Biochemical Evolution or 'Chemical Evolution'

It is a new version of 'Abiogenesis'

1. Biologists believe that present-day life was originated in the remote past from chemical substances by "Biochemical Evolution' or 'Chemical Evolution'.
2. Biochemical Evolution: The formation of complex organic molecules from simpler inorganic molecules through chemical reactions in the oceans during the early history of the Earth.
3. Biochemical evolution was the first step in the development of life on earth.
4. The period of chemical evolution lasted for about a billion years.

Proposed by Oparin & Haldane (1924). Evidences provided by Urey & Miller (1953).

Ø The theory explain that:
1.Life originated from simple inorganic substances.
2.Inorganic substances are transformed to organic substances.
3.Organic substances later transformed into colloidal substances.
4.Colloidal substances acquired life (capacity of propagation).
5.The colloidal substances with life' behaved like present-day prokaryotic cells.

Origin of life on Earth includes following processes in a sequential way:

(1). Origin of Earth
(2). Formation of water, Ammonia and Methane
(3). Formation of Micro-molecules
(4). Formation of Macro-molecules
(5). Formation of Nucleic Acids
(6). Formation of Nucleoproteins
(7). Coacervation
(8). Pre-cell or Pro-cell formation
(9). Cell Formation

(1). Origin of Earth

1.Origin of planet earth occurred before the origin of life. It is believed that earth formed from the sun about 4-54 billion years ago.
2.A piece of sun broken from the sun.
3.The piece then gradually moves away from the sun
4.The broken piece was a firewall.
5.It contains molten mass of gasses and vapours of various elements.
6.The temperature was very high (about 5000 to 6000°C).
7.As the earth moved away from the sun, it gets cooled.
8.Heavier elements such as Ni and Fe occupied at the core of the earth.
9.Lighter elements such as He, H, 0, N and C occupied the atmosphere of the earth.
10.The concentration of Molecular oxygen (O2) was very less or absent.
11.Life originated mainly from elements occupied in the atmosphere.
12.These elements underwent a series of progressive changes for millions of years.
13.These changes resulted in the origin of life on earth.
14.Such a type of origin of life from chemical substances is called chemical evolution.
The energy required for the chemical reactions were obtained from sunlight, lightning and volcanic activities.

(2). Formation of Water, Ammonia and Methane

1.Primitive earth atmosphere contain a large amount of H, N, C and few O.
2.Hydrogen reacted with other atoms to form a variety of molecules.
3.Hydrogen combined with Nitrogen to form Ammonia (NH3).
4.Hydrogen combined with Carbon to form Methane (CH4).
5.Hydrogen combined with Oxygen to form Water (H20).

6.As the years passed, the earth's atmosphere gradually cooled.
7.Steam of water in the atmosphere condensed to liquid water.
8.This resulted in the formation of rain (precipitation).
9.Since the earth was hot when the rainwater reaches the earth surface, it gets re-evaporated immediately.
10.This again produces rain in the next cycle.
11.This process of re-evaporation and raining continued for many years.
12.This gradually resulted in the cooling of the earth surface.
13.Rain water gets accumulated on the earth surface.
14.This resulted in the formation of rivers, streams, lakes and oceans.
15.Compounds like methane, ammonia get dissolved in the rainwater.
16.They reached the earth surface and accumulated in the ocean.
17.Minerals rocks of the earth surface were also dissolved in water.
18.This resulted in the accumulation of minerals in the seawater.

(3). Formation of Micro-molecules

1.Molecules which are of less size are called micro-molecules.
2.Major micro-molecules required for the formation of life on earth are:
1)Amino Acids
2)Monosaccharaides
3)Fatty Acids
4)Purines
5)Pyrimidines
6)Nucleotides.
3.They are formed by the combination of already formed compounds (Ammonia, Methane etc.) and elements (accumulated in the ocean).
4.Their combinations and reactions required high energy input.
5.The energy was obtained from sunlight, lightning and volcanic activities.

(4). Formation of Macro-molecules (Condensation or Połymerization)

1.Macro-molecules were formed by the combination of micro-molecules.
2.Important Macro-molecules of life are:
1) Proteins: Formed by the polymerization of amino acids.
2) Lipids: Formed by the polymerization of alcohols and fatty acids.
3) Polysaccharides: Formed by the polymerization of monosaccharaides.
4) Nucleic acids: Formed by the polymerization of purines, pyrimidines and phosphoric acid and sugars
6) The combination process of micro-molecules to form macro-molecules is called condensation or polymerization.
7) Proteins may be the first macro-molecule formed during the biochemical evolution.
8) First formed proteins were called proto-proteins.

Proto-Proteins

1.Proto-proteins were the first formed proteins during the chemical origin of life on earth.
2.All the present-day proteins were evolved from the 'proto-proteins'.
3.Proto-proteins were small proteins of 15-20 amino acid residues in size.
4.They have some features of the secondary structures.
5.They may have some primitive functions.
6.During the course of time, increasingly stable and more functional proteins arose by adding structural elements to the proto-proteins.

Protenoids

1.They are also called as Thermal Proteins.
2.Protenoid is a polypeptide or mixture of polypeptides obtained by heating a mixture of amino acids.
3. They are protein-like, often cross-linked molecules.
4.They have formed abiotically from amino acids.
5.According to Sidney W. Fox protenoids may have been the precursors to the first living cells (proto-cells).

(5). Formation of nucleic acids

1.Nucleic acids were formed by the condensation of phosphoric acid, sugars, purine and pyrimidine.
2.First nucleosides were formed.
3.Then nucleotides were formed.
4.Nucleotides polymerized to form nucleic acids (RNA and DNA).
5.RNA was formed first, DNA was formed later from RNA.
6.RNA was the first genetic material.
7.Later RNA as the genetic material was replaced by DNA.
8.DNA is more stable than RNA.

RNA World Hypothesis

1.Proposed by Alexander Rich in 1962.
2.Description:RNA world is a hypothetical stage in the evolutionary history of life on Earth, in which self-replicating RNA molecules proliferated before the evolution of DNA and proteins. Like DNA, RNA can store and replicate genetic information; like protein enzymes, RNA enzymes (ribozymes) can catalyze (start or accelerate) chemical reactions that are critical for life. One of the most critical components of cells, the ribosome, is composed primarily of RNA.

(6). Formation of nucleoproteins

1.Proteins and nucleic acids combined to form nucleoproteins.
2.Nucleoproteins started to perform specific functions.
3.Enzymes like activities were acquired by these molecules.
4.Nucleoproteins were more stable than nucleic acid alone.

(7). Coacervates and Coacervation

1.Word meaning: ‘To Assemble Together’
2.Coacervates:They are organic-rich droplets formed via liquid-liquid phase separation, mainly resulting from association of oppositely charged molecules (polysaccharides, proteins, macro-ions) or hydrophobic proteins.
3.Coacervation:A phenomenon that produces coacervate colloidal droplets.
4.When coacervation occurs, TWO liquid phases will co-exist.
5.The TWO phases are:
1)A dense, polymer-rich phase (coacervate phase).
2)A very dilute, polymer-deficient phase (dilute phase).
6.Process of coacervation was proposed by Oparin and Haldane.
7.Macromolecules formed by the polymerization of micro-molecules underwent precipitation in the sea.
8.Precipitation resulted in the aggregation of macro-molecules.
9.The aggregation resulted in the formation of organized structures.
10.These organized structures can be called as Coacervates.
11.Coacervates are distinct colloidal droplets appeared in the sea.
12.Smaller coacervates fused together to form larger coacervates.
13.Coacervates were NOT mixed with the surrounding water.
14.Coacervates contained protein, nucleic acids and other organic and inorganic compounds.
15.The ratio of these compounds in the coacervates highly varies.
16.Surface of coacervates had the ability to selectively absorb other substances from the medium (seawater).
17.According to Oparin, coacervates behaved like living molecule.
18.He also suggested that coacervates may give rise to cell-like structures.

Another Add-on on Coacervates :

In the 1920s, Oparin hypothesised that life would have originated inside tiny phase-separated droplets formed by coacervation [Figure 1].
Complex coacervation is a liquid–liquid phase separation process between two different oppositely charged components in a dilute solution. The polycation and polyanion are attracted to each other by electrostatic interactions and droplet formation is driven by an entropic release from water and counterions from the polymers. This provides a thermodynamically favourable route to droplet-based compartmentalisation on early Earth. The resulting droplets are membrane-free, chemically enriched and in dynamic equilibrium with a polymer-poor outer aqueous phase. Coacervate droplets are intriguing models as protocells as they form between prebiotically relevant molecules such as peptides, RNA, lipids and nucleotides. It has been suggested that coacervate microdroplets are not suitable as protocell models due to their lack of membrane. It was proposed that this would prevent the generation of a chemical gradient generated between the inside of the droplet and the outside. However, it is known that coacervate microdroplets will concentrate reactants (which provides a route to chemical gradient formation); support enzyme reactions; allow the exchange of ions and small molecules with the surrounding media; select for different length of RNA and protect incorporated components from degradation. Moreover, their capability to support complex biochemical reactions suggests that probing and understanding the physical properties of coacervates could reveal how this general phenomena would have facilitated the onset of cellular life on prebiotic Earth.

19.Coacervates were also called as microspheres.but there are few differences between them

20.Microspheres 21.S.Fox heated a mixture of about 18 amino acids in anhydrous condition to about 160 – 2000C. On cooling the amino acids were observed to have linked together to form long chains which resembled to the polypeptide chains of proteins. When these were heated and cooled again they formed small cell like structure which he called proteinoids or microspheres.
22.They show following characterstic properties:
(1) Same size as bacteria
(2) Stable
(3) Divide by fission
(4) They are bounded by double membrane as revealed by electron microscope
(5) Show selective absorption
(6) When Zinc is added to the solution of microsphere it splits ATP with the release of energy which means that microspheres release energy in the presence of metal catalyst. However microspheres are neither precells nor living but they indicate the process that might have occurred in the ocean, resulting in the formation of precells and cells. This process of self assembly in coacervates and microspheres is also visible in the present day molecular structures such as T4 phage and Tobacco Mosaic Virus (TMV). Therefore life is believed to have originated from the organic soup

23.Coacervates or Microspheres show features of a living cell:
1)They were microscopic in size
2)Highly dynamic
3)Spherical in shape
4)Had a uniform diameter
5)They were stable in water
6)Had double-layered boundaries around them
7)Had mobility
8)They showed growth
9)They underwent fission, budding and fragmentation
24.However they lack:
(1) Membrane
(2) Do not show selective absorption
(3) They break off in solution if shaken vigorously
(4) Smaller in size as compared to precells.

(8). Proto-cell Formation

1.First living cell is called Proto-cells or Pro-cell or Proto-biont.
2.Proto-cells originated spontaneously in the seawater.
3.Proto-cells were spherical in shape.
4.They had double-layered membrane around it.
5.Had the capacity to reproduce by fission, budding or fragmentation.
6.Had the capacity to take materials from surrounding.
7.Obtain energy from the fermentation of organic substances. Thus, proto-cells were anaerobes.

(9). Cell(Primitive Cell) formation

1.The proto-cells gave rise to cells.
2.Proto-cells acquired genetic material (RNA or DNA or nucleoprotein).

3.Most probably the genetic material may be RNA.
4.They looked like modern bacteria or virus.
5.RNA or DNA acquired the self-replicating capacity.
6.Genetic material started to assist protein synthesis.
7.Genetic material acquired replication capacity.
8.Earlier cells obtain energy from fermentation.
9.Fermentation released a large amount of CO2 to the atmosphere.
10.CO2 accumulated in the atmosphere.
11.Continuous fermentation resulted in the exhausting of resources.
12.This condition might be responsible for the development of chlorophyll.
13.Chlorophylls started to capture CO2 and fix light energy.
14.Photosynthesis produced O2 as a byproduct.
15.O2 produced by photosynthesis is released into the atmosphere.
16.With this, the heterotrophic cells were transformed into autotrophic cells.
17.When life was originated, the atmosphere was anaerobic.
18.Molecular oxygen was very less or absent.
19.The life process gradually changed the anaerobic condition to aerobic.

Place of origin of life on earth

1.‘Sea is the mother of life’.
2.Majority of scientists believed that life originated in the water in the sea.
3.In the sea, life originated at deep hydrothermal vents.
4.This view is supported by many facts:
1)Most of the simple and primitive animals and plants are aquatic.
2)Protoplasm and body fluids contain salt similar to sea.
3)Earliest fossils were obtained from the sea bed.

Haldane View

Haldane proposed that the primordial sea served as a vast chemical laboratory powered by solar energy. The atmosphere was oxygen free, and the combination of carbon dioxide, ammonia and ultraviolet radiation gave rise to a host of organic compounds. The sea became a

hot dilute soup

containing large populations of organic monomers and polymers. Haldane envisaged that groups of monomers and polymers aquired lipid membranes, and that further developments eventually led to the first living cells. Haldane coined the term

prebiotic soup,

and this became a powerful symbol of the Oparin-Haldane view of the origin of life.

Urey-Miller Exp.

In 1953, when Stanley Miller was a 23-year-old graduate student in the laboratory of Harold Urey at the University of Chicago, he performed experiments that attracted global attention. Miller was the first to show that amino acids and other organic molecules could be formed under conditions thought to simulate those of early Earth. Miller’s experiments were a test of a hypothesis about the origin of life developed in the 1920s by Russian chemist A. I. Oparin and British scientist J. B. S. Haldane. Oparin and Haldane independently proposed that conditions on early Earth could have generated organic molecules. They reasoned that present-day conditions on Earth do not allow the spontaneous synthesis of organic compounds simply because the atmosphere is now rich in oxygen. As a strong oxidizing agent, O2 tends to disrupt chemical bonds. However, before the early photosynthetic prokaryotes added O2 to the air, Earth may have had a reducing (electron-adding) atmosphere. The energy for this abiotic synthesis of organic compounds could have come from lightning and intense UV radiation. Figure is a diagram of the apparatus used in Miller’s experiment. A flask of warmed water represented the primeval sea. The water was heated so that some vaporized and moved into a second, higher flask. The “atmosphere” in this higher flask consisted of water vapor, hydrogen gas (H2), methane (CH4), and ammonia (NH3)—the gases that scientists at the time thought prevailed in the ancient world. Electrodes discharged sparks into the flask to mimic lightning. A condenser with circulating cold water cooled the atmosphere, raining water and any dissolved compounds back down into the miniature sea. As material cycled through the apparatus, Miller periodically collected samples for chemical analysis. Miller identified a variety of organic molecules that are common in organisms, including hydrocarbons (long chains of carbon and hydrogen) and some of the amino acids that make up proteins. Many laboratories have since repeated Miller’s classic experiment using various atmospheric mixtures and produced organic compounds.