The origin and evolution of model organisms
Key Points The number of model organisms has grown in concert with the increasing number of genome-sequencing projects. As a result, the 'tree of life', which was previously based on a ribosomal RNA gene sequence, has been replaced by one that is based on genome-wide sequence data. Eukaryotes are no longer considered to be close relatives of Archaebacteria, but rather genomic hybrids of Archaebacteria and Eubacteria. Large numbers of genes were transferred from prokaryotes to the eukaryote nucleus during symbiotic events. Owing to recent analyses of genomic sequences, phylogenetic support for the presence of hyperthermophiles — species that live at high temperatures — at the base of the tree of life has been weakened. Molecular-clock analyses have supported a more recent origin of Cyanobacteria than indicated by the fossil record. The date is now estimated to be closer to the time at which oxygen levels began to rise in the Earth's atmosphere, as indicated by geological evidence. Some cytological and genomic evidence support the existence of a premitochondrial period in the history of eukaryotes, although this topic has been debated. Whether the ancestors of some amitochondriate eukaryotes once had mitochondria also remains controversial. The basal position of liverworts among land plants has been uprooted, although an alternative phylogeny has not yet been established. Molecular-clock analyses indicate that the main groups of fungi diverged hundreds of millions of years earlier than indicated by the fossil record. The relationships between humans, nematodes and fruitflies continue to be debated, despite the knowledge of their complete genomes.AbstractThe phylogeny and timescale of life are becoming better understood as the analysis of genomic data from model organisms continues to grow. As a result, discoveries are being made about the early history of life and the origin and development of complex multicellular life. This emerging comparative framework and the emphasis on historical patterns is helping to bridge barriers among organism-based research communities.