EARLY EARTH: CONDITIONS AND BEGINNING OF LIFE - BIOLOGY - 2026

The Primitive Earth is a term that is used to refer to what our planet was during its first 1,000 million years of existence. This span encompasses the Hadic Aeon (4,600–4,000 Ma) and the Eoarchic Era (4,000–3,600 Ma) of the Archaic Aeon (4,000–2,500 Ma). In geology, the abbreviation Ma (from Latin, mega annum) means millions of years before the present.

The Hadic, Archaic, and Proterozoic Aeons (2500–542 Ma) make up the Precambrian, referring to rocks formed before the Cambrian Period. The subdivisions of the Precambrian are not formal stratigraphic units and are defined purely chronometrically.

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Formation of the Primitive Earth

The most widely accepted explanation of the origin of the Universe is the Big Bang theory, according to which the Universe expanded from an initial volume equal to zero (all matter concentrated in one place in an instant, which is called a "singularity") to reaching a huge volume 13.7 billion years ago.

The Universe was already almost 9 billion years old when, 4.567 million years ago, our Solar System and the Early Earth formed. This very accurate estimate is based on radiometric dating of meteorites dating back to the Solar System.

The Sun was formed by the collapse of a gas region of the interstellar medium. Compression of matter is the cause of its high temperatures. The rotating disk of gas and dust formed a primitive solar nebula, from which the components of the Solar System come.

The formation of the Early Earth can be explained by the "standard model of planetary formation."

Cosmic dust accumulates through a process of accretion collisions, first between small celestial bodies, then between embryonic planets up to 4,000 kilometers in diameter, finally between a small number of large planetary bodies.

Conditions of the Primitive Earth

During its long history, the Early Earth underwent enormous changes in its environmental conditions.

The initial conditions, qualifying as infernal, were absolutely hostile to all forms of life. The temperatures that made all terrestrial materials part of a sea of ​​magma, the bombardment by meteorites, asteroids and small planets, and the presence of lethal ionized particles brought by the solar wind stand out.

Subsequently, the primitive Earth cooled, allowing the appearance of the earth's crust, liquid water, atmosphere, and physicochemical conditions favorable to the appearance of the first organic molecules and, finally, to the origin and conservation of life.

Hadic Aeon

The knowledge of the Hadic Aeon comes from the analysis of a small number of terrestrial rock samples (formed between 4,031 and 4,0 Ma), complemented with inferences based on the study of meteorites and other celestial materials.

Shortly after the formation of the Earth, already in the Hadic Aeon, a last great accretional collision happened with a celestial body the size of Mars. The energy of the impact melted or vaporized much of the Earth.

Coalescence by cooling and accretion of steam formed the Moon. The molten material that remained on Earth formed an ocean of magma.

The core of the Earth, which is made of liquid metal, comes from deep in the magma ocean. The fused silica that originated the earth's crust constituted the upper layer of that ocean. The great dynamism of this stage led to the differentiation of the core, the mantle, the earth's crust, a protoceano and an atmosphere.

Between 4,568 and 4.4 Ma, the Earth was hostile to life. There were no continents or liquid water, there was only an ocean of magma bombarded intensely by meteorites. However, in this period, the chemical-environmental conditions necessary for the emergence of life began to develop.

It was Eoarchic

Life is generally assumed to have originated at some point in the transition between the Hadic Aeon and the Eoarchic Era, although no microfossils are known to prove this.

The Eoarchic Era was a period of formation and destruction of the earth's crust. The oldest known rock formation, located in Greenland, arose 3.8 billion years ago. Vaalbará, the first supercontinent that Earth had, was formed 3.6 billion years ago.

During the Eoarchic Era, between 3,950 and 3,870 Ma, the Earth and the Moon suffered an extremely intense bombardment of meteorites that ended a period of calm that had lasted 400 million years. The lunar craters (about 1,700 with a diameter greater than 20 km; 15 with a diameter of 300–1200 km) are the most visible result of this bombardment.

On Earth, this bombardment destroyed most of the earth's crust and caused the oceans to boil, killing all life except, probably, certain bacteria, probably extremophiles adapted to high temperatures. Terrestrial life was on the verge of extinction.

Prebiotic processes

In the second decade of the 20th century, the Russian biochemist Aleksandr Oparin, proposed that life originated in an environment like that of the Primitive Earth through a process of chemical evolution that initially led to the appearance of simple organic molecules.

The atmosphere would have been composed of gases (water vapor, hydrogen, ammonia, methane) that would have dissociated into radicals by the action of UV light.

The recombination of these radicals would have produced a shower of organic compounds, forming a primordial broth in which chemical reactions would have produced molecules capable of replicating.

In 1957, Stanley Miller and Harold Urey demonstrated, using an apparatus containing hot water and the Oparin gas mixture subjected to electrical sparks, that chemical evolution could have occurred.

This experiment produced simple compounds present in living things, including nucleic acid bases, amino acids, and sugars.

In the next step of chemical evolution, which has also been experimentally recreated, the previous compounds would have joined together to form polymers that would have aggregated to form protobionts. These are unable to replicate, but have semipermeable and excitable membranes like those of living cells.

Origin of life

Protobionts would have transformed into living beings by acquiring the ability to reproduce, transmitting their genetic information to the next generation.

In the laboratory, short RNA polymers can be chemically synthesized. Among the polymers present in the protobionts there must have been RNA.

When the magma solidified, initiating the formation of the crust of the Primitive Earth, the erosive processes of the rocks produced clay. This mineral can adsorb short RNA polymers onto its hydrated surfaces, serving as a template for the formation of larger RNA molecules.

In the laboratory, it has also been shown that short RNA polymers can function as enzymes, catalyzing their own replication. This shows that the RNA molecules could have replicated in the protobionts, eventually originating cells, without the need for enzymes.

The random changes (mutations) in the RNA molecules of the protobionts would have created variation on which natural selection could have operated. This would have been the beginning of the evolutionary process that originated all forms of life on Earth, from prokaryotes to plants and vertebrates.

References

1. Barge, LM 2018. Considering planetary environments in origin of life studies. Nature Communications, DOI: 10.1038 / s41467-018-07493-3.
2. Djokic, T., Van Kranendonk, MJ, Campbell, KA, Walter, MR, Ward, CR 2017. Earliest signs of life on land preserved in ca. 3.5 Ga hot spring deposits. Nature Communications, DOI: 10.1038 / ncomms15263.
3. Fowler, CMR, Ebinger, CJ, Hawkesworth, CJ (eds). 2002. The early Earth: physical, chemical and biological development. Geological Society, Special Publications 199, London.
4. Gargaud, M., Martin, H., López-García, P., Montmerle, T., Pascal, R. 2012. Young Sun, early Earth and the origins of life: lessons for astrobiology. Springer, Heidelberg.
5. Hedman, M. 2007. The age of everything - how science explores the past. University of Chicago Press, Chicago.
6. Jortner, J. 2006. Conditions for the emergence of life on the early Earth: summary and reflections. Philosophical Transactions of the Royal Society B, 361, 1877–1891.
7. Kesler, SE, Ohmoto, H. (eds.). 2006. Evolution of the early atmosphere, hydrosphere, and biosphere: constraints from ore deposits. Geological Society of America, Boulder, Memoir 198.
8. Lunine, JI 2006. Physical conditions on the early Earth. Philosophical Transactions of the Royal Society B, 361, 1721–1731.
9. Ogg, JG, Ogg, G., Gradstein, FM 2008. The concise geologic time scale. Cambridge, New York.
10. Rollinson, HR 2007. Early Earth systems: a geochemical approach. Blackwell, Malden.
11. Shaw, GH 2016. Earth's early atmosphere and oceans, and the origin of life. Springer, Cham.
12. Teerikorpi, P., Valtonen, M., Lehto, K., Lehto, H., Byrd, G., Chernin, A. 2009. The evolving universe and the origin of life - the search for our cosmic roots. Springer, New York.
13. Wacey, D. 2009. Early life on Earth: a practical guide. Springer, New York.
14. Wickramasinghe, J., Wickramasinghe, C., Napier, W. 2010. Comets and the origin of life. World Scientific, New Jersey.