Aristotle believed in the spontaneous generation of life from nonlife, which means he thought that life could instantaneously form from nonliving matter. He thought that mollusks formed from mud, clams from sand, worms from snow, and bacteria from decaying matter. He stated, “Animals and plants are formed in the earth and in the water because there is water in earth, and spiritus in water, and soul-heat in all spiritus, so that all things are in a way full of soul.” [1]
Spontaneous generation became the accepted paradigm in Ancient Greece and Rome and later in the Middle Ages. Jews, Christians, and Muslims were comfortable with spontaneous generation because scriptures seemed to state that the earth and seas had the capability to naturally bring forth life. They thought that God gave inanimate matter the intrinsic capability to generate life from nonlife.
In 1668, Francesco Redi showed that maggots do not form spontaneously from nonliving matter. Other experiments began to show that other organisms only came from life and did not arise spontaneously. Finally, in 1859, Louis Pasteur proved in an experiment that even microbial life could only come from life, which convinced almost everyone of biogenesis (life can only come from life) and disproved the concept of spontaneous generation. Even Darwin was convinced of biogenesis by this experiment. In 1859, when Darwin published, “On the Origin of Species” ihe stated that God created the first organism or organisms, and that this organism evolved into the great kingdoms of life. However, Darwin later changed his mind and wrote to Hooker in 1871 that life may have begun in a pool of organic chemicals (abiogenesis).
"But if (and oh what a big if) we could conceive in some warm little pond with all sorts of ammonia and phosphoric salts, - light, heat, electricity and etc. present, that a protein compound was chemically formed, ready to undergo still more complex changes."
Darwin referred to an experiment in which Friedrich Wohler created an organic chemical, urea, from ammonium cyanate. He reasoned that if organic chemicals formed from inorganic chemicals, then life could proceed from nonlife (abiogenesis)[2] over an extended period by natural selection, which was different from spontaneous generation. Nevertheless, the majority of the scientific community did not accept the concept of abiogenesis in the late 19th century. Darwinism (natural selection) faded from popularity in the late 19th and early 20th centuries.
Two scientists in the early 20th century reinvigorated the theory of abiogenesis (life comes from nonlife): atheists Alexander Oparin and J.B.S. Haldane. Oparin, a biochemist, lived in the Soviet Union just after communist takeover of Russia. He wrote a theoretical biochemical justification for a natural origin of life in two famous papers published in 1924 and 1938. As with Darwin, Oparin argued that organic chemicals form naturally, and thus life could form from organic chemicals. He envisioned a primordial soup full of organic chemicals on the early Earth. Oparin stated that there was no fundamental difference between living organisms and lifeless matter.
Although Oparin wrote a theoretical justification for the origin of life from chemistry, he provided no experimental evidence. Harold Urey at the University of Chicago was one of America's most brilliant scientists. In 1952, graduate student Stanley Miller and Professor Urey applied electricity for several days to a mixture of H2O, H2, CH4, and NH3 gases and produced amino acids, nucleobases, and fatty acids. It seemed like the discovery of a natural origin of life scenario was just around the corner; however, scientists discovered that amino acids in the environment don’t self assemble into functional biological proteins. Nevertheless, Miller and Urey’s experiment was significant because it was the first demonstration that biological functional groups could form in the natural environment.
A key factor in the history of origin of life research was the discovery of the information system in the cell. Oparin did not appreciate the significant difference between the information driven reactions in cells and the statistical and thermodynamic processes in chemistry. The discovery of the information system in the cell began when an Austrian monk named Gregor Mendel published the first paper on genetics, which is the study of the passing of information from parents to offspring. In 1856, he began an experiment in which he ultimately bred 29,000 pea plants in the monastery garden at St. Thomas' Abbey over an eight-year period. Mendel studied the inheritance of two characteristics: the colors of the flowers and the texture of the peas. He found that there were mathematical relationships between types of parents and numbers of each type of descendent. Mendel presented a paper, Experiments on plant hybridization, in 1865, which showed that there were highly predictable mathematical functions that governed discrete changes in characteristics through succeeding generations.
Although Mendel’s experiments proved that there are discrete changes, the scientific community rejected this conclusion. Darwin argued for continuus and nondiscrete change, which he may have viewed as compatible with the theory of natural selection, but this was incorrect. Shortly after publication of his theory, Mendel became a monastery abbot and became busy with administration so his work in genetics ended. Carl Correns, Erich von Tschermak and Hugo De Vries rediscovered Mendel’s work 35 years later in 1900. They confirmed that strict mathematical laws governed the frequency of inheritance of characteristics from parents. Thomas Morgan and coauthors wrote The Mechanism of Mendelian Inheritance in 1915 in which they described how genes form a line on chromosomes and how combinations of genes from male and female parents govern inheritance, but they did not know how the information was stored in the cell.
Watson, Crick, Wilkins, and Franklin discovered the structure of genes, the double helical structure of deoxyribonucleic acid (DNA) in the 1950s. Margaret Franklin played a key role in the discovery, but only Watson, Crick, and Wilkins won the Nobel Prize. Researchers have since discovered that DNA contains 3,000,000,000 bits of information called base pairs, and they have mapped all of the base pairs in the human genome.
As biologists began to reveal the functions of RNA, Crick, Orgel, and Woese proposed the RNA world in the 1960s; however, most scientists did not consider it plausible until Cech and Altman discovered that RNA polymerase (RNAP) acted as a catalyst (like a protein) for RNA construction.[3] Another essential for the RNA world scenario was that the building blocks of RNA, ribose sugars and nucleic acids, should form naturally in the environment. Miller and others tried for decades to produce stable ribose sugars in chemical reactions that might have taken place on the prebiotic earth.
In 2009, John Sutherland in England produced complete RNA nucleotides (ribose sugar, nitrogenous base, and phosphate group) in a plausible scenario of the early earth that Sutherland calls cyanosulfidic chemistry. [4] In 2015, he also produced lipids and protein building blocks. Sutherland believes that we are now at “the end of the beginning” because we now know that the building blocks of life could form naturally. Once sugars form, a series of processes might lead to nucleotides, lipids, and amino acids. One of the known problems with origin of life scenarios is that useless molecules are just as likely to form as molecules useful for life. Amazingly, Sutherland’s cyanosulfidic chemistry preferentially forms the molecules of life. The next step is to find a natural pathway from RNA nucleotides (monomers) to short strands (>4 mers long) of RNA. Once there are short strands of RNA, experiments in the last decade have demonstrated some amazing self-organization characteristics of RNA strands.
[1] Aristotle, On the Generation of Animals, 3.11, 762a18–21.
[2] Russell, Michael, ed. Origins abiogenesis and the search for life. Cosmology Science Publishers, 2011, p. xii.
[3] Szostak, On the Origin of Life, Medicina (Buenos Aires) 2016, 76: 199-203.
[4] Sutherland, John D. "Opinion: Studies on the origin of life—the end of the beginning." Nature Reviews Chemistry 1 (2017): 0012
The education of Alexander the Great by Aristotle. Credit: Charles Laplante (1866)