Origin of Eukaryotes
Time Period: Proterozoic

The oldest eukaryotic fossil is approximately 1.5 billion years old. The origin of the eukaryotes must have appeared before because the fossil is of a relative complex single-celled organism.

Biologists are almost certain that eukaryotes evolved from prokaryotes because:

1. Both use RNA and DNA are the genetic material
2. Both use the same 20 amino acids
3. Both have ribosomes and DNA and RNA
4. Both have a lipid bilayer cell membrane.
5. Both use L amino acids and D sugars

Biologists are also almost certain that eukaryotes evolved only once (i.e., are monophyletic- descendants of a single common ancestor) because they all share:

1. microtubules (composed of the protein tubulin) and actin molecules-
2. DNA in chromosomes (intertwined with histone protein)

3. membrane-bound organelles.

What steps would have led from prokaryotes to eukaryotes?

There is no fossil record recording the evolution of the eukaryotes. So most hypotheses have been inferred from studying the biology of modern organisms.

Perhaps the most significant difference between prokaryotic and eukaryotic cells, if we want to understand the origin of the later, is that prokaryotes are inclosed in a rigid cell wall, whereas eukaryotes usually are not and can change their cell shape easily.

The absence of the cell wall does, however, mean that the eukaryote needs some other way of supporting and strengthening the cell surface (a wall-less naked cell membrane is very fragile). Eukaryotes have evolved a complex cytoskeleton consisting of two classes of molecules:

1. Actin filaments - resist pulling forces and convert chemical activity into mechanical motion. This is an endothelial cell stained to show actin filaments in the cytoskeleton:

2. Microtubules - resist compression and shearing forces and can be used as rails to slide components through the cytoplasm. This is a picture of the microtubular bundles that support the cell mouths of ciliates (like Paramecium):

Actin makes a process called cytosis possible, while microtubules make mitosis and cell locomotion possible.

Cytosis is the ability of membranes to grow and fuse and it allows cells to both secrete substances efficiently (exocytosis) and to bring them into the cell more efficiently (phagocytosis).

Following the loss of a cell wall and the apearance of the cytoskeleton, there are two different stories to tell about the origin of eukaryotes, one for the origin of organelles such as mitochondria and chloroplasts, and another for the origin of the of other parts of the cell. Cytosis may explain the former:

Endosymbiosis - Origin of Mitochondria and Chloroplasts

One of the most fascinating concepts to gain popularity in recent times is the endosymbiotic theory for the origin of the eukaryotic cell

According to this theory:
€a prokaryotic cell capable of engulfing other prokaryotes, engulfed aerobic bacteria.