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Origin of Life on Earth

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Origin of Life on Earth

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?

Origin of Life on Earth, Timeline, StudySmarterFigure 1: A rough timeline of the origins of life, from the formation of the earth to the present day. Source: Harvard University.

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:

  1. 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.

  2. 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.

  3. 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.

  1. 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.
  2. Smithsonian, Early Life on Earth – Animal Origins, National Museum of Natural History
  3. Harold S Bernhardt, The RNA world hypothesis: the worst theory of the early evolution of life (except for all the others), Biology Direct, 2012.
  4. Cyril Ponnamperuma, Chemical Evolution and the Origin of Life, Nature, 1964.
  5. Joshua Jortner, Conditions for the emergence of life on the early Earth: summary and reflections, Philosophical transactions of the Royal Society of London, 2006.
  6. Cooper GM, The Origin and Evolution of Cells, The Cell: A Molecular Approach. 2nd edition, 2000.
  7. Christie Wilcox, Evolution: Out Of The Sea, Scientific American, 2012.
  8. Ker Than, All Species Evolved From Single Cell, Study Finds, National Geographic News, 2010.

Frequently Asked Questions about Origin of Life on Earth

The earliest lifeforms are suspected to have been simple unicellular microbes which formed at least 3.7 billion years ago from the primordial molecular soup.

There are many theories for the origin of life on earth, though it is widely regarded that the primordial molecular soup and energy from thermal vents allowed for spontaneous chemical reactions and the emergence of RNA.

Biogeochemical analysis dates carbon on rocks, thought to have come from the earliest life forms, back 3.7 billion years ago.

Life on earth started in the ‘primordial molecular soup’. This was an aqueous mixture protected from the suns UV rays containing chemicals which would eventually react to produce the organic molecules crucial to biological life.

The ‘gene first’ hypothesis states the first life form on earth was encapsulated self-replicating RNA. Whereas the ‘metabolism first’ hypothesis claims the first life forms were continuously repeating chemical reactions which eventually gave rise to complex molecules like proteins.

Final Origin of Life on Earth Quiz

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The origin of life is also known as

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Abiogenesis

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The spontaneous theory is...

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disproven

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The panspermia theory is also known as

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The extraterrestrial theory

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The three kinds of panspermia that have been proposed are...

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Litho

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If life is transported throughout space due to an impact on one planet causing rocks to be sent into space, moving to other planers within the same solar system, this would be called...

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Ballistic panspermia

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The clay theory suggests that...

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Self-replicating crystals of clay may have given way to the formation of life by trapping molecules.

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The term “RNA world” was first coined by which of these scientists?

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Walter Gilbert

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Alexander Oparin and J. B. S. Haldane proposed that life arose from inorganic matter mixed with other compounds to form what is commonly called...

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Primordial soup

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______________ was conducted in 1952 in order to test the validity of the Oparin-Haldane theory.

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The Miller-Urey experiment

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At the end of the Miller-Urey experiment, it was found that...

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Organic molecules, such as amino acids and lipids, had formed.

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Oparin and Haldane suggested that, at the time of life's origin, Earth may have been under __________ conditions.

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Oxygen reduced

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Darwin proposed that life likely began in a(n)...

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Small, warm pond.

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True or False: Darwin discussed the origins of life extensively in his published works.

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False

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How old is the Earth estimated to be?

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A little over 4.5 billion years old.

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The oldest fossils date back to...

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3.8-4.3 billion years ago.

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The study of the origin of life is...

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Cross-disciplinary 

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What is the most widely accepted theory on the origin of life?

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The "RNA world" theory

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Where were the world's oldest fossils discovered?

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Hydrothermal vents

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True of False: Like RNA, DNA can catalyze reactions.

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False

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How long ago do scientists think the first life forms emerged? 

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ROughly 3.7 billion to 4 billion years ago. 

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How do scientists commonly refer to the conditions life is thought to have emerged from? 

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Primordial Soup

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What might have provided the energy for early spontaneous chemical reactions within the primordial soup? 

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Hydrothermal vents

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_____ is the name commonly given to the theory that suggested the earliest life forms were not made up of genetic material and instead were self-regulating continuous chemical reactions. 

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The 'Metabolism-first' hypothesis 

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What is the key difference between the metabolism-first and the gene-first hypothesis? 

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The gene-first hypothesis suggests small amounts of genetic material made up the earliest life forms. Scientists who back this theory tend to also support the RNA World Hypothesis. 


The 'Metabolism-first' theory argues genetic material is made up of much too complex molecules to be the initial life forms. Instead scientists in this party believe self-regulating continuous chemical reactions were the earliest life forms. These chemical reactions were thought to produce increasingly complex molecules and eventually give rise to materials for a membrane and DNA & RNA. 

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What does the Chemical Origin of Life refer to? 

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The environmental conditions which made the emergence of life possible, and the chemical reactions and biological processes which may have been possible at the time. 

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The origins of life are thought to have emerged from aquatic environments because....

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Aquatic environments provided easy access to a jumble of molecules that have been proven most likely to give rise to life. 


The early atmosphere is also thought to have lacked an ozone layer, so UV rays would have been highly damaging outside of water. 

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A critical part of life is the ability to _______ . As this allows for genetic material to pass on to the next generation and Darwinian evolution to occur. 


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Reproduce

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The ______________ suggests life arose from simple inorganic molecules which underwent reactions to form more complex molecules within the primordial soup. 


The __________ supports this by showing 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.

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1. Oparin-Haldane Hypothesis


2. Miller-Urey Experiment

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The origins of which complex metabolic pathway are thought to have arisen first? 

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Glycolysis

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Why were cellular metabolic processes beneficial for early life forms? 

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The first cells were unicellular and surrounded by the organic molecules they needed for food and 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.

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What name do we give to the ancestor of all modern day life on earth? 

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LUCA (Last Universal Common Ancestor)

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What evidence is there for the 'universal common ancesor' theory proposed by Charles Darwin? 

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All species of the 3 domains (Archaea, Bacteria, and Eukarya) share 23 universal proteins. Whilst the DNA sequences which encode these proteins vary slightly across the domains they are very similar.


These 23 proteins are essential for life as they underpin many fundamental biological and cellular processes. 


The ‘universal common ancestor’ theory explains the minor differences as 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. 

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Which kingdom is credited with facillitating the biggest changes in earths environment and paving the way for further evolution and migration of other species? 

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The Plant Kingdom / Plantae

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In what year did Haldane first put forward his origin of life through abiogenesis hypothesis? 

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1929

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In what year did Oparin first put forward his origin of life through abiogenesis hypothesis? 

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1924

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Oparin and Haldane proposed their theories on the origin of life through abiogenesis ______

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Together

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What elements were unique to Oparins proposal on the origin of life? 

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Oparin believed the earliest life forms developed from coacervates.

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What is a coacervates?

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A small liquid droplets made up of 2+ different liquids that when mixed will not form a homogenous solution (a solution where the same share of components are found throughout).

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Under what atmospheric conditions did both Oparin and Haldane believe life to have arisen under?

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Reducing Atmosphere

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Where did both Haldane and Oparin believe life to have arisen?

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The Oceans

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Which did Oparin and Haldane believe to be true of early life? 

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Early life was heterotrophic

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How is Abiogenesis defined? 

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Abiogenesis is the act of creating life from non-life. Abiogenesis refers to the idea that life could have evolved from inorganic matter or non-living substances. 

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What is the difference between abiogenesis and spontaneous generation?

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In modern-day terms abiogenesis refers to the creation of very simple life from non-living matter. Spontaneous generation, however, refers to the disproven theory that complex life arises "spontaneously" and "continuously" from non-living matter. 

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What were the main differences in Oparin's and Haldane's proposals for the origin of life?

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At its most basic the difference between the two scientists' proposals boils down to when cells formed. 


Oparin was unfamiliar with Haldanes work on coacervates and proposed pre-cells formed very quickly, held together by electrostatic forces and this proximity drove further complexity of life and molecules. 


Haldane proposed more complex molecules formed first, and then membrane-bound cells evolved.

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When was the Miller-Urey experiment conducted?

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1924

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What four gases were combined in the Miller-Urey experiment to recreate primordial conditions laid out in the Oparin-Haldane hypothesis?

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  1. Water vapour,
  2. Methane,
  3. Ammonia,
  4. Molecular hydrogen. 

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What is significant about the Miller-Urey Experiment?

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Provided the first evidence that some organic molecules (e.g. amino acids, lipids) could arise from inorganic molecules given the right conditions and an energy source. 

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What are the two main weaknesses of the Oparin-Haldane Hypothesis?

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  1. Earth's primordial atmosphere is no longer considered to be reducing. 
  2. As our understanding of genetic material grows, a Coacervates-first model for the evolution of the origin of life on earth appears unlikely. 

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In the modern day, what place does Coacervation hold in the origin of life on earth?

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Coacervates were initially proposed as the first life forms to arise by Oparin. However this was before the discovery of DNA and RNA as genetic material, and their role in the cell and life in general. Nowadays Coacervates are thought unlikely to have been the very first life forms, but the method of Coacervation can be encorporated into many other Origin of Life Theories as further evidence or explanation. 

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Following recent discoveries which of the following are thought to have been abundant components in earth's early atmosphere? 

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Water Vapour

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