The tree of life. This relatively simple tree includes only a few of the many sequenced genomes, but it illustrates some of the complexities of generating a complete tree of life. One crucial complicating factor is the tendency of living systems to occasionally incorporate DNA into their genomes from other sources by horizontal gene transfer (orange arrows). Other factors are the assimilation of bacteria as organelles (mitochondria and chloroplasts; blue and green dashed arrows, respectively) and the subsequent loss of such organelles in some evolutionary lines (red dashed arrows).