What is the argument that, although human mitochondria all descend from a single ancestral ...

By comparing the genomes of organelles with those of proteobacteria, there is a strong degree of conservation between the genes of mitochondria and of proteobacteria. Human-like mitochondria can be traced to a time before the evident evolution of humans. Thus, mitochondria from one human to the next have not diverged. Over time, the nuclear genomes of organisms retained their own informational genes, and many endosymbiont operational genes were transferred to the nuclear genome and often replaced their functional equivalents. Unlike the cases of the mitochondria and chloroplasts, where the endosymbionts can be traced to specific lineages of Bacteria, the putative ancestor for the human nuclear genome may have come from several lineages after the endosymbiosis event.