ABSTRACT
Evolutionarily related (homologous) proteins are very useful in trying to retrace both protein evolution and gene ancestry. Orthologous proteins, which are encoded by genes descending from a unique ancestor, can be used to reconstruct phylogenetic trees of organisms. Such a reconstruction may go as far as the last universal common ancestor, ifhomologues have been detected in the three domains of life (W oese et al., 1990). Paralogous proteins which are encoded by genes descending from copies of an ancestral duplication allow us to progress even further in understanding protein evolution. Since paralogy events occur independently of speciation and may be very ancient, they may help to describe the set of genes carried by ancestral organisms. In this chapter, we retrace the history of presentday carbamoyltransferases which appear to be encoded by paralogous genes, descending from ancestral genes which duplicated at least twice during the early evolution of the last universal common ancestor. This analysis helps to understand how early proteins have evolved and gives us some insights into the complexity of the gene set present in this universal ancestor.
