7.3: Origins of Life

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)?

Several hypotheses address each of these problems.

Video

This 13-minute video provides an overview of the origins of life, exploring along the way "what is life"?
Question after watching: "Life is what living things do" (in contrast to what living things have). What does this mean? Do you agree with this statement based on what you know so far from this course?

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

For the first problem, four hypotheses have been proposed:

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

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

In the fall of 1951, Stanley Miller was a graduate student looking for a research project. He attended a lecture by Nobel Laureate Harold C. Urey. Urey described how the earth’s atmosphere was thought to be in its early formation and how it might have led, through chemical reactions, to the formation of the first organic molecules. Miller decided to test this idea. He created an apparatus that mimicked the earth’s early conditions and atmosphere (or at least, what was thought, in 1951, to be the earth’s early conditions). The apparatus is shown in Figure \(\PageIndex{1}\). He filled it with water (H2O), methane (CH4), ammonia (NH3), and hydrogen (H2) but no oxygen since before life existed no oxygen had been released in the atmosphere by photosynthesis. The mixture circulated in the system by continuously boiling and then condensing/cooling the water. The gases passed through a chamber containing two electrodes with a spark passing between them, simulating lightening. 

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

At the end of the week-long experiment, Miller used paper chromatography to analyze the chemicals found in the flask. He discovered it now contained several amino acids (glycine, alanine, and aspartic acid) as well as other organic molecules, such as gamma-amino butyric acid or GABA (a chemical now used by many animals as a neurotransmitter).

It is now thought that the atmosphere of the early Earth was different than Miller supposed. However, in the years since Miller's work, many variants of his procedure have been tried with different mixtures of gases as our understanding of the early Earth's composition has changed. In nearly all versions of the experiment, organic molecules have almost always been formed, including the following results:

• 17 of the 20 amino acids used in protein synthesis, and all the purines and pyrimidines used in nucleic acid synthesis can be created in experiments similar to Miller's. They are easy to create by chemical reactions outside of living organisms. 
• The synthesis of ribose, and thus of nucleotides, has been much more difficult outside of living organisms. 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. Thus, the synthesis of these molecules outside of a living organism is possible.
• In 2015, chemists in Cambridge, England led by John Sutherland reported that they had synthesized precursors for 12 of the 20 amino acids used in protein synthesis, two (of the four) ribonucleotides used to make RNA, and a precursor of lipids - all from a non-living system. In fact, they created these molecules using only hydrogen cyanide (HCN) and hydrogen sulfide (H₂S) irradiated with ultraviolet light in the presence of mineral catalysts.

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

Video

This 4 min video provides an overview of the hypothesis that the molecules of life may have been brought to earth from outer space.
Question after watching: What is the evidence for the hypothesis of Panspermia?

Astronomers, using infrared spectroscopy, have identified a variety of organic molecules in interstellar gas clouds, including methane (CH₄), methanol (CH₃OH), formaldehyde (HCHO), cyanoacetylene (HC₃N) (which in Miller-type experiments is a precursor to the pyrimidine cytosine), polycyclic aromatic hydrocarbons (PAHs), as well as other important molecules such as carbon dioxide (CO₂), carbon monoxide (CO), ammonia (NH₃), hydrogen sulfide (H₂S), and hydrogen cyanide (HCN). There are organic molecules in space. These could have found their way to earth and provided the building blocks of life.

Several laboratories have reported producing amino acids and other organic molecules by taking a mixture of molecules known to be present in interstellar space such as ammonia (NH₃), carbon monoxide (CO), methanol (CH₃OH), water (H₂O), and hydrogen cyanide (HCN) and exposing it to temperatures close to that of space (near absolute zero) and intense ultraviolet (UV) radiation.  In other words, there is nothing special about creating organic molecules – they arise in the universe through the conditions found in space. They do not require living systems to create them. 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). These could have served as the building blocks for the first living organism.

Alternatively, organic molecules can be transported to Earth via meteorites as demonstrated by the Murchison Meteorite that fell near Murchison, Australia on 28 September 1969 (Figure \(\PageIndex{2}\)). This meteorite contains many organic molecules including purines and pyrimidines and 12 different amino acids. The relative proportion of each amino acid in the meteorite mirrored those found as a result of Miller's experiments. This suggests that abiotic chemical reactions produce these organic molecules easily and reliably, and whether they were formed on the early earth or is space, they could have served to construct the first living organism.

Figure \(\PageIndex{2}\): Murchison meteorite at The National Museum of Natural History (Washington). (CC SA-BY 3.0; Basilicofresco)

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

Another location suspected of serving as a cradle for life are deep-sea hydrothermal vents. These vents would have provided a relatively stable environment for life to begin while the early planet was bombarded by meteorites. Some deep-sea hydrothermal vents discharge copious amounts of hydrogen, hydrogen sulfide, and carbon dioxide at temperatures around 100°C. The gases bubble up through chambers rich in iron sulfides (FeS, FeS₂). The iron sulfide surfaces can facilitate chemical reactions (i.e., serve as catalysts) that lead to the formation of simple organic molecules like acetate. Note also that life today depends on enzymes that have Fe and S atoms in their active (catalytic) site, so these iron sulfide minerals could have been a precursor for today’s living system enzymes.

Video

This 5-minute video provides an overview of the hypothesis that life began at hydrothermal vents.
Question after watching: What are some characteristics of LUCA? How do we know that this organism had these characteristics?

Hypothesis 4: Organic molecules were synthesized when comets or asteroids struck the early Earth

The ingredients for the creation of organic molecules include basic inorganic chemicals like carbon dioxide (CO2) or hydrogen cyanide (HCN), but the rearrangement of their atoms into organic molecules also requires an infusion of energy. In the hypotheses we have investigated thus far, the energy came from lighting, chemical gradients, or UV light. In this hypothesis, the energy comes from the impact of meteorites with 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 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. It could serve as a basis for the creation of many of the organic molecules used by living systems today. Thus, the meteorite impact may have sparked life on earth.

Problem 2: How were macromolecules assembled?

Once the organic molecules of life are created (monomers), the next step in the creation of life is to assemble them into large macromolecules (polymers). For example, nucleotides need to be assembled into RNA or DNA; and amino acids need to assemble into proteins.

Researchers who study this problem think that early polymers were assembled on solid, mineral surfaces These surfaces worked like catalysts, holding the monomers in the right orientation to facilitate their assembly into larger polymers, and once formed the mineral surface held on to the polymer and in so doing, protected them from degradation. In the laboratory, polypeptides and polynucleotides (RNA molecules) containing about ~50 monomers linked together have been synthesized on mineral (e.g., clay) surfaces. 

Today, all living organisms have metabolism to create more of themselves from simpler building blocks and this activity depends on enzymes. Until recently, every enzyme science knew off was 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 on their own 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.

Video

This 3-minute video describes the RNA World Hypothesis.
Question after watching: In your notes, write a short description of the RNA world hypothesis and the argument for it. How could natural selection operate on the molecular level (as demonstrated in the RNA World Hypothesis)?

While no ribozyme in nature has yet been found that can replicate itself, ribozymes have been synthesized in the laboratory (using artificial selection) 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 principle, life might have begun as a self-replicating RNA. 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 the RNA World Hypothesis:

• Many of the enzyme cofactors important for life today are based on a ribose nucleotide; for example:
  • ATP
  • NAD
  • FAD
  • coenzyme A
  • cyclic AMP
  • GTP
• In the cell, all deoxyribonucleotides are synthesized from ribonucleotide precursors.
• Many organisms control the transcription and/or translation of certain genes with RNA molecules, not protein molecules.

Problem 3: How were macromolecules able to reproduce themselves?

Once macromolecules were created, the next problem we must try to understand is how they were able to generate more of themselves, identical in composition and structure. Without this reproductive ability, life would change each generation and there would be no continuity.

The advantage of the RNA World Hypothesis is that it proposes a way in which the creation of the first macromolecule (an RNA with the ability to encode information and replicate itself) and of creating progeny with similar properties. It would explain this step, and provide a mechanism for natural selection to work on this population of "organisms" (RNA ribozymes) to change, with the best replicators emerging from the pool in greater number at each succeeding generation. 

For the other hypotheses about the creation of life, the mechanism for the reproduction of macromolecules is less understood.

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 so that the building blocks are held in greater concentration to facilitate chemical reactions and so that the products do not float away. The chemicals of life need to be separated from the extracellular fluid (ECF). Today, this function is provided by the plasma membrane.

The plasma membranes of all current living organisms 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.

One of the key researchers in this field, Jack Szostak and colleagues from Harvard University, has confirmed 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 charged molecules like nucleotides
• admit 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. 

Other hypotheses have been proposed, like the formation of an iron sulfide cell as a precursor of cells in deep-sea hydrothermal vents. The advantage of these enclosures is that they would have served as catalysts as well as to keep the chemicals created by the early life from floating away. In time, lipids may have come to coat the surface of these metal cells and bud away, seeding life in the ocean.