Devangshu Datta: The ABC of life on Earth

Data from space and some experiments by NASA show that many of the key building blocks of life could arise spontaneously without biological processes

The origin of life is one of the great unsolved questions. Did life arise spontaneously from the primordial soup of the early sea? Or was it imported when the Earth was struck by meteorites (and comet debris) containing organic material?

There are some interesting interpretations of data from space, and interesting results from experiments by the National Aeronautics and Space Administration (NASA) and by others. These demonstrate that life, or rather, many of the key building blocks of life, could arise spontaneously without biological processes. It's even possible that bacteria or viruses could journey through space, inside meteorites. Tallying up, there is evidence that life or its lego bricks, may be imported from meteorites, or comet debris.
 

Life as we know it, requires DNA (deoxyribonucleic acid), RNA (ribonucleic acid) and other proteins. The building blocks of those molecules are nucleobases (often called simply bases) for DNA and RNA, and amino acids for proteins. Comets and meteors contain multiple organic chemicals and water. This material can be used to generate bases and amino acids.

There are key questions as to whether bases and amino acids can arise spontaneously in natural conditions, without biological processes ("abiologically"). For example, the "soup hypothesis" can be backed up by experiments showing that lightning (or electric currents) passing through organic chemicals can generate amino acids. Amino acids have also been found to occur in space, in comets.

In multiple experiments over many years, NASA's Ames Research Center and the Goddard Flight Center have now abiologically generated the nucleobases, uracil (U), cytosine (C) and thymine (T). These are three of the five key bases found in DNA and RNA.

A key precursor chemical called pyrimidine, consists of hexagonal ring-shaped molecules of carbon and nitrogen. Pyrimidine is found on meteorites, though how it originates in space is unknown. It is known that pyrimidine decomposes into constituent elements when hit by ultraviolet (UV) light.

But in NASA's experiments, water, which contained pyrimidine, was frozen in vacuum at extreme cold (-257 degrees celsius). This creates conditions similar to those found in space. This ice was then irradiated with UV radiation (which the sun produces). The ice provides a shield against excessive radiation and a surprising thing happens. The pyrimidine breaks up into the three nucleobases U, C and T.

Other experiments indicate that the other nucleobases may also arise abiologically. For example, a Czech experiment suggests that the bases, guanine (G) and adenine (A) may be generated abiologically by hammering the right chemicals with laser pulses. These pulses mimic the conditions of pressure and temperature caused at meteorite impact. So, if a meteor containing the right mix of organic chemicals hits the Earth, G and A might be generated.

Guanine (G), adenine (A), thymine (T), cytosine (C) and uracil (U) together make up the building blocks of genetic material. DNA contains the bases GATC, while RNA contains AGCU. DNA and RNA are built from millions of nucleotides. A nucleotide contains one nucleobase, a sugar and a phosphate.

DNA is a long molecule, which contains genetic instructions and all the genetic information about an individual creature. It is famously shaped like a double-helix, as determined by James Watson, Francis Crick, Maurice Wilkins, who built on crystallographic research by Rosalind Franklin. DNA gives RNA instructions to make proteins.

Since many amino acids and nucleobases can be synthesised abiologically, there is a strong implication that the key ingredients for DNA, RNA and proteins may all be generated from abiological processes. About 30 tonnes of organic meteorite material is estimated to enter the atmosphere every day, so there is no shortage of raw material.

The theory of evolution explains how simple, small organisms can evolve into larger, more complex organisms. Genome decoding also helps us read the biological record. We can trace developments and timelines to link current flora and fauna to extinct ancestors and cousins.

There are still big questions, of course. How do bases and amino acids turn into RNA, DNA and proteins? What sort of catalysis occurred? Can all amino acids arise abiologically?

But it does appears that life on Earth has extra-terrestrial origins, though these are more prosaic than the Erich Von Däniken fans would like. Je suis alien.