Complex sentence: Tubman began her underground railroad work in 1850 and continued until the beginning of the Civil War, so it's possible Tubman's use of the song predates the origin claimed by Lockwood.
Split into simpler sentences: Tubman began her underground railroad work in 1850, and continued until the beginning of the civil war. So it's possible Tubman's use of the song predates the origin claimed by Lockwood.