Figure 1.9 The Tree of Life The earliest division of life was between the Bacteria and Archaea. Later, one lineage of Archaea incorporated specializations (including a cell nucleus) that allowed the repeated evolution of multicellularity; we call this specialized group Eukarya. The aqua branches within Eukarya represent various groups of microbial eukaryotes, more commonly known as “protists.” The organisms on any one branch share a common ancestor.