One of the first theories to become widely known that described how life may have begun on this planet was proposed in the 1920’s independently by Aleksandr Oparin, a Russian biochemist, and J. B. S. Haldane, a British biologist. They pointed out that some four billion years ago, conditions in the Earth’s shallower seas and oceans would have resembled a chemical vat. This vat must have held a variety of ingredients that would have been warmed by sunlight, constantly stirred by tides and winds, bathed in ultraviolet radiation from the sun, and intermittently subjected to electrical discharges from thunder storms. This so-called called “primordial soup” would inevitably have become more complex over time, as interactions (mostly chemical) between the constituents occurred. Eventually, it would likely contain many of the molecules and compounds needed to create some elemental forms of life.
Stanley Miller, under Harold Urey at the University of Chicago in 1953, recreated many of these conditions in the laboratory. Together they subjected methane, ammonia and hydrogen (gases that very probably existed on this planet in its early years, and that are still present in abundance on Jupiter and Saturn) to electrical sparks in a sealed sterile flask. On later analyzing the flask’s contents, they found many of the amino acids from which life’s building blocks—proteins—are built. Similar experiments have since yielded molecular components of proteins that regulate carbohydrate and fat formation. In other words, some of the major constituents of life have been fabricated from scratch in the laboratory.
However, it is equally possible that life on this planet began thousands of meters beneath the sea’s surface, in total darkness. Clues to this conjecture have been found in north-western Australia, in sulphur-rich rocks that retain micro-fossils of single-celled organisms over three and a quarter billion years old. These rocks possess mineral structures that reveal they originated close to hot springs on the sea floor.
In 1977, a mid-ocean ridge of hot springs was discovered encircling the globe. The wealth of chemicals and nutrients it supplies nourishes a complex ecosystem of over 500 species, from bacteria to tube worms and crabs.
The energy source that sustains this web of life is the oxidization of hydrogen sulphide in a process called chemosynthesis (as opposed to photosynthesis, whereby sunlight powers life on the Earth’s surface). Experiments at the Woods Hole Oceanographic Institution determined that when similar physical (high-pressure, turbulence, completely dark, etc.) and chemical conditions are constructed in the laboratory, large organic molecules containing over thirty carbon atoms form in less than a day. Thus, life on Earth may have first begun in the sunless depths of its oceans.
Alternatively, life may have begun as some form of methanogen. Methanogens are microbes that obtain energy by converting hydrogen and carbon dioxide into methane. Very few such organisms exist in any of Earth’s typical, oxygen-abundant, environments (because methanogens are consumed by the more-efficient carbon life forms that now occupy these niches), but a complete food-chain community of them has been found in Idaho living in 58°C water two hundred meters underground. Presumably these have survived from very early times. Methanogen communities may have been common on this planet before oxygen in gaseous form became abundant (see Development Of Life On Earth) and they may also exist where conditions are similar, such as upon some of the sun’s other planets or moons, for instance.
While several situations and mechanisms might have given rise to life on this planet, it may in fact owe its origins to extraterrestrial events that first happened in water, frozen in space, long ago and far away. Although space temperatures average just 3° above absolute zero, recent experiments have found that amorphous ice (the kind that forms when water vapour freezes in a vacuum) flows when subjected to ultraviolet radiation (as it would be in space). In the laboratory, when carbon monoxide, carbon dioxide, and methanol (gases all abundant in space) are dissolved in water before freezing to form amorphous ice, subsequent ultraviolet radiation produces hundreds of complex organic molecules. Moreover, if this frozen ice flows (or is melted or added to water), membranous vesicles (similar to those found in the 1969 Murchison meteorite) are formed, together with even more complex compounds (some able to convert ultraviolet energy to visible light).
Laboratory findings such as these are reinforced through data collected by the European Space Agency’s satellite Infrared Space Observatory (ISO). When scrutinizing selected objects, the ISO can detect the emission of infrared rays at particular wavelengths, revealing the presence of identifiable atoms, molecules and solids. These data show that complex, ring-structured, aromatic molecules form in the regions surrounding very old stars, over the relatively short period of a thousand years or so. Spectral analysis of interstellar dusts and gases have identified hundreds of different organic compounds, including amino acids of the type needed to build life’s proteins. Recently, sugar molecules (glycolaldehyde) have been spectroscopically detected in the dust clouds near the centre of our Milky Way galaxy. (What makes this finding particularly interesting is that such molecules can combine with other molecules to form ribose and glucose; ribose molecules are utilized in the construction of DNA and RNA.)
All these findings strongly suggest that life’s precursors, including cell-like sacs containing organic compounds, could have been formed many billions of years ago in space, and would therefore be part of all comets, asteroids and planets from their very beginnings.
- J. D. Bernal’s book, The Origin of Life (London: Weidenfeld and Nicolson, 1967) provides a classic account and critical discussion of what was known in the 1960s about the origin of life.
Many books have been written about life’s origin, more recently including:
- David W. Deamer and Gail R. Fleischaker, Origins of Life: The Central Concepts (Sudbury, Massachusetts: Jones and Bartlett, 1994).
- John H. Holland, Emergence From Chaos to Order (Helix Books, 1998).
- Noam Lahav, Biogenesis: Theories of Life’s Origin (Oxford: Oxford University Press, 1999).
- Miniscule microbes (about one thousandth of a millimetre in length) that possess membranes and DNA have also been found living in solid rock, at temperatures over 150º Centigrade, five kilometres underground. These probably developed from life forms that existed when the rocks formed. See “It’s a small world after all,” Discover, January 2001, 58.
- Other theories relating to the origin of life are mentioned in the postscript to this chapter.
- For more detail on this subject, see Michael Gross, Life on the Edge: Amazing creatures thriving in extreme environments (Perseus Publishing, 2001).
- Meteorites are fragments of asteroids that did not become part of the solar system’s planets, and they carry information that depicts what existed at the time of their formation. The Murchison meteorite was extensively examined in 1997 and found to contain an excess of left-handed amino acids—the same bias that life on Earth exhibits.
- Amino acids in space show a slight predominance of left-handedness. (N.B. Miller-type experiments produce equal-handed amino acids.)
- And may still be forming.
- See “Life’s Far-Flung Raw Materials” by Max. P. Bernstein, Scott A. Sandford and Louis J. Allamandola, in the July 1999 edition of Scientific American, 42-49.
Also see: David F. Blake and Peter Jenniskens, “The Ice of Life,” Scientific American, August 2001, 44-51; Jason P. Dworkin et al, “Self-assembling amphiphilic molecules: Synthesis in simulated interstellar/precometary ices,” Proceedings of the National Academy of Science, January 30, 2001. The SETI website (http://www.seti.org) also provides links to other information on this topic.