HiSET Science: Scientific Explanations for the Origin of Life on Earth

Explanations for the Origin of Life on Earth

Panspermia
The word panspermia is a Greek work that means 'seeds everywhere.' Panspermia is one possible explanation for the origin of life on Earth that states that 'seeds' of life exist throughout the universe and can be transferred from one location to another. Three types of panspermia based on the seed-dispersal method have been proposed. Lithopanspermia is described as rocks or dust transferring microorganisms between solar systems. Ballistic panspermia is described as rocks or dust transferring microorganisms between planets within the same solar system. Directed panspermia is described as intelligent extraterrestrials purposely spreading the seeds to other planets and solar systems. The panspermia hypothesis only proposes the origin of life on Earth. It does not offer an explanation for the origin of life in the universe or explain the origin of the seeds themselves.

Abiotic Synthesis of Organic Compounds
Scientists have performed sophisticated experiments to determine how the first organic compounds appeared on Earth. First, scientists performed controlled experiments that closely resembled the conditions similar to an early Earth. In the classic Miller–Urey experiment (1953), the Earth's early atmosphere was simulated with water, methane, ammonia, and hydrogen that were stimulated by an electric discharge. The Miller–Urey experiment produced complex organic compounds including several amino acids, sugars, and hydrocarbons. Later experiments by other scientists produced nucleic acids. Recently, Jeffrey Bada, a former student of Miller, was able to produce amino acids in a simulation using the Earth's current atmospheric conditions with the addition of iron and carbonate to the simulation. This is significant because in previous studies using Earth's current atmosphere, the amino acids were destroyed by the nitrites produced by the nitrogen.

Atmospheric Composition
The early atmosphere of Earth had little or possibly no oxygen. Early rocks had high levels of iron at their surfaces. Without oxygen, the iron just entered into the early oceans as ions. In the same time frame, early photosynthetic algae were beginning to grow abundantly in the early ocean. During photosynthesis, the algae would produce oxygen gas, which oxidized the iron at the rocks' surfaces, forming an iron oxide. This process basically kept the algae in an oxygen-free environment. As the algae population grew much larger, it eventually produced such a large amount of oxygen that it could not be removed by the iron in the rocks. Because the algae at this time were intolerant to oxygen, the algae became extinct. Over time, a new iron-rich layer of sediments formed, and algae populations reformed, and the cycle began again. This cycle repeated itself for millions of years. Iron-rich layers of sediment alternated with iron-poor layers. Gradually, algae and other life forms evolved that were tolerant to oxygen, stabilizing the oxygen concentration in the atmosphere at levels similar to those of today.

Development of Self-Replication
Several hypotheses for the origin of life involve the self-replication of molecules. In order for life to have originated on Earth, proteins and RNA must have been replicated. Hypotheses that combine the replication of proteins and RNA seem promising. One such hypothesis is called RNA world. RNA world explains how the pathway of DNA to RNA to protein may have originated by proposing the reverse process. RNA world proposes that self-replicating RNA was the precursor to DNA. Scientists have shown that RNA can actually function both as a gene and as an enzyme and could therefore have carried genetic information in earlier life stages. Also, RNA can be transcribed into DNA using reverse transcription. In RNA world, RNA molecules self-replicated and evolved through recombination and mutations. RNA molecules developed the ability to act as enzymes. Eventually, RNA began to synthesize proteins. Finally, DNA molecules were copied from the RNA in a process of reverse transcription.