Institute of Botany, University of Düsseldorf, D-40225 Düsseldorf, Germany. tal.dagan@uni-duesseldorf.de
Two significant evolutionary processes are fundamentally not tree-like in nature--lateral gene transfer among prokaryotes and endosymbiotic gene transfer (from organelles) among eukaryotes. To incorporate such processes into the bigger picture of early evolution, biologists need to depart from the preconceived notion that all genomes are related by a single bifurcating tree.