"We shall regard as alive any population of entities which has the properties of multiplication, heredity and variation"; J. Maynard-Smith, 1975

"I suggest that these three properties (mutability, self-duplication, and heterocatalysis) comprise a necessary and sufficient definition of living matter." N. H. Horowitz, 1959

Life must be able to reproduce itself to persist for more than one generation-time, while keeping a recognizable identity. A major problem is then that to reproduce itself, an object must be quite complex, but it is more difficult to reproduce such a complex entity. John von Neumann (a mathematician) defined an automaton that reproduces itself as having the following necessary components (before the discovery of DNA-RNA - his was a completely theoretical model):

A. an automatic factory, that collects raw material, produces it into output, according to written instructions, given to it by something else (in a living organisms, A are the ribosomes in the cell).

B. a duplicator, which takes written instructions and copies them (enzymes RNA and DNA polymerase)

C. a controller, hooked up to A and B. If C receives written instructions, it hands them to B for duplication, then to A to make something according to the instructions, then adds one copy to the product that A made, and keeps one (repressor-depressor control molecules)

D. written instructions; specifications that make A manufacture the combined system A plus B plus C (DNA/RNA).

 Figure 1: Components of living organisms.

Pasteur demonstrated that all life on earth is produced by other life. Origin of living matter from inorganic compounds can not occur spontaneously in the present world, because the organic compounds that make up organisms can not remain in contact with the atmosphere or with sea water with its dissolved oxygen: the organic matter becomes oxidized, it decays. When the idea got established that the early Earth's atmosphere did not contain oxygen, an origin of life on Earth became a tenable scientific theory. This theory was supported by the observation that the organic chemical basic building blocks of life are found in some meteorites. These meteorites contain amino acids, nucleotides, and an assortment of "sugars". These observations can be seen as supporting the theory that life originated elsewhere in space and came to Earth on meteorites. We then still have to formulate a theory as to how life originated in the 'someplace else'. We do not know where 'someplace else' is, so we can assume whatever environments we would like to have for the origin of life - a non-testable hypothesis.

The "classical theory" on the origin of life was proposed during the 1920s through 1950s by Oparin - Haldane - Urey - Miller). This theory was also called the "dilute soup" theory or the "warm little pond" theory (following a remark by Darwin in a letter). At the time of formulation of the "classical theory" scientists thought that the Earth would have had a 'primary' atmosphere. At that same time (1920s to 1950s) the oldest described fossils were not much older than the beginning of abundant multicellular life (about 600 million years ago), i.e., billions of years after the formation of the Earth. There would thus have been billions of years for the development of living cells.

The classical theory held that life originated as a result of such actions as lightning strikes, putting energy into a strongly reducing atmosphere, which would result in the formation of many of the 'building blocks of life' - as shown by the laboratory tests done by Miller. These building blocks (similar to these seen in meteorites) then accumulated in the oceans or ponds, and reacted with each other to form primitive organisms, that could live as heterotrophs on the other 'building blocks' floating around in these same oceans, until they invented 'photosynthesis'. These early organisms were thus seen as living on the organic molecules in the oceans, a sort of dilute soup.

This theory in its simplest form has run into major problems, and has been practically abandoned by scientists mainly because of three lines of evidence.

1. We do no longer think that the Earth had its primary atmosphere.
2. Microfossils have been found which are very much older than 600 million years (the oldest known now, in 1999, are about 3.5 Ga).
3. Molecular biology has shown us the incredible complexity of even very simple living things.

 1. The atmosphere was probably not by far as reducing as had been thought (no free H₂, NH₃, CH₄), but more neutral. The reactions to organic material, fired by electric discharge, will still take place in such an atmosphere, but at a much slower rate than in a more reducing atmosphere. At such slow rates it is very difficult to build up a rather large reservoir of organic building blocks, thus chances of getting enough bits and pieces to react to big molecules are very small: in a very dilute soup - type ocean the molecules never meet in large numbers.

2. The time available for the development of the first living cells has been getting shorter and shorter, much less than the billions of years envisaged. The big meteorite bombardment that also hit the moon stopped only by about 4.0 Ga. Life developed earlier would probably have been obliterated by the impacts. Organic material that carries the carbon isotope signature of having originated by photosynthesis has been dated at 3.85 Ga. See also the text on stromatolites in the web page 'When the world was weird'.

3. Much more is known about how incredibly chemically complex even simple organisms (bacteria) are, and about the complexity of inheritance. All organisms alive today store and transmit hereditary information in two kinds of molecules called DNA (Desoxyribonucleic Acid, a double spiral shape) and RNA (Ribonucleic Acid, a single spiral shape). Both DNA and RNA are made up of four kinds of subunits called nucleotides. Sequences of nucleotides make up the genes, and direct the formation of proteins, on which all life depends. Proteins consist of 20 different subunits called aminoacids, and the sequence of the nucleotides on DNA and RNA determines the sequence of the amino-acids in proteins. There are large parts of DNA, however, that do not code for proteins and appears to have no function ('junk DNA'). Such junk DNA occurs in Archaebacteria and Eukaryotes, not in Eubacteria. The formation of the proteins is helped along by enzymes, which function as catalysts (catalysts help a reaction along without participating in it). We call the nucleotides and aminoacids the building blocks of life; both have been found in meteorites. We have a chicken and egg problem: DNA and RNA "tell" the organism how to make proteins, but these same proteins are needed to make DNA and RNA, by acting as catalysts to form these big, complex molecules. DNA and RNA are very complex molecules and it appears to be very difficult to let these originate from simple amino acids. But there is no way in which we can get proteins to duplicate themselves.

Figure 2:DNA and RNA structure, from 'Science and Creationism: A view from the National Academy of Sciences', second edition, 1999.

Of course, if life originated elsewhere we can always assume that elsewhere had a very reducing atmosphere… Bu while this evidence accumulated against the classical theory, scientists stumbled serendipitously upon evidence that helped in formulating new theories. The most important evidence was all in one way or another related to the discovery that non-photosynthetic ways of autotrophic life were much more important, even on the present Earth, than formerly thought. This evidence was found by coincidence, when geologists studying underseas volcanoes by submarine discovered amazing life forms on the bottom of the oceans which had been unknown before.

Life at the bottom of the ocean usually grows very slowly because there is so little food: everything must trickle down from the surface several miles above, where sunlight makes photosynthesis by floating algae possible. These algae form the base of the food chain in open ocean. But the base of the food chain of the organisms living around volcanic hot springs (called hydrothermal vents) on the sea floor are not algae, but bacteria, which use not sun light for energy, but chemical reactions.

These bacteria get the energy for making organic matter from non-organic components from chemical reactions, which process is called chemosynthesis. Chemosynthetic bacteria were long known, but thought to live only in small, remote corners of the biosphere, (e.g., in water-logged soils) until the recovery in the 1970s of the hydrothermal vents. Around these vents food chains exists of abundant, large and fast-growing animals (huge clams and worms), using chemosynthetic bacteria as the bottom of the food-web. Such faunas have been called 'hydrothermal vent faunas'; their primary producers are all bacteria classified in the group of Archaebacteria. Worms and clams live closest to the vents and have developed enzymes that help them not to get poisoned by the volcanic gas hydrogen sulfide ('rotten eggs'). Some of the worms (called Riftia pachyptila) do not eat and have no mouth, but have an organ called the trophosome, in which millions of bacteria dwell. The bacteria produce organic matter on which the worms live symbiotically.

"Normal" photosynthesis (as used by green plants, protists, some bacteria) proceeds according to (remember?):

H₂O + CO₂ -> CH₂O + O₂.

Note that free O₂ gas is a by-product of the reaction. Some chemosynthetic bacteria use energy from oxidation of H₂S to sulfur (S):

CO₂ + 4H₂S + O₂ -> CH₂O + 4S + 3H₂O.

Carbon dioxides plus sulfurdioxide plus oxygen gives organic matter, sulfur and water. Other chemosynthetic bacteria (such as some Archaebacteria thriving in hot springs, at temperatures of 90-110 ^oC) use the following reaction (and get energy from the oxidation of H2):

CO₂ + 3H₂ + S -> CH₂O + H₂S + H₂O,

carbon dioxide plus hydrogen plus sulfur gives organic matter, hydrogen sulfide and water. Other chemosynthetic bacteria (also surviving at high temperatures, the anaerobic methanogens):

2CO₂ + 6H₂ -> CH₄ + CH₂O + 3H₂O,

carbon dioxide plus hydrogen gives methane, organic matter and water. The idea is not that you remember all these reactions, but to give you an appreciation for the fact that bacteria are chemically peaking extremely diverse, and use many different. All other organisms (and some bacteria) use one reaction only: photosynthesis with energy from sun light.

There is now agreement we should consider chemo-autotrophic ways of life as being the most ancient, instead of heterotrophs (living on a nutrient-rich "broth"). The prokaryotic group called Archaebacteria appears to be the most 'primitive' in its DNA/RNA from all organisms on earth, and this group contains many chemosynthetic forms.

More and more evidence has become available that helps in solving the chicken and egg problem. It is now known that some molecules made of RNA, called ribozymes, can act as catalysts in modern cells. That means, that such RNA molecules could have used bits and pieces of themselves to help them to replicate, without any use of proteins. There may thus have been a 'RNA-world' , in which RNA could have performed the functions of both nucleic acids (DNA, RNA) and proteins. The theory that the first proto-living things were RNA-only organisms is becoming widely accepted (and is called the theory of the RNA-world).

Even so, we keep the problems of a not reducing (neutral, also not oxidizing) atmosphere, and very little time to get to the first photosynthesizing bacteria. There are two main schools of thought (with many variants), often combined in one way or another.

• 1. The original source-material for life was not really carbon-based "life", but some form of self-replicating crystal. We are looking at: different and innovative ways in which we could have used different commonly occurring, natural compounds as some kind of template, or even as a real part of earlier organisms. The most commonly cited possible inorganic compounds are pyrite, FeS₂ (positively charged surface); and clay minerals ( negatively charged surface). The floating bits and pieces of 'building blocks of life' could have become organized in patterns using the crystals as a template, and they could have stuck first to the charged surface (the building blocks commonly have a positively and a negatively charged end). Note that crystals (regularly shaped structures) have the possibilities of carrying "information" in their irregularities (similar to the supermarket bar codes).
• 2. We looked in the wrong place (warm, shallow pond). Reducing circumstances may not have been average in the atmosphere, but may have occurred locally, around volcanic hot springs, and the first forms of life may have been similar to chemosynthetic bacteria. Nice extra: if life originated in rather deep water (no sunlight needed): no problems with high rate of UV-irradiation in the absence of an ozone shield (no O₂). Additionally, many metals that are required in very small amounts because they play a role in complex organic molecules (such as chlorophyll) are present in waters streaming out of hydrothermal vents - e.g., iron, nickel, manganese, magnesium, molybdenum, selenium, copper.

Photosynthesis must have developed early on, because stromatolites have been recognized in rocks of about 3.5 Ga. Organic geochemists have also analyzed ancient rocks for organic compounds, and found evidence for photosynthesis by bacteria in rocks as old as 2.7 Ga. We do, of course, not know how this reaction could have evolved, but recently theories have been offered that center on the idea of pre-adaptation (the use of something for a purpose for which it was not designed - e.g., the use of jaw-bones in reptiles for hearing-purposes in mammals). If we assume that early Archaebacteria were thermophiles (liked high temperatures), and chemoautotrophs, we need to suppose that they lived close to hydrothermal vents. They must have been able to select their environment precisely, with individuals being too close to the vents being boiled (temperatures up to 400 ^oC occur presently), individuals too far away being starved from the sulfur compounds needed for energy as well as getting too cold for active metabolism. It would thus be extremely useful for the bacteria if they could sense heat (i.e., light at infra-red wavelengths). Modern purple and green bacteria (which photosynthesize) do not have the same chlorophyll as plants, but substances called 'bacteriochlorophyll'. These absorb strongly in the infra-red - similar to 'eye-structures' in present-day shrimps which live around hydrothermal vents (in the dark). We could thus envisage that mobile bacteria gained advantage by their ability to sense warmth and hence nutrients, and/or to avoid great heat; this ability would have worked to survive in the hydrothermal vent environment, not to begin photosynthesizing. But if the bacteria were living in shallow water, they could use the same molecule for photosynthesis, although bacteriochlorophyll does not give energy enough to split water H₂S used). It seems a possible first step, however, and in favor of development of life at least close to hydrothermal vents.

Living Archaebacteria have now been found in many environments where there is no light: in cracks in rocks, in oil in oil fields, and so on, in fact in all places where water can exist as a liquid, and where there are sulfur-compounds to use for energy-input. This suggests that many environments that appear to use not to be life-friendly at first sight can sustain large populations of bacteria, including zones deep in the crust of the Earth.

A corollary of theories which conclude that life originated on Earth, and that it originated as soon as it was possible to persist without being whacked out of existence by continually impacting meteorites, is: generation of life must have been quite easy. If it worked in very little time - then the process was probably common. Then it might also have proceeded in places such as Mars - and in fact, may still be functioning deep in that planet's crust. And it might be in existence on other planets, or moons of planets. Remember that life in the universe does not necessarily mean 'intelligent life in the universe'.

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