Does life exist outside of our planet? If so, are there intelligent life forms out there? How did life get started on our World? The Economist makes a quick tour of scientists who have been working on such questions.
Frank Drake in his famous 1961 “Drake Equation” stated that the number of life-bearing planets must be a function of their host stars. How many planets have formed around those stars, what fraction of those planets are suitable for life, on what proportion life has actually begun, and so on.
The Hubble telescope produced a famous image after 23 days of exposure (!) called the Extreme Deep Field, to capture how many stars exist in a narrow section of the Universe. Similarly Keppler telescope have been searching for exoplanets based on dimming of light while planets are transiting in front of their stars. Extrapolating from more than 2,000 confirmed exoplanets we now know that most stars have them. How many of those exoplanets are habitable we are not sure. We first need to understand under what conditions life can start. How did chemistry turned into biology?
Key steps of the Central Dogma of biology that defines life involves three major molecules: DNA, RNA and Proteins. RNA carries the information stored in DNA and facilitates production of proteins which are physical entities that make up functional structures and carry out reactions in cells. How did this three component life get started?
RNA stands out as a molecule that is able is able to carry out the functions of DNA and proteins and could be ancestral to all. The RNA world hypothesis supports this idea. Like DNA, RNA can store genetic information and like proteins, it can carry out chemical reactions. It can even duplicate and modify itself. RNA indeed runs the show in modern cells. The ribosome is a “molecular machine” that chains amino acids together into proteins. Ribosome is so essential that in every living cell the overall shape has not changed (highly conserved) over 3 billion years. The most ancestral part of a ribosome that assembles amino acids into proteins is called “the ratchet”. The ratchet is found on the small subunit and is a modest helical RNA that forms a long hairpin structure. If we can understand how RNA formed in the pre-biotic world then we can explain the beginning of life.
Stanley Miller and Harold Urey carried out a series of exciting experiments in 1952. They filled flasks with mixtures of chemicals that were present in the atmosphere when life began: water, hydrogen, ammonia and methane. They also mimicked early conditions with electric discharges and ultraviolet light and checked if those chemicals combined into longer, more complicated forms. Indeed the final sludgy-tarry stuff that formed at the end of the experiments did contain amino acids. This was a great start. Miller and Urey’s experiments formed the foundations of the “primordial soup” hypothesis and spawned other ideas.
Understanding a group of organisms known as extremophiles can shed light onto events that started life under harsh early Earth conditions. Perhaps life may have started in hydrothermal vents in bottoms of the oceans along the continental ridges where continents separate. Volcanically heated, mineral-laden water gushing up from beneath the ocean floor have structures that could serve as a template for first cells. These hot water vents are loaded with dissolved minerals and form sponge-like vertical tubular structures called chemical gardens. Nobel Prize winner Jack Szostack is one of the scientists researching RNA-based origins of life. His team have already managed to synthesize proto-cells encased in lipid membranes that can contain and protect RNA from degradation.
Our planet provides a template for what to look for about life in the universe. More than half of the minerals now incorporated into the geology of our planet were produced by living organisms. Our world was indeed a very different place when life started. Days were only 8 hours long! Since then the rotation of our planet slowed down to 24 hour. Our Sun was much cooler and thus the early photosynthesizing organisms were dominated by purple pigments. Our Sun is now a red star favoring photosynthesis by green chlorophyll carrying plants (remember this when someone asks you why most plants are green). Today our remote sensing capabilities can be applied to exoplanets. Based on the type of star an exoplanet is orbiting around we can hypothesize what kind of photosynthetic activity (color of plants) we should expect. Morevoer, using spectrophotometric measurements we can understand atmospheric composition of an exoplanet.
We may feel alone in our solar system but we haven’t done with searching yet.