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

Universal Tree of Life

Endosymbiotic Theory

The timeline of life on Earth: First organisms to inhabit Earth was Anaerobic bacteria: 3.8 billion years ago. Earth’s atmosphere did not contain oxygen.

Anaerobic Prokaryotes Thrived

Rise of the photosynthetic bacteria 3.2 billion years ago, photosynthetic bacteria, called cyanobacteria, appeared Used Photosynthesis Oxygen, released as a byproduct, began to accumulate in the atmosphere

Oxygen is poisonous to anaerobic prokaryotes Anaerobic cells were now a disadvantage Aerobic cells appear ~2.5 Billion years ago. Able to use cellular respiration. Organisms that could thrive in an oxygen-containing atmosphere were now 'best suited to the environment'.

Endosymbiotic Theory Proposed that aerobic bacteria ingested by anaerobic bacteria  Each had a survival advantage as long as they continued their partnership.

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

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

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

How does the endosymbiotic theory explain photosynthetic eukaryotic cells?

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

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.