Table schema:
CREATE TABLE table_name_67 (sequence_length__aa_ INTEGER, common_name VARCHAR, divergence_from_human_lineage__mya_ VARCHAR)

Question: 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?
SQL: SELECT SUM(sequence_length__aa_) FROM table_name_67 WHERE common_name = "purple sea urchin" AND divergence_from_human_lineage__mya_ < 742.9