We generated from a single blood sample five independent human monoclonal antibodies that recognized the Sa antigenic site on the head of influenza HA and exhibited inhibitory activity against a broad panel of H1N1 strains. All five Abs used the V_H3-7 and J_H6 gene segments, but at least four independent clones were identified by junctional analysis. High throughput sequence analysis of circulating B cells revealed that each of the independent clones were members of complex phylogenetic lineages that had diversified widely using a pattern of progressive diversification through somatic mutation. Unexpectedly, B cells encoding multiple diverging lineages of these clones, including many containing very few mutations in the antibody genes, persisted in the circulation. Conversely, we noted frequent instances of amino acid sequence convergence in the antigen combining sites exhibited by members of independent clones, suggesting a strong selection for optimal binding sites. We suggest that maintenance in circulation of a wide diversity of somatic variants of dominant clones may facilitate recognition of drift variant virus epitopes that occur in rapidly mutating virus antigens, such as influenza HA. In fact, these Ab clones recognize an epitope that acquired three glycosylation sites mediating escape from previously isolated human antibodies.