chronology of the origin of life
1920s - The primordial soup
Around 1920, the Soviet scientist Alexander Oparin and the English scientist J. B. S. Haldane independently proposed a first hypothesis of some scientific relevance which later became famous under the name of the "primordial soup".
This idea was very successful and thanks to its developments in the early 1950s, the chemist Harold Urey was awarded the Nobel Prize. Let's see this idea schematically: the primordial earth's atmosphere was composed of ammonia, methane, water vapor, hydrogen with virtually no molecular oxygen.
Lightning, ultraviolet radiation from the sun and volcanic heat triggered reactions that led to the formation of carbon molecules. These molecules accumulated in the primordial oceans giving them the consistency of a soup diluted at a high temperature. Subsequent changes led to the formation of life.
Criticism to the theory: the aforementioned composition of the earth's primordial atmosphere which subsequent studies have shown to be rich in nitrogen, carbon dioxide and water vapor. Even admitting a composition like the one proposed by Oparin Haldane, the molecules with carbon atoms would not last long in the prebiotic soup and one would return to the starting compounds.
1953 - The Miller and Urey experiment
In 1953 Stanley Miller, a young graduate of the University of Chicago, conceived, under the guidance of Professor Harold Urey, an experiment that captured the imagination of the biological world.
Miller based on Oparin-Haldane's ideas regarding the composition of the primordial earth's atmosphere and created an apparatus to test whether the formation of basic molecules for life could actually be achieved.
The apparatus was a large glass tube bent to form a sort of rounded rectangle. Miller introduced inside the pipe water and a mixture of the gases of the "primordial soup", ammonia, hydrogen and methane. He then connected electrodes to create sparks inside the tube. He left the tube in this situation for a week until he realized the tube was getting a patina. Analyzing this patina he found among its components small quantities of two simple amino acids, alanine and glycine. The scientific world immediately gave great prominence and support to Miller's achievement.
Criticisms: again the initial mix was that of Oparin - Haldane, therefore with little or no chance of being realistic.
Furthermore, Miller had not presented any evidence on how the amino acids formed should have replicated. Without auto replication, life could not have existed.
It should be keep in mind that just in those years (1953) Watson and Crick presented their work (subtracting it from Rosemborg) on the structure of DNA.
The replicator RNA
Leslie Orgel, born in 1927, graduated from Oxford in 1949, Phd in 1951, brought the idea of the RNA as a replicator to the attention of the scientific community. According to the scientist, RNA was somehow formed in the prebiotic world.
RNA can be represented as a long side of a hinge. Orgel had managed to experimentally demonstrate that RNA was self-duplicating, that is, that the other side of the hinge was formed, perfectly mirroring the first. In this way the first stage of duplication was completed.
Criticisms: also in this case the Oparin-Haldane atmosphere is assumed to be the primordial terrestrial atmosphere. Furthermore, the molecules of RNA, and therefore of DNA, are so huge and so complicated, that using them as a starting point for life on earth seems to be risky. Enzymes were needed to get the starting side of the hinge (not the mirror side). Enzymes (biological catalysts) are nothing more than proteins. And the proteins are synthesized thanks to the sequences of information reported by the RNA. So we're at the point where the cat bites its own tail.
1960s - The microspheres
As it was unlikely for Sidney Fox that RNA, and therefore DNA, could be synthesized randomly in the primordial atmosphere, he thought of proteins as precursors of life on earth. He imagined large lagoons filled with bubble-like structures called microspheres. Each microsphere was made up of a substance called a proteinoid.
In his model, Fox hypothesized that proteinoids were able to catalyze chemical reactions and thus form outermost surfaces that behave like a cell membrane. The amino acid chains could self-duplicate and then evolve up to the formation of DNA and RNA. The microspheres possessed, among other things, the ability for a kind of reproduction. Furthermore, during the fusion of the microspheres, an exchange of internal material took place
Criticism: Several attempts to synthesize proteinoids in the manner described by Fox have led to the formation of amino acid sequences very far from conventional proteins. Attempts to synthesize the microspheres have proved to be only partially effective, as they tend to be very fragile and unstable in the type of environment considered.
1980s - Silica crystals
Graham Cairns-Smith hypothesized in the early 1980s that the first living organisms on earth were not based on a carbon atom structure, but on a sort of clay crystal formed from the mud.
A living specie made from silicon dioxide crystals. Crystals of silicon dioxide (silica) form quartz. Mica is a mineral present in volcanic lava made up of silicon dioxide crystals. Crystals are interesting because they are the result of the natural and spontaneous organization of atoms in very different geometries and designs.
We know that mineral crystals have the ability to grow simply by agglomerating around an original crystal. If the structure breaks, the fragments of the crystals can continue to grow independently of each other, in a sort of self-replication process. Some particular types of clays called kaolinites have the property of growing in thin layers. Smith argues that the clay crystals began to grow by adding layers to themselves. The primeval earth proposed by this idea would have been populated by communities of clays competing with each other to grab as much material as possible for growth. At some point these clay beings would have begun to incorporate molecules with a carbonic structure, and subsequently they would have synthesized DNA or RNA.
The positive aspects of this theory lie in not making assumptions about the primordial atmosphere of the earth. It is also based on one of the simplest properties of crystals, the ability to grow and reproduce by fragmentation.
Criticism: the way in which DNA or RNA should have formed from clay is not presented.
Extraterrestrial origins
Swante Arrhenius from Sweden was one of the earliest supporters of the idea that life on earth may have come from some distant planet.
Crick and Orgel also proposed an idea related to the origin of life from other planets: "DIRECTED PANSPERMIA". They thought of a distant civilization that developed outside our solar system. Finding themselves in danger of extinction, they sent primitive life forms in every direction of the universe in the hope that they would reach other planets suitable for life.
Fred Hoyle and Chandra Wickramasinghe also proposed that the first forms of life originated outside our planet.
Today
NASA scientists recently announced (year 2002) the creation of amino acids, necessary for life, in an environment that reproduces the same conditions as deep space. In a laboratory at NASA's Ames research center in California's Silicon Valley, a group of astrobiologists irradiated ultraviolet light onto space "ices", simulating conditions typical of interstellar space.
Space ice is common water ice containing simple molecules. The researchers, after irradiation, verified the presence of amino acids, in particular glycine, alanine and serine. This testifies that amino acids can have formed anywhere in the universe, wherever there are stars and planets. To confirm this, the presence of the same amino acids has been verified in some meteorites that have fallen to earth. The fact that amino acids could have been formed anywhere increases the chances that life could have developed in other parts of the universe as well.
