This video segment adapted from NOVA scienceNOW examines how a team of chemists led by Dr. John Sutherland at the University of Manchester discovered a way in which parts of the first RNA molecules could have formed in the primordial soup of early Earth and given rise to life. DNA and RNA are complex molecules that make possible the inheritance of genetic information, an essential component of life. Dr. Sutherland's discovery of a series of inorganic reactions—that used simple chemicals available on early Earth and would proceed naturally under the environmental conditions at the time—solved the long-standing question of how complex molecules such as DNA and RNA could have first formed nonbiologically.
Nearly 4.6 billion years ago, the primordial Earth condensed out of the swirling gas and dust remnants of the Sun's formation. Ancient zircon crystals found on Earth indicate that oceans and continents formed after just a few hundred million years. However, because of geologic processes that destroy the crust—by erosion or subduction into the mantle, for example—very few of Earth's earliest rocks still exist. The oldest known fossils are the microscopic imprints of photosynthetic bacteria in 3.5 billion-year-old Australian rocks. Some chemical traces of life, called biological markers, have been found in rocks up to 3.8 billion years old, leading scientists to estimate that life originated about 4 billion years ago.
Because all life on Earth today shares the same basic genetic material—DNA, or deoxyribonucleic acid—scientists believe it has evolved from a common ancestor. The building blocks of DNA are phosphates, sugars, and four organic bases: adenine (A), guanine (G), cytosine (C), and thymine (T). A sugar, phosphate, and base combined form a nucleotide, a unit of DNA. The unique sequence of these nucleotides forms a genetic code, which carries instructions for protein synthesis and cell processes and functions. During protein synthesis, RNA (ribonucleic acid) carries a copy of DNA from the cell's nucleus to ribosomes, which string together amino acids to make proteins. Unlike DNA, with its famed double helix, RNA is single stranded and uses the base uracil (U) in place of thymine.
In 1989, the Nobel Prize was awarded for the discovery that RNA can act like an enzyme. Until then, it was believed that all enzymes were proteins. The discovery that RNA not only carries genetic code, like DNA, but can also function like proteins lends support to the RNA World hypothesis, which holds that the first life was based on RNA, with DNA and proteins evolving later. But how RNA could have arisen abiotically (without life) out of the chemical soup of Earth's early environment was unknown.
Based on the chemistry and geology of early Earth, scientists propose that the early atmosphere contained water vapor, carbon monoxide, carbon dioxide, nitrogen, methane, and ammonia. Volcanic activity produced lightning that electrified the atmosphere, and with no oxygen to form an ozone layer, intense ultraviolet (UV) light penetrated to the surface. Simple chemicals present on the surface included glyceraldehyde, cyanimide, cyanoacetaldehyde, and cyanoacetylene. The ingredients of RNA, including the base cytosine, could have formed naturally from these. However, once formed, cytosine does not easily join to the other components.
Organic chemist John Sutherland recreated these conditions, including UV light and electricity (to mimic sunlight and lightning), and then allowed natural physical processes, such as evaporation and condensation, to proceed. After 10 years of testing different combinations of ingredients, Dr. Sutherland finally discovered a recipe for the cytosine nucleotide that included some half reactions not previously considered. The finding supports the idea that RNA could have formed abiotically, potentially giving rise to the first life on Earth.
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