Chapter 6-4

The View from Earth

illustration of 4 test tubes and a  red horizon
The role of clays.
Chemists studying possible mechanisms for the origin of life have emphasized the likely role of clays in providing a suitable environment for the necessary reactions. Experiments with various clay solutions demonstrate the validity of this system as a model for the synthesis of polymers essential for life under conditions simulating the primordial ocean environment.

We are actively exploring the origin of life on the surface of our own planet. Several recent discoveries have provided new insights into the mechanisms which may have acted to synthesize biomolecules on the primitive Earth. We have discovered that, under geologically reasonable conditions, the surfaces of certain clay minerals can select specific biomolecules (molecules necessary for life) from a dilute solution like sea water, can concentrate them, and can cause them to polymerize, or grow into chains of more complex molecules. Model systems based on this research simulate a primitive tidal basin and provide for the first time an attractive sequence of events to explain how large molecules, like proteins and nucleic acids, could form from simpler molecules present in small amounts in the primordial oceans. These findings indicate that clays could have played a vital role in the origin of life on Earth.

We have also come to understand better the puzzling data sent to us from Mars by the Viking landers. During 1979, experiments conducted in several university laboratories finally reproduced the controversial measurements from one of the life detection instruments. Specially prepared clays, when mixed with salts in an amount determined by other Viking instruments to be present in the Martian soil, released carbon dioxide in a fashion like that observed in the Viking labeled release (LR) experiment. More recently, it has been shown that the activity of these same clay-salt mixtures can be destroyed by heat sterilization. This strong sensitivity to heat, also observed in the LR experiment, was the most lifelike response observed in the Martian soil, and it had been the most difficult feature to simulate on Earth up to this time. The discovery on Mars that a non-biological system could exhibit life-like behavior was a significant finding with implications for the exobiological history of Mars that remain to be determined. These findings highlight the need for continued exploration of Mars, including the return of samples for detailed study on Earth. The data also reinforce the central role that clays may have played in generating chemical reactions of exobiological importance, both on Earth and on the other planets.

Our present knowledge of the Mars environment indicates that it is quite hostile as far as the survival of Earth-like microorganisms is concerned. Mars is cold, dry, and bombarded by ultraviolet radiation from the Sun. However, Earth-based research continues to discover microorganisms with unique abilities to thrive in environments previously considered to be too severe or extreme for life. The interior of Antarctica was once believed to be devoid of indigenous life forms because of its cold and dryness. But recent work in the dry valleys of the Antarctic has revealed native microbes of three different kinds. Algae, bacteria, and fungi were discovered living comfortably, embedded just below the surface of rocks strewn over more than 100 locations in these frozen deserts. The finding of life in Antarctica's dry valleys extends the known limits of life on Earth to its driest and coldest climates. At the same time, the discovery suggests that life may exist in similar environments on a planet like Mars, where the climate closely resembles that of the dry valleys of Antarctica. The surface of Martian rocks could provide an effective shield from the harmful radiation of the Sun for microbes dwelling inside.

chart illustrating the geological events in earth's history
Down through the ages.
This geologic "clock" diagram summarizes the history of life on Earth and its relation to the geologic eras. (After Prof. J.W.Schopf, UCLA.)
Another new line of investigation was initiated in an attempt to decipher the long series of events that took place in early biological history after the formation of the first living cell. A multidisciplinary, international team of specialists assembled for a fifteen-month project to discover the details of some of the most significant events in the early biological evolution of the Earth. Some of these events include the origin of early microorganisms that did not use oxygen; the development of the photosynthetic capability to produce food-energy from sunlight; and the advent of oxygen-using or aerobic microrganisms.

This group is searching the Earth's ancient rocks for evidence of the chemical reactions which preceded the appearance of the first life forms. They have already succeeded in pushing back in time the direct evidence for life on Earth by discovering microfossils in rocks that are 3.5 billion years old. This unique research team approach should have a major impact on future investigations of the organic geochemistry of early Earth.

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