3.3.2: Origins of Life

Unit 3.3.2 - Origins of Life

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Learning Objectives

• Explain the different problems to solve and scientific hypotheses on the origins of life 

The Origins of Life - A Mystery to Solve

To account for the origin of life on our earth requires solving several problems:

1. How did the organic molecules that define life, e.g. amino acids, nucleotides, originate?
2. How were macromolecules assembled, e.g. proteins and nucleic acids, - a process requiring catalysts?
3. How were macromolecules able to reproduce themselves?
4. How were these macromolecules assembled into a system separate from their surroundings (i.e., a cell)?

A number of theories address each of these problems.

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Problem 1: How did the organic molecules that define life, e.g. amino acids, nucleotides, originate?

As for the first problem, four scenarios have been proposed. 

• Scenario 1: Organic molecules were synthesized from inorganic compounds in the atmosphere,
• Scenario 2: Organic molecules rained down on the Earth from outer space,
• Scenario 3: Organic molecules were synthesized at hydrothermal vents on the ocean floor,
• Scenario 4: Organic molecules were synthesized when comets or asteroids struck the early earth, AND/OR

Scenario 1: Organic molecules were synthesized from inorganic compounds in the atmosphere

Stanley Miller, a graduate student in biochemistry, built the apparatus shown in Figure \(\PageIndex{1}\). He filled it with water (H₂O), methane (CH₄), ammonia (NH₃) and hydrogen (H₂), but no oxygen. He hypothesized that this mixture resembled the atmosphere of the early earth. The mixture was kept circulating by continuously boiling and then condensing the water. The gases passed through a chamber containing two electrodes with a spark passing between them.

Figure \(\PageIndex{1}\): Miller experiment. (CC BY-SA; Yassine Mrabet).

At the end of a week, Miller used paper chromatography to show that the flask now contained several amino acids as well as some other organic molecules. However, it is now thought that the atmosphere of the early Earth was not rich in methane and ammonia - essential ingredients in Miller's experiments. In the years since Miller's work, many variants of his procedure have been tried. Virtually all the small molecules that are associated with life have been formed:

• 17 of the 20 amino acids used in protein synthesis, and all the purines and pyrimidines used in nucleic acid synthesis.
• But abiotic synthesis of ribose - and thus of nucleotides - has been much more difficult. However, success in synthesizing pyrimidine ribonucleotides under conditions that might have existed in the early earth was reported in the 14 May 2009 issue of the scientific journal Nature.
• And in 2015, chemists in Cambridge England led by John Sutherland reported that they had been able to synthesize precursors of 12 of the 20 amino acids and two (of the four) ribonucleotides used by life as well as a precursor of lipids. They created all of these molecules using only hydrogen cyanide (HCN) and hydrogen sulfide (H₂S) irradiated with ultraviolet light in the presence of mineral catalysts.

Scenario 2: Organic molecules rained down on the Earth from outer space

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Astronomers, using infrared spectroscopy, have identified a variety of organic molecules in interstellar space, including methane (CH₄), methanol (CH₃OH), formaldehyde (HCHO), cyanoacetylene (HC₃N) (which in spark-discharge experiments is a precursor to the pyrimidine cytosine), polycyclic aromatic hydrocarbonsas well as such inorganic building blocks as carbon dioxide (CO₂), carbon monoxide (CO), ammonia (NH₃), hydrogen sulfide (H₂S), and hydrogen cyanide (HCN).

There have been several reports of producing amino acids and other organic molecules in laboratories by taking a mixture of molecules known to be present in interstellar space such as ammonia (NH₃), carbon monoxide (CO), methanol (CH₃OH) and water (H₂O), hydrogen cyanide (HCN) and exposing it to a temperature close to that of space (near absolute zero) and intense ultraviolet (UV) radiation. Whether or not the molecules that formed terrestrial life arrived here from space, there is little doubt that organic matter continuously rains down on the earth (estimated at 30 tons per day).

Alternatively, organic molecules can be transported to Earth via meteorites as demonstrated with the Murchison Meteorite that fell near Murchison, Australia on 28 September 1969. This meteorite turned out to contain a variety of organic molecules including purines and pyrimidines and 12 different amino acids, including some not found on Earth. The amino acids and their relative proportions were quite similar to the products formed in Miller's experiments.

Murchison meteorite at the The National Museum of Natural History (Washington). (CC SA-BY 3.0; Basilicofresco).

Scenario 3: Organic molecules were synthesized at hydrothermal vents on the ocean floor

Some deep-sea hydrothermal vents discharge copious amounts of hydrogen, hydrogen sulfide, and carbon dioxide at temperatures around 100°C. (These are not "black smokers".) These gases bubble up through chambers rich in iron sulfides (FeS, FeS₂). These can catalyze the formation of simple organic molecules like acetate. (And life today depends on enzymes that have Fe and S atoms in their active sites.)

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This 4-minute video

Scenario 4: Organic molecules were synthesized when comets or asteroids struck the early earth

Researchers in the Czech Republic reported in 2014 that they had succeeded in the abiotic synthesis of adenine (A), guanine (G), cytosine (C), and uracil (U) — the four bases found in RNA (an RNA beginning?) and three of the four found in DNA. They achieved this by bombarding a mixture of formamide and clay with powerful laser pulses that mimicked the temperature and pressure expected when a large meteorite strikes the earth. Formamide is a simple substance, CH₃NO, thought to have been abundant on the early earth and containing the four elements fundamental to all life.

Problem 2: How were macromolecules assembled?

Another problem is how macromolecules, the basis of life itself, could be assembled.

This has led to a theory that early polymers were assembled on solid, mineral surfaces that protected them from degradation, and in the laboratory polypeptides and polynucleotides (RNA molecules) containing about ~50 units have been synthesized on mineral (e.g., clay) surfaces.

All metabolism depends on enzymes and, until recently, every enzyme has turned out to be a protein. But proteins are synthesized from information encoded in DNA and translated into mRNA. So here is a chicken-and-egg dilemma. The synthesis of DNA and RNA requires proteins. So proteins cannot be made without nucleic acids and nucleic acids cannot be made without proteins. The discovery that certain RNA molecules have enzymatic activity provides a possible solution. These RNA molecules — called ribozymes — incorporate both the features required of life: storage of information and the ability to act as catalysts. This leads to what is known as the RNA World Hypothesis.

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While no ribozyme in nature has yet been found that can replicate itself, ribozymes have been synthesized in the laboratory that can catalyze the assembly of short oligonucleotides into exact complements of themselves. The ribozyme serves as both the template on which short lengths of RNA ("oligonucleotides" are assembled following the rules of base pairing and the catalyst for covalently linking these oligonucleotides.

In principal, the minimal functions of life might have begun with RNA and only later did proteins take over the catalytic machinery of metabolism and DNA take over as the repository of the genetic code. Several other bits of evidence support this notion of an original "RNA world":

• Many of the cofactors that play so many roles in life are based on ribose; for example:
  • ATP
  • NAD
  • FAD
  • coenzyme A
  • cyclic AMP
  • GTP
• In the cell, all deoxyribonucleotides are synthesized from ribonucleotide precursors.
• Many bacteria control the transcription and/or translation of certain genes with RNA molecules, not protein molecules.

Problem 3: How were macromolecules able to reproduce themselves?

Perhaps the earliest form of reproduction was simple fission of the growing aggregate into two parts - each with identical metabolic and genetic systems intact.

Problem 4: How were these macromolecules assembled into a system separate from their surroundings (i.e., a cell)?

To function, the machinery of life must be separated from its surroundings - some form of extracellular fluid (ECF). This function is provided by the plasma membrane. Today's plasma membranes are made of a double layer of phospholipids. They are only permeable to small, uncharged molecules like H₂O, CO₂, and O₂. Specialized transmembrane transporters are needed for ions, hydrophilic, and charged organic molecules (e.g., amino acids and nucleotides) to pass into and out of the cell.

However, the same Szostak lab that produced the RNA World Hypothesis discovered that fatty acids, fatty alcohols, and monoglycerides - all molecules that can be synthesized under prebiotic conditions - can also form lipid bilayers and these can spontaneously assemble into enclosed vesicles.

Unlike phospholipid vesicles, these

• admit from the external medium charged molecules like nucleotides
• admit from the external medium hydrophilic molecules like ribose
• grow by self-assembly
• are impermeable to, and thus retain, polymers like oligonucleotides.

Here, then, is a simple system that is a plausible model for the creation of the first cells from the primeval "soup" of organic molecules.