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:

For this eventual emergence, three ingredients were necessary:
 * lipids (cell membranes) in protocells
 * carbohydrates (sugars, cellulose)
 * amino acids (protein metabolism)
 * nucleic acids (DNA and RNA)


 * energy
 * organic molecules
 * liquid water.

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.

Lightning may have provided the spark needed to generate amino acids and sugars from volcanic clouds in an early atmosphere loaded with water, methane, ammonia and hydrogen. Over millions of years, larger and more complex molecules could form.

Ice might have covered the oceans 3 billion years ago and facilitated the birth of life. At normal temperatures compounds such as amino acids are sparsely populated in water, but when frozen become concentrated and facilitate reactions into more complex molecules. Ice also might have protected fragile organic compounds in the water below from ultraviolet light and destruction from cosmic impacts. The cold might have also helped these molecules to survive longer, enabling key reactions to happen.

Global warming is an enhanced greenhouse effect due to human contributions to greenhouse gases, such as the burning of fossil fuels. However, the greenhouse effect is natural and necessary. The atmosphere contains gases that enable sunlight to pass through to the earth’s surface but hinder the escape of reradiated heat energy. Without this natural greenhouse effect which regulates the temperature of the earth's surface, the earth would be much too cold to sustain life. Carl Sagan once wrote, “A little greenhouse effect is a good thing.”

Before RNA, a nucleic acid was possibly the first to emerge as a self-reproducing molecule, yielding sugars and cellulose. Nucleotides are organic molecular units of nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecules within all life-forms on Earth. Nucleotides also play a central role in cell metabolism and provide chemical energy.

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. Life started when RNA chains as DNA molecules began to self-replicate, initiating the selection mechanisms: heritability, variation of type, and differential reproductive output. The main role of DNA is to store information on how other molecules should be arranged. Genetic sequences in DNA are essentially instructions on how amino acids should be arranged in proteins. Mineral crystals in clay could have arranged organic molecules into organized patterns. After a while, organic molecules took over this job and organized themselves.

The first living organism was a prokaryotic cell. They were unicellular and lacked a nucleus or a plasma membrane which protects cells from harm. A nucleus contains chromosomes that house DNA. Mitochondria are the powerhouses of cells that convert glucose to energy. Eventually unicellular cells multiplied into eukaryotic cells, which have a nucleus and reproduce via mitosis where a cell divides into identical cells. These cells combine to form tissue, which in turn forms organs, muscles and a nervous system which sends stimuli and signals through the body. Fungi, animals, plants and protozoa are multicellular organisms built from eukaryotic cells and the first such organism was a sponge likely created in oceanic hydrothermal vents. The flatworm was the first vertebrate with bilateral symmetry, a developed nervous system, and a simple brain.

Bacteria are also unicellular but have cell walls. A virus replicates within cells and because of its ability to infiltrate DNA led to complex life.

Cyanobacteria are the first organisms known to have produced oxygen. By producing and releasing oxygen as a byproduct of photosynthesis, cyanobacteria are thought to have converted the early oxygen-poor atmosphere into an oxidizing one, causing the Great Oxidation Event about 2.4 billion years ago. This created an excess of oxygen which was toxic to most lifeforms, causing the first mass extinction.

Earth was born 4.5 billion years ago. The carbon dioxide that volcanos produced warmed Earth and was a catalyst for habitability. The ozone layer has protected Earth from ultraviolet radiation otherwise no life could have formed. Microbes formed about 4 billion years ago, multicellular organisms about 3.5 billion years ago, protozoans about 2.2 billion years ago, and multicellular animals about 600 million years ago. Multicellular organisms and animals made extinct by the Ordovician–Silurian event 450 million years ago. Sponges and trilobites emerge. Sponges made extinct by the Late Devonian event 375 million years ago. Trilobites became dominant. Trilobites made extinct by the Permian–Triassic event 252 million years ago. Archosaurs became dominant. Archosaurs and amphibians made extinct by the Triassic–Jurassic event 201 million years ago. Dinosaurs became dominant. Dinosaurs made extinct by the Cretaceous–Paleogene event 66 million years ago. Mammals and birds became dominant, and eventually humans.

Trivia
Cell to Singularity is game about evolution, and human civilization, and technology. Start as a single cell organism and progress through the tree of life. Each biological upgrade brings you closer to engulfing an entire planet with a civilization on the brink of technological singularity.