The ancestors of eukaryotes developed mitochondria and chloroplasts through a process known as endosymbiosis. This theory suggests that ancient prokaryotic cells engulfed other prokaryotes, which eventually became the organelles we now recognize in eukaryotic cells.
Initially, these engulfed cells were likely free-living bacteria. The most widely accepted scenario involves a symbiotic relationship where the early ancestors of mitochondria were proteobacteria, while the ancestors of chloroplasts were cyanobacteria. The engulfed organisms provided essential functions, such as energy production and photosynthesis, in exchange for protection and access to resources within the host cell.
Over time, the engulfed cells and their hosts became highly integrated. The engulfed cells transferred much of their genetic material to the host’s genome, leading to the loss of their autonomy. This mutual dependence resulted in a profound evolutionary innovation, allowing eukaryotic cells to exploit new ecological niches.
As a result, the incorporation of these organelles enabled the development of complex multicellular life and paved the way for the vast diversity of eukaryotic organisms we see today.