Here's why:

* Endosymbiotic Theory: This theory proposes that mitochondria and chloroplasts, organelles found in eukaryotic cells (like plants and animals), were originally free-living bacteria that were engulfed by larger cells. Over time, these bacteria developed a symbiotic relationship with the host cell, eventually becoming integral parts of the cell.

* Shared Proteins: The presence of the same protein in mitochondria, chloroplasts, and bacteria supports the idea that these organelles were once independent organisms. The protein likely served a similar function in the ancestral bacteria and was retained when they became integrated into the host cell.

In summary, the shared protein is a molecular "fossil" that points to the common ancestry of mitochondria, chloroplasts, and bacteria.