Which came first, the chicken or the egg?

The chicken or egg riddle is an apt analogy when considering the origins of life.  Just as it's difficult to imagine eggs without chickens or chickens without eggs; what gave birth to the first living thing?  Evidence suggests that the earliest lifeforms on Earth were single-celled organisms that reproduce asexually by dividing in two -- but where did the first cell capable of replicating itself come from?

"Understanding the origin of life may be profitably explored by decoupling the origins of different features of life." - Leo Buss

A living system can be described as having three essential components: a metabolism, an information program, and a boundary dividing it from the environment.  All living things need to eat, to take in nutrients from the environment and generate the chemicals needed to maintain cellular function.  Life requires some way to copy itself, to transmit information about its specific chemical makeup and processes to the next generation.  The collection of metabolic function and genetic instructions are set apart from the outside environment, contained within a semi-permeable membrane.


Origins of life scientists can generally be described as "information first" or "metabolism first." (full disclosure: I work with self-replicating RNA, which puts my primary research in the information camp.)  And here we come back to the chicken and egg paradox...  How were information containing molecules capable of copying themselves without a system for extracting energy and transforming chemicals?  How was a complex set of chemical reactions able to develop without a set of instructions to orchestrate the operation?


I imagine the answer will be found in the synthesis, as that would mark the true origin of life as we know it.  In the meantime, much insight can be gained by attempting to study the systems in isolation.


There is also the notion of "membranes first," in that a separation from the outside is required to maintain the chemicals of life in high enough concentration.  Cells are mostly bags of water and the first life probably formed in water; a semi-permeable membrane allows the inside to take in nutrients and prevent useful chemicals from diffusing away.  It's difficult to imagine "naked chemicals" drifting about in a dilute primordial soup as life.




My apologies if you were expecting a final answer to the origin of life.  Nobody knows.  The best scientific answer is that it probably occurred about 3.5-4 billion years ago.  There's debate over a deep hot origin, with life beginning at geothermal vents on the ocean floor, and surface origins in a warm lagoon with the tides helping mix the first chemical cycles.  The earliest fossil evidence is debatable, since single-celled organisms don't necessarily fossilize well nor preserve the same shape after 3.5 billion years buried in rock.  From phylogenetic assays, LUCA (the Last Universal Common Ancestor) is presumed to be a hyperthermophilic (high temperature dwelling) bacteria.  But nobody knows if that represents the first life on Earth, or merely what was able to survive a horrible cataclysm during the Late Heavy Bombardment of planet sterilizing meteor collisions.


I should also mention panspermia, the idea that Earth was seeded from life elsewhere.  Totally plausible in my opinion, but that just begs the question -- how did the first life on another planet come about?