Text of http://www.evsc.virginia.edu/~lkb2e/101/lec22.htm - placed here for reference from “Abiogenesis for the Birdwatcher” because original link was broken during site re-organisation.

There are two basic ideas about how life on Earth originated. The first is that life (or at least the chemical precursors of life) arose on other planets followed by a diaspora (migration) to various parts of the galaxy including Earth. This idea is know as panspermia (see BP p. 398-399). Recent support for this idea comes from Mars rocks found in the Antarctic that appear to contain fossilized organisms resembling bacteria. There is also some evidence from Martian probes suggesting the possibility of life existing either in the past or even currently on Mars or Europa.

The alternate hypothesis about how life arose on Earth is that of Spontaneous Generation. Not the spontaneous generation of the17^th century that Reddi, Leeuwenhoek, Spallanzalli, Tyndall, and Pasteur put to rest, but the 1936 spontaneous generation of Oparin, a Russian biochemist, that non-cellular macromolecular precursors developed into cells.

Oparin's argument goes something like this:

1. The Universe formed approximately 20 billion years ago, followed by our solar system (the Sun and planets) about 4-5 billion years ago. Our solar system formed from a cloud of dust and gas which condensed into a single compact mass resulting in tremendous heat and pressure. Because of the heat and pressure, thermonuclear reactions were initiated creating the Sun. Lesser centers of condensation occurred to the Earth and other planets.

Earth's condensation (and other terrestrial plants too) resulted in stratification of the components giving rise to Fe + Ni at the Earth's center (the heavy metals) and H₂ + He + other gases (the lighter elements) as the primordial atmosphere. But, because of the size of the Earth and its weak gravitational field, these gases escaped nearly immediately. As a result the Earth became a bare, rocky globe with no oceans or atmosphere.

Time passes and gravity compresses the earth more. Radioactive decay occurs to produce heat and create a molten interior. More stratification occurs to produce a core of molten Fe + Ni and a mantel of Fe and magnesium silicates. The heat of the core forces gases and water out by volcanic action. These gases formed a second atmosphere, the evolutionary atmosphere.

As the Earth cooled, water vapor would have condensed and torrential rains would have fallen. The rain would have dissolved minerals (i.e., mineral weathering would have occurred) and the run-off from topographic highs would have collected in topographic lows to form oceans. If CH₄ and NH₃ were present in the atmosphere, then they would have also dissolved to some extent in the rain and ended up in the forming oceans.

2. The next step in the evolution of life would have been the formation of small organic molecules. The combination of minerals, NH₄⁺, CH₄ , H₂, and H₂O that formed the oceans is a very stable mixture so how could the building blocks of life arise? Some sort of energy source was necessary. An abundance of energy sources was available on early Earth in the form of solar radiation (visible light, ultra-violet light, x-rays), lightning, heat, cosmic rays, radioactive decay, or volcanic explosions.

In 1953, Stanley Miller did an experiment in which he combined an atmosphere containing CH₄ , H₂, H₂O, and NH₃ in a reaction vessel. Using an electrical spark, he was able to generate several small organic molecules including urea (CO(NH₂)₂), hydrogen cyanide (CHN), acetic acid (CH₃COOH), and lactic acid (CH₃COH₂COOH). To eliminate the criticism that microbial activity was the source of the organic compounds, Miller ran the experiment under sterile conditions and without an electrical spark. When no spark was used, no organic compounds were formed. When the experiment was conducted under sterile conditions, no organic compounds were formed.

Since Miller's experiment many other similar experiments have been done with various atmospheric component combinations with similar results. All 20 of the most common amino acids have been produced by these types of experiments. In fact the experiments have been so successful under so many conditions that it is unlikely that organic compounds weren't formed in this manner!!!!

Some have argued that the organic compounds formed by these means would have been destroyed as fast as they formed. It has been suggested the small organic compounds would have oxidized, but there was no O₂ in the early atmosphere for this to happen. It has been suggested that the small organic compounds would have decayed, but there were no microorganisms to decay them. It has been suggested that the high levels of UV light would have destroyed the small organic compounds. This is true to some extent, but because UV doesn't penetrate water very well, small organic molecules formed in water would have been shielded from UV destruction by the water. Soooo, it seems highly likely that small organic molecules would have formed and survived on early Earth. But is the formation of small organic molecules sufficient for the beginning of life? NO!

3. The next step in evolution of life on Earth would have been the formation of macromolecules or polymers. It is unlikely that sufficient concentrations of small organic molecules would have formed in the early oceans. A concentration process would have been necessary to encourage formation of polymers like polypeptides (precursors of proteins) or polynucleotides (precursors of DNA and RNA). Several processes easily could have occurred on early Earth to allow for polymer formation. For example, small organic molecules could have been concentrated by adsorption on clay particles or adsorption on pyrites in black smokers (see BP p. 398). Another concentration mechanism would have been evaporation in small pools or puddles on beaches or in lagoons. Additionally, UV light could have served as an energy source to produced polymers. To demonstrate that these mechanisms have the potential to produce polymers from small organic molecules, Sidney Fox heated dry mixtures of amino acids to produce a variety of polypeptides in the laboratory.

4. Once polymers were formed the next step in creating life on Earth would have been the formation of molecular aggregates and primitive "cells". The term cells is used very loosely here. When polypeptides or polynucleotides are combined in solution they form one of two types of complex units: one that Oparin called coacervate droplets and the other that Stanley Fox called proteinoid microspheres. Coacervate droplets are macromolecules that are surrounded by a shell of water molecules which are rigidly oriented relative to the macromolecule forming a "membrane". The coacervate droplets will adsorb and absorb chemicals from the surrounding medium and can se highly selective (lets only certain things in and certain things out) like a cell membrane. The coacervate droplets can become complex and show internal structure which becomes more and more pronounced as more materials pass the "membrane" and are incorporated into the droplet.

Proteinoid microspheres form when hot aqueous solutions of polypeptides are cooled. Proteinoid microspheres are much more stable than coacervate droplets and have the following characteristics: swell in a high salt solution, shrink in a low salt solution, have a double-layered outer boundary which is very similar to a cell membrane, show internal movement similar to cytoplasmic streaming "grow" in size and complexity, "bud" in a manner superficially similar to yeast cell reproduction, have electrical potential differences across the outer boundary which is necessary for cell membranes to generate ATP, and aggregate into clusters.

In either case (coacervate droplets or proteinoid microspheres) these "prebionts" are structurally complex and sharply separated from their environment creating a situation in which chemical reactions can take place inside the prebionts that would not happen in the surrounding medium.

On early Earth, many types of prebionts would have formed. Some would have been unstable. Others may have contained especially favorable combinations of chemicals. If they were to continue to exist, they must have had synthetic ability and thus a mechanism for handling energy. Energy demands probably would have been handled through ATP since all living things use ATP as their energy currency. Sidney Fox showed that proteinoid microspheres can use ATP to make polypeptides and nucleic acids. As a result proteinoid microspheres can increase in size. They are also susceptible to fragmentation to produce smaller droplets of similar composition - or they demonstrate a kind of reproduction.

There is a third hypothesis about formation of prebionts called the naked gene hypothesis. The basis for this hypothesis is the observation that self-replicating macromolecules (nucleotides) can accumulate a shell of other substances in a manner analogous to a modern day virus. Modern day viruses contain either DNA or RNA and are surrounded by a protein shell. BUT, modern day viruses are parasites that require living cells to reproduce and so are thought to be recent evolutionary products. Evidence for the naked gene hypothesis comes from discovery of prions, which are naked pieces of DNA that have been associated with cancer.

The difference between the naked gene hypothesis and the coacervate droplet or proteinoid microsphere hypotheses is that the later two prebionts showed primitive reproduction first followed by development of a genetic control system while the naked gene prebiont evolved a genetic control system first followed by cytoplasm and a membrane. Regardless of which hypothesis is correct, CELLS HAPPEN HERE!!!!

The oldest cell fossils are all procaryotic (bacterial) and are 3.5 billion years old. Procaryotes (lacking a "true" nucleus) are cells in which the chromosome is not surrounded by a nuclear membrane. They also lack the specialized cellular organelles that other types of cells have with the exception of ribosomes. There are many other distinguishing characteristics of procaryotic cells but these characteristics, while fascinating, are outside the scope of EVSC 101.

5. Once cells arrived on the scene, they would have had to evolve biochemical pathways. The oceans of early Earth would have been a soup that contained lots of energy rich organic matter the first cells could have used to produce ATP. However, as the number of organisms increased the supply of organic compounds would have decreased, becoming limiting, because the natural rate of organic matter formation is very slow. Thus, these early organisms were faced with a minor crisis - limited energy sources. To overcome this, some organisms began to use alternate forms of organic matter, converting it to chemical compounds similar to those they used previously to generate energy. As the process continued, the chain of reactions cells were capable of carrying out would have become longer and longer leading to the very complex series of chemical reactions all cells carry out today. Because these early cells all used pre-formed organic materials, they were all heterotrophs.

As the number of cells increased even further, the demand for abiotically formed organic materials would have out striped the rate at which they could be formed leading to a major crisis in the evolution of life on Earth - the first famine.

5. By the time the first famine occurred, cells had evolved very complex biochemical pathways. The lack of available energy sources to these early organisms would have created selective pressure for evolution of an alternative to small organic molecules as a way to obtain energy. That alternative was to capture light energy and convert it to chemical energy in the form of ATP. This process is what we know as photosynthesis, but not the type of photosynthesis that green plants carry out. The first photosynthetic organisms were bacteria that used H₂S instead of H₂O as a source of reducing power or H⁺. As a result, the end products of bacterial photosynthesis were organic matter and sulfide, not O₂ like green plants produce. However, because the supply of H2S on early Earth would have also been limited, selective pressure would have existed for the evolution of an alternative to H₂S. That alternative was H₂O, which was very abundant and serves as an excellent source of H⁺ for photosynthesis. The first organisms to make the leap from using H₂S to H₂O were the blue-green bacteria or the Cyanobacteria (you will often hear these organisms referred to incorrectly as the blue-green algae).

This new type of photosynthesis (based on H₂O) created the second major crisis for life on Earth. One of the products of H₂O-dependent photosynthesis is O₂ - a highly toxic gas. Oxygen is toxic to many critical biochemicals in cells and as a result early organisms had to either avoid O₂ or evolve mechanisms to detoxify or protect the cell biochemicals from O₂. In addition to the affect on cells, accumulation of O₂ in the atmosphere lead to the formation of the ozone (O₃) layer, which reduced the rate of mutation and significantly slowed the rate of evolution. Development of oxygenic photosynthesis is an excellent example of organisms altering their environment.

6. The accumulation of O₂ in the atmosphere forced the next major evolutionary step - the evolution of aerobic organisms. The procaryotic organisms that evolved prior to this were all anaerobes - that is they didn't require O₂ for cellular respiration or were fermentors. Aerobes require O₂ for cellular respiration. The advantage of respiration using O₂ is that it is very efficient compared to fermentation and somewhat more efficient than anaerobic respiration. Consequently aerobic organisms had a selective advantage over organisms that ferment or respire anaerobically.

So at this point, after about 3 billion years in the story of evolution of life on Earth the organisms that have evolved are all procaryotic (bacteria) and are:

A. anaerobes

i. fermentors

ii. respirers

iii. photosynthesizers

B. aerobes

i. photosynthesizers

7. The rest of evolution on Earth is simple - the eucaryotes evolve. The first eucaryotic fossils are 1.5 billion years old. From these very first eucaryotes (cells with a "true" nucleus - or one enclosed in a nuclear membrane) all other eucaryotes (protozoa, fungi, plants, and animals) evolved. Several hypotheses have been proposed to explain how eucaryotic cells arose: the invagination hypothesis and the symbiont hypothesis (see BP figure on p. 402). The invagination hypothesis proposes that a procaryotic cell enclosed a portion of itself to become an organelle to form either a mitochondrion or a chloroplast like those typical of eucaryotic cells. The symbiont hypothesis suggests that independent, free-living procaryote cells formed a symbiotic relationship with one another in which one of the procaryotes was resident inside the other procaryotic. As the relationship evolved the "invader" lost its ability to live independently and became either a mitochondrion ("invader" was a heterotroph) or a chloroplast ("invader" was a photosynthetic autotroph).