No evidence exists today of their presence some 3.5 to 4 billion years ago. Likewise, their demise is obscured in the geological record. But the case for chemosynthetic life forms being first comers to the biosphere is still strong. If they existed, then their disappearance into the void constitutes the first great mass extinction on the planet.
Chemosynthetic bacteria derive their energy from converting inorganic molecules into organic substances. They don’t need sunlight, and they don’t require existing organic materials. Technically their energy is derived from the chemical oxidation of inorganic molecules.
These bacteria thrive in some of the most inhospitable environments on the earth. They call hydrothermal vents at the bottom of the ocean home. The bone-crushing pressures and hellish temperatures around these vents make them happy. They also thrive deep in the polar ice, hidden in remote caves, and even in petroleum deposits.
Chemosynthetic organisms come with a set of skills that make them ideal for the harsh conditions on the early earth. Because they feed on substances such as ammonia, molecular hydrogen, sulfur, hydrogen sulfide, and ferrous iron, their “food” would have been readily available in the deep oceans around volcanic and hydrothermal vents.
Oxygen
One way to split up organisms is between aerobic and anaerobic. Aerobic microorganisms thrive in the presence of oxygen, but anaerobic microorganisms don’t require oxygen to survive. Also, many anaerobic bacteria die in the presence of oxygen. Free oxygen is toxic to them. The oxygen issue is essential when considering early life since the atmosphere and oceans of early Earth were anoxic. This lack of oxygen in the earth’s earliest biosphere, therefore, means that oxygen would probably have been toxic to early chemosynthetic life.
Chemosynthetic organisms stayed at the top of the evolutionary pyramid until cyanobacteria learned the trick of photosynthesis some 3.5 billion years ago. The atmosphere at the beginning of the Archean Eon was anoxic, lacking any significant component of free oxygen. But the cyanobacteria thrived under the eye of an ancient sun. They used their photosynthesis trick to convert free solar energy into chemical energy.
For a billion years, algae dominated the biosphere. Ever so slowly, molecule by molecule, the algae took CO2 + H2O + sunlight and produced the sugars needed for life. However, all actions have unintentional consequences, and a byproduct of photosynthesis was the free oxygen (O2) that the algae discarded. This slow leakage of oxygen, first into the oceans and later into the atmosphere, was fatal to the existing chemosynthetic life.
The first wave of life
No direct evidence exists of an early Archean world populated by chemosynthetic bacteria. But an evolutionary jump from raw DNA directly to cyanobacteria seems unlikely. How did life start in a world devoid of oxygen and organic molecules? The first organisms to develop a cellular structure needed energy. However, the only raw materials they had to work with were inorganic molecules. Chemosynthetic bacteria are the most likely answer to this riddle.
The rise of oxygen-producing cyanobacteria was also the demise of an entire ecosystem of chemosynthetic life. First-life became an anaerobic ecosystem laid to waste by evolution as the ocean oxygenated. This event was probably the first mass extinction on the planet earth.
ArcheanWeb:
Mass Extinction Events: Life’s Struggle for Survival (Source: ArcheanWeb) – https://archeanweb.com/2019/12/01/mass-extinction-events-lifes-struggle-for-survival/ Also:
Sources:
Chemosynthesis Definition and Examples (By Anne Marie Helmenstine; ThoughtCo.) – https://www.thoughtco.com/chemosynthesis-definition-and-examples-4122301 Also:
What Are Chemosynthetic Bacteria? (By Jose Juan Gutierrez; Owlcation) – https://owlcation.com/stem/What-are-Chemosynthetic-Bacteria Also:
Feature Image: Chemosynthetic Microbial Mats (Source: Submarine Ring of Fire 2004 Exploration, NOAA Vents Program) (Modified) – https://commons.wikimedia.org/wiki/File:Chemosynthetic_Microbial_Mats.jpg – This work is in the public domain in the United States because it is a work prepared by an officer or employee of the United States Government as part of that person’s official duties
