RNA (on left) compared with DNA (on right); courtesy Wikimedia
The abiogenesis problem
Exactly how life first emerged on Earth (the “abiogenesis” problem) remains a critical unsolved question in biology. Was it inevitable, given a favorable environment, or was it a fantastically improbable event? All we know for sure is that it occurred at least 3.8 billion years ago and possibly more than 4 billion years ago. The fact that life arose relatively soon after the surface of the Earth solidified indicates to some that abiogenesis was inevitable, but there is no way to know for sure. For further details, see this Math Scholar blog.
The RNA world hypothesis
One leading hypotheses is that ribonucleic acid (RNA), which operates in biology alongside its more familiar cousin DNA, played a key role in the earliest abiogenesis events — a notion known as the RNA world hypothesis. For example, researchers recently found that certain RNA molecules can greatly increase the rate of specific chemical reactions, including, remarkably, the replication of parts of other RNA molecules. Thus perhaps RNA, or an even a more primitive molecule similar to RNA, could have “self-catalyzed” itself in this manner, perhaps with the assistance of some related molecules. Perhaps then some larger conglomerates of such compounds, packaged within simple hydrophobic compounds as membranes, could together have formed some very primitive cells.
Nonetheless, the RNA world hypothesis faces major challenges as an explanation of abiogenesis.
In May 2009, a team led by John Sutherland of the University of Cambridge solved one problem that had perplexed researchers for at least 20 years, namely how the four basic nucleotides (building blocks) in RNA chains could have spontaneously assembled. Sutherland and his team first discovered one combination of chemicals assumed to be present on the primordial Earth that formed the RNA nucleotides cytosine and uracil. Then in May 2016, a team led by German chemist Thomas Carell found a plausible way to form adenine and guanine, the other two nucleotides. Finally, in November 2018, Carell’s team announced that they had found a single set of plausible reactions that could have formed all four RNA nucleotides on the early Earth. See this Nature article for additional details.
Nonetheless, researchers in the abiogenesis arena are still stuck with a stubborn unanswered question: How could large chains of RNA, sufficiently long to be the basis of primitive self-replicating evolutionary life, have spontaneously formed in the primordial Earth’s water-rich environment, which is thermodynamically unfavorable for the formation of such chains? The current consensus is that any such self-replicating RNA molecule would need at least 40-60 nucleotide bases (rungs in the chain), and most likely over 100, to possess even a minimal self-replicating function. What’s more, a pair of such molecules may be necessary, if one is to serve as a template for replication. Yet the largest number of bases that have been reproducibly demonstrated in laboratory experiments is 10, and the probability of successful formation drops sharply as the number of bases increases.
Abiogenesis in an inflationary universe
In a new paper published in Nature Scientific Reports, Japanese astronomer Tomonori Totani proposes a solution to this conundrum. He first reviews the relevant RNA world literature and analyzes the process of RNA formation and the prospects for this happening on a given planet in considerable detail. Then he calculates the probability of spontaneous assembly of a sufficiently long RNA chain to be the basis of life.
Interestingly, Totani finds that this probability is negligibly small on our planet, and minuscule even in the observable universe to which we belong, which contains approximately 1022 stars. But Totani finds that this probability would be virtually 100% in the much larger universe created in the inflationary epoch just following the Big Bang, which is estimated to contain approximately 10100 stars, most of which are beyond the “horizon” visible from Earth. Under this hypothesis, the fact that we reside on such an exceedingly fortunate planet to have been a home for RNA-based life is merely a consequence of the anthropic principle — if we did not reside on such a fortuitous planet, we would not be here to discuss the issue.
By way of background, the inflationary Big Bang cosmology is the theory, first proposed in the 1980s by physicist Alan Guth, that in the first tiny fraction of a second after the Big Bang, the fabric of space exploded by a factor of roughly 1036 [Guth1997]. The inflation hypothesis explained two paradoxes: the “flatness problem” (in the very early universe, the ratio of the actual mass density of the universe to the critical density must have been exceedingly close to one), and the “horizon problem” (the fact that different regions on opposite sides of the universe appear to have identical characteristics, even though no physical force, even light rays, could have communicated between them). The inflation theory is now widely accepted in the field, although some demur, as we will see later.
Fermi’s paradox
In previous Math Scholar articles (see article A and article B), we discussed the nagging conundrum known as Fermi’s paradox: If the universe or even just the Milky Way is teeming with life, why do we not see evidence of even a single other technological civilization? After all, if such a civilization exists at all say in the Milky Way, almost certainly it is thousands or millions of years more advanced, and thus exploring and communicating with habitable planets in the Milky Way would be a relatively simple and inexpensive undertaking, even for a small group of individuals.
Numerous solutions have been proposed to Fermi’s paradox, but almost all of them have devastating rejoinders. Arguments such as “extraterrestrial (ET) societies are under a strict global command not to disturb Earth,” or “ETs have lost interest in space research and exploration,” or “ETs are not interested in a primitive planet such as Earth,” or “ETs have moved on to more advanced communication technologies,” all collapse under the principle of diversity, a fundamental feature of evolution-based life (even assuming a very general, not-necessarily-carbon-based definition of life). In particular, it is hardly credible that in a vast, diverse ET society, and much less credible if there are numerous such societies, that not a single individual or group of individuals has ever attempted to contact Earth, using a means of communication that an emerging technological society such as ours could quickly and easily recognize. And note that once such a signal has been sent to Earth, it cannot be called back, according to known laws of physics.
Some researchers (see this PBS television show for instance) have claimed that since only 70 years or so have elapsed since radio/TV and radio telescope transmissions began on Earth, this means that only ETs within 70 light-years of Earth, if any such exist, would even know of our existence. But this is clearly groundless, because networks of lights have been visible on Earth for hundreds of years, other evidences of civilization (Egyptian pyramids, etc.) have been visible for thousands of years, large animal species, including early hominins, have been visible for millions of years, and atmospheric signatures of life have been evident for billions of years.
Arguments that exploration and/or communication are technologically “too difficult” for an ET society immediately founder on the fact that human society is on the verge of launching such technologies today, and ET societies, as mentioned above, are almost certainly thousands or millions of years more advanced. As a single example, since we now have rapidly improving exoplanet detection, analysis and imaging facilities, surely any ET society in the Milky Way galaxy has far superior facilities that can observe Earth. Also, within a few decades it will be possible to launch “von Neumann probes” that land on distant planets or asteroids, construct extra copies of themselves (with the latest software beamed from the home planet), and then launch these probes to other stars, thus exploring the entire galaxy within at most a million years or so [Nicholson2013]. Such probes could beam details of their discoveries back to the home planet and, importantly, also initiate communication with promising planets. Along this line, gravitational lenses, which utilize a star’s gravitational field as an enormously magnifying telescope, could be used to view images of distant planets such as Earth and to initiate communication with these planets [Landis2016].
So why have we not seen any such probes or communications? There are no easy answers. See this previous Math Scholar article for more discussion of proposed solutions and rejoinders to Fermi’s paradox.
The “rare Earth” explanation
One plausible resolution of Fermi’s paradox, although it is sternly resisted from many quarters, is the “rare Earth” explanation: Earth is a unique planet with characteristics fostering a long-lived biological regime leading to intelligent life [Ward2000,Gribbin2018]. Under this explanation, the reason we have not seen any evidence of the existence of ET civilizations, or any unmistakable attempts by an ET civilization to contact us, is simply that they do not exist, at least not within a vast distance from Earth. Clearly Totani’s analysis fits with the rare Earth explanation.
There are other arguments as well that suggest that Earth is significantly more special than typically recognized, in spite of promising observations of extrasolar planets (see this earlier Math Scholar article for details and references):
- To form more complex compounds, the RNA world scenario requires ultraviolet light at a certain moderate energy level, which the early Earth provided. Out of some 4000 recently discovered exoplanets, only one has both a moderate temperature regime for liquid water and satisfies the UV light criterion.
- Virtually all exoplanets orbiting red dwarf stars, which are much more numerous in the Milky Way than are planets orbiting our type of star, are unlikely to harbor life because of frequent flares of sterilizing X-ray radiation, and any atmosphere would be quickly stripped away.
- Many recently discovered exoplanets that have a solid crust, and thus are candidates for life, are “toffee planets,” with surface rocks hot enough to stretch like toffee candy, and are unlikely to feature plate tectonics, which is known to be essential for a long-lasting moderate regime for life.
- Our solar system is also quite special, in that it includes small planets like Earth but also large planets like Jupiter, which have cleared out debris and reduced asteroid impacts on Earth. Also, the solar system’s position in the Milky Way is rather special — close enough to the galaxy’s center to have sufficient concentrations of heavier elements for complex chemistry, yet not so close as to be bathed in sterilizing radiation.
For additional discussion why Earth and our solar system are quite possibly unique in the Milky Way for harboring life, see this 2018 Scientific American article by John Gribbin.
Looking forward
There are, of course, some significant qualifications and rejoinders to Totani’s analysis. To begin with, although the inflationary scenario of the early Big Bang offers elegant solutions to several vexing paradoxes of the observed universe, and is widely accepted in the field, it has significant difficulties that do not seem to be going away, such as how the inflation process started and stopped. Paul Steinhart, one of the early proponents of inflation, more recently has expressed his doubts. At the least, it now seems likely that the inflation theory will need to be significantly modified, although various suggested modifications do not appear to affect Totani’s central conclusion. See this earlier Math Scholar article for details.
Secondly, Totani’s analysis only applies to a carbon-based (RNA-based) biology. But in his defense, although one can imagine living organisms based on other elements, carbon is by far is the most suitable element for the construction of complex molecules, as required for any conceivable form of living or sentient beings.
Finally, Totani’s calculations, although very well documented and based on the latest published research in the field, still are relatively tentative, and could easily be upset by a breakthrough in laboratory studies of the RNA world hypothesis. Totani himself offers at least one potential refutation of his analysis:
If extraterrestrial organisms of a different origin from those on Earth are discovered in the future, it would imply an unknown mechanism at work to polymerize nucleotides much faster than random statistical processes.
So was the origin of life on the early Earth an inevitable albeit remarkable event, bound to happen within a few tens or hundreds of millions of years after the formation of the early Earth? Or was it a freak accident of nature, and are we, as descendants of that exceedingly improbable event, the only sentient beings within a vast volume of the observable universe able to comprehend this astounding fact? Time will tell.
Either way, we await further research in this arena. It is an exciting time to be alive!