Results: Evidence of Endosymbiosis

- Modern scientists believe that certain organelles found exclusively in eukaryotic cells may have arisen from various prokaryotic ancestors that initiated endosymbiotic relationships with host cells that consumed them. This belief is supported by evidence such as the discovery of DNA and ribosomes within organelles like mitochondria and chloroplasts. Examining the DNA has led scientists to find similarities in the sequences to those of modern living bacteria.

- Furthermore, the existence of double membranes over many of these organelles suggests the possibility that the inner membrane may have belonged to the original prokaryote while the outer membrane may have formed from food vacuoles as the host cell devoured the prokaryote. The inner membrane of these organelles contains enzymes and transport systems that are similar to the plasma membrane of prokaryotes. In addition, certain species of modern organisms such as amoebas have been known to live, via endosymbiosis, with aerobic prokaryotes.

- The timeline of life on Earth:

a. 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. b. Photosynthetic bacteria: About ~3.2 billion years ago, fossil evidence of photosynthetic bacteria, or cyanobacteria, appears. These bacteria use the sun's energy to make sugar. Oxygen, released as a byproduct, began to accumulate in the atmosphere. However, oxygen is actually pretty toxic to cells, even our cells! As a result, anaerobic cells were now a disadvantage in an oxygen-containing atmosphere, and started to die out as oxygen levels increased. [Image] c. Aerobic cells appear in the fossil record shortly after that (~2.5 Billion years ago). There cells were 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'.

- Organelles have their own DNA, and divide independently of the cell they live in:

When Margulis initially proposed the Symbiotic Theory, she predicted that, if the organelles were really bacterial (prokaryotic) symbionts, they would have their own DNA. If her theory DID best explain the origin of eukaryotic cells, she reasoned, organelles would have DNA that resembled bacterial DNA and be different from the cell's DNA (located in the nucleus membrane). Amazingly, in the 1980's this was proven to be the case for two classes of organelles, the mitochondria and chloroplasts. Further, in the late 1980's a team of Rockefeller University investigators announced their similar discovery regarding centrioles, structures that provide the eukaryotic cell with the ability of locomotion and cell division.