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How and when did life begin?

When we think of life on Earth today, we imagine diverse sets of animals and plants. Life also includes tiny unicellular organisms called bacteria, some of which reside in our bodies. The three domains of life are Archaea, Bacteria and Eukarya.

The domain Archaea is made up of prokaryotic cells. These are tiny, unicellular organisms that don’t have a nucleus or other membrane-bound organelles. These kinds of organisms live in extreme environments, including very salty conditions and/or very hot areas.

The domain Bacteria is made up of prokaryotic cells. Bacteria is similar to Achaea, but they have different evolutionary histories. Bacteria can be found almost anywhere.

The domain Eukarya is made up of eukaryotic cells, which have a nucleus and organelles. It is also made up of several kingdoms.

The protista kingdom is made up of simple, mostly unicellular organisms. Some scientists actually want this kingdom to be broken down because of the vast differences among species in the group. Protozoans and algae are examples of species in this kingdom.

The fungi kingdom has unicellular and multicellular organisms. They get their nutrients through absorption. Yeast is an example of a species belonging to this kingdom.

The plantae kingdom is made up of multicellular organisms that gain nutrients through photosynthesis (the process of converting light and water to glucose — a simple sugar — and oxygen). Examples include flowers and mosses.

The animalia kingdom is made up of multicellular organisms that get nutrients through eating other organisms. We are an example of a species that belongs to this kingdom.

But how did all these archaea, bacteria, protists, fungi, plants and animals appear on Earth in the first place?

Although the earliest fossils are from around 3.5 billion years ago, it is estimated that life began around 3.8 billion years ago. The basic elements and compounds needed for any form of life, including the very first one, are hydrogen, phosphate, nitrogen, carbon dioxide, methane and ammonia. For these elements and compounds to interact with each other, they need the universal solvent: water. And life in general needs a source of energy.

Organisms can be thought of either as autotrophs or heterotrophs. Autotrophs make energy for themselves by themselves. Heterotrophs have to  consume other organisms in order to get energy. Thus, the first form of life had to have been an autotroph, because there weren’t any other forms of life for it to consume.

Amazingly, the first life formed  when Earth didn’t seem to have any environments suitable for life to make energy. In fact, the only  suitable places were hydrothermal vents: fissures on the ocean floor where seawater seeps into magma chambers and then ejects back out.

The Last Universal Common Ancestor, or LUCA, also points to the first life’s home being hydrothermal vents. It isn’t the first life form, but it is the earliest form of life that we can trace back to.

We don’t know what LUCA looked like (we don’t have fossil evidence), but we know it existed because scientists looked at the DNA of all living organisms  and saw a commonality. The commonality told scientists that LUCA lived in a really hot area that didn’t have any oxygen and got its energy from a chemical gradient.

This sounds exactly like the environment of hydrothermal vents. Hydrothermal vents on the Mid-Atlantic Ridge called Lost City are really alkaline (very high on the pH scale), don’t have much carbon dioxide and are rich in methane, which makes them hospitable for early forms of life.

And we can thank early forms of life for the rich diversity we see today.

VAAGEESHA DAS is a senior at Morgantown High School.

Information comes from: 

  • Kaiser, G. (2021, January 3). 1.3: Classification – The three domain system. Biology LibreTexts.; Marshall, M. (2019, February 27). 
  • Timeline: The evolution of life. New Scientist.; 
  • TED-Ed. (n.d.). The mysterious origins of life. TED.

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