The Origin of Life on Earth
Episode #2 of the course The evolution of life in the universe by Silvano P. Colombano Ph.D.
From the origins of the universe, we saw the emergence of galaxies from visible and invisible matter, the emergence of nebulae within galaxies, star systems from nebulae, and planets from star systems. The story continues for the origin of life on Earth, but there is an important and often not appreciated difference.
There Are No Rigorous Models of the Origin of Life
There are clear laws of physics that, when applied to known or postulated objects present in the early universe, allow scientists to build mathematical models that can predict with reasonable accuracy at least the important features of the known universe. This has been much harder to do for the origin of life. Life of course does not violate the laws of physics, but the laws of physics alone cannot yet predict the emergence of life. The result is that there is no well-established “model” for the origin of life. What we do have is a vast number of possible scenarios, none of which has been definitely proven, and which are mostly limited to the formation of bits and pieces of the elements that were necessary. Let’s see what the necessary elements are.
The Necessary Building Blocks
The necessary elements are clear from the part of the evolution of life we do understand, and that is Darwinian evolution. For that to be possible we need the ability to separate an organism from the outside world, simply a cell wall at the start, so that it will be possible to build and contain other necessary internal elements (metabolism). We need to have something that maintains the memory of what the system looks like, a kind of “blueprint”, and we need to have a system that is able to read the blueprint instructions and to build all the objects that are necessary to the whole system, including copies of itself. We will see in the next lesson how these elements are necessary for Darwinian evolution to get going.
Just like stars emerged from galactic clouds, scientists have tried to identify systems that were stable and had the time to form the objects from which “live” organisms could emerge. So far only “conceptual” and unproven models of such systems have been proposed. Yet a number of such intermediate stages would be necessary because the original organism we just described would be far too complex to have emerged by chance out of a “soup” of random molecules. To keep this in perspective, the simplest known organism (Carsonella Ruddi bacterium) has 160,000 base pairs in its DNA (the “blueprint”) and codes for 160 proteins (the system “machinery”). By comparison, the human genome comprises 3 billion DNA base pairs and codes for about 35,000 proteins, but even that very simple organism would be much too complex to be formed in some random process.
Origin of Life Experiments
The emergence of life was certainly contingent on the geologic and atmospheric conditions of early earth. The possible time windows for favorable conditions range from 4.2 to 3.8 Billion (or “Giga”) Years Ago (Gya).
There has been much experimental work that has tried to match postulated conditions with the possibility of formation of the needed building blocks. Perhaps still the most famous remains the original work by Harold Urey and Stanley L. Miller, in the 1950’s. They showed that conditions postulated for an early atmosphere could produce amino acids, the building blocks of proteins.
Forming the building blocks of RNA-DNA has been a much greater challenge, but, just recently (2015), at NASA, a group was able to produce some of the essential nucleotides that form DNA and RNA from chemicals found in meteorites, raising the intriguing possibility that meteorites may have played a role in the origin of life.
The last essential element was some form of stable “container” that could allow for some primitive metabolism to take place. Early ideas date back to the 1920’s when Oparin and Haldane identified colloidal droplets called Coacervates as possible sites of early metabolism. A related idea is that of self-assembling vesicles formed of lipid (fats) layers.
LUCA
Eventually, a system emerged where a stable container could support the metabolism necessary to sustain the necessary machinery of nucleotides (RNA/DNA) and amino acids (proteins). This hypothetical system has been called “LUCA” for “Last Universal Common Ancestor”. All life on Earth derived from LUCA.
This type of system can absorb materials from the environment to maintain its metabolism and it can build copies of itself (reproduction). But how can it become the common ancestor of all life on Earth? The simplest mechanism that explains this is “mutations”: mistakes are made while copying the blueprint part of the system (organism). Most mistakes will kill the new copy of the organism, but some can actually make it better, in some way, than the previous one. When this happens, the new copy will have an advantage and will eventually take over as a new form of the organism. There are other important “invention” mechanisms I’ll mention in the next lessons, but conceptually this suffices to understand how biological evolution gets going.
Conclusion
To conclude, we know what elements need to come together to get life started, but we are still far from any rigorous model for how this could have happened. For now, we will acknowledge that gap, assume that at some point LUCA a stable organism capable of biological evolution was formed, and we’ll proceed on our journey. Next stop: Homo Sapiens!
Recommended book
Genesis: The Scientific Quest for Life’s Origin by Robert M. Hazen
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