On primitive Earth, chemistry was driven by contact between atoms and molecules. With sufficient energy and proper orientation of molecular collisions, chemistry proceeded to initiate and/or evolve life. To qualify, life requires the following criteria: ability to replicate, metabolize and evolve.
In the 1860's Pasteur was one of the first to address origin of life questions experimentally showing that it could not arise from a closed vessel with nutrient broth. During this period of time Darwin was questioning the origin of life in a warm reducing pond. Haldane took Darwin's idea and coined the term "primordial soup." In 1953 Stanley Miller conducted landmark experiments in prebiotic chemistry assuming that primitive Earth had a reducing environment like the big planets do now (little oxygen since not yet existence of oxygen producing plants or ocean dwelling bacteria). With a flask of early Earth rich molecules (formaldehyde, ammonia, cyanide) Miller subjected the flask to electrical discharge (simulate lightning that provides energy to drive reaction) and famously produced 14 of 20 amino acids found in biological proteins. Eight years later, Joan Oro showed spontaneous formation of adenine, a nitrogenous base found in RNA, DNA and a major energy currency for cell metabolism (see image at bottom of page). Even though an ability to generate some essential-for-life molecules had been documented, some wonder if those events may not have been necessary to jump-start construction of the molecular building blocks required for replicating molecules.
In 1969 a carbonaceous meteorite containing molecular hitchhikers hit Earth that astoundingly contained amino acids, like those found by Miller, and nitrogenous bases, like those made by Oro. One hypothesis: we are stardust. Did the nitrogeneous bases and amino acids arise on Earth, or were they transported from distant reaches of space via meteorites and comets?
Once in place, what conditions were necessary to allow favorable thermodynamics to promote assembly of amino acids and nitrogeneous bases into proteins, RNA and DNA? Were they clays, primordial soups or deep-sea hydrothermal vents? Who knows... But, once assembled how did these molecules acquire catalytic function? In the 1980's, experiments done independently by Cech and Altman ultimately revealed that RNA could act as a biological catalyst, a ribozyme. Since then, catalytic RNA has undergone tremendous investigation. Hammerhead, a catalytic RNA found in organisms as diverse as plant viruses, newts, schistosomes and crickets was initially observed in 2001 by Szostak (image of Hammerhead shown in upper right).
Yet beyond just an ability to replicate, mutate and be naturally selected, life requires metabolism. And metabolism it seems is best contained within membranes or vesicles. So how in the world did chemical thermodynamics manage the assembly of protocells, their assembly and division?
