By Michelle Hampson
One of the most important questions humans have ever pondered is: how did life first evolve? Fish are perfectly adapted to turn on a dime in the ocean; birds can effortlessly harness the turbulent currents of the sky; and us humans have mastered manipulation of the land. In total, there may be as many as one trillion species on Earth, each highly specialised to its own environment. Where did this all of this complex and diverse life first come from? What were the ingredients, the lucky combination of events in the universe that triggered this transition from basic atoms, simply reacting to the environment, into organised genetic material and proteins that transfer information and energy with purpose?
This is no easy question to address, and one with perhaps multiple answers if life has been created from non-life more than once. Before discussing what those early evolutionary steps of life may be, however, it’s important to define life itself.
The Definition of Life
All life as we know it involves carbon-based chemistry and relies on water. But there could be other organisms that thrive on a different set of basic requirements, meaning a much broader definition is needed. The question of what constitutes life is very subjective and remains widely debated though. In general, most definitions argue that life has most or all of the following features:
- Has a metabolism (harvests energy for use)
- Reacts to is environment (changes behaviour)
- Can evolve (grows or adapts to its environment)
- Can reproduce offspring.
There are other life-like forms that come close to fulfilling the features defined above, but which are not widely accepted as living organisms. For example, viruses cannot replicate on their own (they must infect a cell to so) and are not capable of metabolising energy, so these snippets of DNA and RNA are not universally considered living organisms. Just like crystals and snowflakes that grow but do not demonstrate the metabolic properties of life, viruses do not meet enough of the criteria.
All known lifeforms are cellular by nature, and can be classified into three overarching domains: bacteria, archaea, and eukarya. The first two of these domains are purely made up of single-cell organisms, while the latter includes some single-celled organisms but also all multi-cellular organisms.
Structurally, the cells of organism across these three domains can vary.
Bacteria: These cells have no cell nucleus, and contain ester-linked lipids in their membranes.
Archaea: These single-celled organisms also lack a nucleus, but contain ether-linked lipids in their membranes. Despite lacking a nucleus like bacteria, archaea cells are said to be somewhat more like eukaryotic organisms, since the two domains share some very similar genetic and metabolic pathways (for example, similar enzymes that process energy and more complex RNA polymerases than bacteria).
Eukarya: These species are distinct because their cells contain a nucleus, whereas cells in the former two domains do not. These cells are also more complex because they contain additional inner components within their membranes, called organelles. Eukaryotes also have multiple, linear chromosomes (packaged compartments of proteins and DNA), while the other two domains predominantly have circular chromosomes.
These three domains capture all cellular life as we know it, meeting the requirements listed further above. This is not to say, however, that other domains of life don’t exist – they may just need to be discovered and described.
How Life May Have Begun on Earth
The Earth has existed for about 4.54 billion years, and the earliest evidence of life on the planet is fossilised single-cell organisms estimated to have lived between 3.7 billion and 4.2 billion years ago.
But before cells came into existence, the smaller components that make up a cell must have arisen and combined. Cells are made up of various proteins, with a layer of lipids (fats) forming a protective outer membrane, while proteins inside the cell perform specialised functions. DNA and RNA hold the recipe for protein production, and yet proteins help RNA and DNA replicate. This raises a major conundrum. Which evolved first: the protein or the genetic code that prescribes how a protein is made? Much like the chicken-or-the-egg dilemma, it is hard to determine which one came into existence first.
Although which one evolved first remains disputed, scientists on both sides of the debate have shown how the respective subcomponents of a cell can sporadically form.
The Case for Amino Acids and Proteins
All proteins are made up of various combinations of carbon-based (organic) molecules called amino acids. Amino acids are molecules with a main “backbone” containing amine and carboxyl functional groups, but which vary by the nature of their sidechains, made up almost exclusively of carbon, oxygen, nitrogen and hydrogen. In total, 22 amino acids exist across all known forms of life. In recent years, scientists have proven that about half of these amino acids can spontaneously form folded proteins, without instructions from DNA or RNA, under very hot and extremely salty (halophile) conditions that likely took occurred during the Earth’s early years.
The Case for Nucleic Acids and RNA
Evidence supporting the prospect that RNA evolved first is less convincing and abundant than that for the evolution of proteins, but not completely lacking. A primary supporting argument for the “RNA First” concept is that a recipe is needed to produce the proteins and other subcomponents of a cell.
RNA and DNA is made up of oligonucleotides: short sequences of nucleic acids. Some scientists propose that these precursor building blocks combined to form longer genetic sequences.
But a major question is, if RNA evolved first, how did it replicate itself without the help of proteins? In the 1980s, some scientists discovered a type of RNA molecule that also acts like an enzyme: a protein that catalyses reactions. These unusual types of RNA, dubbed ribozymes, could theoretically reproduce themselves, if blessed with the right machinery. Scientists have succeed in creating a ribozyme from founding population of random sequence RNAs; however, no ribozyme has yet been found to be able to sustain its own replication.
Other major concerns in the “RNA First” branch of astrobiology include:
- whether RNA was preceded by some other nucleic acid easier to polymerise;
- whether RNA or its predecessor could have polymerised in an unenclosed environment;
- and how RNA could spontaneously arise at all.
Some progress on this last point has been made in recent years. For example in 2009 chemists found that by using two precursor compounds, called acetylene and formaldehyde, they could compel non-living compounds to undergo a sequence of reactions to produce RNA without the help of enzymes.
Other Key Processes That Need to Be Considered
Along with how amino acids and nucleic acids first formed, it’s critical to consider:
- the polymerisation of these molecules (i.e., how they link up to form larger, more complex molecules);
- the formation of membranes, which offer a protective environment for these molecules;
- and the development of metabolic networks for power (photosynthesis and the use of ATP are just some examples that are used by living organisms today, but the early metabolic pathways for cells may no longer exist having been superseded by the pathways that exist today)
Some theories suggest that metabolism and/or membranes had to evolve first. For example, membranes offer a controlled environment, providing protection and selective access to nutrients. A lining of phospholipids that form a membrane would enable an emerging protocell to utilise transmembrane ion gradients, which is essential for metabolism in all existing cells. But this is based on an assumption that life requires water, which appears to be true on Earth, but may not be universal.
Different Pathways to Life & the Role of Symbiosis
This debate is further muddled by the possibility that the very first proteins and nucleic acids may have since gone extinct, replaced by the ones we see today. As well, life could have evolved multiple times, via different routes. Thus a combination of the options outlined above are a possibility. As it stands today, we have only the organisms whose records and ancestors survive now, 4.5 billion years after the formation of our Solar System, to base our conjectures on.
The various combinations are endless, and it would be near impossible to pinpoint with absolute certainty the recipe by which life evolved. Scientists are thus faced with finding the parameters within which life most likely evolved.
Many biologists have underscored the fact that all cell components rely on each other to function. In a way this solves our chicken-and-the-egg dilemma. As Paul Jarrett and collaborators wrote, “No self-replication of any cell component has ever been achieved in isolation, and the alternative may be that self-replication is impossible on a molecular self-sufficient basis, but rather dependent upon the interplay of disparate molecular assemblies.”
Where on Earth Did It All Start？
By narrowing down the range of conditions that can give rise to life, scientists may be able to find the most likely candidates for the first nucleotide and precursor amino acid combinations. There are a number of possible places of origin, each discussed further below in detail.
First, however, it’s important to know that early life on our planet experienced extremely different conditions than what living organisms today face. In its early stages, Earth was hot and its atmosphere lacked much oxygen. About 2.5 billion years ago, the global environment shifted dramatically with the Great Oxidation Event, when Earth moved from a reductive or neutral to an oxidative atmosphere around 2.5 billion years ago. So the first organisms to emerge on our planet were born into was a very different world than the one we see today.
Among the top possible locations for where life first began is along the hydrothermal vents that lie at the bottom of the world’s oceans. Ever since hydrothermal vents were discovered near the continental shelves of our planet, these hotspots have been considered as a place where life may have started, for several good reasons:
- Although hydrothermal vents reach scalding hot temperatures (up to 400°Celsius) that could make organic molecules unstable, some very primitive archaea organisms can be found in these extreme environments today, demonstrating that it is possible for organisms to withstand these extreme environments
- Hydrothermal vents offer energy gradients for budding organisms, when alkaline vent water mixes with more acidic, carbonic acid-rich seawater (since the early oceans were thought to contain larger amounts of carbon dioxide than they do at present).
- Hydrothermal vents create huge chimneys of calcium carbonate, which contain spores that could have served as a template for cells, creating a thin outer layer that separates the vent and sea water
There are some hurdles that life evolving in these extreme environments would face, though. These include:
- Relatively high sodium and low potassium ion concentrations of seawater compared to what’s typically observed inside cells; this underscores the question of how cells originating in seawater contain 10 times more potassium than sodium, when seawater contains has 40 times more sodium than potassium.
- Some scientists say that, at these locations, there may not be enough of a flux in conditions for the diverse components of cells to form (for example, wet-dry cycles that allow for water to be removed from the protocell)
- RNA nucleotides tend to be stable in the presence of UV light, suggesting that life could have first emerged in the presence of UV light, found on the earth’s surface but not in the depths of the sea.
One scenario that overcomes many of the issues that arise with the hydrothermal vent hypothesis is the occurrence of clay-lined fresh-water pools near volcanic land masses, which are often referred to as hydrothermal fields. In a number of ways, hydrothermal fields provide advantages over hydrothermal vents, including:
- How these pools experience wet–dry cycles, which would allow for increasingly complex interactions between emerging cell components
- The pools are confined, allowing for the right ingredients for cellular life to accumulate
- Exposure to light, which can act as a source of energy and power chemical reactions
Whether light helps or harms budding lifeforms has been somewhat debated. Some argue that light could destroy fragile, emerging molecules of life, yet recent experiments show that UV light can cause the right ingredients, such as hydrogen cyanide and hydrogen sulphide, to react and form nucleic acid precursors; what’s more, scientists involved in these experiments say that similar reactions could give rise to amino acids and lipids, hinting at a single set of reactions that could have given rise to most of life’s building blocks simultaneously.
Like the thermal pools discussed above, tidal pools exhibit some characteristics that could give rise to life. These pools yield evaporation–concentration cycles, a gradient in water activity, and high porosity. However, more recent research has suggested that the Earth’s early atmosphere may have been neutral, rather than reductive, which has prompted scientists to focus more on hydrothermal vents and fields.
What if Life Didn’t Start on Earth？
Despite the fact that our planet supports life so well, allowing organisms to flourish in many different forms, there’s still a good chance that life – or at least its precursors – originated from outside the boundaries of our world.
Many of the conditions discussed above that are amenable to the origins of life also occur in other areas of our solar system, such as Jupiter’s icy moon Europa and Saturn’s moon Enceladus. This raises the possibility that the precursors for life, or living organisms, were delivered to our planet from another source. This possibility falls within the reign of the panspermia hypothesis.
The Panspermia Hypothesis
Molecules that have the potential to found life have been found on meteorites, asteroids and even Mars. Many scientists suspect that certain moons within our solar system also harbour such organic molecules. Therefore, it’s not unreasonable to consider that the ingredients for life are abundant and widespread throughout our solar system, and even beyond.
According to the panspermia hypothesis, life is indeed abundant and travels from different planetary bodies, and is delivered, for example, via meteorites. Indeed, amino acids, as well as traces of water have been found in the interstellar medium, throughout our solar system and even in meteorites that impacted Earth. By hitching a ride on meteorites and other planetary bodies, life could move from one location to another throughout the solar system and even between galaxies, seeding new branches of life in some encounters.
Indeed, scientists are increasingly finding exoplanets (those outside of our solar system) that have the potential to support life. One such example is TRAPPIST-1, a collection of seven Earth-sized planets where up to three may possess conditions suspected to support life, albeit around a star that’s cooler and lower in mass than our Sun. Perhaps life as we know it originated on a faraway planetary body, such as the TRAPPIST-1 system, and was transferred to Earth via natural space debris.
In such a scenario, some life forms could remain dormant under the extreme conditions of space, and if delivered to a new home with life-supporting properties (e.g., an ocean) could once again become active and colonise their new environment.
In fact, some microbes have been found that support this hypothesis. It has been demonstrated that tardigrades, water-dwelling, eight-legged, micro-animals, are able to survive exposure to solar radiation while in space. However, organisms that are found deeper within rocks traveling through space have the best chance of survival, because they are shielded from ultraviolet radiation.
If some organisms are able to travel through space and seed life in new locations, this very well means that life could have begun outside of our solar system, in a galaxy far, far away. If this is the case, life could have begun before our solar system had been created, with our ancestor en route to Earth before it formed.
In fact, based on how life grows exponentially – from feedback factors such as gene cooperation, duplication of genes with their subsequent specialisation, and emergence of novel functional niches associated with existing genes – it’s conjectured that life may have originated about 9.7 billion years ago, if complexity of the non-redundant genome increases logarithmically with time. If the math stands true, life must have evolved before our solar system was formed, supporting the panspermia hypothesis.
The ways in which life may have first originated are vast and complex, making it difficult to pinpoint one method or location that kick started evolution. The number of possibilities highlighted in this chapter alone suggests that there are multiple pathways that could theoretically lead to life as we know it, and perhaps that is indeed the case. Furthermore, scientists suspect that all life on Earth came from one common ancestor, but what we see today are the survivors; therefore other forms of life could have come into existence but since went extinct, and hence remain undiscovered.
Scientists are studying the building blocks of life – how these membranes, proteins and nucleic acids possibly combine and work together to create life as we know it. Although these experiments have yielded valuable clues and insights into abiogenesis in recent years, scientists have yet to create life from non-living ingredients. They are also working backwards, by identifying the genes that are found across all known forms of life, to find the “common denominator.” But we will likely never be able to say with 100% certainty that life evolved in a specific manner. All we can do is continue to search for the most likely scenarios, combination of events, that created the ~1 trillion species that alive on Earth today.
- Dodd, MS. (2017). Evidence for early life in Earth’s oldest hydrothermal vent precipitates http://eprints.whiterose.ac.uk/112179/
- Florida State University. (2013). How life may have first emerged on Earth: Foldable proteins in a high-salt environment https://www.sciencedaily.com/releases/2013/04/130405064027.htm
- Jowett, Paul (2107). Myth and fact in the origins of cellular life on Earth https://academic.oup.com/biohorizons/article/doi/10.1093/biohorizons/hzx017/4693744
- Powner, Matthew (2009). Synthesis of activated pyrimidine ribonucleotides in prebiotically plausible conditions https://www.nature.com/articles/nature08013.epdf?referrer_access_token=LOGkVF2ZbVHAu8GT9sS9m9RgN0jAjWel9jnR3ZoTv0Ns48sWjoiZjOrCF2DQ96eJQBexsQ84KSPuVz83Vh8EHNwlGzbFiLh_NDkYk9FoVG0OVhZM-GQLaVaRNXBw54EDSNSyD3IO_6PVQyOPZQyrr33-czIGE_noH_dyL-dSRrCgBs6q8cs9nyyD
- Brazil, Rachel. Hydrothermal vents and the origins of life https://www.chemistryworld.com/feature/hydrothermal-vents-and-the-origins-of-life/3007088.article
- Sutherland, John D. (2015). Common origins of RNA, protein and lipid precursors in a cyanosulfidic protometabolism https://www.nature.com/articles/nchem.2202
- Kitadai, Norio (2017). Origins of building blocks of life: A review https://www.sciencedirect.com/science/article/pii/S1674987117301305
- Gordon, Sharov (2013). Life Before Earth https://arxiv.org/abs/1304.3381