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Evolution of Evolution — Text-only | Flash Special Report
Is There a Chemical Origin of the Species?

Astrochemists search for precursors to DNA in outer space

By Anthony J. Remijan

Charles Darwin teaches us that by natural selection, the most fit and robust species survive and this leads to a complex chain of eventsthe formation and evolution of the complex organic systems present on the Earth.

“On the Origin of Species” opened up a new realm of scientific knowledge, arguing that all life-forms seen on the planet today developed through natural selection and evolution.  However, even the simplest of organisms must have had a more humble and fundamental beginning.  Before life ever organized into a single cell, it first needed proteins; and to assemble proteins, it needed to assemble the correct types and number of amino acids. RNA and DNA, the genetic instructions used to create amino acids, are also critical in the development and functioning of all living organisms.

But to truly understand “On the Origin of Species” and the formation of the organic molecules that lie at the heart of life, an even more basic question must be asked: was there a similar “evolution” from smaller, more robust molecules that eventually lead to the molecules that we now call RNA and DNA?  Searching for answers has some scientists looking in a most atypical place: outer space. 

Researchers in a discipline called astrochemistrythe study of chemical elements and molecules in space—are using the most powerful telescopes ever built to search for our molecular origins, led there by a fascinating scientific accomplishmentthe famous Miller-Urey experiment of 1953.  Miller-Urey showed that large, organic molecules that form the basis of life on Earth can be formed in an atmosphere of little or no free oxygen.  The result presents an interesting dilemma because it is becoming more evident that the early Earth did not have the low oxygen environment necessary to synthesize these biologically important molecules. 

Unlike the oxidizing atmosphere of primitive Earth, interstellar space is a highly reducing environment, one that is rich in hydrogen and is conducive to the formation of large, organic species that may be the chemical precursors to RNA and DNA.  Evidence of this comes from infrared views of the space dust around newly forming stars.  Looking at these sources of material, we find the most complex of interstellar molecules.  In parts of the Orion nebula, as well as regions of high-mass star formation in our own Milky Way Galaxy, lies a chemistry that is producing molecules from vinegar (acetic acid) to antifreeze (ethylene glycol).

From spectroscopic research conducted via remote sensing at radio telescopes, we now know that interstellar spacethe space between starswhich only a few decades ago was widely believed to be hostile to organic chemistry and chemical bonding, contains an astonishingly rich inventory of both familiar and exotic molecules. Furthermore, these molecules are found throughout the universe in giant gas clouds relatively close to Earth in our own Milky Way galaxy, and in external galaxies billions of light years away.

The inventory of interstellar molecules currently stands at 154.  Many are common terrestrial compounds such as water and ammonia, and larger species including alcohols, aldehydes, ethers, carboxylic acids, amines and nitriles.  But an equally large number are exotic by terrestrial standards, generally unknown or unfamiliar to chemists, including both positively and negatively charged molecular ions, radicals, carbenes and their energetic isomers, unusual metal-bearing species, and highly unsaturated organic species, including bare carbon clusters.

It is apparent there is a complex chemistry in space leading to a suite of organic material.  However, the question remains: how do larger, organic species that are the possible precursors to amino acids, or even amino acids themselves, assemble into a self-replicating molecule like RNA?  Is there a preferred “natural selection” that will not only lead to the formation of larger molecules but also eliminate smaller, less robust molecules that cannot survive the harsh environments of interstellar space?

For example, did the first polyatomic molecule detected in space, formaldehyde, evolve into acetone—the main solvent in household nail polish removers or urea (found in the excretion of all terrestrial vertebrates) via the principles of evolution outlined by Darwin?  As intricate as these species seem, they are exceedingly simple compared to a strand of RNA.

One idea, called the “RNA world hypothesis,” proposes that life in our world first was based on RNA, which evolved into current life based on DNA.  It argues that RNA is the evolutionary remnant of the RNA world.  DNA, through its greater chemical stability, took over the role of data storage and proteins became the specialized catalytic molecules that fuel the development of life.  Yet there are nearly infinite possibilities of arranging atoms, then molecules, before a species resembling RNA is ever formed.  So while advances in astrochemistry continue to find even more molecular complexity in the universe, we still may never determine how these benign and humble beginnings to organic chemistry manifested themselves into the complex biological systems of today.  However, we continue to search for the answers to our molecular origins by looking out into the universe.

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Anthony J. Remijan is an assistant scientist at the National Radio Astronomy Observatory in Charlottesville, Va.  He is a commissioning scientist who helps devise procedures to test and verify the Atacama Large Millimeter/submillimeter Array, a system of 66 radio telescopes that combine their signals to simulate a single, expansive telescope, located on the Chajnantor plateau in the Atacama Desert of northern Chile.  The project is the most ambitious ground-based telescope currently under construction.  Remijan has taught in several educational settings, including the University of Maryland-University College, Prince George's Community College, Parkland College and Illinois State University.  He promotes the sciences of astrochemistry and astrobiology through frequent public speaking and holds a Ph.D. in astronomy from the University of Illinois at Urbana-Champaign.

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Please see the Resources section for the Bibliography/Additional Reading list for this essay.