7.3 - Early Chemical Evolution on Earth

Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

First Edition

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

Chemical evolution refers to the period in Earth’s history during which the chemical components on the surface changed from simple forms into complex substances from which the first living organisms -- protobionts -- could develop. The primary investigative tool in abiogenesis research has been the prebiotic synthesis experiment. Plausible primitive Earth conditions are arranged in a closed laboratory apparatus, and the changes that take place are carefully monitored.

The argument has long been made that since no geological record of the origin of life exists, the course of events leading up to the creative event is fundamentally unknowable. While most biochemists today would dispute this supposition, how close to reality are the simulated prebiotic experiments?

It is unnecessary for scientists to heat together water, methane, ammonia and hydrogen (components of the primitive atmosphere), irradiate the mess with various forms of energy, and then sit back to wait for a recognizable lifeform to reach its slimy paw over the edge of the beaker and crawl out onto the lab desktop. We won’t ever achieve this kind of completeness, because that takes evolution and the secret to evolution is time.²²⁵ (But it has been seriously suggested that a complete artificial seashore be set up to test some of the proposed mechanisms in the origin of life.¹⁶³⁰)

From chemical equilibria we know the kinds of substances that had to be floating around in the primitive atmosphere and seas. Protein molecules ultimately consist of different combinations of only twenty different amino acids. Nucleic acids are composed of one of five bases, one of two sugars, and a single type of phosphate group. As Cyril Ponnamperuma of the NASA/Ames Exobiology Division once remarked: "The alphabet of life is extremely simple; the wide variety of life observed today may be traced to a mere handful of chemicals."⁸⁵

Abiogenesis research differs markedly from most other scientific work, in that an unverifiable historical process is being reconstructed. It probably is not practical to run through an entire origin of life "from scratch," so different criteria must be used to evaluate hypotheses. For instance, postulates must at least be consistent with known astronomical, geophysical, and biochemical principles insofar as this is possible. And stepwise experiments, in which only one step of abiogenesis at a time is simulated, are reasonable if plausible and appropriate prebiotic conditions are maintained.

It is believed that the origin of life may have happened very fast, certainly less than a billion years⁵²¹ and possibly less than a hundred million years.^225,305,2160 Most estimates today place the creative event in the primitive seas, roughly 4.2 to 3.6 eons ago.