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Earth formed about 4.5 billion years ago, and fossil evidence of microbial life, such as stromatolites, dates back about 3.5 billion years. Scientists study how life could have arisen from nonliving chemistry through a series of hypothesized steps: abiotic synthesis of small organic molecules, joining of those molecules into polymers, packaging of molecules inside membrane-bounded protocells, and the origin of self-replicating molecules that made inheritance possible. Laboratory experiments under conditions meant to mimic the early Earth have reproduced parts of each step, and the RNA world hypothesis proposes that RNA served as both genetic material and catalyst before DNA and proteins took over those roles. How life actually began remains an open question.
Present the problem as four engineering challenges (make building blocks, link them, enclose them, copy them) and match each to an experiment. Discuss what Pasteur's flasks did and did not show. Compare the properties of DNA, RNA and protein in a table to see why RNA is the leading candidate for the first genetic molecule.
Every living cell today comes from another cell. But the first cells must have come from nonliving chemistry. Because no one can watch that event, scientists study it by testing each step that it would have required.
Start with the timeline. Radiometric dating puts Earth's age at about 4.5 billion years. Layered rock structures called stromatolites, built by mats of microbes, are found in rocks about 3.5 billion years old, and some chemical traces may be older. So life appeared within the first billion years.
The first step is making small organic molecules. In the 1920s Alexander Oparin and J. B. S. Haldane independently proposed that the early atmosphere lacked free oxygen and that energy from lightning and ultraviolet light could drive the synthesis of organic compounds. In 1953 Stanley Miller and Harold Urey tested this by passing sparks through methane, ammonia, hydrogen and water vapor. They obtained several amino acids. Geologists now think the early atmosphere may have been less favorable than the one Miller used, but similar reactions may have occurred near volcanoes and deep-sea hydrothermal vents. Meteorites, such as the Murchison meteorite that fell in Australia in 1969, contain amino acids too.
The second step is linking small molecules into polymers. In water, polymers tend to break apart. Experiments show that hot rock surfaces and clay minerals can hold monomers close together and help link them, producing short chains of amino acids or RNA nucleotides.
The third step is enclosure. Lipids placed in water form vesicles with a bilayer membrane. Such vesicles can grow by absorbing more lipid, split into smaller vesicles, and keep an internal chemistry separate from the outside.
The fourth step is heredity. A system must copy information so that variations can be passed on and acted on by natural selection. The RNA world hypothesis proposes that early life used RNA for both jobs. RNA stores information in its sequence, and certain RNA molecules, called ribozymes, catalyze reactions. Thomas Cech and Sidney Altman shared the 1989 Nobel Prize in Chemistry for discovering catalytic RNA. In modern cells the ribosome's peptide-forming center is RNA, a possible relic of that time. Later, DNA, which is more stable, may have taken over storage, and proteins, which are more versatile, took over most catalysis.
None of these results shows how life actually began. Together, they show that parts of each step can happen without cells, and they turn the question into one that experiments can address.