Article Writer- Sudipta Biswas, Int. MSc., School of Biological Sciences, NISER, Odisha(India)
strobiology is one of the most intriguing subjects of humanity. Simply put, it can be considered as an area that is concerned with the origins and the distribution of life in the universe . It should be noted that when people started discovering new theories, laws, rules that govern nature, they came across the very fundamental question every time: How life began at ﬁrst? This led to the advancement of astrobiology as a scientiﬁc ﬁeld. From that period, many scientists came up with several theories describing the origins of life on Earth. However, none of the theories appeared to ﬁt well with the equation of the origin. Hence, trying to ﬁnd out how life started on Earth became one of the most sorts after enigmas in the astrobiology society. The people who work under this are called the OoL(Origin of Life) researchers. Here, in this piece, we will try to address the most recent advances in the ﬁeld and look at how it can lead our civilization to have better growth ahead in the future.
Let’s ﬁrst review some of the theories concerning origins that conclusively got dropped or are still under huge debate. Panspermia is one of them which states that life forms were distributed to diﬀerent galaxies by traveling meteorites, asteroids, comets, and space dust . This simply means that life didn’t originate on Earth. Also, it does not explain the origins; it just throws light on the pattern of life, which is not the particular question that we are intending to ask. Well, due to the lack of satisfactory shreds of evidence, the theory got rebuﬀed. However, recent studies have shown that some comets contain life-building molecules like amino acids(which produce proteins) . For that purpose, still, now the theory of panspermia thrives in the scientiﬁc community but not that enormously. The second theory will be that of special creation. It asserts that life and all of the universe originated in its present form by some unusual divine decree . Due to the absence of enough proof, this theory was also not favored. Similarly, there are several others which still hasn’t got any suitable endorsement or approval by the scientists, like the biogenesis and others.
Leaving those theories aside, the hypothesis of abiogenesis has gained the most acclaim. Although it’s still controversial, many scientists are thriving to ﬁnd a probable mechanism to prove the hypothesis. It aﬃrms that life arose from the non-living matter like the simple organic compounds which were speculated to be present on the primitive Earth. To give a more plausible way of ascertaining how life must have started, scientists have abandoned the idea that life has appeared from a true cell, which is way more chemically complex/heterogeneous. According to them, on the primitive Earth, the environment contained an extensive assortment of chemicals that might have reacted with each other randomly. Then life must have emerged from such chemical mess when one of these reactions led to the formation of a basic cell-like structure by linking some arbitrary molecules .
With this approach, investigators have started proposing and trying to model a primitive cell or protocell. A protocell is a cell-like structure circumscribed by a membrane that contains a piece of information-containing molecule needed for replication. It can be recognized as a simple, enclosed chemical system that is self-sustainable. It can form its own membrane, self-assemble, replicate, use energy for performing several metabolic tasks, and evolve . Because of these inferences, the protocells are recognized as the right candidate for being the object from which true cells might have developed. However, it diﬀers from the present-day cell as it does not own any practical evolutionary characteristics. Most of the OoL research focuses on how the primitive environment produced the molecules in that era and then assembled them into structures that led to the ﬁrst cells. Numerous chemical systems are being proposed every year, which can be acknowledged as a potential model for protocell, like the vesicles or micelles made up of amphiphilic molecules, membrane-less droplets, coacervates, and others . All of these systems display protocellular characters, every one having its pros and cons. The systems are modeled to analyze their formation, capacity to replicate, and stability.
The vesicles formed by fatty acids were one of the ﬁrst protocells which were developed in the lab. Fatty acids are a type of carboxylic acids with a long aliphatic chain, which is either saturated or unsaturated. Various abiogenesis synthesis experiments have also validated their presence on the primitive Earth. This makes them an accurate ﬁt for being the molecules from which cells might have arisen. The acids can form vesicular structures by using hydrogen bonds at pH near the apparent pKa of their carboxylic head groups, which lies between pH 7 to 9 for most fatty acids . These single chained fatty acids have less hydrophobicity than the double-chained phospholipids(which is present in the modern cells), making the vesicles exceedingly ﬂexible. Hence, it can readily change its shape as compared to the present-day cells. The vesicles are also permeable, which helps them to intake several molecules across the fatty acid membranes. Supporting this property, they are capable of encapsulating RNA-like molecules that contain the information for the replication of the protocells, some amino acids, and other primitive metabolic molecules .
The reproduction/replication of such vesicles is also of immense signiﬁcance besides their formation and permeability. It is based on a process called pearling instability. According to which the vesicles will take the shape of a tubule when fully grown up. And then, due to the instability of this tubule, it breaks simultaneously to bead-shaped daughter cells with a random distribution of the vesicle’s materials. In addition to its own replication, the vesicles also allow the replication and oligomerization of the several bio-molecules. It is proposed that some highly activated nucleotide monomers would have been present, which were able to polymerize with non-enzymatic template-dependent polymerization inside the vesicles . All of this assemblage of molecules in the vesicles and the process of their unequal division during the vesicular replication could have led to the emergence of variation in protocells leading to Darwinian evolution. And this evolution through increasingly complex stages might have given rise to the structures found in modern cells.
Another model for protocell is the polyester-based membrane-less droplets. Molecules like alpha-hydroxy acids, like glycolic and lactic acid, when dehydrated at low temperatures followed by rehydration, can form gel-like phases that self-assemble into polyester microdroplets . They don’t possess a membrane (hence, called the membrane-less droplets). But even, they able to compartmentalize many important molecules like the RNA and amino acids. These droplets are stable to coalescence/merging in the presence of agitation, and RNA, along with proteins, could still function in these droplets.
Several other systems are also used to simulate the conditions of the primitive cells. Even when they seem very promising to be accepted, they have some shortcomings. For example, the vesicles, as discussed above, always need a particular pH range for their formation along with the present low ionic concentrations in the surrounding . However, many enzymes present inside the vesicles need higher concentrations of ions like Mn[II] for their work, which includes the oligomerization of RNA. Then looking at the polyester droplets, they also have some disadvantages up their sleeves. They can trap and concentrate RNA but, the exchange rate of these RNA molecules from the droplets is very rapid than the fatty acid vesicles . Hence, it can be argued that a lot of discoveries are still required to unravel the conundrum of the origins.
Even if a lot of questions remain in OoL, the answers that are available to us now can resolve a lot of problems that we as a community are trying to ﬁgure out. A lot of new life-detection missions are going on, which focus on ﬁnding life on other planets and celestial bodies. Here, the OoL research will be able to give further structure to these missions by furnishing the information about the biomarkers which are needed to search for life or their origins in outer space. Also, the knowledge about the origins of life can broaden our understanding of how new forms of life can start again in other reaches of the universe, and maybe that can help us to ﬁnd conceivable ET life. Scientists have also begun using OoL for vaccine development for ﬁghting COVID-19. They have bleakly proposed the idea about the use of primitive activated nucleotides, which are recently discovered for stopping the replication of the virus inside the host cell . Now, with a lot of mysteries still remaining and many things being developed, the ﬁeld of astrobiology and particularly OoL will need a whole new bunch of scientists to traverse new possibilities and applaud the research that has been already done. Hopefully, it will help us appreciate life in the universe and maybe give us a new perspective on how we are created.
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