There are many theories of how life began on Earth. The "primordial soup" theory suggests that from Earth's inorganic molecules amino acids were formed. These amino acids gave way to more complex organic molecules which evolved. The "panspermia hypothesis" holds that life on Earth was seeded by molecules that arrived on Earth via meteorites. The chemoautotrophic hypothesis suggests that life began in the depths of the oceans at hydrothermal vents where hydrogen sulfide was the energy source. Whichever hypothesis is correct, one thing is certain; the oldest fossils found are the remains of microbes
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The oldest fossils on Earth are around 3.8 billion years old. They are in the form of stromatolites, which are dome shaped rock structures. Through exhaustive research, scientists found evidence of the microorganism cyanobacteria, also known as blue-green algae. Cyanobacteria are self-feeding organisms that use inorganic molecules, such as carbon dioxide and light, in the metabolic process of photosynthesis. This discovery has helped to understand the transformation of Earth's carbon dioxide rich atmosphere into one with enough oxygen to support other life forms
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