Origin of Eukaryotic Cells

What theory explains the origin of eukaryotic cells?

One of the most important events in the history of life was the evolution of eukaryotic cells from prokaryotic cells. Remember that eukaryotic cells have nuclei, but prokaryotic cells do not. Eukaryotic cells also have complex organelles. Virtually all eukaryotes have mitochondria, and both plants and algae also have chloroplasts. How did these complex cells evolve?

Endosymbiotic Theory Researchers hypothesize that about 2 billion years ago, some ancient prokaryotes began evolving internal cell membranes. These prokaryotes were the ancestors of eukaryotic organisms. Then, according to endosymbiotic (en doh sim by AHT ik) theory, prokaryotic cells entered those ancestral eukaryotes. These intruders didn't infect their hosts, as parasites would have done, and the host cells didn't digest them, as they would have digested prey. Instead, the small prokaryotes began living inside the larger cells, as shown in Figure 19–18.

The endosymbiotic theory proposes that a symbiotic relationship evolved over time, between primitive eukaryotic cells and the prokaryotic cells within them. This idea was proposed more than a century ago. At that time, microscopists saw that the membranes of mitochondria and chloroplasts resembled the cell membranes of free-living prokaryotes. This observation led to two related hypotheses.