Final answer:
Mitochondria and chloroplasts both originated from endosymbiosis with gram-negative bacteria, rather than evolving separately. Evidence for this includes their double membranes, prokaryote-like DNA, and similarities to bacterial ribosomes. The theory that mitochondria evolved before chloroplasts is supported by their presence in all eukaryotes, while plastids are found only in some.
Step-by-step explanation:
The statement that mitochondria and chloroplasts evolved separately, each from a common ancestor, is false. The prevailing scientific understanding is that mitochondria and chloroplasts originated from a process called endosymbiosis. This is when one cell engulfs another cell, and rather than being digested, the engulfed cell becomes a part of the host cell, eventually evolving into an organelle. In the case of mitochondria and chloroplasts, they likely evolved through endosymbiosis with gram-negative bacteria. This theory is supported by several lines of evidence.
Both chloroplasts and mitochondria share similar features with prokaryotic cells, such as having their own DNA, which is similar to that of bacteria, and having double membranes, which is characteristic of gram-negative bacteria. Indeed, the outer membrane of these organelles is thought to originate from the host cell's plasma membrane, and the inner membrane from the engulfed gram-negative bacteria's plasma membrane. Additionally, mitochondria and chloroplasts have their own ribosomes, which are more similar in size to prokaryotic ribosomes than to eukaryotic ones. These facts provide evidence for an endosymbiotic origin.
Moreover, mitochondria likely evolved before plastids, given that all eukaryotes possess mitochondria or mitochondria-like organelles, while plastids are only found in a subset of eukaryotes, such as terrestrial plants and algae. Therefore, it is believed that the ancestral eukaryote initially engaged in endosymbiosis with the ancestor of mitochondria, and subsequently, another endosymbiotic event led to the incorporation of the ancestor of chloroplasts, but only in certain lineages.
The idea that mitochondria and chloroplasts might have come from archaea or gram positive bacteria, or that they have a single membrane is incorrect, as evidence strongly supports their origin from endosymbiosis with gram-negative bacteria.