Can you explain the endosymbiotic theory of the origin of eukaryotic cells?

Question

What is an example of evidence supporting this theory?

Answer 1

First of all the eukaryotic domain includes animals, plants, fungi, and protists. So my counter question would be which eukaryotic cell. In general the endosymbiotic theory states that organelles of eukaryotic cells developed from acquired of proteobacteria (-> mitochondria) and cyanobacteria (-> chloroplasts). Plants host chloroplasts and mitochondria. Animals have only mitochondria.

Answer 2

The endosymbiotic theory addresses the origins of certain intracellular organelles.  Both mitochondria (animals and plants) and chloroplasts (plants) have strucutres that resemble those of bacteria.  It has been hypothesized that early on in eukaryotic evolution that an phagocytised bacterial cell somehow evaded digestion and survived in a symbiotic relationship inside the eukaryotic cell.  After many millions of years, the internalized bacteria eventually became an integral part of the eukaryotic cell.  

Answer 3

Both mitochondria and chloroplasts contain their own circular DNA, double membranes, and ribsomes that are similar in structure to prokaryotic cells.  This leads us to believe that they once were cells which became part of a larger cell and stayed because of mutualism.  They thus evolved together.

Answer 4

The short answer is mitochondria.  I'm too tired to answer in full, but google should show you.  ;-)