Abiogenesis Vs Panspermia - Why Life is Extremely Rare in the Universe?

The very first or one of the very first candidates for the great filter is the first instance of the genesis of organic matter. In case of earth, two prominent candidates that come out on top as the answers to this question are 'panspermia' and 'abiogenesis'.

Both of these questions answer many questions and give rise to many others. One important thing to keep in mind is that these two are competing theories only in case of earth. If we consider the universe as a whole abiogenesis has the potential to answer the question how does life start anywhere? This will be more clear as we discuss these in more detail in this article.

In this article we will briefly see both the concepts and some of the intriguing possibilities arsing from them. Also, we will see how this ties up with the great filter hypothesis and the rarity of life in the universe. 

Panspermia

The word 'panspermia' is derived from the Greek words "Pan" meaning all and "Sperma" meaning seed. This is a school of thought which proposes that the life exists throughout the universe and is distributed everywhere by comets, asteroids, planets, meteoroids, and space dust. 

According to Jamie Carter of Forbes, it is an untested, unproven and rather wild theory. But is it really so? I don't think it is. Though not conclusive there are interesting (may be circumstantial) evidences for this theory. A doctoral thesis by Jamie Hamilton Wallis at Cardiff University titled as 'Evidence of Panspermia: From Astronomy to Meteorites' is a good read for in depth research on such evidences. However, the point of this article is not to look into the proofs of the either concepts and therefore, we will continue with a premise that both the theories hold certain merits and are based on sound scientific evidences and rigor.  

From, the available evidences there is a good reason to believe that panspermia may have played a significant part in the origin of life here on earth. However, there are certain points which I find thought provoking. They are;

1. Randomness - The contaminated object is negligible in size as compared to the entire universe which is dominated by 'absolute nothingness'. The chance of the organic material reaching a planet with right conditions to harbor life is minuscule considering the size of a contaminated object and the number and size of the right planetary bodies in this vast universe. The probability would be less than winning a lottery and getting struck by lightning on the same day.
2. Pro-life conditions - If the conditions favorable to originate life are similar or same across the universe, then it give rise to another interesting proposition and that is the life is similar everywhere. If earth like conditions are required to initiate life from the organic material brought in by the contaminated objects then theoretically the life originating elsewhere will also be carbon based DNA-centric life just like earth. It will show variations only as per the atmosphere and environment of the planet, but the basic structure would be remarkably similar to our.
3. Basic building blocks of the life - If point 2 is true, then the basic building block of life must be of one specific type. This may explain why the genesis of living matter is so rare in the universe. Though we have found many exoplanets similar to earth, we haven't found any evidence of life (microbial or otherwise) on any of the planets. This may be because the life bringing celestial bodies have not had an encounter with them to kick start the life process. 
4. Other form of life - If point 2 and 3 both are wrong, then the basic building blocks of life may be of various types. Some may give rise to a carbon based life, some may give rise to a silicon based life and some may be altogether different. All these will require different initial conditions to start life. Consequently, the life on that planet will be drastically different owing to its initial conditions.  

In the light of these points, panspermia not only offers a solution to the problem of genesis on earth but may also explain the rarity of life elsewhere. However, it still doesn't answer the real question, how the required building blocks or seeds of life are created from an inorganic matter. The theory to answer that question is 'abiogenesis'.

Abiogenesis

In evolutionary biology, abiogenesis, or informally the origin of life (OoL)is the natural process by which life has arisen from non-living matter, such as simple organic compounds. In the perspective of universe this is the only theory that explains origin of life. However, in case of the earth, abiogenesis is considered to be the alternative to panspermia. 

From the definition it may appear that is some kind of a trigger or singular event but, in reality it is a series of events evolving over a period of long time. It involves number of steps. The steps will vary based on the initial conditions and involved chemical components. 

This is not one single theory, but involves many possible ways of originating life from inorganic compounds. Before looking into abiogenesis and some of the intriguing questions that rise from it, it is necessary to understand the definition of life. In spite of being present everywhere on earth, defining life is extremely difficult. To limit diversifying the flow of this article, we will consider the definition of life given by Neil Campbell and Jane Reece. They say, "by more or general consensus nowadays, an entity is considered to be "alive" if it has the capacity to carry out three basic functional activities: metabolism, self-repair, and replication."

Abiogenesis tries to explain origin of such life from inorganic matter. The Miller-Urey experiment performed in 1952 showed that the early building blocks for life on earth i.e. amino acids can be synthesized from inorganic matter in early earth like conditions. The requirements for for this to happen are the presence of correct inorganic matter and a source of energy for the reaction to happen. 

Without delving much into the details of abiogenesis, let us focus on the intriguing possibilities that arise due to abiogenesis and view it from the perspective of the great filter hypothesis. I find the following points difficult to wrap my head around;

1. Series of Events - Since it is a series of events, there remains a high possibility of going one or many steps wrong or not happening altogether. In any such event the life will not arise or the abiogenesis will not take place. This helps in explaining the rarity of life. In the universe one or few steps of abiogenesis may take place on many planets but the possibility of all steps taking place precisely one after the other is mindbogglingly small. 
2. Source of Energy - For abiogenesis to happen one crucial requirement is the availability of the energy source. Here two things are very important, the energy source should be present at the 'right' time and in the 'right' amount. If any of these conditions are not fulfilled the abiogenesis will not happen. If we think about it, it is just amazing that all these conditions were fulfilled when the life on earth began. 

Abiogenesis has a potential to explain life not only on the earth but in the entire universe. Potentially, it also accounts for varied forms of life and not just a carbon based, DNA-centric life. If the correct inorganic matters are present in correct initial conditions life can start and evolve in a manner entirely different than our own. 

Conclusion

In my opinion, in earth's case, not just one but both of these phenomena played a role in the origin of life. Also, both of these phenomena involve many steps which have astonishingly small probabilities of happening. This indicates towards the reason of the glaring rarity of life in the universe. At the same time, it almost guarantees that the life must have originated elsewhere as well. Until any of these prospects are proven or written off with conclusive evidence, all options are still on the table. 

Further reading

Like how the life began, another intriguing question is 'when' the life began. Though this question seems relatively simple, let me assure you it is not. To begin with you may like to watch this video or if you are interested please read this paper by Dr Abraham (Avi) Loeb