Origin of Eukaryotes

Chapter Six: Major Life Transitions

There is more to be said about evolution than the evolution of single-celled organisms so this is the topic of this chapter. Eukaryotes are infinitely more complex than prokaryotes—even those that are unicellular. Many eukaryotic organisms are multicellular; for this reason, the evolution of multicellularity is discussed in this chapter. Because evolution happens to populations rather than to individuals, it is important to also talk about the evolution of individuality. There are advantages to evolving in a social environment, which is also covered in this chapter.

ORIGIN OF EUKARYOTES

As you remember, the eukaryotic organisms, also called Eukarya or Eukaryota, are one of three domains of life that also include Bacteria and Archaea. Most of the life that can be seen without a microscope represents eukaryotic cells. Plants and animals are both eukaryotic. These are more complex, have linear strands of DNA, have DNA within a nucleus, and have membranebound organelles.

Most plants are characterized by the presence of chloroplasts. Chloroplasts are entirely used for photosynthesis. Plants still have mitochondria for energy production but need chloroplasts to capture light energy in order to make complex organic nutrients.

Both plants and animal cells have mitochondria in the cells. Mitochondria are double-walled organelles that make the majority of the ATP necessary for cellular energy. The processes involved in oxidative phosphorylation take place within the mitochondria. Figure 9 shows the structure of a typical mitochondrion:

Figure 9.

Eukaryotic cells are likely the descendants of symbiotic prokaryotic cells that were once separate cells. It is believed that mitochondria were once separate prokaryotic cells that were taken up by larger prokaryotic cells. They may have been engulfed by the larger cell types but remained as symbiotic partners to create eukaryotic cells. This type of symbiosis is called endosymbiosis.

The evidence for endosymbiosis, including the following: 1) There are double membranes in both mitochondria and chloroplasts. No other organelles have a double membrane. 2) Mitochondria and chloroplasts have their own DNA, which happens to be circular. This is passed in humans from mother to child because only egg cells bring their mitochondria with them. 3) Mitochondria divide through binary fission or by pinching into two halves, which is how this is done with bacteria.

Chloroplasts are similar to mitochondria but, because of the way that evolution works, mitochondria likely were endosymbiotic in eukaryotes first and chloroplasts did this second. This is because all eukaryotes have mitochondria but only plant cells have chloroplasts. Neither chloroplasts nor mitochondria can live independently because the necessary proteins to make and operate them are found in the host genome and not in the mitochondrial or chloroplast genome.