User:Aflucke/Heterotroph

Origin and Diversification
The chemical origin of life hypothesis suggests that life originated in a prebiotic soup with heterotrophs. The summary of this theory is as follows: early Earth had a highly reducing atmosphere and energy sources such as electrical energy in the form of lightning, which resulted in reactions that formed simple organic compounds, which further reacted to form more complex compounds and eventually result in life. Alternative theories of an autotrophic origin of life contradict this theory.

The theory of a chemical origin of life beginning with heterotrophic life was first proposed in 1924 by Alexander Ivanovich Oparin, and eventually published “The Origin of Life.” It was independently proposed for the first time in English in 1929 by John Burdon Sanderson Haldane. While these authors agreed on the gasses present and the progression of events to a point, Oparin championed a progressive complexity of organic matter prior to the formation of cells, while Haldane had more considerations about the concept of genes as units of heredity and the possibility of light playing a role in chemical synthesis (autotrophy).

Evidence grew to support this theory in 1953, when Stanley Miller’s conducted an experiment in which he added gasses that were thought to be present on early Earth– water (H2O), methane (CH4), ammonia (NH3), and hydrogen (H2)--  to a flask and stimulated them with electricity that resembled lightning present on early Earth. The experiment resulted in the discovery that early Earth conditions were supportive of the production of amino acids, with recent re-analyses of the data recognizing that over 40 different amino acids were produced, including several not currently used by life. This experiment heralded the beginning of the field of synthetic probiotic chemistry, and is now known as the Miller Urey experiment.

On early Earth, oceans and shallow waters were rich with organic molecules that could have been used by primitive heterotrophs. This method of obtaining energy was energetically favorable until organic carbon became more scarce than inorganic carbon, providing a potential evolutionary pressure to become autotrophic. Following the evolution of autotrophs, heterotrophs were able to utilize them as a food source instead of relying on the limited nutrients found in their environment. Eventually, autotrophic and heterotrophic cells were engulfed by these early heterotrophs and formed a symbiotic relationship. The endosymbiosis of autotrophic cells is suggested to have evolved into the chloroplasts while the endosymbiosis of smaller heterotrophs developed into the mitochondria, allowing the differentiation of tissues and development into multicellularity. This advancement allowed the further diversification of heterotrophs. Today, many heterotrophs and autotrophs also utilize mutualistic relationships that provide needed resources to both organisms. One example of this is the mutualism between corals and algae, where the former provides protection and necessary compounds for photosynthesis while the latter provides oxygen.

Heterotrophs are currently found in each domain of life: Bacteria, Archaea, and Eukarya. Domain Bacteria includes a variety of metabolic activity including photoheterotrophs, chemoheterotrophs, organotrophs, and heterolithotrophs. Within Domain Eukarya, kingdoms Fungi and Animalia are entirely heterotrophic, though most fungi absorb nutrients through their environment. Most organisms within Kingdom Protista are heterotrophic while Kingdom Plantae is almost entirely autotrophic, except for myco-heterotrophic plants. Lastly, Domain Archaea varies immensely in metabolic functions and contains many methods of heterotrophy.