Complex sentence: Distance-transitive graphs were first defined in 1971 by Norman L. Biggs and D. H. Smith, who showed that there are only 12 finite trivalent distance-transitive graphs.
Split into simpler sentences: Distance-transitive graphs were first defined in 1971 by Norman L. Biggs and D. H. Smith. After introducing them in 1971, Biggs and Smith showed that there are only 12 finite trivalent distance-transitive graphs.