Theories of the Origin of Life

Living things are composed of the same elements as the inanimate universe - the 92 elements of the periodic table. But the arrangements of these atoms into molecules in biological systems are unique. You cannot find DNA in rocks unless it came from a once-living organism. How life began on Earth sometime during the 600 million ears of the Hadean is impossible for us to know for certain, given the vast amount of time that has passed. There are two theories of the origin of life: life from extraterrestrial sources, and chemical evolution.

Comets probably brought Earth most of its water. The meteorites described at the beginning of this chapter are evidence that molecules characteristic of life may have travelled to Earth from space. Taken together, these two observations suggest that some of life’s complex molecules could have come from space. Although the presence of such molecules in rocks may suggest that those rocks once harboured life, it does not prove that there were living things in the rocks when they landed on Earth. Claims that the spherical objects seen in ALH 84001 are the remnants of ancient Martian organisms are far from accepted by all scientists in the field. Most scientists find it hard to believe that an organism in a meteorite could survive thousands of years travelling through space followed by intense heat as it passed through Earth’s atmosphere. But there is some evidence that the heat inside some meteorites may not have been severe. When weakly magnetized rock is heated, it reorients its magnetic field to align with the magnetic field around it. In the case of ALH 84001, this would have been Earth’s powerful magnetic field which would have affected the meteorite as it approached our planet. Careful measurements indicate that, while reorientation did occur at the surface of the rock, it did not occur in the inside. The scientists who took these measurements, Benjamin Weiss and Joseph Kirschvink at the California Institute of Technology, claim that the inside of ALH 84001 was never heated over 40oC on its trip to Antarctica, making a long interplanetary trip by living organisms more plausible.

Both Earth and Mars once had the water and other simple molecules that could, under the right conditions, form the large molecules unique to life. The second theory of the origin of life on Earth, chemical evolution, holds that conditions on the primitive Earth led to the emergence of these molecules. Scientists have sought to reconstruct those primitive conditions. Early in the twentieth century, researchers proposed that there was little oxygen gas (O₂) in Earth’s first atmosphere (unlike today when it constitutes 21 percent of our atmosphere). O₂ is thought to have accumulated in quantity about 2.5 billion years ago as the by-product of the metabolism of single-celled life forms. In the 1950s, Stanley Miller and Harold Urey set up an experimental “primitive” atmosphere, containing hydrogen gas, ammonia, methane gas and water vapour. Through these gases, they passed a spark to simulate lightning, and then cooled the system so the gases would condense and collect in a watery solution or “ocean”. Within days, the system contained numerous complex molecules, including amino acids, purines and pyrimidines— some of the building blocks of life. In science, an experiment and its results must be constantly reinterpreted and refined as more knowledge accumulates. The results of the Miller-Urey experiments have undergone several such refinements. In living organisms, many molecules have a unique three-dimensional “handedness”. The amino acids, for example, are all in the L-configuration. But the amino acids formed in the Miller-Urey experiments were a mixture of the D- and L-forms. Recent experiments show that natural processes could have selected the L-amino acids from the mixture. Some minerals, especially calcite-based rocks, have unique crystal structures that selectively bind to D- or L-amino acids, separating the two. Such rocks were abundant during the Archean. Scientists’ views of Earth’s original atmosphere have changed since Miller and Urey did their experiment. There is abundant evidence of major volcanic eruptions 4 billion years ago that released carbon dioxide (CO₂), nitrogen (N₂), hydrogen sulphide (H₂S) and sulphur dioxide (SO₂). Prebiotic chemistry experiments using these molecules in addition to the ones in the original “soup” have led to more diverse molecules.

Long polymers had to be formed from simpler building blocks called monomers. Scientists have used model systems to try to simulate conditions under which polymers could be made. Solid mineral surfaces, such as finely divided clays, seem to provide the best environment to bind monomers and allow them to polymerize.

A. Oparin, as well as much later Miller and Urey, suggested that life originated in hot pools at the edges of oceans. Because life has been found in many extreme environments on earth, scientists have proposed that such environments— found beneath ice, in deep-sea hydrothermal vents and within fine clays near the shore—could be the original site of life’s emergence. In whatever way the earliest stages of chemical evolution occurred, they resulted in the emergence of monomers and polymers that have probably remained unchanged in their general structure and function for 3.8 billion years. We now turn our attention to these large molecules.