedimentary rock with cyanobacteria at its surface. Stromatolites’ appearances in shallow saltwater reveals that the cyanobacteria that formed them had begun to surface at least 3.5 billion years ago in search of light to use for photosynthesis.

Ancient ocean fossils also inform researchers of the Cambrian explosion, a time when organisms became more complex and began to develop hard body parts, such as shells made of calcium carbonate, a compound of calcium and carbon. Fossils inform us of this because the hard body parts evolved by organisms significantly increased the amount of fossils waiting to be unearthed by paleontologists. Fossil dating, which determines the age of a given fossil, also allows us to use the age of Cambrian fossils to determine when the Cambrian explosion occurred: between 541 and 530 million years ago.

Ocean fossils reveal creatures that went extinct long before humans emerged, and how they evolved. Fossil examination tells palaeontologists that the first vertebrates evolved during the Ordovician period, between 485 and 443 million years ago - the age of the first fossils containing backbones. It also shows us what animals that lived before the Permian extinction wiped out 90% of all marine life looked like. For example, fossils have informed palaeontologists of bony-plated fish called placoderms and of a primitive shark species with a spiralled set of teeth called Helicoprion, which are both extinct today.

Fossils have sparked intense interest in evolution - researchers have recreated the chemical reactions between hydrogen and carbon dioxide that formed the first cells in acidic hydrothermal vents, and discovered that the Cambrian explosion was enabled by the oxygen that photosynthesis created, which led organisms to develop traits such as vision and protective shells that aided their survival when organisms began preying on each other, by expanding on fossil discoveries.

This knowledge is applicable in the search for extraterrestrial life. Enceladus, a moon of Saturn, may have hydrothermal vents according to NASA’s Cassini spacecraft, which discovered activity exceeding 90 degrees Celsius beneath its oceans. Extraterrestrial moons Titan, Europa, Ganymede, and Callisto are also known to have oceans, as was ancient Mars, as evidenced by signatures of H2O and HDO - water, and a heavier form of water in which one hydrogen atom has an additional neutron, forming deuterium - on its surface.

Could these celestial bodies hold, or have held, life? And could this be the next discovery that ancient ocean fossils lead us to?


[1] “Cambrian Explosion | Paleontology.” Encyclopedia Britannica, Accessed 12 Nov. 2021.

[2] “What Sparked the Cambrian Explosion?” Nature, 16 Feb. 2016,

[3] “What Are Stromatolites?” Bush Heritage Australia, Accessed 12 Nov. 2021.

[4] “Fluid Chemistry of Archean Seafloor Hydrothermal Vents: Implications for the Composition of circa 3.2 Ga Seawater.” ScienceDirect, 1 Oct. 1997,

[5] “Life at the Hydrothermal Vents | AMNH.” American Museum of Natural History, Accessed 12 Nov. 2021.

[6] Smithsonian Ocean. “Ocean Through Time.” Smithsonian Ocean, 20 May 2019,

[7] Ucl. “Deep Sea Vents Had Ideal Conditions for Origin of Life.” UCL News, 7 Nov. 2019,

[8] Choi, Charles. “Hydrothermal Vent Experiments Bring Enceladus to Earth.” NASA Astrobiology, Accessed 12 Nov. 2021.

[9] “NASA Research Suggests Mars Once Had More Water Than Arctic Ocean.” NASA, Accessed 12 Nov. 2021.

[10] Dodd, Matthew. “Evidence for Early Life in Earth’s Oldest...” Nature, 2 Mar. 2017,

[11] Homann, Martin. Earliest Life on Earth: Evidence from the Barberton Greenstone Belt, South Africa. European Institute for Marine Studies, 2019.

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