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The origin of life

By 3.5 billion years ago, the Earth’s magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind.

The first photosynthetic organisms probably evolved about 3,500 million years ago, early in the evolutionary history of life, when all forms of life on Earth were microorganisms and the atmosphere had much more carbon dioxide. They most likely used hydrogen or hydrogen sulfide as sources of electrons, rather than water.

The current model of the evolution of the first living organisms is that these were some for m of prokaryotes. The oldest known fossilized prokaryotes were laid down approximately 3.5 billion years ago, only about 1 billion years after the formation of the Earth’s crust. Even today, prokaryotes are perhaps the most successful and abundant life-forms.

The oldest undisputed evidence of life on Earth from fossilised cyanobacteria, dates to 3,000 million years ago. Other finds in rocks dated to about 3,500 million years ago have been interpreted as bacteria, with geochemical evidence also seeming to show the presence of life 3,800 million years ago. However these analyses were closely scrutinized, and non-biological processes were found which could produce all of the “signatures of life” that had been reported. While this does not prove that the structures found had a non-biological origin, they cannot be taken as clear evidence for the presence of life.

Life on Earth is based on carbon and water. Carbon provides stable frameworks for complex chemicals and can be easily extracted from the environment, especially from carbon dioxide.

Research on how life might have emerged unaided from non-living chemicals focuses on three possible starting points: 1. self-replication, an organism’s ability to produce offspring that are very similar to itself; 2. metabolism, its ability to feed and repair itself; and 3. external cell membranes, which allow food to enter and waste products to leave, but exclude unwanted substances.

Biologists reason that all living organisms on Earth (including you) must share a single last universal ancestor, because it would be virtually impossible that two or more separate lineages could have independently developed the many complex biochemical mechanisms common to all living organisms. As previously mentioned the earliest organisms for which fossil evidence is available are bacteria, cells far too complex to have arisen directly from non-living materials. The lack of fossil or geochemical evidence for earlier organisms has left plenty of scope for hypotheses, which fall into two main groups: 1) that life arose spontaneously on Earth or 2) that it was “seeded” from elsewhere in the universe.

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