by Aw Swee-Eng

Profound advances in the fields of molecular biology in recent years have enabled the elucidation of cell structure and function in detail previously unimaginable. The unexpected levels of complexity revealed at the molecular level have further strained the concept of the random assembly of a self-replicating system. At the same time, the recent discovery of fossil algae and stromatolites (primitive colonies of cyanobacteria) from as early as the Precambrian, have reduced the time for development of the first cell as much as tenfold. Together with implications of this for the oxidative state of the primitive atmosphere, these developments will force researchers to rethink many fundamental ideas pertaining to current models of the origin of life on Earth. The evidence for the nature of the primitive atmosphere is examined and the possibility of ribonucleic acid (RNA) as the first self-replicating molecule is evaluated. The focus is then on DNA, proteins and the first cells.

The early atmosphere

The nature of the atmosphere under which life arose is of great interest. The high oxygen content of the Earth’s atmosphere is unique among the planets of the Solar System and could have been tied up with the composition of the core and its crust. It has to be said that none of the hypotheses of core formation of the Earth survives quantitative scrutiny. The gross features of mantle geochemistry, such as its redox state (FeO) and its iron–sulphur systems, apparently do not agree with experimental data.1,2 There are outstanding questions relating to the formation and recycling of the Archaean crust.3

Figure 1. Simplified apparatus for abiotic synthesis of organic compounds as performed originally by Miller and Urey. By varying the mixture of gases, including using volcanic gases of today, experimenters have been able to produce many types of organic compounds.

Interesting organic molecules such as sugars and amino acids can be formed from laboratory ‘atmospheres’ of different proportions of CO2, H2O, N2, NH3, H2, CH4, H2S and CO. This happens only in the absence of free O2. Oxygen is highly reactive, breaking chemical bonds by removing electrons from them. A reducing gas (H2, CH4 or CO) is therefore thought to be essential for the successful synthesis of prebiotic organic molecules.

It has been generally accepted that at about 1.5 Ga [Giga annum = billion years ago] the oxygen content of the air rose at least 15-fold. (Note that evolutionary/uniformitarian ‘ages’ are only used here for argument’s sake.) Before this, the oxygen had been reduced by Fe(II) in sea water and deposited in enormous bands as oxides or hydroxides on the shallow sea floors. The source of the ferrous iron was hydrothermal vents in the company of reducing gases such as hydrogen sulphide (H2S).

In 1993 Widdel and his team cultured non-sulphur bacteria from marine and freshwater muds. These anoxygenic, photosynthetic bacteria use ferrous iron as the electron donor to drive CO2 fixation. It was a signal discovery that oxygen-independent biological iron oxidation was possible before the evolution of oxygen-releasing photosynthesis. Quantitative calculations support the possibility of generating such massive iron oxide deposits dating from Archaean and Early Proterozoic times, 3.5–1.8 Ga.4

In 1992 Han and Runnegar made a discovery which impinged on discussions of oxygen evolution during the Precambrian. To everyone’s surprise they reported the spiral algal fossil Grypania within banded iron formations (BIFs) in Michigan, USA. Algae require oxygen, so their existence at this juncture shows banded iron formations do not necessarily indicate global anoxic conditions.5

Indeed, as early as 1980 two reports appeared on the discovery of stromatolites in the 3.4–3.5 Ga Warrawoona Group sediments from the Pilbara Block, Australia.6,7 Similar remains were also discovered in Zimbabwe8 and South Africa.9

It is fair to conclude that the Earth’s early atmosphere before 3.5 Ga could have significant quantities of oxygen. This should discourage the sort of hypothesising on abiotic monomer and polymer syntheses so often assumed to have occurred in Archaean times. Robert Riding says that the Grypania discovery

“ … could spell the end of BIF-dominated models of oxygen build-up in the early atmosphere … The cat really will be put among the pigeons, however, if [further] fossil discoveries extend the eukaryote record back much beyond 2200 million years ago, into what is still widely perceived to have been an essentially anaerobic world.”10….

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