Traditional, Non Spiritual, View Of Origin of Life

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Referenced From

Origin of Life

 

Evidence for an Ancient Earth:  The evidence for an ancient earth come from many sources.  We have already talked about two of these sources of evidence. 

 

1.       Deduced from the principle of Uniformitarianism (i.e. rates of geological processes, such as sedimentation rates, are constant) and the thickness of the layers in sedimentary rock.

 

2.       From dating of rocks using density of radioactive isotopes

 

Early Earth:  The earth formed by the aggregation of materials into a planetary body.  Initially the earth was molten with heaviest elements (e.g. lead, nickel) closer to the core.  It began to cool and to become differentiated into the molten core, cooler semiliquid mantel, and the coolest crust.  As the earth cooled the atmosphere began to form.  The major gases in the atmosphere were not always the same as they are today.  There were greater amounts of volcanic activity than we experience today and the gasses produced by volcanoes probably made up a major portion of the early atmosphere.  Water vapor condensed and gradually formed the oceans.  .   

Early Atmosphere:  N₂ a major component (as now), also H₂O and CO₂.  Volcanoes would also be emitting H₂S, NH₃, CH₄, H₂, and CO.  From the chemical forms of the ancient rocks, we know that the ancient earth did not have high amounts of atmospheric Oxygen (O₂).  We wouldn’t have survived more than a couple of minutes in the place.  And without Oxygen, there would not have been an ozone layer which filter’s out solar radiation and therefore, there was a punishing amount of radiation hitting the earth.   

These differences in the earth provide clues as to the processes that contributed to the origin of life. 

The steps that preceded the origin of life on earth is not completely knowable, but we can explore these processes by recreating the environmental conditions on earth and then evaluating the rates of the processes.   

Characteristics of Life:

1.      Cellular organization (cytoplasm in a membrane)

2.      Growth and metabolism (chemical changes inside cells)

3.      Reproduction

4.      Heredity

 

What about viruses? 

Four steps in the origin of life 

1. The abiotic synthesis and accumulation of small organic molecules or monomers (e.g. amino acids and nucleotides)

Miller-Urey experiment of conditions of early earth.  Fig. 26.4.  With the input of energy, the basic building blocks of complex chemicals spontaneously formed.  The energy in the early earth would have come from lightning strikes, heat from volcanic activity and earth’s crust, high solar radiation.   In the presence of free oxygen, they would have been immediately broken down, but without atmospheric oxygen and without living creatures, these chemicals can accumulate.    

2. The joining of these monomers into polymers (e.g. proteins and nucleic acids)

Polymerization is facilitated by the concentrating of the monomers, such as would occur in drying droplets.  Clay could have played an important role because the monomers bind to clay’s charged surfaces and clay also binds to metals that can catalyze the uniting of polymers.  Possibly, other metals such as iron pyrite could also have played this role.   

3. The aggregation of molecules into droplets (protobionts)

The abiotically produced polymers could have been concentrated in protobionts.  One type of protobiont (called the coacervate by Oparin) self assemble when mixtures of macromolecules are shaken.  The molecules are hydrophobic and they therefore have higher affinity towards each other than with water molecules.  Lipids can also spontaneously form liposomes.  These protobionts can exhibit some of the characterisitics of life such as metabolic activity, and growth through incorporation of the similar chemicals from the environment, and division into daughter protobionts.    

It is possible that there could be differential success among the probionts that would select for more and more cell-like characteristics.  But without a way to reliably reproduce similar types (ie heritability), the selection would not result in sustained evolution.   

4. The origin of heredity

The origin of the modern genetic system, DNA -> RNA -> Protein (central dogma), is one of the most uncertain of the steps.  RNA does self assemble from RNA nucleotide bases.  And then a single RNA strand can self-replicate, so steps in the process can be reproduced in the lab.  The currently favored idea stems for the discovery in the 1980’s by Tom Cech of U. Colorado, that modern cells use RNA catalysts called ribozymes.  These ribozymes help catalyze the synthesis of new RNA.  Thus RNA molecules simultaneously possess a genotype (the sequence of nucleotides), and a phenotype (the three dimensional structure and ability to interact with other chemicals).    In laboratory experiments involving serial transfer of a crude ribozymes and RNA nucleotides, the ribozymes evolved to become more efficient replicators of pieces of DNA.     

Once life started, biological evolution also began.  The sexual recombination was probably very crude so that lineages exchanged and recombined genetic material in a haphazard fashion.  As a result the early lineages would have been mixed and this would limit divergence over the first 100’s of millions of years.   

There remain many questions on the origin of life, but we do know that

1.      All life currently arises from existing life—the conditions that allowed life to start are gone.

2.      All life is descended from a common ancestor.   

Coevolution of Life and Earth.  Note that the evolution of life had profound effects on the physical earth itself.  Initially, all metabolism was anaerobic--there was probably several chemical bases for them including sulfur and carbon.  With the evolution of the cyanobacteria with photosynthetic apparatus that produce oxygen, the atmosphere filled with O₂.  This was initially poison for most of life, then aerobic respiration evolved.  Also, with the increasing levels of O₂, the Ozone layer formed which reduced the level of radiation hitting the earth’s surface—an essential condition for the colonization of land.  With the colonization of land by plants and fungi, living things had large array of effects on a surface that was previously colonized by bacteria alone.