Final answer:
1) Plant cells evolved from an endosymbiotic relationship, where larger prokaryotic cells ingested bacteria, leading to the development of mitochondria and chloroplasts, giving rise to eukaryotic cells according to the endosymbiotic theory.
Step-by-step explanation:
The organism that is believed to have evolved from the endosymbiotic relationship between a host cell and bacteria is plant cells. This evolutionary process occurred when a larger prokaryotic cell ingested, but did not destroy, aerobic bacteria that became mitochondria and autotrophic bacteria (cyanobacteria) that became chloroplasts. Throughout millions of years, this mutualistic relationship led to the origin of eukaryotic cells with these bacteria becoming specialized organelles. Scientists like Lynn Margulis have contributed significantly to the development of the endosymbiotic theory, which has now gained wide acceptance thanks to substantial genetic evidence and DNA sequencing that support the hypothesis.