Theories of How Life Began
Did the First Cells Form in a Soup, Sandwich, or Pizza?
Theories about the first life forms tend to agree on what had to happen, but differ in their explanations of where and how. There are three strong contenders.
When and How Did Life Begin?
Scientists believe that the earliest forms of life appeared about four billion years ago. No one knows exactly how it happened, but scientists do know what the basic steps would be. Hydrogen linked nucleotides (molecules composed of an organic base, a sugar, and a phosphate group) together, in chemical reactions, to create chains of nucleotides called nucleic acids. Nucleic acids linked together to form RNA (ribonucleic acid) and DNA (deoxyribonucleic acid), the complex molecules that contain the code for carbon-based life—life on Earth.
At least three theories draw from this basic understanding to propose what circumstances supported the necessary chemical reactions for life to begin. They are charmingly called the soup theory, the pizza theory, and the soup and sandwich theory.
The Soup Theory of the Origins of Life
In the soup hypothesis, all the elements needed for the construction of nucleic acids were drifting around in the early waters—primordial soup—and atmosphere of earth. The addition of energy, in the form of lightning, catalyzed the chemical reaction that joined them together.
While nucleic acids have been produced in the laboratory in this fashion, the biggest problem with the soup theory is that the primordial soup would have been too thin: the elements were not present in sufficient numbers to come together and remain together, eventually forming larger more complex molecules.
The Pizza Theory of the Origins of Life
The pizza hypothesis solves the problem of elements being too scarce and too easily torn apart by proposing that they came together on a solid surface. The negatively charged molecules would be attracted to a positively charged mineral surface and so they’d be more likely to remain together.
Variations on the pizza theory describe it happening on dry land (which might not have enough water for the chemical reactions to proceed), on aquatic surfaces, and even on hot wet surfaces, such as those near deep sea hydrothermal vents.
The Soup and Sandwich Theory of the Origins of Life
The most recent hypothesis of the life’s beginnings, proposed by biochemist Helen Hansma of the U. S. National Science Foundation, has the necessary elements and molecules trapped in solution between layers of mica, a mineral that forms in very thin sheets.
When mica is underwater, the thin layers move in relation to each other because of ocean currents and temperature changes—molecules trapped in liquid between the layers are continually rearranged and will thus come into contact with each other repeatedly, increasing the likelihood that they will react with each other and grow larger and more complex, while being sheltered in an enclosed environment.
Energy for the reactions would come from the movement of the mica and possibly from sunlight.
Interestingly, this theory might explain the high potassium levels in living cells: mica is rich in potassium.
We are probably a long way from knowing which of these theories, if any, is correct.
Further reading:
How did Life Begin: An Interview with Andy Knoll. NOVA: Science Programming on Air and Online http://www.pbs.org/wgbh/nova/origins/knoll.html
Sources:
"Did Life Originate in a Mica Sandwich Sitting in Primordial Soup?" National Science Foundation Dec 04, 2007
The Cassell Dictionary of Chemistry. Waites, Gillian and Percy Harrison. London: Cassell, 1998.
