Likewise, Mammalia is a subgroup of Amniota, whose ancestor had an amnion, is a subgroup of Tetrapoda, whose ancestor had four limbs, is a subgroup of Vertebrata, whose ancestor had vertebrae, and so on. In the left column below is the entire detailed lineage for Eutheria and to the right are siblings and children of Eutheria. Evolution generally results in lineage splitting. The human lineage is forever separated from the mouse lineage because each has evolved for many millions of years along separate paths. If separated lineages could commonly fuse back together again to form a single lineage, then we would not expect patterns of similarities that are hierarchical.
Languages are mostly hierarchical.
French, Italian, and Spanish words can usually be traced back to a root word in a presumed common ancestral language. However, melding together of long separate languages can also occur. When the Romans invaded England, the necessity to communicate with each other had lasting impacts on each language. This phenomenon complicates the historical reconstruction of word origins. Judging from the neat hierarchical arrangement of similarities in organims, it is likely that such fusion is rare in lineages of organisms.