THE BIOLOGY BOOK

Origin of Life

c. 4 Billion BCE

The remains of fossils of microorganisms reveal that life on Earth began perhaps as early as 4–4.2 billion years ago. But how did life begin? The notion that life could arise from nonliving matter (spontaneous generation) can be traced back to ancient Greece, and it wasn’t until 1859 that Louis Pasteur conducted a series of experiments that apparently disproved this concept. But in the mid-1920s, spontaneous generation reappeared, now relabeled as abiogenesis. The Russian biochemist Alexander Oparin and British evolutionary biologist J.B.S. Haldane, working independently, suggested that conditions on the primordial Earth were very different from those that now exist and favored chemical reactions leading to the synthesis of organic molecules from inorganic starting materials. The scientific literature abounds with theories postulating how the origin of life occurred, and while none have gained universal acceptance, most use some variation of the Oparin-Haldane hypothesis as their basis.

The process of abiogenesis (or biopoiesis), by which life arises from self-replicating abiotic (nonliving) simple organic molecules, occurs in several stages: Small organic molecules, such as amino acids and nitrogen-containing bases, are synthesized from atmospheric carbon dioxide and nitrogen, with energy provided by intense sunlight or UV (ultraviolet) radiation. These small organic molecules are linked to form macromolecules, such as proteins and nucleic acids. The macromolecules are brought in proximity within protocells, vesicles that are precursors of living cells and surrounded by a membrane controlling the cell’s internal chemical contents. Under these conditions, reproduction and energy-producing and energy-utilizing chemical reactions can occur. In the final stage, a self-reproducing ribonucleic acid (RNA) is formed that is required for protein synthesis and that can perform enzyme functions needed for RNA replication. The unique chemistry of these newly formed RNA molecules makes them most successful in self-replication, allowing them to pass their favorable traits to daughter RNA molecules. This may represent the earliest example of natural selection.

SEE ALSO: Prokaryotes (c. 3.9 Billion BCE), Eukaryotes (c. 2 Billion BCE), Metabolism (1614), Refuting Spontaneous Generation (1668), Fossil Record and Evolution (1836), Darwin’s Theory of Natural Selection (1859), Enzymes (1878), Miller-Urey Experiment (1953), Domains of Life (1990).

The question of how life first began on Earth has challenged scholars and philosophers for thousands of years. Conditions on our planet some one billion years after its formation were very different and conducive to the formation of simple organic molecules from the raw materials in the primordial atmosphere.