Final answer:
Mitochondria and chloroplasts in eukaryotic cells originated through the process of endosymbiosis, where ancestral eukaryotic cells engulfed free-living prokaryotes, leading to a permanent symbiotic relationship.
Step-by-step explanation:
Origin of Mitochondria and Chloroplasts in Eukaryotic Cells
The origin of mitochondria and chloroplasts in the first eukaryotic cells is best explained by the endosymbiotic theory. This theory posits that the mitochondria and chloroplasts were once free-living prokaryotes that were engulfed by larger ancestral eukaryotic cells. The enduring symbiotic relationship between the host cell and the engulfed prokaryotes eventually led to the prokaryotes becoming permanent organelles within the eukaryotic cells. Specifically, an ancestral cell absorbed an aerobic bacterium, giving rise to mitochondria. Similarly, the absorption of a photosynthetic bacterium, likely a cyanobacterium, led to the development of chloroplasts.
According to the endosymbiotic theory, mitochondria likely appeared first, followed by chloroplasts as the eukaryotic cells evolved. These organelles contain their own DNA, which closely resembles that of prokaryotes, providing strong evidence for their prokaryotic origins. This relationship has been incredibly successful and is a defining characteristic of eukaryotic cells, leading to the vast diversity of eukaryotic life forms we observe today.