What do scientists understand about the origin of life?

Updated 1 October 2024 (c) 2024

Credit: Walker, Packer, Cody, “Re-conceptualizing the origins of life,” https://royalsocietypublishing.org/doi/10.1098/rsta.2016.0337

Introduction

In the past few decades, modern science has uncovered a universe that is far vaster and more awe-inspiring than ever imagined before, together with a set of elegant natural laws that deeply resonate with the idea of a cosmic lawgiver. In spite of these exhilarating developments, some writers, principally of the creationist and intelligent design communities, prefer a highly combative approach to science, particularly to geology and evolution.

In addition to citing deeply flawed probability-based arguments (see Probability), these evolution skeptics often assert that science has yet to understand the origin of life, and that this is a fatal flaw in evolutionary theory [Behe1996; Dembski1998]. For example, in the 2005 Dover, Pennsylvania intelligent design trial, the Dover Area School Board passed a resolution saying that “The Theory [of evolution] is not a fact. Gaps in the Theory exist for which there is no evidence.” It continued, “Intelligent Design is an explanation of the origin of life that differs from Darwin’s view.” [Lebo2008, pg. 62].

So to what extent does modern science understand the origin of life? What are the latest developments in this arena?

It is certainly true that as of the above date, scientists do not yet fully understand abiogenesis (the formal term for the origin of life on Earth — see [Abiogenesis2022]). In particular, the origin of the first self-reproducing biomolecules, on which evolutionary processes could operate to produce more complicated systems, remains unknown. What’s more, unlike bony structures that leave fossil records, the early stages of biological evolution on Earth very likely have been completely erased, so that we may never know with certainty the full details of what transpired. If anything, the very rapid appearance of life on Earth after it first formed suggests that the origin of life was a reasonably likely event, given a few tens of millions of years (see below), but we have no way to know for sure. It may have been an extraordinarily improbable event.

The question of the origin of life on Earth is deeply intertwined with Fermi’s paradox: If the origin of life is inevitable on a planet such as ours, and if life inexorably increases in complexity and intelligence to a technology-capable society, then why after 50+ years of searching have we not yet found any evidence of any extraterrestrial civilization? There is no easy answer here — see Fermi’s paradox for further analysis.

Is research into the origin of life a proper scientific pursuit?

It should be emphasized that research in abiogenesis is fundamentally no different, philosophically or methodologically, than research in any other field of science — the fact that this event occurred approximately four billion years ago makes no difference whatsoever. Scientists routinely study phenomena at the atomic and subatomic level that are far smaller than what can be viewed by eye, or even via optical microscopes, and they also study phenomena in distant galaxies that are far beyond current technology to visit in person; note also that the events viewed in these galaxies occurred millions or billions of years ago, since these objects are typically millions or billions of light-years away. Yes, there are unanswered questions in abiogenesis, but there are also unanswered questions even in areas of science that one would think are extremely well established, such as gravitational physics [Grossman2012a], cosmology [Barnes2013; Susskind2005] and reproductive biology [Ridley1995]. Thus claims by evolution skeptics that unknowns in the origin of life arena “prove” that scientists do not have all the answers are only met with puzzled stares by real research scientists — after all, exploring unknown, unanswered questions is what science is all about and what researchers explore in approximately two million peer-reviewed papers published each year [Ware2012].

The Miller-Urey experiment

The first major result in the field of abiogenesis was a 1953 experiment by Stanley Miller and Harold Urey. In this experiment, they placed water plus some gases in a sealed flask and passed electric sparks through the mixture to simulate the effects of sunlight and lightning. Over the next week or so, the mixture slowly turned a reddish-brown color; analysis revealed that it contained several amino acids, the building blocks of proteins [Davies1999, pg. 86-94]. Researchers have more recently observed that in current models of early Earth’s atmosphere and oceans, carbon dioxide and nitrogen would have reacted to form nitrites, which quickly destroy amino acids. Thus the Miller-Urey experiment might not be truly representative of what really happened on the early Earth, although some variation of the experiment might still be valid [Fox2007].

The RNA world hypothesis

Going beyond the synthesis of basic amino acids, one leading hypotheses is that ribonucleic acid (RNA) played a key role. 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 a molecule like RNA could “self-catalyze” itself in this manner, perhaps with the assistance of some related molecules, and then larger conglomerates of such compounds, packaged within simple membranes (such as simple hydrophobic compounds), could have formed very primitive cells [NAS2008, pg. 22]. Nonetheless, even the “RNA world” hypothesis [Cafferty2014], as the above scenario is popularly known, faces challenges. As biochemist Robert Shapiro notes, “Unfortunately, neither chemists nor laboratories were present on the early Earth to produce RNA.” [Shapiro2007; Service2019].

But research on the RNA world hypothesis continues, with some remarkable recent discoveries. In May 2009, a Cambridge University team led by John Sutherland solved a problem that has perplexed researchers for at least 20 years, namely how the basic nucleotides (building blocks) of RNA could spontaneously assemble. As recently as 1999, the appearance of these nucleotides on the primitive Earth was widely thought to be a “near miracle” by researchers in the field [Joyce1999]. In the 2009 study, Sutherland and his team used the same starting chemicals that have been employed in numerous earlier experiments, but they tried many different orders and combinations. They finally discovered one order and combination that formed the RNA nucleotides cytosine and uracil, which are collectively known as the pyrimidines [Wade2009]. Then in March 2015, Sutherland’s team announced that two simple compounds can launch a cascade of chemical reactions that can produce all three major classes of biomolecules: nucleic acids, amino acids and lipids. Biologist Jack Szostak described the result as “a very important paper” [Service2015]. Among other things, Sutherland’s group has been able to generate 12 of the 20 amino acids that are used in living organisms [Wade2015a].

More recently, in May 2016, a team led by Thomas Carell, a chemist at Ludwig Maximilian University of Munich in Germany, reported that his team had found a plausible way to form adenine and guanine, the two building blocks that Sutherland’s team had not been able to produce [Service2016], and subsequently found a simple set of reactions that could have formed all four RNA bases on the early Earth. Ramanarayanan Krishnamurthy, a biochemist at Scripps Research, said “This paper has demonstrated marvellously the chemistry that needs to take place so you can make all the RNA nucleotides,” although he cautioned that this scenario might not be the actual pathway that was taken on the early Earth [Service2018; Castelvecchi2019].

When did life start?

One key question is when this abiogenesis event occurred. A recent consensus picture is as follows: the solar system was created 4.568 billion years ago in the aftermath of a stellar explosion; this was followed, 4.53 billion years ago, by the formation of the Earth; the Moon formed 4.51 billion years ago from a collision of some other celestial body with the Earth; by 4.46 billion years ago, Earth had cooled enough to have both land and water. Then the first RNA formed about 4.35 billion years ago. By 4.1 billion years ago, zircon crystals from that era show a ratio of carbon 12 and carbon 13 isotopes that is typical of photosynthetic life, and very recent results suggest an even earlier origin (see below). It is thought that the Earth continued to be bombarded by meteorites fairly heavily until about 3.8 billion years ago (although see below). The earliest fossils attributed to microorganisms have been dated to 3.43 billion years ago. See this 2019 Science article for overview of the latest research in the area as of 2019 [Service2019].

Other 21st century developments

In light of these results, there is a sense in the abiogenesis field that progress is accelerating. Here is a brief summary of some other recent (since 2015) developments, listed in chronological order, most recent listed last.

1. In October 2015 geochemists at the University of California, Los Angeles announced evidence that life very likely existed 4.1 billion years ago on Earth, which is at least 300 million years earlier than the previous research consensus. The research consisted of studying 10,000 zircon crystals from the Jack Hills area of Western Australia. Zircons, which are related to the synthetic cubic zirconium that is used for imitation diamond, are extremely durable and virtually impermeable, so that material trapped inside them constitutes a very reliable picture of the environment when they formed. One of these zircons contained graphite (carbon) in two locations. These carbon dots had a ratio of carbon-12 to carbon-13 that is a characteristic signature of photosynthetic life. Researchers are very confident that this carbon is at least 4.1 billion years old, because the zircon that contains them is that old, based on careful measurements of its uranium-to-lead ratio [SD2015c].

2. In July 2016, researchers at Heinrich Heine University in Dusseldorf, Germany searched DNA databases for gene families shared by at least two species of bacteria and two archaea (even more primitive biological organisms). After analyzing millions of genes and gene families, they found that only 355 gene families are truly shared across all modern organisms, and thus are the most probable genes shared by the “last universal common ancestor” (LUCA) of life. Further, these researchers found that the probable LUCA organism was an anerobe, namely it grew in an evironment devoid of oxygen. These results suggest that LUCA originated near undersea volcanoes [Service2016a].

3. In August 2016, researchers at the Scripps Research Institute created a ribozyme (a special RNA enzyme) that can both amplify genetic information and generate functional molecules. In particular, it can efficiently replicate short segments of RNA and can transcribe longer RNA segments to make functional RNA molecules with complex strictures. These features are close to what scientists envision was an RNA replicator that could have supported life on the very early Earth, before the emergence of current biology, where protein enzymes handle gene replication and transcription [SD2016b].

4. In March 2017, British geologists announced that they had found fossil specimens from a geological formation near Hudson Bay, Canada, with dates measured as between from 3.77 billion years to 4.22 billion years old, earlier than any previous claim for biological organisms on Earth. These specimens are similar in appearance to tube-like structures found in modern-day undersea hydrothermal vents. If these findings are upheld, particularly at the 4.22 billion-year-old level, this will be very quickly after the earliest oceans formed [Zimmer2017a].

5. In May 2018, a team of researchers led by Philipp Holliger at the Medical Research Council Laboratory of Molecular Biology in Cambridge, UK, produced RNA enzymes that can copy RNA molecules even longer than themselves. The problem with many of these RNA experiments is that while RNA can only act as an enzyme if it is in a folded shape, RNA enzymes can only copy RNA molecules that not folded. Holliger’s solution was to build RNA out of “trinucleotides,” slighter larger nucleotide molecules, and to conduct the experiments in an icy solution. Nonetheless, biologists consider his work to be a major step towards completely “natural” synthesis and enzymatic processing of RNA [LePage2018].

6. In April 2019, a team of researches at Scripps Research Institute found evidence that both RNA and DNA could have arisen from the same precursor molecules. In particular, they showed that the compound thiouridine, which could have been a precursor to RNA, could also be converted into a DNA building block, namely deoxyadenosine (the “A” of the DNA four-letter code) [SD2019a]. A well-written account of these results, with detailed background, is available in a Quanta Magazine article by Jordana Cepelewicz [Cepelewicz2019]. In a December 2020 update to these results, the Scripps team demonstrated that the relatively simple compound diamidophosphate, which plausibly existed on the early Earth, could have acted as a catalyst to connect links of RNA and DNA “letters.” What’s more, the process works better when the RNA or DNA chain has distinct letters [Scripps2020].

7. In March 2020, a team of researchers at the University of Groningen in the Netherlands discovered that mixtures of simple compounds based on carbon can spontaneously form relatively elaborate molecules that can construct copies of themselves. The researchers combined amino acids and nucleobases of DNA and RNA, which (as noted above) are now known to form spontaneously, given the right settings. When the compounds were mixed together, the amino acids and nucleobases linked to form ring-shaped molecules and then assembled into cylindrical stacks. These stacks then attracted other rings to stack together, in effect copying the stack [Marshall2020].

8. Also in March 2020, at team of German researchers found that certain metals, plentiful near ocean hydrothermal vents, could catalyze the reaction of water and CO2 to form acetate and pyruvate, key components of the “acetyl-CoA” pathway, a process that feeds organic compounds that drive the production of proteins, carbohydrates and lipids in biological energy metabolism. Thomas Carell, the researcher mentioned above who found a pathway to produce key RNA nucleotides, called the result “thrilling” [Service2020].

9. In March 2022, a group of researchers at the Max Planck Institute for Astronomy in Germany announced that they had demonstrated experimentally that the condensation of carbon atoms on the surface of cold cosmic dust particles forms aminoketene molecules, which then can polymerize to produce peptides (molecular subunits of proteins) of different lengths. The process involves only carbon, carbon monoxide and ammonia, which are known to be present in star-forming clouds, and proceeds via a pathway that skips the stage of amino acid formation [Saplakoglu2022a].

10. In June 2022, a team of researchers at the Ludwig-Maximilians University in, Munich, Germany found that peptides such as those described in the previous item could have co-evolved with RNA molecules — the RNA molecules may have enabled peptides to grow directly on them, and the peptides in return could have helped stabilize the RNA molecules, which otherwise decompose quickly. Andro Rios, a researcher at NASA Ames Research Center, called the result a “highly intriguing demonstration” [Saplakoglu2022b].

11. Also in 2022, researchers at the University of Tokyo found that a network of five RNA populations formed a replicator network with diverse interactions, including cooperation to help the replication of other members: “These results support the capability of molecular replicators to spontaneously develop complexity through Darwinian evolution, a critical step for the emergence of life” [Mizuuchi2022].

12. In March 2024, researchers at the Salk Institute for Biological Studies in California developed an RNA molecule that managed to make accurate copies of a different type of RNA. The work was hailed as one step in constructing an RNA molecule that can make accurate copies of itself. John Chaput of U.C. Irvine, who did not participate, called the achievement “monumental” [Johnson2024].

13. In July 2024, a team led at the University of Bristol in the UK announced solid evidence that the last universal common ancestor (LUCA) must have arisen at least 4.2 billion years ago. Note that this means the 4.1 billion year figure mentioned above for the origin of life may have to be revised even higher, in other words that life arose within the first 250 million years of Earth’s existence. This result also seriously draws into question the long-held assumption that a “late heavy bombardment” sterilized Earth until 3.8 billion years ago [LePage2024; Sarchet2024].

Many of these results and others are outlined in a newly published (2024) book by Mario Livio and Jack Szostak, titled Is Earth Exceptional?: The Quest for Cosmic Life [Livio2024].

Remaining unknowns in abiogenesis

In spite of these advances, it must be acknowledged that there are some significant unknowns in the RNA world hypothesis, as it is currently understood [Cafferty2014; Livio2024].

In particular, 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 [Totani2020].

In a 2020 paper published in Nature Scientific Reports, Japanese astronomer Tomonori Totani proposes a solution to this conundrum [Totani2020]. Using a fairly straightforward analysis, he found the probability of spontaneous assembly of a sufficiently long RNA chain to be the basis of life is negligibly small on our planet or galaxy, or even in the larger observable universe to which we belong, but it would be virtually 100% in the much larger universe created in the inflationary epoch just following the big bang. 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 (see Anthropic principle, Big bang and Inflation).

In any event, it must be kept in mind that continuing research into biochemical pathways for abiogenesis could invalidate such reckonings — the origin of life may yet prove to be entirely solvable via straightforward laboratory experimentation.

What is the impact on the larger theory of evolution?

It must be kept in mind that the process of evolution after abiogenesis is very well attested in fossils, radiometric measurements, DNA, and numerous other lines of evidence, completely independent of how the first biological structures formed. In other words, those unknowns that remain in abiogenesis theory have no bearing on the central hypothesis of evolution, namely that all species are related in a family tree, having proliferated and adapted over many millions of years. Thus there is no substance to the claims by evolution skeptics that unknowns in the origins area are a fatal flaw of evolutionary theory.

This line of reasoning, namely that the absence of a full explanation of abiogenesis invalidates the whole of evolutionary theory, is a classic instance of the “forest fallacy” — picking a flaw or two in the bark of a single tree, and then trying to claim that the forest doesn’t exist. Indeed, to the extent that evolution skeptics continue to emphasize the abiogenesis issue as a premier flaw of evolution, they risk being discredited, even in the public eye, as new and ever-more-remarkable developments are publicly announced.

Along this line, the origin of life with respect to the larger theory of evolution is in perfect analogy to the origin of the universe at the big bang with respect to the Standard Model of modern physics (the over-arching theory that describes interactions of forces and matter from the smallest scales of time and space to the largest). It is undeniably true that scientists do not yet fully understand the big bang, and it is also true that scientists recognize that the current Standard Model is not the final word. But this is hardly reason to dismiss or minimize the enormous corpus of research in modern physics, astronomy and cosmology and the impressive levels of precision to which these theories have been tested. For additional details, see Big bang.

Summary

It is undeniably true that scientists do not yet have a complete chain of evidence, or even a fully-developed biochemical scenario for the origin of life — no knowledgeable scientist has ever claimed otherwise. Numerous scenarios have been explored, but there are still some unanswered questions. Nonetheless, thousands of scientific papers, documenting countless experimental studies, have been published on these topics, and several previous show-stopping obstacles, such as the formation of certain building blocks of RNA, have been overcome. Indeed, some of the above results, such as the recent results by Sutherland and Carell on the synthesis of RNA nucleotides, represent remarkable progress in the field. Almost certainly even more remarkable results will be published in the next few years. It would be utter folly to presume that no additional progress will be made.

Given these developments, most observers believe that it is extremely unwise to reject out of hand the possibility that research eventually will discover a complete natural process that could satisfactorily explain the origin or early development of life on Earth. Along this line, it may well be the case that all traces of the first self-reproducing systems and the earliest unicellular life may have been destroyed in the chaotic chemistry of the early Earth, so that we may never know for certain the precise path that actually was taken. But even if scientific research eventually demonstrates some plausible natural path, this would already defeat the claims of evolution skeptics that life could only have originated outside the realm of natural forces and processes. And, frankly, such a discovery could be announced at any time.

On the other hand, it may turn out that the origin of the very first self-replicating strands of RNA, to mention one unsolved aspect of this theory, is fantastically improbable, and present-day humans are descendants of this remarkably unlikely event, as suggested by Totani’s research mentioned above. But even here, nothing suggests that this origin event was the result of anything beyond the operation of known laws of physics and biology.

Either way, the line of reasoning by evolution skeptics, namely that the absence of a full explanation of abiogenesis invalidates the whole of evolutionary theory, implying that creationism and intelligent design must be considered on an equal par with evolution, is a classic instance of the “forest fallacy,” and has no validity whatsoever.