The Origin of Life on Earth—Life from Nonlife

Geologists estimate from radioisotope dating that the earth is approximately 4.6 billion years old.  The earliest direct evidence of life on earth are microfossils of 3.5 billion-year-old rock formations known as stromatolites from Australia and South Africa. Thus, it took no more than 1 to 1.5 billion years for life to originate.

In trying to explain how life may have arisen, scientists first needed to know the conditions that existed on the earth after its formation.  In 1929, J. B. S. Haldane described the atmosphere of primordial earth as a reducing atmosphere (with little free oxygen present) containing primarily hydrogen, water, ammonia and methane.  In 1953, Stanley Miller and Harold Urey constructed a reaction vessel in which they duplicated the atmosphere Haldane described.  They heated the mixture to 80^oC and provided the atmosphere with an electrical spark to simulate lightning.  Over the course of a week, they removed samples from their system and found a variety of common amino acids and other organic acids.

The previous scenario for the origin of the first organic compounds, although once widely accepted, is under increasing scrutiny.  Recent evidence suggests that carbon dioxide and nitrogen gas, not methane and ammonia, were the major components of the earth’s primitive atmosphere.  These conditions are much less favorable for the formation of organic compounds using the Miller/Urey apparatus.

Scientists have begun to look for new explanations of the origin of the first organic chemicals and at older explanations, which are being revived.  One of these is that life may have begun deep in the oceans, in underwater hot springs called hydrothermal vents.  These vents could have supplied the energy and raw materials for the origin and survival of early life forms.  A group of organisms, called Archaea, that tolerate temperatures up to 120^oC and seem to have undergone less evolutionary change than any other living species, support this vent hypothesis.

Another explanation for the origin of the earth’s first organic molecules is that they came from outer space.  Astronomers are detecting an increasing diversity of organic compounds (such as amino acids and other hydrocarbons) in meteorites that have collided with the earth.  Investigations of the most recent pass-by of Halley’s Comet revealed that comets might be relatively rich in organic compounds.  Even though many scientists think that the first organic compounds could have come from space, no microbial life forms have been detected in space, and conditions in outer space are incompatible with life as we know it. 

A second step in the origin of life must have been the hooking together of early organic molecules into the polymers of living organisms: polypeptides, polynucleotides and carbohydrates.  Organic molecules may have become isolated in tide pools or freshwater ponds, and as water was lost through evaporation, condensation reactions could have occurred.  Alternatively, reacting molecules may have been concentrated by adsorption on the surfaces of clay or iron pyrite particles, where polymerization could occur.

The final steps in the origin of life are the subject of endless speculation.  In some way, organic molecules were surrounded by a membrane-like structure, self-replication occurred and DNA became established as the genetic material.  A “chicken-or-egg” paradox emerges if DNA was the first genetic material.  DNA codes for proteins, yet proteins (enzymes) are required for DNA replication, transcription and translation.  Thomas R. Cech from the University of Colorado at Boulder suggested a possible way around this paradox in the early 1980s.  He discovered a certain type of RNA that acts like an enzyme, cutting and splicing itself into a functional molecule.  The first organisms could have been vesicles of self-replicating RNA molecules.  Other scientists think that proteins may have been the first genetic material and that DNA was then established as the code-carrying molecule. (click here for more details)

What were the first living cell like? No one knows for sure because we don't have good data from the first 2 billion years of Earth's history—a period known as the Archaean. Most likely they were heterotrophic microbes that found enough energy from the acid and base molecules in the rich broth of the early seas to continue living and reproducing. These organisms extracted energy by fermentation, which is the same process used by unicellular microbes (mostly yeast) to make beer or bread.

As Earth cooled, geologic and atmospheric activity declined, and as gases thinkened the air, smaller amounts of solar radiation reached the surface. Lab experiments show that these changing conditions would have caused the organic soup to gradually thin, which is why we don't see a thick film of nutrients floating on today's oceans and rivers. The 'juicy' organic molecules on which heterotrophs could feed became more scarce, creating a crisis for the multiplying cells. The organic production of acids and bases via lightning, volcanoes, or solar radiation simply couldn’t satisfy the voracious appetite of the growing population of heterotrophs.

If life were to continue, another source of nutrients was needed.  Photosynthesis, which is the production of organic molecules using solar energy and inorganic compounds, solved the problem of a dwindling supply of nutrients.  The first photosynthetic organisms probably used hydrogen sulfide as a source of hydrogen for reducing carbon dioxide to sugar.  Later, water served this same purpose, and oxygen liberated by photosynthetic reactions began to accumulate in the atmosphere.  Earth and its atmosphere slowly began to change.  Ozone in the upper atmosphere began to filter ultraviolet radiation from the sun, the reducing atmosphere slowly became an oxidizing atmosphere, and at least some living organisms began to utilize oxygen.  About 420 million years ago enough protective ozone had built up to make life on land possible.  Ironically, the change from a reducing atmosphere to an oxidizing atmosphere also meant that life could no longer arise abiotically.