Alternatives to Chemosynthesis

Introduction

In the preceding lecture, the chemosynthesis model was discussed as a means for life to have developed on Earth. In this lecture, possible objections to this model are briefly reviewed, and an alternative model is examined.

Objections to Chemosynthesis

A number of objections can be raised against the chemosynthesis model. Some of these include:

1. Early simple organic compounds formed through processes similar to the Miller-Urey experiment would have been destroyed as quickly as they were formed, primarily by oxidation and UV radiation. This would have prevented their assembly into more-complex compounds.
2. Many biochemical reactions which are important to life will not occur spontaneously without the presence of enzymes (biological catalysts). These enzymes have the task of reducing the energy required to initiate a chemical reaction; but how did they come into existence before the reaction did?
3. The chances against forming a given macromolecule (e.g., a protein) from random chemical reactions are astronomically small.
4. There is no explanation for the formation of lipids (fats), which are vital ingredients in cell membranes.

However, answers exist to all of these objections:

1. As discussed in Lecture 4, there would have been no free oxygen on Earth's until the appearance of photosynthetic autotrophs. By then, life was sufficiently advanced to be able to cope with the problems of oxidation. Likewise, many of the simple organic compounds would have formed in the the oceans; to quote from Miller (1982), in discussion of the Miller-Urey experiment:

   [The amino acids] were not formed directly in the electric discharge, but were the result of reactions in aqueous solution of smaller molecules produced in the discharge - including HCN and aldehydes.

   Water is a good absorber of UV radiation; therefore, those simple compounds formed in the oceans would have been shielded, and could avoid being destroyed by radiation.
2. A number of the macromolecules (including RNA, the precursor of DNA) are both self-replicating and can act as enzymes. Therefore, these molecules would have first formed in abundance, and through their role as catalysts would have then permitted new biochemical reactions.
3. Indeed, the probability of generating a specific macromolecule through random events is very small. However, it should be noted that there are many different macromolecules which could have fulfilled the same role in the development of life; so to require the formation of a specific macromolecule is unnecessary.
   
   In addition, the random events leading to macromolecules were occurring in countless locations on the primordial Earth; therefore. Even though the chance of forming a macromolecule was small in each location, the cumulative chance of forming a macromolecule anywhere on Earth was not. There is an interesting discussion of this issue at http://www.talkorigins.org/faqs/abioprob/abioprob.html, which contains these particularly-relevant passages:

   Okay, you are looking at that number again, 1 chance in 4.29 x 10^40, that's a big number, and although a billion starting molecules is a lot of molecules, could we ever get enough molecules to randomly assemble our first replicator in under half a billion years?

   Yes, one kilogram of the amino acid arginine has 2.85 x 10^24 molecules in it (that's well over a billion billion); a tonne of arginine has 2.85 x 10^27 molecules. If you took a semi-trailer load of each amino acid and dumped it into a medium size lake, you would have enough molecules to generate our particular replicator in a few tens of years, given that you can make 55 amino acid long proteins in 1 to 2 weeks [14,16].

   So how does this shape up with the prebiotic Earth? On the early Earth it is likely that the ocean had a volume of 1 x 10^24 litres. Given an amino acid concentration of 1 x 10^-6 M (a moderately dilute soup, see Chyba and Sagan 1992 [23]), then there are roughly 1 x 10^50 potential starting chains, so that a fair number of efficent peptide ligases (about 1 x 10^31) could be produced in a under a year, let alone a million years. The synthesis of primitive self-replicators could happen relatively rapidly, even given a probability of 1 chance in 4.29 x 10^40 (and remember, our replicator could be synthesized on the very first trial).

4. Indeed, the Miller-Urey experiment did not produce any lipids. However, this does not mean that these compounds could not have formed on primordial Earth. In fact, experiments have demonstrated that fatty acids (precursors to lipids) can form from carbon monoxide, under the right conditions. In the presence of kaolin clay, which acts as a catalyst, these fatty acids then combine to form lipids. Hot springs appear to be the best location for such reactions to have taken place.

Panspermia

An alternative to the chemosynthesis model for the development of life on Earth is the panspermia model. Originally, panspermia was a 19th century concept:

The hypothetical doctrine of the omnipresence of minute forms and spores of animal and vegetable life, thus accounting for apparent spontaneous generation. Origin: pan-+ G. Sperma, seed.

Obviously, this sounds decidedly non-scientific. In fact, panspermia is usually taken to mean:

The theory that microorganisms or biochemical compounds from outer space are responsible for originating life on Earth and possibly in other parts of the universe where suitable atmospheric conditions exist.

This idea was first put forward in 1974 by Hoyle and Wickramasinghe; the essential elements of their panspermia model are:

• Life could not have began on Earth because of heavy meteorite and comet bombardments.
• Therefore, life on Earth was started by viruses and bacteria, which were delivered by cometary impacts.
• Microorganisms are still arriving today; this continual arrival contributes to evolution, and the interactions between native lifeforms and cometary bacteria could lead to epidemic diseases.

This model caused quite a bit of controversy; one of the foremost objections was that microorganisms would have a difficult time surviving during transit from one solar system to another, due to damage caused by UV radiation and cosmic rays (very high-energy subatomic particles).

Modifications to Panspermia

One significant point regarding the panspermia theory, as put forward by Hoyle and Wickramasinghe, is this: if life (or its building blocks) did not form on Earth, then where did it first form? A number of variant ideas have been put forward to answer this question:

• Macromolecules formed in large molecular clouds situated in space, and were incorporated during the formation of the Earth, thereby supplying the necessary starting materials for life. This pseudo-panspermia concept was lent support in 1994, when a group of astronomers at the University of Illinois discovered a signature of the amino acid glycine in the Sagittarius B2 cloud. However, more recent observations have failed to reproduce their findings.
• Life formed on other planets, presumably by chemosynthesis. It was subsequently ejected into space after a meteor impact, where it then traveled to Earth on a comet or asteroid. The discovery of the meteorite ALH84001 in Antarctica, which was originally from Mars, and contained possible signatures of fossilized life, has lent support to this impact panspermia idea.
• Life never developed from non-living materials; there has always been life in the Universe, and it has been continually raining down on Earth via cometary impacts. This cosmic ancestry theory is the most radical of all panspermia variants, since it calls into question the very beginning of the Universe via the Big Bang. One of its principal claims is that, during the process of evolution, new genes were not created from already-existing ones via random mutations plus natural selection; instead, these genes were delivered from space. Cosmic ancestry is often toted as the 'modern theory of panspermia' (see http://www.panspermia.org/).

Clearly, there is still much debate about the panspermia theory, and indeed about chemosynthesis; the origin of life on Earth remains one of the biggest scientific questions awaiting an answer.

Rich Townsend
Last Modified Date: 31 January 2003