The origin of Life
The subject of how life originated is an interesting one on several levels. Its remains one of the key areas of scientific endeavour and it being actively worked on by scientists and research institutions the world over. The fact that science has to date been unable to fully replicate the process in a laboratory is often used by theists as a good place to insert their respective intervening deities as possible “causes” for the generation of self replicating organic life.
In this article I will attempt to briefly summarise the history, evidence for and current hypotheses for abiogenesis, and investigate whether or not the abiogenesis hypothesis is a viable explanation for how life on earth (or some other planet) began. I will also briefly touch on the “creation/intelligent design” hypothesis and comment on the likelihood thereof.
Firstly we need to get a few definitions and misconceptions out of the way.
Abiogenesis or biopoesis is defined as the natural process by which life arose from non-living matter such as simple organic compounds.
Evolution is the change in the inherited characteristics of biological populations over successive generations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins (“ToE”).
At this point it is important to note a common misconception. The origin of life abiogenesis has nothing to do with the ToE. The ToE would stand as a theory given the preponderance of evidence without a single abiogenesis hypothesis being verified. Even if it was, by some miracle, to be demonstrated that a theistic entity did indeed create the first living replicating organism, the ToE would still remain the most plausible explanation for how life on Earth changed over time. Scientific theories are designed to explain a specific set of facts. One would for instance not claim that the theory of general relativity is invalid because it does not explain germs, as explained by germ theory.
It is also important to note that some abiogenetic hypotheses invoke evolutionary-like mechanisms in the development and formation of replicating organic material.
For a long time humanity has attempted to explain how living matter arises. Classical notions of spontaneous generation held that complex living organisms are generated by decaying organic material. Belief in the spontaneous generation of life forms from non-living matter can be traced back to Aristotle and ancient Greek philosophy and continued to have support in Western scholarship until the 19th century.
The first experimental evidence against spontaneous generation came in 1668 when Francesco Redi showed that no maggots appeared in meat when flies were prevented from laying eggs. It was gradually shown that, at least in the case of all the higher and readily visible organisms, the previous sentiment regarding spontaneous generation was false. The alternative, at the time, seemed to be biogenesis: that every living thing came from a pre-existing living thing. By the middle of the 19th century, the theory of biogenesis had accumulated so much evidential support, due to the work of Louis Pasteur and others, that the alternative theory of spontaneous generation had been effectively disproved.
In a letter to Joseph Dalton Hooker on February 1, 1871, Charles Darwin addressed the question of the generation of first life, suggesting that the original spark of life may have begun in a "warm little pond, with all sorts of ammonia and phosphoric salts, lights, heat, electricity, etc. present, so that a protein compound was chemically formed ready to undergo still more complex changes".
The Oparin-Haldane theory
Very little work was conducted in the field of biogenesis until the 1920s, when British scientist J.B.S. Haldane and Russian biochemist Alexander Oparin independently set forth similar ideas concerning the conditions required for the origin of life on Earth. Both suggested that organic molecules could be formed from non-organic materials (hence the term abiogenesis) in the presence of an external energy source (e.g., ultraviolet radiation) and that the primitive atmosphere was reducing (having very low amounts of free oxygen) and contained ammonia and water vapour, among other gases. Both also proposed that the first life-forms appeared in the warm, primitive ocean and were heterotrophic (obtaining nutrients from the compounds in existence on early Earth) rather than autotrophic (generating food and nutrients from sunlight or inorganic materials).
Oparin believed that life developed from coacervates, microscopic spontaneously formed spherical aggregates of lipid molecules that are held together by electrostatic forces, possibly the precursors of cells. Oparin’s work with coacervates confirmed that enzymes fundamental for the biochemical reactions of metabolism functioned more efficiently when contained within membrane-bound spheres than when free in aqueous solutions. Haldane believed that simple organic molecules formed first and in the presence of ultraviolet light became increasingly complex, ultimately forming cells. Haldane and Oparin’s ideas formed the foundation for the later research on abiogenesis.
The Miller-Urey experiment
In 1953 American chemists Harold C. Urey and Stanley Miller tested the Oparin-Haldane theory and successfully produced organic molecules from some of the inorganic components thought to have been present on prebiotic Earth. In what became known as the Miller-Urey experiment, the two scientists combined warm water with a mixture of four gases (water vapour, methane, ammonia, and molecular hydrogen) and pulsed the “atmosphere” with electrical discharges. One week later Miller and Urey found that simple organic molecules, including amino acids (the building blocks of proteins), had formed under the simulated conditions of early Earth. A recent investigation into their experiment, with modern scientific equipment, has revealed that a far larger variety of organic material was formed during their experiment than they reported at the time.
Modern abiogenesis hypotheses are based largely on the same principles as the Oparin-Haldane theory and the Miller-Urey experiment. The subtle differences between the several models that have been set forth to explain the progression from abiogenic molecule to living organism, revolve around the question of whether complex organic molecules first became self-replicating entities without metabolic functions or first became metabolizing protocells that then developed the ability to self-replicate.
The environment for abiogenesis is also still a subject of debate. While some evidence suggests that life may have originated from nonlife in hydrothermal vents on the ocean floor, it is possible that abiogenesis occurred elsewhere, such as deep below Earth’s surface, where newly arisen protocells could have subsisted on methane or hydrogen, or even on ocean shores, where proteinoids may have emerged from the reaction of amino acids with heat and then entered the water as cell-like protein droplets. Significant improvements have been made and are constantly being improved in understanding early conditions on Earth through the science of geology.
An additional hypothesis, gaining some traction with the publication of recent papers, following findings of the NASA's Curiosity Mars rover and investigations of other moons in our solar system, is that of Panspermia, which is the hypothesis that life exists throughout the Universe, distributed by meteoroids, asteroids, comets and planetoids. The idea is that this mechanism is the source for initial life on Earth and that abiogenesis occurred on other bodies/planets such as Mars before they were present on Earth.
Since the 1950s, researchers have found that amino acids can spontaneously form peptides (small proteins) and that key intermediates in the synthesis of RNA nucleotides (nitrogen-containing compounds [bases] linked to sugar and phosphate groups) can form from prebiotic starting materials. The latter evidence may support the RNA world hypothesis, the idea that on early Earth there existed an abundance of RNA life produced through prebiotic chemical reactions. RNA in addition to carrying and translating genetic information, also acts as a catalyst that increases the rate of a reaction without being consumed. The RNA world hypothesis is one of the leading self-replication-first conceptions of abiogenesis.
Some modern metabolism-based models of abiogenesis incorporate Oparin’s enzyme-containing coacervates but suggest a steady progression from simple organic molecules to coacervates, specifically protobionts, aggregates of organic molecules that display some characteristics of life. Protobionts presumably then gave rise to prokaryotes, single-celled organisms lacking a distinct nucleus and other organelles because of the absence of internal membranes but capable of metabolism and self-replication and susceptible to natural selection. Examples of primitive prokaryotes still found on Earth today include archaea, which often inhabit extreme environments with conditions similar to those that may have existed billions of years ago, and cyanobacteria (blue-green algae), which also flourish in inhospitable environments and are of particular interest in understanding the origin of life, given their photosynthetic abilities. Stromatolites, deposits formed by the growth of blue-green algae, are the world’s oldest fossils, dating to 3.5 billion years ago.
There are still unanswered questions concerning abiogenesis. Experiments have yet to demonstrate the complete transition of inorganic materials to structures like protobionts and protocells and, in the case of the proposed RNA world, have yet to reconcile important differences in mechanisms in the synthesis of purine and pyrimidine bases necessary to form complete RNA nucleotides.
There are also hypotheses that abiogenesis occurred more than once. In one example of this hypothetical scenario, different types of life arose, each with distinct biochemical architectures reflecting the nature of the abiogenic materials from which they developed. Ultimately, however, phosphate-based life (“standard” life, having a biochemical architecture requiring phosphorus) gained an evolutionary advantage over all non-phosphate-based life (“non-standard” life) and thereby became the most widely distributed type of life on Earth. If life is discovered on other stellar bodies will it necessarily have the same architecture as we find on Earth?
Creationism is the religious fundamentalist belief that living organisms, the Earth, and the universe did not come to be through some physical process but were created by a supernatural being in a single creation event. Some variations of this view include Intelligent Design, which while accepting many of the findings of science, maintains that complex systems are necessarily designed and intimately guided, and that the designer is the Christian version of a theistic deity.
Typically creationism manifests itself as a “god-of-the-gaps” where scientific unknowns are replaced with actions of the relevant theistic entity. Currently no evidence or experimentation has been collated or conducted in testing this hypothesis and therefore remains untested, with a low likelihood of being converted to a scientific theory. Not a single unrefuted peer reviewed scientific paper or journal has been presented to the scientific community in support of this hypothesis.
This hypothesis has one serious flaw. Needless to say it’s fairly obvious and requires no further discussion.
Science has demonstrated that systems in the universe evolved by self-organization of matter towards increasing complexity. Atoms, stars and galaxies are self-assembled out of the fundamental particles produced by the Big Bang. In first-generation stars, heavier elements like carbon, nitrogen and oxygen were formed. Aging first-generation stars then expelled them out into space. The heaviest elements were born in the explosions of supernovae. Humans and all other organic life are comprised of these elements, are thus literally born from stardust. The forces of gravity subsequently allowed for the formation of new stars and of planets. Finally, in the process of biological evolution from bacteria-like tiny cells to all life on earth, including humans; complex life forms arose from simpler ones.
In this article I have very briefly presented some of the scientific evidence and experimentation in the field of abiogenesis. The field is truly massive and complex. The experimental study of the origin of life kick-started with Miller’s ‘prebiotic soup’ experiment (Miller-Urey experiment) which produced amino acids, essential to life. In the following decades, a lot of impressive chemistry on the building blocks of life has been performed.
Conceptually, the genesis of the protein translation system (DNA) has posed a fundamental problem. Yet in the last decades significant progress has been made. Proposed reactions in the ‘metabolism-first’ model, which assumes metabolism, not genes, at the origin of life, have also been rendered more promising by recent findings.
The pieces of the abiogenesis puzzle are starting to come together in such a way that the scientific assumption origin of life from non-living matter finally has achieved plausibility on the level of experimental evidence.
While research in the field now appears vastly more promising than just a few decades ago, the science on the origin of life is, compared to the science of biological evolution, still considerably underdeveloped in its explanatory power, however the hard work continues with the body of knowledge growing daily.
Given the alternative creation/Intelligent Design hypothesis, the current scientific knowledge and hypotheses on abiogenesis are orders of magnitude more robust and present a far more probable solution.
Is it now just a matter of time before science comprehensively answers the abiogenesis question or will we never know the details for sure?
Sources and Further Reading
Many of these links have further links to scientific publications and books on the subject.