The Birth of Cells

Life has existed on Earth for more than 4 billion years, yet the precise origin of the first cells remains a mystery. Our understanding of what Earth was like when this occurred has allowed us to develop a few theories regarding the birth of cells. Firstly, most of the surface of the planet was covered in water. Moreover, the atmosphere did not have molecular oxygen or an ozone layer. As such, the surface of the Earth was exposed to considerably more UV radiation from the sun. Given these conditions on the planet, our best guess of where simple single-cellular life may have first flourished is somewhere in oceanic depths near hydrothermal vents, or beneath the surface of the Earth. There are specific requirements that had to be fulfilled for cells to have arisen—structures that permitted molecules to come together to form a cell. In particular, hereditary material such as DNA and RNA, and a structure for compartmentalization such as a cell wall or membrane, were needed. The formation of these structures is the result of prebiotic chemical and physical processes.The RNA world hypothesis is a widely accepted theory that suggests that self-replicating ribonucleic acid molecules are the precursors to prokaryotic cells. Over time, RNA developed enzymes allowing it create polymers of amino acids, eventually leading to the more stable DNA. In the first cells, however, it is believed that a more primitive version of RNA acted as the hereditary material.

Fatty acids in water, given a level above a threshold concentration, spontaneously form a bilayer, a physically stable, low-energy configuration. This bilayer takes on a spherical shape. As such, this create a structure that envelopes water within its walls, a sort of vesicle. Again, this occurs as a direct result of the tendency of matter to organize itself into its most stable state. Supplying the vesicle with more fatty acids (in the form of organic compounds and the energy needed for the formation of said fatty acids) allows it to grow and divide. When the size of the bilayer increases by a fixed amount, its volume increases more than its surface area. And if the contents within the vesicle are not changed, the bilayer will naturally cave around its equator, resulting in a dumbbell-shaped vesicle, a first step towards cell division. Furthermore, hereditary material can become embedded in phospholipid bilayers, and with energy from hydrothermal vents, it is possible that this union of the two structures led to the first cell. Lastly, RNA within the cell, capable of replicating, would also split into both parts of the vesicle when a the bilayer begins to divide. This results in a cell with a membrane, hereditary material, and the ability to reproduce. The bilayer composed of fatty acids and the hereditary material are some of the most important structures needed for simple single-cellular life to have developed.

Sources:

Lane, Nick. The Vital Question: Energy, Evolution, and the Origins of Complex Life. New York: W.W. Norton, 2015. Print.

- Ricardo Roche