Life: What It Is And How It Forms

 

Biology emerged from biochemistry which emerged from chemistry which emerged from physics. Sounds simple, no? Not quite. How the first protocell emerged on the early Earth to give way to humans, reptiles, insects, and all other forms of life is a complex and interesting problem that requires various areas of science, from astrobiology to biochemistry to physics in order to answer. But did life actually predate this protocell? According to NASA, life is a "self-sustaining chemical system capable of Darwinian evolution." Let's learn more about such systems. 

Origin Of Life 

It was once thought that chemical systems were disorganized and random. As chemistry and biochemistry in particular advanced, we realized that chemical and molecular systems exhibit features of Darwinian selection. Some molecules gravitate toward self-assembly and eventually form membrane-like structures or other biologically important constructs. Selection predated biology. The subfield of Chemistry that studies selection on a chemical and molecular level is called Systems Chemistry. This subfield is essential for studying the origin of life on Earth and other planets because it can help us discover how these systems generated the first protocell on Earth about 3.8 billion years ago, which then evolved and complexified. 

Dr. Gerald Joyce, a prominent figure in the origin of life research, conducted an experiment that made significant strides in elucidating chemical evolution. The experiment centered on the in vitro evolution of RNA enzymes, also termed ribozymes, which have the capacity to catalyze RNA replication.

Joyce and his research team commenced with a diverse pool of random RNA sequences. They subjected these sequences to a process known as in vitro selection or systematic evolution of ligands by exponential enrichment (SELEX). During SELEX, RNA molecules demonstrating even rudimentary abilities to catalyze self-replication were preferentially amplified and isolated. Subsequent rounds of mutation and selection were applied to these chosen molecules, progressively enhancing RNA replicase activity over time. By showing that RNA molecules could evolve in the laboratory to perform a function as complex as self-replication, Joyce provided experimental evidence for the plausibility of chemical evolution. 

Along with this (and other) experimental support, the RNA world hypothesis for the origin of life on Earth is most prominent compared to others (such as metabolism first and protein first) because RNA uniquely contains catalytic as well as informational functions. It is able to catalyze a variety of biochemical interactions as well as store genetic information.

Geochemistry On The Early Earth 

In order to begin understanding the complex process by which this occurred, we must understand the geochemistry. The early Earth was very different than our current beautiful planet. Billions of years ago, Earth had very low levels of oxygen (less than one part per billion) while having carbon dioxide levels likely exceeding 70%. 

In order to replicate conditions on the early Earth and hopefully generate genetic material, Stanley Miller and Harold Urey conducted an experiment simulating early Earth conditions to demonstrate the synthesis of organic compounds from inorganic constituents. They used methane (CH4), ammonia (NH3), hydrogen (H2), and water (H2O) in a ratio of 2:2:1, along with an electric arc to simulate lightning. They were able to produce simple organic compounds, including amino acids, which are the building blocks of proteins and other macromolecules. Although this experiment was an exciting leap for the study of the origin of life, it wasn't without it's inaccuracies. Later research suggests that the Earth's early atmosphere likely consisted of about 97% carbon dioxide (CO2) and 3% nitrogen (N).  In addition, the type of glassware they used, borosilicate glass, was crucial in catalyzing organic compound synthesis. This may not be a major issue, because the first protocell on Earth was generated through mineral and geochemical interactions. 

The most likely settings for where life got started on Earth are hydrothermal vents, tidal pools, and hot springs. There are multiple reasons for this, including them being a rich source of chemical energy, serving as mineral catalysts, and being able to concentrate a high volume of organic (carbon-based) molecules. Another theory is Panspermia, which posits that life emerged on Earth through extraterrestrial delivery (through a meteorite or similar object). Panspermia was more popular among origin of life scientists in years past, but seems to have gone down in popularity, probably because many settings on early Earth were seemingly great for abiogenesis (formation of life from inorganic material) to occur. 

Sources:

The antiquity of RNA-based evolution | Nature

RNA-catalyzed evolution of catalytic RNA - PubMed (nih.gov)

Geochemistry and the Origin of Life: From Extraterrestrial Processes, Chemical Evolution on Earth, Fossilized Life’s Records, to Natures of the Extant Life - PMC (nih.gov)