Origin of Life Theories
How did life on Earth begin? How did a world of inorganic molecules give rise to the first living thing?
No one really knows. There are many uncertainties and controversies. But a general picture is emerging as researchers from diverse fields put together different pieces of the puzzle.
Origin of life theories
Scientific theories about the chemical steps leading to the origin of life fall broadly into two categories—genes first and metabolism first.
A groundbreaking experiment
In a 1953 experiment suggested by chemist Harold Urey, graduate student Stanley Miller simulated an atmosphere containing inorganic chemicals thought to have been present on early Earth. To replicate lightning, an electrical charge was passed through this atmosphere. Within a week, a host of simple organic molecules had formed, including amino acids.
The Miller-Urey experiment didn’t show how life on Earth arose; in fact, early Earth conditions were very different from those in the experiment. It did, however, demonstrate that biochemicals necessary for life could arise out of a nonbiochemical stew. The experiment also sparked intense scientific interest in the origin of life that continues to this day.
Inspired by Miller-Urey, scientists have conducted extensive research on sugars and nucleobases—the molecules that encode information in DNA and RNA. RNA has drawn a lot of attention: although its function now is to transmit genetic information carried by DNA and to manufacture proteins, many researchers suspect RNA or its precursor was the original carrier of genetic information.
Genes first: Was it chance?
Since Miller-Urey, the prevailing theory has been that life emerged by chance: powered by lightning or sunlight warming a tidal pool, a group of molecules happened to join together into a complex molecule that could reproduce itself. This set off the chain reaction of reproduction, variation, and evolution that constitutes life on Earth. In the years following the discovery of DNA and RNA, it was hypothesized that an unidentified forerunner to RNA was that first replicating molecule.
The probability that the necessary sequence of chemical reactions to produce this molecule would occur at any given time was miniscule, but proponents of the theory thought that, over a span of millions of years, it could have happened.
Metabolism first: Was it inevitable?
According to the metabolism first theory, life began with the development of metabolic processes out of combinations of naturally occurring chemical reactions. These processes evolved into the chemical pathways and cycles now shared by every living creature. Thermal vents on the ocean floor could have provided the right conditions for this process; archaea and bacteria flourish under these extreme temperatures and pressures.
According to this theory, life on Earth may have been inevitable.
The Citric Acid Cycle
Scientists Harold Morowitz and Günter Wächtershäuser, among others, have homed in on the citric acid cycle and the fact that all living things—without exception—depend on all or portions of it to metabolize carbon. The citric acid cycle is intriguing for another reason: it can run in two directions. In oxygen-rich environments like ours, it runs one way and is called the oxidative citric acid cycle or the Krebs cycle. In oxygen-deprived environments like early Earth, it runs in the opposite direction and is called the reductive citric acid cycle. Some deep-ocean bacteria continue to use this version.
The suspicion is that the reductive citric acid cycle was primordial—that every other biochemical cycle evolved from it. In short, it’s viewed as a strong candidate for the bridge between a non-living and a living planet.
It came from outer space
There are other, quite different origin-of-life theories. Proponents of Panspermia suggest that life on Earth—or its essential ingredients—came from outer space. They note that the Murchison meteorite in Australia was found to contain over 90 different amino acids, including 19 of the 20 deemed essential to life. Even if this theory explains where life on Earth came from, however, it still fails to answer how life began before it arrived.