The human nuclear genome includes hundreds of genes that are orthologs of bacterial genes, and hundreds of other genes that are orthologs of archaean genes. This finding can be explained by proposing that
Answer
neither archaea nor bacteria contain paralogous genes.
horizontal gene transfer did not occur to any significant extent among the prokaryotic ancestors of humans.
the infection of humans by bacteriophage introduced prokaryotic genes into the human genome.
the eukaryotic lineage leading to humans involved at least one fusion of an ancient bacterium with an ancient archaean