The Origin of Life

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To account for the origin of life on our earth requires solving several problems: A number of theories address each of these problems.

Abiotic Synthesis of Organic Molecules

As for the first, three scenarios have been proposed: organic molecules
  1. were synthesized from inorganic compounds in the atmosphere — the "primeval soup" theory;
  2. rained down on earth from outer space;
  3. were synthesized at hydrothermal vents on the ocean floor;
  4. were synthesized when comets or asteroids struck the early earth.

1. Miller's Experiment

Stanley Miller, a graduate student in biochemistry, built the apparatus shown here. He filled it with

He hypothesized that this mixture resembled the atmosphere of the early earth. (Some are not so sure.) 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.

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.

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:

One difficulty with the primeval soup theory is that it is now thought that the atmosphere of the early earth was not rich in methane and ammonia — essential ingredients in Miller's experiments.

2. Molecules from outer space?

The Murchison Meteorite

Representative amino acids found in the Murchison meteorite. Six of the amino acids (blue) are found in all living things, but the others (yellow) are not normally found in living matter here on earth. The same amino acids are produced in discharge experiments like Miller's.
Glycine Glutamic acid
Alanine Isovaline
Valine Norvaline
Proline N-methylalanine
Aspartic acid N-ethylglycine
This meteorite, that fell near Murchison, Australia on 28 September 1969, turned out to contain a variety of organic molecules including:

The question is: were these molecules simply terrestrial contaminants that got into the meteorite after it fell to earth?

Probably not:

The ALH84001 meteorite

This meteorite arrived here from Mars. It contained a variety of organic molecules.

Furthermore, there is evidence that its interior never rose about 40° C during its fiery trip through the earth's atmosphere. Live bacteria could easily survive such a trip.

Link to a discussion of the possibility of life on Mars and more on the ALH84001 meteorite.

Organic molecules in interstellar space

Astronomers, using infrared spectroscopy, have identified a variety of organic molecules in interstellar space, including

Laboratory Synthesis of Organic Molecules Under Conditions Mimicking Outer Space

There have been several reports of producing amino acids and other organic molecules by taking a mixture of molecules known to be present in interstellar space such as: and exposing it to

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).

3. Deep-Sea Hydrothermal Vents

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.)

4. Laboratory Synthesis of Nucleobases Under Conditions Mimicking the Impact of Asteroids or Comets on 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.

Assembling Polymers

Another problem is how polymers — the basis of life itself — could be assembled.

Link to a discussion of enantiomers.

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.

An RNA Beginning?

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 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:
Link to a discussion of ribozymes.

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 figure is based on the work of Green and Szostak, Science 258:1910, 1992.)

In principal, the minimal functions of life might have begun with RNA and only later did Several other bits of evidence support this notion of an original "RNA world":

Reproduction?

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

The First 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 finding described above reported in the 3 July 2008 issue of Nature 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

These workers loaded their synthetic vesicles with a short single strand of deoxycytidine (dC) structured to provide a template for its replication. When the vesicles were placed in a medium containing (chemically modified) dG, these nucleotides entered the vesicles and assembled into a strand of Gs complementary to the template strand of Cs.

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.

From Unicellular to Multicellular Organisms

This transition is probably the easiest to understand.

Several colonial flagellated green algae provide a clue. These species are called colonial because they are made up simply of clusters of independent cells. If a single cell of Gonium, Pandorina, or Eudorina is isolated from the rest of the colony, it will swim away looking quite like a Chlamydomonas cell. Then, as it undergoes mitosis, it will form a new colony with the characteristic number of cells in that colony.

(The figures are not drawn to scale. Their sizes range from Chlamydomonas which is about 10 µm in diameter — little larger than a human red blood cell — to Volvox whose sphere is some 350 µm in diameter — visible to the naked eye.)

The situation in Pleodorina and Volvox is different. In these organisms, some of the cells of the colony (most in Volvox) are not able to live independently. If a nonreproductive cell is isolated from a Volvox colony, it will fail to reproduce itself by mitosis and eventually will die. What has happened? In some way, as yet unclear, Volvox has crossed the line separating simple colonial organisms from truly multicellular ones. Unlike Gonium, Volvox cannot be considered simply a colony of individual cells. It is a single organism whose cells have lost their ability to live independently. If a sufficient number of them become damaged, the entire sphere of cells will die.

What has Volvox gained? In giving up their independence, the cells of Volvox have become specialists. No longer does every cell carry out all of life's functions (as in colonial forms); instead certain cells specialize to carry out certain functions while leaving other functions to other specialists. In Volvox this process goes no further than having certain cells specialize for reproduction while others, unable to reproduce themselves, fulfill the needs for photosynthesis and locomotion.

In more complex multicellular organisms, the degree of specialization is carried much further. Each cell has one or two precise functions to carry out. It depends on other cells to carry out all the other functions needed to maintain the life of the organism and thus its own.

The specialization and division of labor among cells is the outcome of their history of differentiation. One of the great problems in biology is how differentiation arises among cells, all of which having arisen by mitosis, share the same genes. Link to a discussion of the solution.

The genomes of both Chlamydomonas and Volvox have been sequenced. Although one is unicellular, the other multicellular, they have not only about the same number of protein-encoding genes (14,516 in Chlamydomonas, 14,520 in Volvox) but most of these are homologous. Volvox has only 58 genes that have no relatives in Chlamydomonas and even fewer unique mRNAs.

At one time, many of us would have expected that a multicellular organism like Volvox with its differentiated cells and complex life cycle would have had many more genes than a single-celled organism like Chlamydomonas. But that turns out not to be the case.

How to explain this apparent paradox? My guess is that just as we have seen in the evolution of animals [Examples], we are seeing here that the evolution of organismic complexity is not so much a matter of the evolution of new genes but rather the evolution of changes in the control elements (promoters and enhancers) that dictate how and where the basic tool kit of eukaryotic genes will be expressed .

The evidence is compelling that all these organisms are close relatives; that is, belong to the same clade. They illustrate how colonial forms could arise from unicellular ones and multicellular forms from colonial ones.

The Last Universal Common Ancestor (LUCA)?

The 3 kingdoms of contemporary life — archaea, bacteria, and eukaryotes — all share many similarities of their metabolic and genetic systems [Link]. Presumably these were present in an organism that was ancestral to these groups: the "LUCA". Although there are not enough data at present to describe LUCA, comparative genomics and proteomics reveal a closer relationship between archaea and eukaryotes than either shares with the bacteria. Except, of course, for the mitochondria and chloroplasts that eukaryotes gained from bacterial endosymbionts [Link]. Whether the endosymbionts were acquired before or after a lineage of archaea had acquired a nucleus — and thus started the lineage of euaryotes — is still uncertain.

Creating Life?

When I headed off to college (in 1949), I wrote an essay speculating on the possibility that some day we would be able to create a living organism from nonliving ingredients. By the time I finished my formal studies in biology — having learned of the incredible complexity of even the simplest organism — I concluded that such a feat could never be accomplished.

Now I'm not so sure.

Several recent advances suggest that we may be getting close to creating life. (But note that these examples represent laboratory manipulations that do not necessarily reflect what may have happened when life first appeared.)

Examples:

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20 December 2014