Timeline of Events in Life’s Origins

4.55 billion years ago: Formation of the Solar System, including the Sun and Earth, from a giant, rotating, disk-shaped nebula. The Sun and planets gradually formed as gravity pulled matter together into clumps.

4.5 billion years ago: Formation of the Moon by a giant impact of a Mars-sized object with Earth.

4.1 to 3.9 billion years ago: The gradual end to the great bombardment of Earth’s surface and the last of the globe-sterilizing giant impacts. End of the Hadean Eon, beginning of the Archean Eon.

3.85 billion years ago: The oldest rocks on Earth, which occur in outcrops along the coast of Greenland. Some of these rocks contain small amounts of carbon that have the diagnostic signature for life. It is possible, therefore, that primitive life had established a foothold by this early date.

3.45 billion years ago: The oldest purported fossils, which occur in western Australia. Some of these microscopic cell-like structures have been the subject of much recent debate.

2.7 billion years ago: The oldest known molecular biomarkers, which are distinctive biological organic molecules that have been recovered from ancient rocks.

2.5 billion years ago: Continent-sized landmasses first coalesced. End of the Archean Eon.

Key Events in Research on Life’s Origins

1828: German chemist Friedrich Wöhler demonstrates that the molecules of life are no different in principle from any other chemicals. This discovery shows that life is a chemical process.

1859: Publication of British naturalist Charles Darwin’s On the Origin of Species by Means of Natural Selection, which introduced the concept of evolution by natural selection. This evolutionary principle applies to collections of molecules, as well as to living cells.

c.1860: French biochemist Louis Pasteur refutes the theory of spontaneous generation, thus pushing the origin of life back to an unknowably remote time and place. Ironically, any chemical theory of life’s origin must involve spontaneous generation.

1906: German chemist Walter Löb synthesized amino acids by using sparks in a gas mixture, thus anticipating the Miller-Urey experiment by almost half a century. However, his experiment was not designed to mimic a prebiotic environment.

1924: Publication of Russian chemist Aleksandr Oparin’s The Origin of Life, which elaborated on the idea that life arose from a body of water that gradually became enriched in organic molecules.

1929: Publication of British biochemist J. B. S. Haldane’s essay “The Origin of Life” in The Rationalist Annual.

1952: University of Chicago chemists Harold Urey and Stanley Miller perform the Miller-Urey experiment, the first experiment designed to mimic the production of organic molecules under plausible prebiotic conditions. The initial publication, with Stanley Miller as sole author, appeared in Science in 1953 under the title “A Production of Amino Acids Under Possible Primitive Earth Conditions.”

1953: Cambridge University crystallographers James Watson and Francis Crick discover the double-helix structure of DNA.

1958: Protein chemist Sidney Fox presents the first version of his Protenoid World theory of life’s origin, by which self-replicating collections of amino acids have lifelike qualities. The origins community largely rejected the theory, but it was the first model to treat life’s origin as a deterministic process.

1960: Chemist John Oró of the University of Houston synthesizes adenine by heating a concentrated hydrogen cyanide solution.

1966: Scottish chemist Graham Cairns-Smith presents the first version of his Clay Life hypothesis, in which evolving clay minerals are the first life-form on Earth. This model was subsequently revised and expanded in a series of books and articles.

1967: Sol Spiegelman and colleagues at the University of Illinois demonstrate that a virus called Qβ can be induced to evolve in carefully controlled laboratory experiments.

1968: Leslie Orgel of the Salk Institute contributes his now-classic paper entitled “Evolution of the Genetic Apparatus.” Orgel argues in favor of a genetics-first origin of life, based on an RNA-like polymer.

1970s: Development of the theory of emergent systems by Russian-born chemist Ilya Prigogine. Prigogine recognized that complex patterns arise when energy flows through a system of many interacting particles. The origin of life is a dramatic example of emergence.

1976: NASA’s two Mars Viking landers provide ambiguous evidence regarding the presence of microbial life on Mars.

1977: Professor Carl Woese of the University of Illinois proposes a dramatic revision of the tree of life, with three domains—the Bacteria, the Archaea, and the Eukarya. The first two of these domains are exclusively single-celled organisms, many of which are extremophiles.

1977: Professor Thomas Gold of Cornell University proposes that petroleum arises from non-biological processes in Earth’s deep interior. He follows up this speculation in 1992 with descriptions of the “deep, hot biosphere.”

1979: Publication of “An Hypothesis Concerning the Relationship Between Submarine Hot Springs and the Origin of Life on Earth,” by Oregon State University scientists Jack Corliss, John Baross, and Sarah Hoffman. This paper was a significant challenge to the prevailing view of a surface origin of life.

1981: The independent discovery by Sidney Altman of Yale University and Thomas Cech of the University of Colorado of ribozymes, which are strands of RNA that act as catalysts. They shared the Nobel Prize in 1989 for this finding.

1985: Publication of Belgian chemist Christian de Duve’s Vital Dust: Life as a Cosmic Imperative, which elaborated on the Thioester World scenario for life’s origin.

1986: Nobelist Walter Gilbert proposes the RNA World scenario for life’s origin, by which a self-replicating strand of RNA represents the first living entity. Leslie Orgel, Gerald Joyce, and others quickly elaborated on the theory.

1988: Initial publication of the Iron-Sulfur World scenario for life’s origin by German chemist Günter Wächtershäuser. Subsequent papers, notably in 1990 and 1992, greatly expanded on the chemical details of this model.

1988: James Ferris and coworkers at Rensselaer Polytechnic Institute discover the ability of clay minerals to induce RNA nucleotides to link together. Subsequent experiments document chains of more than 50 RNA units.

1989: David Deamer and coworkers at the University of California demonstrate the self-assembly of amphiphilic molecules that were extracted from the Murchison meteorite, thus demonstrating a possible prebiotic pathway to primitive cell membranes.

1991: Swiss chemist Albert Eschenmoser and colleagues synthesize more than a dozen variants of RNA with different sugar-phosphate backbones. Of special note was their synthesis of TNA, a polymer with the simple 4-carbon sugar threose instead of the 5-carbon ribose of RNA.

1991: Danish chemist Peter Nielsen and colleagues synthesize a novel genetic molecule with a backbone of amino-acid molecules. This variant has been given the name peptide nucleic acid, or PNA.

1993: Paleontologist William Schopf of UCLA publishes descriptions of 11 purported new species of fossil microbes from the 3.45-billion-year-old Apex Chert—supposedly the oldest fossils on Earth.

1996: The publication in Science of “Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001” by a team of NASA scientists led by David McKay. The article immediately creates intense controversy over the authors’ claims.

1997: Formation of NASA’s Astrobiology Institute (NAI), which is headquartered at the NASA Ames Research Center at Moffett Field, California. The NAI becomes a primary funding organization for origins-of-life research.