The Archean -- which means "ancient" -- period is believed to be the first time there was life on Earth. Taking place during Precambrian Time, after the Hadean era, the Archean era was 3,800 to 2,500 million years ago. The Earth's atmosphere at that time would have been toxic to most organisms alive today. The life forms that thrived in that environment are the predecessors to all life on Earth.
Eubacteria, commonly called bacteria, are thought to be the very first organisms on Earth, formed between 3,700 and 3,500 million years ago. Eubacteria are monocellular, prokaryotic organisms that lack a nucleus holding DNA. They are necessarily organisms that can survive in very hostile settings, such as thermal vents and hypersaline water. Today, their descendants are called extremophiles. Eubacteria use chemosynthesis, the ingestion of organic molecules, to acquire energy. They also produced fossils that date back to 3,500 million years ago, the oldest recorded fossils.
The first known bacteria to produce oxygen, Cyanobacteria, commonly called blue-green algae, also came about during the Archean period. Cyanobacteria form stromatolites, which are microbes that emit oxygen as a result of their metabolism. At that time, oxygen was a new and toxic gas, and its creation killed off many primitive cells. Like Eubacteria, Cyanobacteria are prokaryotes. The prokaryotes that successfully survived and evolved were able to generate their own food and energy, many of them through the use of photosynthesis.
Approximately 2,700 million years ago, the first eukaryotes came in existence. Eukaryotes are organisms whose cells include a nucleus. Modern day organisms like plants, animals, fungi and amoeba that are also eukaryotic organisms descend from the first eukaryotes.
As the Archean period came to a close, the first multicellular, soft-bodied organisms started to appear. These organisms included jellyfish, sea pens and worms.
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