1. How did eukaryotic cells evolve from unicellular organisms?
Endosymbiotic Theory
How did eukaryotic cells evolve from unicellular organisms?

2. Universal Tree of Life

3. Endosymbiotic Theory
Explains the rise of eukaryotic cells from simple prokaryotic organisms.
Focuses mainly on the origins of chloroplasts and mitochondria

4. The timeline of life on Earth:
First organisms to inhabit Earth was Anaerobic bacteria: 
Scientists have fossil evidence of bacterial life on Earth ~3.8 billion years ago.
At this time, the atmosphere of the Earth did not contain oxygen, and all life (bacterial cells) was anaerobic.

5. Anaerobic Prokaryotes Thrived

6. Rise of the photosynthetic bacteria
About ~3.2 billion years ago, fossil evidence of photosynthetic bacteria, called cyanobacteria, appeared
These bacteria use the sun's energy to make sugar. 
Oxygen, released as a byproduct, began to accumulate in the atmosphere

7. Oxygen is poisonous to anaerobic prokaryotes
As a result, anaerobic cells were now a disadvantage in an oxygen-containing atmosphere, and started to die out as oxygen levels increased.
Aerobic cells appear in the fossil record shortly after that (~2.5 Billion years ago). Their cells were able to use that 'toxic' oxygen and convert it into energy (ATP) and water. Organisms that could thrive in an oxygen-containing atmosphere were now 'best suited to the environment'.

8. Endosymbiotic Theory
First proposed by former Boston University Biologist Lynn Margulis in the 1960's
Proposed that aerobic bacteria were preyed upon and ingested by anaerobic bacteria 
Each had a survival advantage as long as they continued their partnership.

9. Symbiosis ~ “living together, mutually beneficial”
The aerobic bacteria would have handled the toxic oxygen for the anaerobic bacteria
The anaerobic bacteria would provide food and protection the aerobic bacteria

10. Over time, the aerobic bacteria lost the ability to live on its own

11. Which organelle do you think the ingested aerobic bacteria would eventually evolve into?

12. How does the endosymbiotic theory explain photosynthetic eukaryotic cells?

13. Explain how eukaryotic cells evolved from prokaryotic cells
Talk to a Neighbor
Explain how eukaryotic cells evolved from prokaryotic cells

14. Evidence to Support Endosymbiosis Theory
Organelles similar size of smaller bacterial cells
Organelles are separated from the cytoplasm by membranes similar to cell membrane of bacterial cells.
Organelles reproduce by simple fission like bacterial cells
Some organelles have simple (often circular) DNA
DNA of the organelles are more closely related to particular types of bacteria than they are to the nucleus of the cell in which they reside.