CREATE TABLE table_75250 ("common name" CLOB, "protein name" CLOB, "divergence from human lineage  ( MYA ) " FLOAT, "accession number" CLOB, "sequence length  ( aa ) " FLOAT, "sequence identity to human protein" CLOB)

What is the sequence length (aa) of the protein with the common name Purple Sea Urchin and a divergence from human lineage less than 742.9?
SELECT SUM("sequence length (aa)") FROM table_75250 WHERE "common name" = 'purple sea urchin' AND "divergence from human lineage (MYA)" < '742.9'
SELECT SUM("sequence length (aa)") FROM "table_75250" WHERE "common name" = 'purple sea urchin' AND "divergence from human lineage (MYA)" < '742.9'