How did life begin? This is not a philosophical question. Or a theological one. It is purely scientific. It is a question on my own mind of late due to the publication of a new paper, already widely derided, suggesting that we contain DNA from extra-terrestrial sources. It also says that the octopus most likely is completely from space, possibly crashing here as fertilised eggs on asteroids (link). It is a startling paper, not just because of its claims, but because it passed peer review. It also sports an astonishing list of co-authors, but with some delving, they appear to be mostly fringe elements with a history of unsupported claims in this area. The debates around its publication are highly entertaining, but I will not discuss them further here. You can read a nice review of the kerfuffle at this link.
The paper nevertheless is yet another attempt to answer the greatest of ancient human questions: Why and how are we here? More specifically, what were the mechanisms that brought together the fundamental building blocks to form the first viable cell? And where did this happen? And how long ago? And how many times?
There are four principle theoretical arenas on which researchers currently focus on; the primordial soup theory; the life from an asteroid theory; the hydrothermal vent theory; and the clay crystals theory. All are intellectually active today, all overlap with each other in some way, and none have disproved another. Though, none have been decisively proven either.
What appears likely is that life arose soon after the aggregation of the first oceans circa 3.8 billion years ago near the end of, or after, what is known as the Hadean period. The Hadean eon describes a world unknown today any distance away from a volcanic disaster zone, and much worse. Temperatures of the first seas were believed to be up to 100oC, and quite acidic when compared to today’s more neutral to alkaline seas (link). The atmosphere generated ferocious storms, and the nascent planet, lacking a protective ozone layer, was lashed with sterilising solar radiation from a youthful sun. It also suffered from frequent meteor and asteroid strikes, especially during what is known as the “late heavy bombardment” sometime around 3.8 – 4.1 billion years ago. Some of these collisions are believed to have involved objects as big as 200km in diameter (link). Impacts of this magnitude would have plunged vast amounts of dust and gases high into the earth’s atmosphere and vapourise large areas of early ocean. This would probably kill any form of life that might have developed at that point. After each strike, it would be a few thousand years before the gases and debris settled back again, and the oceans returned to prestrike depths from 3000 years or so of rain. So, in light of this, it was probably not a good time to assemble your first set of amino acids.
Around this time life did begin to exist for the first time on earth. But it is at this point that the debate begins to rage. When, how and why?
The time of first emergence has been narrowed somewhat to between 3.48 and 4.3 billion years ago. Microfossil evidence has been unearthed suggesting life existed in what is now Australia 3.48 billion years ago (link); fossils in Greenland are suggested as being from 3.7 billion ago; and again, in Australia, microfossils have been uncovered that are thought to be 4.1 Billion years old (link).
But recently, reported evidence from rocks in Northern Quebec in Canada suggest that life evolved between 3.77 and 4.28 billion years ago (link). These could be the oldest, and most exciting, microfossils we have found thus far.
Before 3.8 billion years ago, it is believed that the surface of the earth was inhospitable to life. It’s been suggested that if life originated on the surface it could have happened when the earth began to cool, and the land masses began to form (link). But if it originated in the relatively protective deep oceans, in say hydrothermal vents, then it could have happened sooner, maybe 4 – 4.2 billion years ago – very soon after the formation of the earth. This means that life managed to survive in the violent and lethal conditions described above. There are claims that the meteor bombardment of Earth did not end 3.8 billion years ago, but in fact, went on for another billion years in reducing intensity (link 1, link 2). Life had almost certainly evolved by then and the theory suggests that life on early earth was a slow struggle to become established with recurrent setbacks due to celestial impacts (link). And this may extend back to an emergence of life around 4-4.2 billion years ago.
Within the debate about the timeline and nature of abiogenesis (emergence of first cellular life) are the theories attempting to describe it.
THE PRIMORDIAL SOUP THEORY
The theory of a primordial soup refers to a what Charles Darwin called “warm little ponds” occurring on early earth, containing all the necessary ingredients allowing for the spontaneous emergence of the first cells. After Darwin, a Russian scientist in 1924 (Alexander Oparin) put forward ideas describing how, in the probable reducing atmosphere of early earth, and in the absence of oxygen, a primitive soup of organic materials could form in liquids in the presence of sunlight and other energy forms (link). This idea was tested in the laboratory for the first time in Britain in 1953. Researchers, using a highly reducing mixture of gases (methane, ammonia, hydrogen) and water, percolating through a system with zaps of electricity, managed to develop what can be called the basic building blocks of life, including 5 amino acids (link). It was groundbreaking work and described for the first time experimental evidence for the role of energy in the development of life beyond Frankenstein stories. It is known as the Miller-Urey experiments.
In recent years, the experimental materials from some related but unreported studies from 1958 by Miller (this time using Hydrogen sulfide as a hydrogen source) was tested using more advanced techniques. It was found that these experiments produced 23 amino acids, substantially more than the five reported by Miller-Urey in 1953 (Link).
LIFE FROM AN ASTEROID THEORY (PANSPERMIA)
This is the oldest theory, with first records of the idea found in the writings of 5th century Greeks (link). It postulates that life exists throughout the universe, in some form or another, most likely in a single-celled form or as viral type organisms. The dominant narrative states that these zip around the cosmos on moving bodies such as meteors, asteroids, comets that facilitate their delivery to different planets post-collision. It differs from other abiogenesis theories by not placing much emphasis on explaining how life actually originated but rather places the origin of life onto another world.
Although experimental and observational evidence supports the idea of the presence of an abundance of organic molecules in the universe (link), it remains a contentious theory. It is generally regarded as not 100% impossible that life might have originated from off-planet sources, but there is zero evidence to support that. Proponents of Panspermia often cite backing from prominent scientists, such as none other than Francis Crick (link). But even people who do great things appear to sometimes harbour ridiculous notions (link). The general scientific consensus for the role of panspermia is that it provided for nothing more than the basic building blocks of life. Panspermia is especially held in suspicion due to its propensity to be hijacked and misunderstood by pseudo-panspermia theorists who appear to add 2 and 2 to get 5 (again see this paper).
THE CLAY CRYSTALS THEORY
This theory is one which may overlap with the primordial soup hypothesis, in that it might have occurred within a “warm little pond”. It proposes an inorganic beginning of life, as opposed to an organic one (link). Briefly, it supposes that clay silicate crystals may have facilitated the development of complex organic molecules such as nucleotides. The theory emerged from the observed behaviour of the growth of clay crystals, which grow and layer and snap off. They develop layers that appear to copy the patterns of the previous layers. This process was determined by observing the copying and maintenance of “defects” in layer structure that are passed on faithfully to following clay crystal layers, which then snap and emerge as a new crystal structure. If a particular clay crystal structure develops a defect (read here ‘mutation’) that allows a more adhesive surface than previous “generations”, then it could, in theory, increase its chances of successfully silting a pond bed or the bottom of a stream. This adhesive structure may then grow further and hypothetically dry out during drought events. Forming dust particles, it may then be carried by wind motion to other streams thereby growing once again following the same crystal patterns of its’ “parent” in the stream bed of origin. At this point, the theory holds, we now have a reproducible non-organic structure using physical processes; a type of inorganic hereditary mechanism.
It then follows that such a sticky silicone clay platform would inevitably be collecting a whole range of other molecules, some of which, when concentrated via the forces of drying, may form complex proto-organic molecules.
So far, so credible. The theory hits at this point, a great scientific wall of China, as the leap from reproducible silicate matrix structures to eventual complex reproducible information-carrying nucleic acids requires imagination, and a lot of it. It is not so popular a theory presently, although aspects of it are often considered plausible. The clay matrix certainly could have been a catalyst, or platform, for abiogenic reactions.
HYDRO-THERMAL VENTS THEORY
Scientific theories that are difficult to prove tend to turn around every few decades, depending on the credibility of evidence thus far presented. There is no exception for the theories of abiogenesis. All the above had their day, and now mostly linger in the background supported by a few die-hard adherents, and a large crowd of cranks, in some cases. The theory now most popular is that life formed in an alkaline hydrothermal vent. I feel though this one is different and is the most compelling.
Among most theories of abiogenesis, the prevailing consensus requires some form of concentrating factor to bring all the relevant materials together in order to interact. Primordial soup and clay matrix theories suggest desiccation, but the hydrothermal vent theory suggests pressure. Here, hot sulphide-rich liquids emerging from under the ocean floor at an alkaline hydrothermal vent, interacting with iron-rich seawater, via a redox, pH and temperature gradient inside the micro-passages of the mono-sulphide precipitate vent system, may have acted as a compartmentalisation mechanism that was a prototype of the first cell walls and membranes. This mechanism, maintained for long enough, could have spurred further abiogenic developments leading to the eventual emergence of the first living prokaryotic cells, the last universal common ancestor (LUCA), and eventually life as we know it today.
This is one of the few theories where evidence is actually emerging to strengthen the argument, especially recently (link 1, link 2, link 3, link 4, link 5, and here for an excellent book on the matter Nick Lane – The Vital Question).
Among those references I have just given is the most recent one, for the oldest microfossils yet discovered, mentioned at the beginning of this rambling post. That brings a nice circular end to this parsimonious tour of the state of what we currently know about the beginnings of life. And I really do not feel that we can say that our DNA came from space. The octopuses are our own.
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