From Nicholas L. Sharp
There are many theories as to how and where life originated in the universe. Conventionally, biologists claim that life originated on Terra independently of any other life in the universe. They also believe that extraterrestrial life is highly unlikely. However, unorthodox theories like panspermia have become increasingly more accepted by biologists in the last several decades.
Exogenetic theories state that life could not have possibly originated on Terra in the time allotted and must have originated extraterrestrially and been subsequently transported to our planet in the form of panspermia. Geogenetic theories claim that life must have originated on Terra and that life could not possibly survive the interplanetary radiations, vacuum conditions, and subsequent bolide impacts necessary in exogenetic theories. However, there is much evidence that living organisms, even if only in a dormant state, can survive interplanetary vacuum conditions.
There are several similarities between the exogenetic and geogenetic theories of the origin of life. Firstly, despite heated debate and criticisms from both sides of argument, there is no compelling evidence to support or contradict either theory. Also despite the fundamental importance the origination of life has to the field of biology, it is not an active or popular area of research. Very little research has been done on testing the various hypotheses on the origin of life. Neither exogenesis nor geogenesis states exactly as to how life originated, only where and somewhat when. Most geogenetic theories claim that life continues to originate, whether in hydrothermal vents or deep under the crust. Some exogenetic theories also claim that Terra is being continuously seeded with life from outer space, possibly via interplanetary debris entering the atmosphere.
Both exogenesis and geogenesis have several different versions. Theories of exogenesis usually revolve around the concept of panspermia, where life is present throughout the galaxy or solar system on board small bodies that seed planets. The lithopanspermia of William Thomson, who proposed the Thomson model of the atom and is also known as Lord Kelvin, was founded in 1871 and states that Terra was seeded with life via the bombardment of bolides. Svante August Arrhenius (who predicted the greenhouse effect), founded a theory of radiopanspermia in 1903 which claims that Terra was seeded with life via small interplanetary or interstellar particles being pushed by radiation pressure. The directed panspermia of Francis Crick (codiscoverer of DNA) and Leslie Orgel was founded in 1973 and states that Terra was seeded with life somehow through the actions of an extraterrestrial intelligence.
The theory of biopoiesis or chemical evolution of John Burdon Sanderson Haldane and Alexandr Ivanovich Oparin originated in the 1920s. It postulates that protocells evolved from self-replicating but nonliving molecules. It was confirmed by the Miller-Urey experiment in 1953, which produced simple amino acids by firing simulated lightning into a container containing what they thought constituted a primordial atmosphere. Another theory is the RNA world hypothesis of Carl Richard Woese, which was developed in 1967. RNA world models claim that the first life was made from RNA and not DNA. Ultraviolet light causes RNA to polymerize while breaking down other molecules that might break apart the fragile RNA. Another alternative to RNA is the use of PNA, or peptide nucleic acid, for its increased stability. The iron-sulfur world theory of Günter Wächtershäuser, developed in the 1980s, claims that life didn’t form in the oceans as the "soup theory" claims, but rather on mineral surfaces like iron pyrites. Protocells would have evolved as lipid bubbles on the mineral surface. The clay theory of Alexander Graham Cairns-Smith, developed in 1985, claims that life evolved in silicate clays in solution. Thomas Gold’s deep-hot biosphere model, postulated in 1992, claims that life didn’t even evolve on the surface of Terra, but several kilometers beneath the surface. Life has been found up to five kilometers beneath the surface already, and is thought to be able to survive larger depths.
The fundamental difference between exogenesis and geogenesis, at least terrestrially, is that exogenesis is a biogenetic theory, while geogenesis is abiogenetic. This is unusual, for one would think that exogenesis would be more mainstream because it invokes the old concept of cosmic ancestry, that life has always existed and was never created or formed at any point (similar to the creationist or ID argument). The abiogenetic geogenesis, however, refutes the foundation of modern biology and cell theory: omne vivum ex ovo ("all life from an egg"), or the idea that all cells originate from previously existing cells and cannot spontaneously generate.
Geogenesis assumes that life is rare. If it is, however, then why did it form in such a short time on Terra? Exogenesis assumes that life is prevalent throughout the universe, a much more sensible hypothesis. It also extends the Copernican principle to life, saying that Terra is not unique in the fact that it has life, just as it said that Terra is not unique in its location in the universe. If Terra’s location is not unique, and it has life, then life must be nearly everywhere.
One of the major arguments against geogenesis is that if it is true, then life would have to form in around a hundred million years. Proponents of exogenesis claim that this isn’t possible, and that if life originated extraterrestrially, then life has a period of nine billion years to form. A fundamental, though possibly flawed argument against exogenesis is Occam’s razor. Occam’s razor states that when there are multiple explanations for a problem, then the simplest one tends to be the best. In geogenesis, there is one step involved, namely that life originates on Terra. In exogenesis, there are two steps: that life originates and that it is subsequently transported to Terra.
One would think that the more controversial theory originated first. However, while biopoiesis originated in the 1920s, exogenesis has roots dating back to Anaxagoras, who lived from 500 to 428 BC. The theory was then put on a firm scientific basis by Jöns Jackob Berzelius (discoverer of selenium, silicon, thorium, and the periodic table) in 1834 and further developed into radiopanspermia by Svante August Arrhenius in 1903.
There is very little major research being done on this field of biology. Perhaps that is why it is riddled with mysteries and there is such debate over the issue. To understand this mystery could be said to find a grail of biology, just as the theory of everything is a grail to physics. Since we are organic creatures of life, to understand life is to understand ourselves.