Prokaryotic Cell and Eukaryotic Cell Evolution

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

explained is how to get these polypeptides to be self-replicating. Only nucleic acids can selfreplicate so these were necessary for life.

Because it has been shown that RNA can catalyze reactions and serve as a template in order to catalyze its own self-replication, it is widely believed that RNA was the original nucleic acid in early forms of life. The time period where this happened is called the RNA world. As we have mentioned, DNA later replaced RNA as the main genetic material in cells.

When RNA became enclosed by a phospholipid bilayer, this would have become the first cell. A phospholipid bilayer is necessary for a cell to actually be a cell. Plasma membranes made of lipid bilayers are part of both prokaryotic and eukaryotic cells. Phospholipids are considered amphipathic, which means that they have a water-loving end and a water-hating end. They arrange themselves so that the water-loving end is exposed to the exterior and interior of the cell.

Cells also need to metabolize food to make energy. This is something that also had to evolve. All cells of any type use ATP energy as their main energy source. ATP is also used for movement of the cell. Some early cells only used a pathway called glycolysis to make ATP energy and some of these fermented things like sugars. Later cells used photosynthesis, while more advanced cells use oxygen in the oxidative phosphorylation pathway that makes the most ATP energy. These pathways altered the atmosphere of the earth.

Glycolysis and fermentation do not require oxygen so these pathways were present in cells when there was little oxygen in the atmosphere. Photosynthesis was the next evolutionary step, which uses energy gotten from the sun. The first photosynthetic cells came about 3 billion years ago. Probably, hydrogen sulfide and CO2 were used in these early cells. Using H2O or water did not come until there was plenty of water on earth.

When oxygen became prevalent, oxidative metabolism evolved. This makes use of oxygen to make ATP energy, with water and CO2 as end-products. This process is much more efficient in oxidative phosphorylation than it is in glycolysis, which gave these organisms an evolutionary advantage over glycolytic cells.

There are two types of prokaryotes in present time. The first is archaebacteria or Archaea and the second is eubacteria or just bacteria. Archaea are known for their extreme environments but it doesn t have to be the case. Thermoacidophiles live in certain hot sulfur springs that are acidic and hot. Some are also human pathogens.

Bacteria can be of several different shapes with small to larger genomes, depending on the organism. Cyanobacteria participate in photosynthesis as their major feature. Most bacteria are not human pathogens; however, there are some that are always pathogenic in nature.

Eukaryotes have ribosomes and a plasma membrane like prokaryotes but all parts of eukaryotes are considered more complex than the same parts seen in prokaryotes. Eukaryotes are larger than prokaryotes and they are largely more interesting than prokaryotic cells. Figure 8 shows what a eukaryotic cell looks like:

Figure 8.

Eukaryotic cells are compartmentalized, which isn t as much the case in prokaryotic cells. We will talk more about eukaryotic cell evolution in the next chapter but, for now you should know that the chloroplasts and mitochondria have interesting origins from an evolutionary perspective. Mitochondria are where much of the ATP is made, while chloroplasts are responsible for photosynthesis. Lysosomes and peroxisomes are present in the cell in order to break down and digest other molecules. Plant cells contain vacuoles that have a variety of functions for the cell.

Eukarytoic cells also have a cytoskeleton that helps to give the cell its structure and that helps to organize the different organelles so they stay in place. Some parts of the cytoskeleton are involved in cell movement.

Eukaryotes were first on earth about 2.7 billion years ago, which is more than a billion years after prokaryotes. It was once thought that Archaea and Eubacteria were very similar but DNA analysis has shown they are actually very different. Some early evolutionary event must have happened to separate these lines of descent. Archaea are actually more similar to eukaryotes than eubacteria are related to eukaryotes. This indicates that the ancestors of archaea and eukaryotes were probably more similar to these cells than they were to bacteria.

We will talk more about how multicellular organisms evolved in a later chapter. Many eukaryotic cells are unicellular but some are more complex than others. Yeast organisms, for example, are more complex than bacteria and contain more DNA per cell. Some unicellular eukaryotes have pseudopodia or false feet”, such as amoeba, which help them move from one place to another.

Multicellularity developed about 1.7 billion years ago. Cells of algae for example will form multicellular colonies and will share resources. These are precursors to actual multicellular organisms. Ultimately, cellular specialization and division of labor took place in order to make what are complex organisms today. Many multicellular organisms have highly specialized cells that do many different things. Animal cells tend to have more complexity than plant cells.