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Origin of Life Theories

Origin of Life Theories

The Earth is estimated to be a little over 4.5 billion years old and organic life is estimated to have existed on Earth for much of that time, with recently discovered fossils of microorganisms dating back to between 3.8 and 4.3 billion years.

The origin of life, also known as abiogenesis, describes how living organisms first evolved from inorganic matter! The exact process by which this occurred is not yet known, though there are several theories.

We will cover these theories, including some that have been disproven and others that are currently being studied experimentally!

Origin of Life Theories in Biology

Studying the origin of life requires understanding how and why certain chemical reactions within inorganic matter could be capable of producing life. This is a cross-disciplinary study, encompassing portions of biology, chemistry, and physics. In the following, we will look at several theories on the origin of life, what is required for life, and what Charles Darwin's thoughts were on the origin of life.

Life Arose Spontaneously

The, now disproven, spontaneous theory states that life was produced from inorganic matter, such as mud, in the absence of any reactions. This was a very early theory on the origin of life, championed by Aristotle. Hundreds of years ago, experiments were conducted to determine the validity of this theory.

An experiment in 1668 by a scientist named Francesco Redi (1626-1697) utilized raw meat in order to determine the origin of maggots. He placed raw meat into two jars- one sealed and one unsealed (Fig. 1). He then waited a week and found that no maggots were produced from the sealed jar, but that maggots were found in the unsealed jar. Flies had access to the unsealed jar, thus revealing that maggots were produced from the eggs of flies. Several different experiments by other scientists followed, all providing further evidence that the theory was incorrect.

Origin of Life Theories Francesco Redi's experiment testing the spontaneous theory of life's origin. Study SmarterFigure 1: Francesco Redi's experiment testing the spontaneous theory of life's origin. Source: Wiki Commons

The Panspermia Theory of the Origin of Life

The panspermia theory, also known as the extraterrestrial theory, states that life may have begun elsewhere in the universe and brought to Earth via microorganisms present in a meteor or some other method of transportation. Panspermia means “all seeds” or “seeds everywhere”, implying that the seeds of life are present throughout the universe. Three kinds of panspermia have been proposed:

  1. Litho: life is transported throughout space when an impact on one planet causes rocks to be sent into space, moving to other planets in different solar systems.

  2. Ballistic: life is transported throughout space when an impact on one planet causes rocks to be sent into space, moving to other planets within the same solar system (Fig. 2).

  3. Directed: life is spread to other planets and solar systems intentionally by extraterrestrial civilizations with more advanced technology.

Origin of Life Theories The panspermia theory suggests that life arrived on Earth from elsewhere. Study SmarterFigure 2: The panspermia theory suggests that life arrived on Earth from elsewhere, potentially from ejected debris following the impact of a meteor on another planet. Source: Wiki Commons

Life Began in Ice

It is believed that, billions of years ago, much of the Earth was frozen and covered in thick ice. Such cold temperatures could have allowed for many of the compounds implicated in the formation of life to maintain stability. This stability could have then allowed these compounds to exist for much longer periods of time than under warmer conditions, potentially allowing for reactions to occur and life to emerge.

Life Began in Clay

The clay theory suggests that self-replicating crystals of clay may have given way to the formation of life by trapping molecules and enhancing their ability to replicate and catalyze reactions. This could have resulted in further replication, the emergence of life, and evolution to complex organisms. The theory was first proposed by a scientist named Alexander Graham Cairns-Smith in the 1960s.

The “RNA world” Theory of the Origin of Life

The “RNA world” theory states that life may have begun with molecules of RNA (Ribonucleic acid), which are able to perform self-replication and catalyze reactions. Over time, these molecules evolved to become more complex. This theory has some evidence to support it.

Like DNA, RNA can contain and process genetic information, but it can also catalyze reactions, which DNA cannot. Thus, RNA may have had the ability to independently support organic life. RNA could also, theoretically, pass genetic information down to the following generations and evolve. While the term “RNA world” was first coined by an American scientist named Walter Gilbert in 1986, the theory was originally presented by American biologist Alexander Rich in 1962. The "RNA world" theory is the most widely accepted theory on the origin of life today.

The Oparin-Haldane Theory of the Origin of Life

In 1924 a Russian scientist named Alexander Oparin and, in 1929, an English scientist named J. B. S. Haldane, proposed that life arose from inorganic matter mixed with other compounds (known as the primordial soup) under an oxygen-deprived (reducing) atmosphere and gradually evolved into more complex organisms over time. This is known as the Oparin-Haldane or heterotrophic theory of the origin of life. They suggested that this inorganic matter may have experienced reactions caused by lightning that resulted in the formation of amino acids and other important building blocks for the formation of life, thus creating the primordial soup. Reactions within this primordial soup could have then allowed for the formation of molecules of greater complexity, such as proteins, and eventually evolution into complex organisms.

The Miller-Urey Experiment

The Miller-Urey experiment was conducted in 1952 in order to test the validity of the Oparin-Haldane theory. The experiment, conducted by chemists Stanley Miller under the supervision of Harold Urey, revealed that the spontaneous production of organic molecules was possible under the oxygen-deprived conditions Oparin and Haldane suggested Earth may have been under at the time of life's origin.

They ended up building an enclosed environment containing heated water and various compounds (which may have been present in the Earth’s atmosphere at the time), and sent electrical currents through it, in order to initiate reactions through simulated lightning (Fig. 3). After a week, they discovered that organic molecules, such as amino acids and lipids, had formed. Although no complex molecules had formed, this experiment did show that such conditions could result in the formation of organic molecules.

Origin of Life Theories Diagram of the Miller-Urey experiments Study SmarterFigure 3: Diagram of the Miller-Urey experiment. Source: Wikipedia

Unfortunately, it is now believed that the Earth’s atmosphere was not oxygen deprived at the time life emerged, thus rendering the results of Miller-Urey’s experiment questionable in regard to its applicability to life's origins and casting doubt on Oparin-Haldane’s theory, at least the idea that oxygen deprivation played a role. However, since it was shown that organic molecules could indeed form from abiotic matter, this suggests that some portions of the Oparin-Haldane theory could be correct.

Life Began in Deep-Sea Vents

The deep-sea vents theory involves deep-sea hydrothermal vents, which are geologic structures that spew molecules that have abundant hydrogen. Billions of years ago, these molecules may have then clumped together and experienced chemical reactions, which may have resulted in the emergence of life. The world's oldest fossils, containing microorganisms and dating to between 3.8 and 4.3 billion years old, were discovered in hydrothermal vents in Quebec in 2017.

Timeline of Origin of Life Theories

As you can see above, there have been numerous theories on the origin of life. The following (Fig. 4) is a brief timeline looking at the timeline of some of the theories on the origin of life.

Origin of Life Theories A timeline of theories on the origin of life.  Study SmarterFigure 4: A timeline of theories on the origin of life. Source: Susanna O’Brien

Darwin’s Theory of the Origin of Life

While Charles Darwin’s theory of evolution, published in his book On the Origin of Species in 1859, is the scientist’s most famous contribution to science, he also had some ideas concerning the origin of life on Earth. Darwin never published his thoughts on the origin of life in any of his books, but he briefly discussed his ideas on the subject in a letter to naturalist Joseph Dalton Hooker in 1871. In a mere four paragraphs, he proposed that life likely began in a warm pond in which a protein was formed due to chemical processes and, eventually, evolved into more complex organisms.

This theory is, therefore, somewhat similar to the primordial soup theory proposed by Oparin and Haldane. Unfortunately, Darwin’s thoughts were limited by the scientific knowledge of the time, as it was not yet known that proteins were comprised of amino acids, and little was known about genetic processes or cellular structures. Despite this, his thoughts were very prescient, and a high density of chemicals in a small pond would, indeed, require a source of energy, such as light or heat, to drive reactions.

Origin of Life Theories - Key takeaways

  • The origin of life, also known as abiogenesis, describes how living organisms first evolved from inorganic matter.
  • The, now disproven, spontaneous theory states that life was produced from inorganic matter, such as mud, in the absence of any reactions.
  • The panspermia theory, also known as the extraterrestrial theory, states that life may have begun elsewhere in the universe and brought to Earth via microorganisms present in a meteor or some other method of transportation. There are three kinds of panspermia- litho, ballistic, and directed.
  • The “RNA world” theory states that life may have begun with molecules of RNA (Ribonucleic acid), which are able to perform self-replication and catalyze reactions. Over time, these molecules evolved to become more complex. The "RNA world" theory is the most widely accepted theory on the origin of life today.
  • The Oparin-Haldane theory states that life arose from inorganic matter mixed with other compounds (known as the primordial soup) under an oxygen-deprived (reducing) atmosphere and gradually evolved into more complex organisms over time.
    • The Miller-Urey experiment was conducted in 1952 in order to test the validity of the Oparin-Haldane theory.

Frequently Asked Questions about Origin of Life Theories

Studying the origin of life requires understanding how and why certain chemical reactions within inorganic matter could be capable of producing life. This is a cross-disciplinary study, encompassing portions of biology, chemistry, and physics. The “RNA world” theory states that life may have begun with molecules of RNA (Ribonucleic acid), which are able to perform self-replication and catalyze reactions. Over time, these molecules evolved to become more complex. This theory has some evidence to support it. The "RNA world" theory is the most widely accepted theory on the origin of life today.

The main theories are the panspermia theory, the theory that life began in ice, the theory that life began in clay, the "RNA world" theory of the origin of life, the Oparin-Haldane theory of the origin of life, and the theory that life began in deep-sea vents. 

The Oparin-Haldane theory proposes that life arose from inorganic matter mixed with other compounds (known as the primordial soup) under an oxygen-deprived (reducing) atmosphere and gradually evolved into more complex organisms over time. 

The "RNA world" theory is the most widely accepted theory on the origin of life today.

The “RNA world” theory states that life may have begun with molecules of RNA (Ribonucleic acid), which are able to perform self-replication and catalyze reactions. Over time, these molecules evolved to become more complex. This theory has some evidence to support it. 

Final Origin of Life Theories 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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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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What are the key steps for the origin of life, as outlined by the Oparin-Haldane hypothesis and adopted by many widely regarded origin of life on earth theories? 

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  1. Simple inorganic molecules abundant in the primordial soup mixed to form more complex molecules.
  2. Longer polymers formed. 
  3. Proto-cells or pre-cells formed as concentrations of organic material carrying out early metabolic processes.

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What is the main philosophy of the Oparin-Haldane Hypothesis? 

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Early life forms arose from chemical evolution. Where stepwise reactions from inorganic molecules to the organic building blocks of life occurred in the primordial soup. 

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What did the Miller-Urey experiment provide evidence for?

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The Miller-Urey Experiment provided the first evidence that organic molecules could come from inorganic molecules which is important in research on the origin of life on Earth.

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Prior to the findings of the Miller-Urey Experiment it was thought that ____ could only be produced by life forms. 

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organic compounds

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In 1953 American chemists Harold C. Urey and Stanley Miller set out to test the Oparin-Haldane Hypothesis. What did the Oparin-Haldane Hypothesis say about the evolution of life on earth?


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The Oparin-Haldane Hypothesis suggested life emerged from a series of step by step reactions between inorganic matter driven by a large energy input. These reactions initially produced the 'building blocks' of life (e.g., amino acids and nucleotides), then more and more complex molecules until primitive life forms arose.

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___ is defined as an oxygen-deprived atmosphere where oxidation can't occur, or occurs at very low levels. 

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reducing atmosphere

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___ is defined as an oxygen-rich atmosphere where molecules in the form of released gases and surface material are oxidized to a higher state. 

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oxidizing atmosphere

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How did Miller and Urey recreate the reducing primordial atmospheric conditions laid out by Oparin and Haldene?

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Miller and Urey attempted to recreate the reducing primordial atmospheric conditions laid out by Oparin and Haldane by combining water vapor, methane, ammonia, and molecular hydrogen in an enclosed environment. 

Then, they stimulated their faux atmosphere with electrical pules to simulate energy provided by lightning, UV rays or hydrothermal vents and left the experiment running to see if the building blocks for life would form. 

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The results of Miller and Urey's experiment ___ that

organic molecules could form under the conditions laid out in the Oparin-Haldane hypothesis. 

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proved

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The Miller-Urey experiment was modelled on, and recreated conditions laid out under what hypothesis?

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The Oparin-Haldane Hypothesis

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How has the Miller-Urey experiment been challenged in recent years?

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The Miller-Urey experiment was modelled on, and recreated conditions laid out under the Oparin-Haldane Hypothesis, primarily recreating the reducing atmospheric conditions the previous pair stipulated was crucial for the formation of early life. Scientists now think the earth's primordial atmosphere was comprised mainly of carbon dioxide and nitrogen, which is very different from the atmospheric makeup (with heavy ammonia and methane) that Miller and Urey recreated. 

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What happened when other scientists attempted to recreate the Miller-Urey experiment using more accurate gaseous mixtures?

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Whilst their experiments similarly produced amino acid, they noticed nitrates forming in the product. These nitrates were able to break down amino acids as quickly as they formed, yet in the conditions of primordial earth, iron and carbonate minerals would have reacted with these nitrates before they had the chance to do so.

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Why are some scientists unconvinced that life originated from chemical evolution alone?

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The Miller-Urey Experiment failed to produce all the building blocks needed for life - some complex nucleotides have yet to be produced even in subsequent experiments. 

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The Miller-Urey experiment led to the birth of what field in chemistry?

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Pre-biotic chemistry

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