Viruses cannot form fossils but the genomes of many organisms do contain certain endogenous viral elements that are the remnants of ancient viruses that have inserted themselves into cellular genomes. There are hundreds or thousands of retrovirus sequences in most vertebrates. These help us study the evolution of viruses. They exhibit Darwinian natural selection. RNA viruses mutate more than other viruses because there are no good replication repair processes. Some mutations are lethal, most are silent, but a few are beneficial. Viruses can shuffle their genes so that they can exhibit genetic shift, making them more virulent. Other viruses change slowly over time in what is called antigenic drift. Each of these contribute to the emergence of new viruses. Several virus types have evolved to infect more than one species. Viruses evolve in order to become more infectious. This is enabled by their rapid response to natural selection and rapid mutation rates. Viruses are transmitted through droplets, like sneezing and coughing, airborne transmission, passed through breathing, waterborne transmission, vector transmission, and viruses that can live outside the host. Viruses that are transmitted from mother to child in utero, called vertical transmission, will have lesser virulence than those that are transmitted horizontally from one host to another.
PROKARYOTIC CELL AND EUKARYOTIC CELL EVOLUTION We have talked about prokaryotic cells and eukaryotic cells but have not officially defined them. Prokaryotic cells or prokaryotes are more primitive. They have nucleic acids (DNA) but do not have any enveloped or lipid-bound organelles. Eukaryotic cells or eukaryotes are more complex. They have a cell nucleus that contains their genetic material and have multiple types of membrane-bound organelles. Even with their differences, they did evolve from a single common ancestor that was likely some type of prokaryote. The first cell on earth first occurred about 3.8 billion years ago, which was about 750 million years after the earth itself was formed. No one knows exactly how this happened and it has not been reproduced in a laboratory system. When life first emerged, there was little oxygen on earth but there was plenty of CO2 and nitrogen along with carbon monoxide, hydrogen gas, and hydrogen sulfide. These are conditions that are most optimal for photosynthetic cells. We know that electrical discharges in the presence or inorganic molecules can make amino acids and simple organic molecules. The next step in forming life would have been the making of macromolecules. This can happen under certain conditions of heat in the presence of amino acids. The trick that cannot be
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