Lineages of Archaea and Bacteria with the NifD sequences listed in Tables 1 and 2.

*Lineage data were obtained from the NCBI Taxonomy Browser at http://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi. Information on the lineage for the each of the organisms examined in this study is provided in the Supplementary Material in Tables S1 and S2.