I was rereading Tom Sawyer the other day and Tom trades a fish hook to another boy for something. What would they have used as line in that age?
Fishing with hook and line is ancient, at least 4-7 thousand years old. This is evident from bone fishhooks found in archaeological sites in both the old and new worlds. Fishing with a variety of methods including hook and line is depicted in Egyptian tombs dating to 4000 years ago. Modern fishing is documented by the late 17th century. There were a variety of materials being used for leaders including gut, braided horsehair and braided silk. Thomas Barker in 1667 mentions a salmon-fishing line of 26 yards suggesting that the organic materials could be wound or braided to extend a considerable distance. That they were being used for salmon fishing suggests considerable durability. A photograph of George Washington's fishing kit is in the Library of Congress here. His kit included horsehair and silk lines. Gut, silk and braided horsehair in various combinations persisted to the mid 20th century when artificial materials like nylon, dacron and polyethylene were developed.
Izaak Walton, The Compleat Angler (1653)
Thomas Barker, The Art of Angling Augmented (1664)