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18.5 Origin of Complex Molecules

A quick glimpse at the world around us reveals a complexity that is mind-blowing. From macroscopic living systems such as the rainforest to microscopic single-celled organisms, the biological world is a sobering example of how even the most simple forms of life on Earth exemplify a mystifying level of complexity. Even the molecules that comprise the simplest life forms are themselves very complex. How did life on Earth arrive at this complexity? Certainly the first molecules were not that different from what we see distributed through the universe in stars, nebulae, and the interstellar medium. So how did we get the complex molecules needed for life as we know it?  

Scientists who study the origin of life on Earth strive to understand the chemistry and conditions that resulted in the first life forms on our planet. One question they specifically strive to answer is how complex molecules such as amino acids, lipids, proteins, and DNA originated. All of these molecules are carbon based and are quite complex. Certainly there was a ready supply of carbon on early Earth in the form of carbon dioxide, methane, and carbon monoxide. But the process from simple to complex is still under debate.

Perhaps the first laboratory experiments conducted in response to this question was that of Stanley Miller in 1953. Now more famously known as the Miller-Urey experiment, this week long experiment simulated early Earth's atmospheric conditions and resulted in the spontaneous formation of certain complex carbon compounds including certain amino acids. Although this was the first real evidence that complex carbon molecules could have been arisen on Earth from basic chemistry, it did not account for all the complex molecules necessary for life. It didn't even create all 20 of the amino acids commonly used by living organisms. (Although after Miller's death in 2007, the sealed vials of his experiment were opened and analyzed, and a total of more than 20 amino acids were identified.) This historical experiment has been reproduced in its original and altered form for over 50 years. Despite arduous efforts, scientists have been unable to create all the molecules needed for life in laboratory simulations of early Earth conditions.

Now we are faced with more questions. Is it chemically impossible to obtain all the necessary molecules for life starting from the simple list of ingredients and conditions provided by early Earth? Or have scientists not engineered a laboratory environment that has given us a true representation of this time period? Or have scientists made incorrect predictions about the conditions on early Earth? The inability of scientists to artificially create the multitude of molecules represented by life on Earth today has stimulated the search for other explanations.