Origin of life

The origin of life or abiogenesis is the natural process by which life has arisen from non-living matter, such as simple organic compounds. There are many theories but the prevailing scientific hypothesis is that the transition from non-living to living entities was not a single event, but an evolutionary process of increasing complexity that involved molecular self-replication, self-assembly, autocatalysis, and the eventual emergence of four key families of chemicals that support life:


 * lipids (cell membranes) in protocells
 * carbohydrates (sugars, cellulose)
 * amino acids (protein metabolism)
 * nucleic acids (DNA and RNA)

Panspermia
Panspermia is the hypothesis that life exists throughout the universe, distributed by meteoroids, asteroids, comets and planetoids. Life likely started in a single location, and then spread about the galaxy to other star systems via cometary or meteorite impact.

An extreme speculation is that the biochemistry of life could have begun as early as 17 million years after the Big Bang, during a habitable epoch, and that life may exist throughout the universe.

Primordial soup
The hypothesis that the first molecules constituting the earliest cells were synthesized under natural conditions by molecular evolution, and these molecules then organized into the first molecular system. This "primeval soup" of organic molecules could be created in an oxygen-less atmosphere through the action of sunlight. These would combine and grow in ever more complex ways until they have a metabolism with cell integrity. The Earth's prebiotic oceans would have formed a "hot dilute soup" in which organic compounds could have formed.

In summary:


 * The early Earth had a chemically reducing atmosphere.
 * This atmosphere exposed to energy produced simple organic compounds called monomers.
 * These compounds accumulated in a "soup".
 * More complex organic polymers, and ultimately life, developed in the soup.

Darwin's little pond
In 1871 Charles Darwin said that the original spark of life may have begun in a warm little pond, with ammonia and phosphoric salts, light, heat, electricity, etc., and that a protein compound was chemically formed ready to undergo still more complex changes. More recent studies in 2017 support the notion that life may have begun right after the Earth was formed as RNA molecules emerging from "warm little ponds". Life may also have begun in other warm watery areas such as submarine hydrothermal vents or hydrothermal pools on volcanic islands.

From protocells to DNA and RNA
A protocell is a collection of lipids which is a stepping-stone to the origin of life. The second law of thermodynamics requires that non-living matter is bound by entropy, yet life is distinguished by its great degree of organization. Therefore, a boundary is needed to separate life processes from non-living matter. Elementary protocells can give rise to cellular behaviours like reproduction and energy storage, autocatalyzing a transition from simple replicating molecules to true cells and eventually to the three properties of life: metabolism, self-replication, and a bilipid membrane.

Thermosynthesis is the bioenergetic process of osmosis, which plays an essential role in cellular respiration and photosynthesis, leading to enzymes which is related to the first metabolic process, and is the primordial first protein. Thermal cycling in germination and cell division is a relic of primordial thermosynthesis. The primordial protein synthesized daughters with similar capabilities, consisting of amino acid sequences.

Before RNA, a nucleic acid was possibly the first to emerge as a self-reproducing molecule, yielding sugars and cellulose.

There are hypotheses about the formation of RNA. Early cell membranes could have formed spontaneously from proteinoids, which are protein-like molecules produced when amino acid solutions are heated while in the correct concentration of aqueous solution. Other possible means of producing more complicated organic molecules include chemical reactions that take place on clay substrates or on minerals.

RNA is important for life due to its abilities to store information, catalyze chemical reactions, and maintain genetic information (in the form of DNA). Life started when RNA chains began to self-replicate, initiating the selection mechanisms: heritability, variation of type, and differential reproductive output.