What is simian crease?

Palm lines: Most folks have two lines across the palm joining the side below the little finger with the space between the thumb and index finger. If only one is present, as in some monkeys ("simians") & in 1% of normal folks, it's a tip-off that the person may have another birth defect, especially trisomy 21, but ultimately it's of no use as the other defects will be obvious.