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The earliest self-reproducing cell on Earth, our common ancestor, was probably as small as present-day bacteria. It gave rise to a very large and durable clone whose descendants must have been the only living occupants of the oceans for about one thousand million years. They reached astronomical numbers of separate, disjunct cells, and synthesized many new genes. Their small volume could not accommodate ever larger genomes and useful new genes replaced resident, less successful sequences, thus increasing diversity and the number of strains with highly specialized, distinct, bioenergetic potentialities. Also, selective pressure favored strains able to participate successfully in division of labor and in the sharing of diverse abilities in mixed communities, counterbalancing the limited capacities of individual genomes. Lateral gene transfer mechanisms appeared and were progressively improved, furthering the development of diversity. The prokaryotes' constructive evolution resulted in the formation of a worldwide web of genetic information, and a global bacterial superbiosystem (superorganism). By contrast, eukaryotic evolution of organisms has been typically Darwinian. Diversi®cation of eukaryotic organisms was, however, considerably enriched and accelerated by symbioses with prokaryotes.The more broadly diversi-®ed bioenergetic potential of prokaryotes considerably increased the diversity of eukaryotes. Without their participation, our biosphere would have remained much less diverse and less dynamic. Environmental homeostasis has been maintained all along by guided bacterial evolution.

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Nine hemin-deficient mutants of Salmonella typhimurium LT2 were isolated as neomycin-resistant colonies. Five of these mutants could be stimulated by A-aminolevulinic acid (A-ALA), thus representing hemA mutants. Since S. typhimurium LT2 is not able to incorporate hemin, the identification of the mutants not stimulated by A-ALA was made on the basis of the simultaneous loss of catalase activity and cytochromes. The hemA gene was mapped by conjugation in the trp region, probably in the order purB-pyrD-hemA-trp; the episome FT71 trp does not carry the hemA gene. Transductional intercrosses by phage P22 indicate that hemA 11, 12, 13, and 37 are at very closely linked sites, whereas hemA14 is at a more distant site in the same or an adjacent gene. No joint transduction was detected between hemA and trp or pyrF. The loci affected in the other hemin-deficient mutants were linked in conjugation to the pro+ marker (frequency of linkage, 88 to 97%), but cotransduction of the two markers could not be obtained. The episome F lac hem purE, which originates from Escherichia coli K-12, could complement these hemin-deficient mutants of S. typhimurium LT2. As a result, the sequence of the markers on the chromosome of S. typhimurium LT2 is probably pro heme purE, analogous to the sequence found in E. coli K-12. Thus, the chromosome of S. typhimurium also possesses two hem regions, with a location similar to that described in E. coli K-12. Research of the last years has shown a great similarity of chromosomal structure between Salmonella typhimurium LT2 and Escherichia coli K-12, two organisms for which we possess wellelaborated chromosomal maps (3, 11). This is not surprising since the two organisms are closely related and probably originated from a common ancestor, or one species evolved first, with the other having originated from the first by evolution. Recently we described the position of the hem loci on the chromosome of E. coh K-12 (9). The