Will we ever know how life began? Whilst we will probably never know with absolute certainty how the origins of life came to be, scientists are continuously finding new evidence to support existing theories or propose new ones, shedding light on early life. Whilst the origins of life are very hypothetical, these hypotheses and theories can help us ask the right questions and explore aspects of evolution and answer the big question: are we alone in the universe?
Timeline of the origin of life on Earth
How did we go from the simple unicellular microorganisms found when life originated to the plethora of biodiversity we see on earth today? By examining the fossil record and examining specimens of DNA and RNA, scientists can make educated hypotheses to explain the age-old question of how life came to be. In fact, biochemical analysis of carbon left behind on rocks suggests the first life appeared ~3.7 billion years ago. But how did this very first life form come to be?
A rough timeline of the origins of life from the formation of the earth till the present day | StudySmarter Original
After earth formed ~4.5 billion years ago, the atmosphere’s chemical composition underwent vast fluctuations until it became stable enough to conceive the beginnings of life. In the beginning stages of life there was a jumble of molecules and chemicals within water sources often referred to as the ‘primordial molecular soup’. Between 4.5 and 3.7 billion years ago within the mess of primordial soup there is thought to have been enough energy, perhaps from hydrothermal vents or lightning, to cause spontaneous chemical reactions allowing for the emergence of the first RNA molecules.
Time continued to pass and the RNA and chemicals within the molecular soup gradually became more complex, eventually becoming enveloped within a membrane and forming the first cells. If we look again to the fossil and geochemical records, these first unicellular organisms are thought to have emerged at least 3.7 billion years ago.
There are competing theories and hypotheses describing the nature of the first life forms, and the emergence of complex molecules and RNA. The ‘gene first’ hypothesis stipulates that ‘self-replicating’ RNA made up the first life forms with additional components and chemicals incorporated later.
Whereas the ‘metabolism first’ hypothesis claims metabolic or chemical reactions, which could continuously occur thanks to their self-sustaining nature and abundance of reactants, may have been simple life forms prior to the emergence of RNA.
From this point, life began to diverge. Different life forms gained the ability to undertake different chemical reactions and biological processes. The processes and reactions each organism could undertake would ultimately determine its characteristics, structure and growth factors, which in turn determined the environments it could inhabit. It is in this way that life progressed from the very first microbe to the plethora of biodiversity visible on our planet today (Fig. 1).
Chemical origin of life on Earth
When asking how life on earth originated, it's useful to think about the environmental conditions that may have made life possible. This is known as the chemical origin of life and provides us with clues to the chemical and physical reactions which may have occurred.
Life began in water
Early life is thought to have originated in very anaerobic conditions with little to no ozone layer. In these early days of the earth's geological history, UV rays would have caused severe radiation damage to anything they touched. Hence, the origins of life on earth are thought to have occurred in the oceans, or at the very least under a couple of centimeters of water which would deflect most of the harmful UV rays.
Simple molecules as a precursor for life
A critical part of life is the ability to reproduce. Whether through sexual reproduction or self-replication, all cells and living organisms can reproduce. Therefore, the ability to replicate oneself is essential for the formation of initial life on Earth, and the molecules life sprung from. Chemical experiments have shown that organic molecules, complex carbon-containing molecules found within living systems, can spontaneously form in conditions similar to earth's early anaerobic atmosphere with a little bit of energy. This energy could have been provided by sunlight, lightning or heat from hydrothermal vents.
This hypothesis, known as the Oparin-Haldane Hypothesis and backed up partially by the Miller-Urey Experiment, suggests a spontaneous stepwise transformation of atoms and molecules to the more complex chemicals which underpinned early life.
The Oparin-Haldane Hypothesis discussed life originating from an oxygen-deprived environment. However, more recent geochemical analysis has shown this is probably not a match for earth's primordial atmosphere. This has cast doubt on the accuracy of the Oparin-Haldane hypothesis, and the applicability of the Miller-Urey experiment (which was carried out under the conditions set out by the Oparin-Haldane hypothesis).
The Miller-Urey experiment was however the first of its kind to prove organic molecules could form from inorganic matter, as suggested in the Oparin-Haldane hypothesis. Many scientists now think the chemical evolution suggested in the hypothesis at least is correct, even if it occurred under different atmospheric conditions.
From the resulting ‘molecular soup’ RNA nucleotides are thought to have emerged. Crucially, RNA can be self replicated. Over millions of years of mixing around, RNA is thought to have given rise to DNA. This theory of the origin of life is known as the RNA World Hypothesis, and is the most widely accepted origin of life theory by the scientific community. The first cell is thought to have simply been a jumble of self-replicating RNA contained within a membrane.
The nature of the origins of life on Earth
The first cells were unicellular and surrounded by the organic molecules they needed for energy. These required molecules were abundant in the environment and could simply diffuse through the membrane of the cell. As life evolved and became more complex, systems were needed for cells to produce their own energy, rather than sourcing it straight from their environment. This is thought to have happened in three key stages:
Under primordial anaerobic conditions, early cells needed to produce energy without the use of oxygen. It is at this stage initial pathways for glycolysis were laid out. Glycolysis converts organic molecules into ATP which can be used as an energy source for other metabolic and cellular processes.
Cells developed the ability to perform photosynthesis, allowing them to harness sunlight for energy without the need for external organic molecules. Photosynthesis is thought to have evolved in bacteria.
The development of photosynthesis increased the amount of available O2 in the atmosphere. This gave rise to the evolution of oxidative metabolism and cellular respiration, which is far more efficient at converting organic molecules into ATP than glycolysis, but does require oxygen.
Evolution since the origins of life on Earth
Whilst the origins of life itself are hotly contested throughout the scientific community, it is mostly agreed that all life we see today stems from a single common ancestor. This common ancestor formed roughly 3.5 billion years ago as a single-celled microorganism commonly referred to as LUCA (Last Universal Common Ancestor).
The ‘universal common ancestor’ theory was first proposed by Charles Darwin in his book ‘On the Origin of Species’. Whilst this has several adversaries in the form of the ‘multiple ancestry hypothesis’, the ‘universal common ancestor’ theory is the most widely backed due to supporting statistical and computation analysis’ which highlights this theory as much more likely, statistically speaking.
This is because all species of the three domains (Archaea, Bacteria, and Eukarya) share 23 universal proteins. The DNA sequences which encode these proteins vary slightly across the domains, though for the most part, they are very similar. These 23 proteins are essential for life, as they underpin many fundamental biological and cellular processes. Through the ‘universal common ancestor’ theory, the minor differences can be explained by a couple of mutations. However, if these 23 proteins were to have evolved independently, through convergent evolution, many more mutations would be required and there would likely be far more variation between the proteins than there are.
From LUCA life on earth flourished in the oceans, and eventually moved to land. This boom in life was enabled by the development of the complex metabolic processes described above. These processes allowed early life forms to expand their niches and occupy new habitats, and the gases released contributed to the change in the earth's atmosphere through time.
The biggest changes to the earth's environment, climate and atmosphere were facilitated by plant evolution. Once on land, plants paved the way for animals and other life forms to follow by changing the terrain to be more habitable. As autotrophs, plants provided a source of energy in dry terrestrial environments where crucial nutrients could not be obtained through osmosis. Unlike plants, animals and other life forms made several jumps back and forth from aquatic to dry land.
Origin of life on Earth summary
The many competing hypothesis of the origins of life on earth can be hard to wrap your head around, and we will likely never know for sure which if any of them are true. However it is now widely believed amongst the scientific community that some form of chemical evolution, as laid out in the Oparin-Haldane Hypothesis, produced the building blocks for RNA to form, as noted in the RNA world hypothesis. The conditions under which this occurred have been greatly contested through time, though many now believe life's origins began under an oxygen rich environment within a body of water.
Origin of Life on Earth - Key takeaways
- The earth formed roughly 4.5 billion years ago, but life wasn’t to emerge for at least another half a billion years.
- The gene first hypothesis claims the earliest life forms were self replicating RNA, whilst the metabolism first hypothesis were self sustaining chemical reactions which gave rise to more and more complex molecules.
- The first life form is presumed to have formed in an aquatic environment, like the primordial molecular soup.
- All life found on earth today is descended from LUCA.
- After the emergence of the first life forms and LUCA, different descendants developed the ability to undergo more complex metabolic processes.
- Addy Pross and Robert Pascal, The origin of life: what we know, what we can know and what we will never know, Open Biology, 2013.
- Smithsonian, Early Life on Earth – Animal Origins, National Museum of Natural History
- Harold S Bernhardt, The RNA world hypothesis: the worst theory of the early evolution of life (except for all the others), Biology Direct, 2012.
- Cyril Ponnamperuma, Chemical Evolution and the Origin of Life, Nature, 1964.
- Joshua Jortner, Conditions for the emergence of life on the early Earth: summary and reflections, Philosophical transactions of the Royal Society of London, 2006.
- Cooper GM, The Origin and Evolution of Cells, The Cell: A Molecular Approach. 2nd edition, 2000.
- Christie Wilcox, Evolution: Out Of The Sea, Scientific American, 2012.
- Ker Than, All Species Evolved From Single Cell, Study Finds, National Geographic News, 2010.
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