Evolutionary hypothesis for the early diversification of the biotin-dependent carboxylases. Colored blocks represent functional protein domains. A biotin-dependent carboxylase content similar to that of modern archaea was present in the cenancestor. It was conserved in Archaea but in Bacteria these enzymes were duplicated and fused several times, bearing to the very diverse modern biotin-dependent carboxylase family found in these organisms. Eukaryotes received their enzymes by horizontal gene transfer (HGT).