The lines a chiromant reads are flexion creases, and they have an anatomy of their own that is independent of anything read from them. The primary creases form in the womb, around the twelfth and thirteenth weeks, as the hand begins to fold; they mark where the skin is tethered to the fascia beneath so that it hinges cleanly instead of bunching. They are set before birth and change very little afterward, which is the first thing a reader should know about them.
What the tradition calls the Life Line is the thenar crease, curving around the base of the thumb, the third primary. It follows the edge of the thumb muscle and its shape is largely a consequence of how that muscle sits, which is worth weighing against any reading of its length as a span of years.
It is worth separating chiromancy from dermatoglyphics, which is often confused with it and is a different subject. Dermatoglyphics studies the ridge patterns of the skin, the loops and whorls used in fingerprinting, and it has a genuine research literature including work on developmental correlates. Palmistry reads the flexion creases, which are a different structure entirely, and there is no body of evidence connecting crease form to character, career or lifespan. The atlas records what the chiromantic tradition says about this line because that tradition is a real object of study. It does not endorse the reading.