The theory and evidence for the endosymbiont (endosymbiotic) theory.

1. Theories about origin of cells
Endosymbiont theory
•Proposed by Lynn Margulis in 1966
•The first cells probably were prokaryotic and
there were no eukaryotic cells
•This theory explains how eukaryotic cells
might have evolved from prokaryotic cells

2. •it is believed that eukaryotic cells came
about through endosymbiosis and evolution
•at one time all cells were prokaryotic and
very simple: no nucleus, no mitochondria,
no chloroplasts, simpler DNA, etc
•at some point in time we think some
prokaryotes started living inside other
prokaryotes

3. How could this happen?
•probably a larger cell tried to eat a smaller
cell and the smaller cell didn’t die. Over
many generations this became a mutualistic
relationship where the two cells could not
live without each other
•the smaller cell became a mitochondrion
•we also think the same process happened
with chloroplasts

4. What’s the evidence?
•both mitochondria and chloroplasts have their own
circular DNA
•mitochondrial DNA is similar to a type of bacteria called
alpha proteobacteria and chloroplast DNA is similar to
cyanobacteria. This suggests they have a common
ancestor
•mitochondria and chloroplasts have inner membranes
with transport systems similar to ones found in some
bacteria
•we already know symbiosis is possible (we see it in other
organisms, even ourselves such as E. coli bacteria that live
in our guts)