I rather like reading and pondering about life’s origins.  It’s really interesting to me as a synthetic organic chemist to consider how the complex array of life came to exist from simple building blocks, and how the building blocks we have today arose from the generally-accepted conditions of early Earth at the time life began.  Perhaps we’ve all heard of the famous Miller-Urey experiment which attempted to recreate those conditions.  Methane, ammonia and hydrogen were circulated over boiling water.  Electrodes were introduced to mimic lightning.  After a time, the composition of the mixture was analyzed, and a number of amino acids were detected.

But did that really recreate the conditions of early Earth?

This paper includes some interesting conditions I hadn’t really thought about.  Consider: it is generally accepted that the reducing conditions of early Earth did not include significant amounts of oxygen.  But no oxygen means no ozone.  No ozone means no protection from high energy UV light.  This UV light perhaps provided a significant amount of energy to drive the early reactions.

Also consider: even as more (and more complex) molecules were created, the concentration of these molecules remained small – even if you allow that all of these molecules might be dissolved in the early oceans.  For these molecules to randomly collide and ultimately react to form polymers would be unlikely.  But the authors contend that perhaps, over time, those early molecules adhered to mineral-rich clay particles in the water.  The clay would serve to concentrate reagents, and the minerals (aluminum, iron, magnesium, calcium, sodium, and potassium being the most abundant) would serve to catalyze the early reactions.   Eventually, more complex molecules were synthesized.  The authors postulate that saccharides were among the first complex molecules to form in significant amounts.  But how?

Given that only the simplest molecules were present on early Earth, it is likely that formaldehyde was one of them present in non-trivial amounts.  Perhaps formaldehyde self-polymerizes on these clay surfaces to form small saccharides, which eventually undergo aldol condensation to more advanced hexoses and pentoses.  The authors argue this pathway for the formation of a number of monosaccharides on early Earth.  Note that in the cyclic isomer of glucose – β-D-glucopyranose (left) – all 5 substituents on the pyran ring are in the low-energy equatorial position (actually, the lowest-energy conformation of glucose is α-D-glucopyranose, where one of the -OH substituents is in the axial position. It is stabilized by what is known as the anomeric effect).  Perhaps it is not surprising that glucose is the most abundant organic compound on Earth today.

Additionally, assume the authors are correct that a carbohydrate polymer world existed as a precursor to the RNA and DNA/protein world.  This propensity for the newly-forming carbohydrates to favor stereoisomers which existed in low-energy cyclic forms could produce disproportionate amounts of glucose and other low-energy carbohydrates.  If these carbohydrates somehow formed the scaffold involved in the template for the mass production of the known amino acids, might it not be so coincidental that L-amino acids were created with near exclusivity?

It might also not be difficult to see how the carbohydrate polymers world gave way to an RNA world.  The authors suspect carbohydrates polymerized along with abundant phosphates to form longer chain polymers.  There are several possible polymers that can form, but one of them might have looked like RNA without the base pairs: a series of ribofuranose units linked together by phosphate groups (right, click for larger).  The highly oxidized purine and pyrimidine bases probably came about later than sugars.  When they did arrive, only a substitution was needed to create the first strands of RNA (below).

The authors continue to discuss how the first cells may have come to be and how the first unicellular organisms might have appeared, but I’ll let the curious reader find that out by reading the paper.  One issue that often comes up in talks of this nature is the creation/evolution issue (am I really going there in only my second post?) (Yes).  I’ll refrain from offering my take on that issue (maybe after we get to know each other a bit better…), but I will say that it is interesting that people often talk about the two issues as if they were two mutually exclusive options.  Are they?  Does the acceptance of one theory necessarily preclude the option of the other theory?  Discuss.