The Origin Of Life

Scientific models_

There have been many scientific models for the origin of life, some of them now rejected by the evidence, and others still available as potentially valid hypotheses:

1 Spontaneous generation.

2 Inorganic model.

3 Extraterrestrial origins.

4 Biochemical model.

5 Hydrothermal model.

Medieval scholars believed that many organisms sprang into life directly from nonliving matter, a form of spontaneous generation. For example, frogs were said to arise from the spring dew and maggots were said to come to life in rotting flesh. However, careful tests proved that there was no truth in these ideas. Louis Pasteur in 1861 enclosed pieces of meat in airtight containers, and maggots did not appear. He showed that flies laid their eggs on rotting meat, the eggs hatched as maggots and the maggots then turned into flies. So, the idea of the origin of life by spontaneous generation is a scientific hypothesis because it may be tested, but it turns out to have been wrong. It is important to realize that scientific and non-scientific do not mean "right" and "wrong": science is about testing and rejecting alternate hypotheses until one remains that is not rejected.

The inorganic model for the origin of life is that complex organic molecules arose gradually on a pre-existing, non-organic replication platform - silicate crystals in solution. Silicate crystals, clay minerals, were subject to selection pressures on the ancient seabed, and then organic molecules became involved and the inorganic selection became organic. This view has been championed vigorously by Graham Cairns-Smith of Glasgow University, but it has not gained widespread support. The first experiments to test the model were carried out in 2007, but they were not conclusive.

The extraterrestrial model is that the building blocks for life were seeded on Earth from outer space. Simple molecules, such as hydrogen cyanide, formic acid, aldehydes and acetylenes are found in certain classes of meteorites called carbonaceous chondrites, as well as in comets, and these chemicals might have been delivered to the surface of the Earth during a phase of massive meteorite bombardment about 3.8 Ga. In other, more extreme, forms of this hypothesis, DNA might even exist in space, or life in its entirety might have evolved elsewhere in the universe, and was seeded on the Earth during the Precambrian.

Collectively, these views have sometimes been called "panspermia", meaning "universal seeding". The panspermia model received

Figure 8.1 The biochemical theory for the origin of life, as proposed by I. A. Oparin and J. B. S. Haldane in the 1920s. Biochemists have achieved steps 1-3 in the laboratory, but scientists have so far failed to create life. ATP, adenosine triphosphate.

a boost in 1996 when David McKay and a team from NASA announced that they had identified fossil bacteria and organic chemical traces of former life in a Martian meteorite. These findings have, however, been disputed vigorously, and the initial excitement has waned. It is hard to see how extraterrestrial/ panspermia models for the origin of life could be tested decisively and, in any case, positing the origin of life on another planet still leaves open the question of how that life originated.

The biochemical model for the origin of life was developed in the 1920s independently by a Russian biochemist, A. I. Oparin, and a British evolutionary biologist, J. B. S. Haldane. They argued that life could have arisen through a series of organic chemical reactions that produced ever more complex biochemical structures (Fig. 8.1). They proposed that common gases in the early Earth atmosphere combined to form simple organic chemicals, and that these in turn combined to form more complex molecules. Then, the complex molecules became separated from the surrounding medium, and acquired some of the characters of living organisms. They became able to absorb nutrients, to grow, to divide (reproduce) and so on.

The hydrothermal model is a recently proposed modification to the Oparin-Haldane biochemical model (Nisbet & Sleep 2001). According to this view, the last universal common ancestor of life (sometimes abbreviated as LUCA) was a hyperthermophile, a simple organism that lived in unusually hot conditions. The transition from isolated amino acids to DNA (Fig. 8.1) may then have happened in a hot-water system associated with active volcanoes. There are two main kinds of hot-water systems on Earth today, hot pools and fumaroles fed by rainwater that are found around active volcanoes, and black smokers in the deep ocean. Black smokers arise along mid-ocean ridges, where new crust is being formed from magma welling up as major oceanic plates move apart (see p. 42). Seawa-ter leaks down into the crust carrying sulfur as sulfate, mixes with molten magma and emerges as superheated steam, with the sulfur now concentrated as sulfide. As minerals precipitate in the cooler sea bottom waters, they color the emerging hot-water plume black. Black smokers are too hot as a site for the origin of life, but the other kinds of hydrothermal systems are less extreme.

This leaves us the Oparin-Haldane biochemical model as a broad-brush picture of how life might have originated, and the hydrothermal model as a specific aspect. How far have scientists been able to test the biochemical model?

Testing the biochemical model_

In cartoons and pop fiction, the white-coated scientist is seen in a laboratory full of mysterious bubbling glass vessels, and he declares, "I've just created life". Could this be true? How far have the experiments gone along the chain of organic synthesis that is postulated in the biochemical model for the origin of life (see Fig. 8.1)?

It took some years before the first laboratory results were obtained. The Oparin-Haldane biochemical model was proposed in the 1920s, but nobody tested it seriously until the 1950s. In 1953, Stanley Miller, then a student at the University of Chicago, made a model of the Precambrian atmosphere and ocean in a laboratory glass vessel. He exposed a mixture of water, nitrogen, carbon monoxide and nitrogen to electric sparks, to mimic lightning, and found a brownish sludge in the bottle after a few days. This contained sugars, amino acids and nucleotides. So, Miller had apparently recreated step 2 in the sequence (see Fig. 8.1). However, nowadays most researchers consider the mixture of gases that Miller used (with high percentage concentrations of H2 and CH4) to have been too strongly chemically reducing to represent a likely atmosphere for the early Earth. Atmospheric hydrogen is ultimately replenished from the mixture of gases released from the solid Earth, but the geochemistry of the subsurface means that the mixture generally should contain the oxidized form of hydrogen (i.e. water vapor, H2O) rather than the large proportion of H2 in Miller's atmosphere.

Further experiments in the 1950s and 1960s led to the production of polypeptides, poly-saccharides and other larger organic molecules (step 3). Sidney Fox at Florida State University even succeeded in creating cell-like structures, in which a soup of organic molecules became enclosed in a membrane (step 4). His "protocells" seemed to feed and divide, but they did not survive for long.

Could scientists ever show how non-living protocells could become living? Did this happen in one jump or was there an intermediate stage?

RNA world_

Biochemists and molecular biologists have worried about the transition from non-living to living; it is hard to see how bacterial cells could form from non-living chemicals in one step. What then could have been the transitional form of "precellular" life? The most widely accepted view today is that RNA is the precellular entity, and the time between nonlife and life has been termed the "RNA world".

RNA, or ribonucleic acid, is one of the nucleic acids and it has key roles in protein synthesis. Proteins are manufactured within the nucleus of eukaryotic cells, and within the cell mass of prokaryotic cells. The genetic code, the basic instructions that contain all the information to construct a living organism, is encoded in the DNA (deoxyribonucleic acid) strands that make up the chromosomes. There are several different forms of RNA that have different functions: one type acts as the template for the translation of genes into proteins, another transfers amino acids to the ribosome (the cell organelle where protein synthesis takes place) to form proteins, and a third type translates the transcript into proteins.

In 1968, Francis Crick (1916-2004), who co-discovered the double-helix structure of DNA in 1953 with James Watson, suggested that RNA was the first genetic molecule. He argued that RNA must have the unique property of acting both as a gene and an enzyme, so RNA on its own could act as a precursor of life. When Harvard molecular biologist Walter Gilbert first used the term "RNA world" in 1986, the concept was controversial. But the first evidence came soon after when Sidney Altman and Thomas Cech independently discovered a kind of RNA that could edit out unnecessary parts of the message it carried before delivering it to the ribosome. Because RNA was acting like an enzyme, Cech called his discovery a ribozyme. This was such a major discovery that the two were awarded the Nobel Prize for Chemistry in 1989; Altman and Cech had confirmed part of Crick's prediction.

Since 1990, numerous labs have been chasing evidence for the RNA world. For example, Jack Szostak and colleagues at Massachusetts General Hospital in Boston argued that the first RNA molecules on the prebiotic ("before life") Earth were assembled randomly from nucleotides dissolved in rock pools (Szostak et al. 2001). Among the millions of short RNA molecules, there would have been one or two that could copy themselves, an ability that soon made them the dominant RNA on the planet. To take this forward to create a living cell, Szostak identified two stages: (i) the production of a proto-cell by the combination of an RNA replicase and a self-replicating vesicle; and (ii) the production of a cell by the addition of a living function (Fig. 8.2).

Simply proving that RNA could act as gene and enzyme was one thing; however, a single self-replicating vesicle replicase / \

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