The heterotrophic hypothesis is a proposal from the branch of evolutionary biology that maintains that the first living organisms were heterotrophs; that is, those incapable of synthesizing their own energy.
The term heterotroph comes from the Greek "heteros" (others) and "trophes" (eat). Heterotrophs obtain their energy and raw material by ingesting organic molecules, or other organisms.
The heterotrophic hypothesis was first named by Charles Darwin.
Origin of the hypothesis
The heterotrophic hypothesis was first mentioned by scientist Charles Darwin in one of his letters with JD Hooker. In the letter, Darwin wrote:
«… How great if we could conceive in some small hot pond with all kinds of ammonia and phosphoric salts, light, electricity, that a protein compound was chemically formed today such matter would have been eaten or absorbed, which would not have been the case before of which living creatures were formed «.
In the 20th century, scientists Aleksandr Oparin and John Haldane proposed similar theories in favor of the heterotrophic hypothesis, becoming known as the Opadin-Haldane hypothesis.
According to this proposal, the sea became a hot and diluted soup of organic compounds. These compounds aggregated to form coacervates, until organic compounds were assimilated in a manner similar to metabolism.
Stanley Miller and Harold Urey experiments
It wasn't until 1950 that biochemists Stanley Miller and Harold Urey managed to recreate the atmosphere of Earth's origin over a body of water, known as the Miller-Urey experiment.
Urey and Miller created a gas chamber with electrodes to recreate the atmosphere of the time, and they ran the experiment for a week. At the end of the experiment, they found the formation of organic compounds from inorganic compounds previously in water.
This experiment corroborated the existence of coacervates, proposed by Oparin at the beginning of the century.
The Miller and Urey experiment has created skepticism in the scientific community. This one proposed a window of evolutionary research, and has been recreated by other scientists.
A recent experiment found a higher number of amino acids than those reported by Miller and Urey.
Urey and Miller created a gas chamber with electrodes to recreate the atmosphere of the time, and they ran the experiment for a week.
The question about the possibility of accurately recreating the atmosphere of bygone times in the laboratory remains unanswered.
Heterotrophic organisms
Life on earth dates back 3.5 billion years. During this period, the atmosphere was composed of hydrogen, water, ammonia, and methylene. Oxygen was not part of it.
Today, scientists are studying the atmosphere and its importance in creating the first biological molecules, such as proteins, nucleotides, and adenosine triphosphate (ATP).
A possible proposal explains the union of molecules to form complex compounds, and thus be able to carry out metabolic processes. This joint work brought the first cells, specifically heterotrophs.
The heterotrophs are unable to produce their own source of energy and food, so they consumed other organisms from the hot soup described by Haldane.
The metabolic processes of heterotrophs released carbon dioxide into the atmosphere. Eventually, carbon dioxide in the atmosphere allowed the evolution of photosynthetic autotrophs, capable of synthesizing their own food through energy and carbon dioxide.
References
1. Flammer, L., J. Beard, CE Nelson, & M. Nickels. (199). Ensiweb. Evolution / Nature of Science Institutes: Heterotroph Hypothesis. University of Indiana.
2. Darwin, Charles (1857). Darwin Correspondence Project, “Letter no. 7471, ”University of Cambridge.
3. Gordon-Smith, C. (2002). Origin Of Life: Twentieth Century Landmarks.
4. Miller, S., & Urey, H. (1959). Organic Compound Synthesis on the Primitive Earth. Science, 130 (3370), 245-251. Retrieved from jstor.org
5. Haldane, JBS (1929/1967). "The origin of life". The Rationalist Annual. Reprinted as an appendix in JD Bernal 1967, The Origin of Life. Weidenfeld & Nicolson, London
6. McCollom, T. (2013). Miller-Urey and Beyond: What Have We Learned About Prebiotic Organic Synthesis Reactions in the Past 60 Years? Annual Review of Earth and Planetary Sciences 2013 41: 1, 207-229