ASTROBIOLOGY PART -V LIFE STARTED SMALL

We are well aware that life on our planet originated with single celled organisms and later became more and more complex with mutations. It may sound unbelievable but everything started with that first single celled organism. All the species including us have evolved from it with a series of mutations over billions of years. People say we have evolved from monkeys, but they came up much later and have originated from that single celled organism 400 crore years ago after so much of evolution.

Now if we look around we find hundreds of life forms all around us. But those we see are multicellular organisms and but a fraction of the living species. They represent the domain of “Eukaryotes” which mostly contain microscopic organisms which can be a single cell like the Amoeba. There are 2 other domains of life Bacteria and Archaea together referred to as the “Prokaryotes”

There is one thing that is common in all forms of life and that is DNA. It is an enormously complex carbon based molecule made up of billions of atoms. It is present in every cell and is capable of making copies of itself. Mostly the duplicates are perfect but sometimes mutations occur. These mutations lead to evolution of a completely new species.

By tracing the similarities and differences between the species, it is clear that they evolved from a single ancestor. I give below the family tree of the life on earth.

Life Eukaryotes on earth started about 2 billion years ago and the first 2 billion years belonged to the Prokaryotes and there were no Eukaryotes at all. It was another billion years before the Prokaryotes evolved into non microscopic forms. Even today the Prokaryotes outnumber Eukaryotes in all the environments.

The Prokaryotes interest us in astrobiology because some Prokaryotes are able to survive in environments which the Prokaryotes find very inhospitable. There is a word called “Extremophile” coined in the 1970’s for organisms that love extreme environments. So even if the environment in some planets looks extremely inhospitable for us, it does not look so for the Extremophiles. They live in extremes of acidity or Alkalinity, high or low temperatures, in the complete absence of light or in the presence of radiation.

The Extremophile “Deinococcus Radiodurans” can live happily inside a nuclear reactor. Other species are found in salt lakes, in rocks deep within the earth’s crust or around Hydrothermal vents on the ocean floor where the pressure of water is crushingly high and the water is superheated to hundreds of degrees of Celsius.

What is Biochemistry and when does biology meet chemistry and combine into the double science? For Chemistry the DNA is another cell but it is self-replicating and therefore satisfies the definition of life given by NASA (“A self-sustaining chemical system capable of Darwinian Evolution”). Essentially there is no non bridgeable difference between inorganic matter and living systems and that under suitable physical conditions the emergence of life is highly probable.

An Oxygen free early earth looks a better place for life to evolve in the initial stages because it requires Ultraviolet rays that would be blocked by Ozone if Oxygen had been present. In 1952 a Phd student Stanley Miller set up an experiment to duplicate the evolution of complex organic molecules on earth. His apparatus consisted of a flask of water connected to a flask of gases-methane, ammonia & hydrogen which were present in the primordial earth. The lightning flash for ionization was provided initially by an electrical flash but the reruns were done by Ultraviolet rays. That experiment gave him 20 Amino acids the chemical building blocks of life. The experiment however, did not yield the 4 nucleotides that make the building blocks for DNA.       

In 2017, the Miller Experiment was replicated but with powerful laser discharges to simulate the plasmas entering from asteroid impact shock waves on earth and the 4 nucleotides got synthesized. Unlike today 4 billion years ago, the solar system was still young and contained lots of debris that made asteroid hits very common on earth. These impacts played a role in getting the chemical reactions for life going.

A NASA funded team has created the “Hachimoji” DNA in the laboratory very recently  in 2019.  Hachi means eight and moji means letter. Our 4 DNA bases have the standard codes A,G,C & T (Adenine, Guanine, Cytosine and Thymine). The Hachimoji DNA in addition has another 4; B,P,S & Z. It is capable of storing and transmitting information like the ordinary DNA, but it does that in a different way.

On the earth the bacteria produced Oxygen that led to the formation of the atmosphere and a natural screen from ultraviolet rays. That made earth hospitable for advanced forms of life. However, this took billions of ears.