Many people in ancient times were illiterate and uneducated. Although some people may be uneducated, there are many occasions that need commitment. If you want to promise, you have to leave some evidence, as the saying goes? Say it without evidence? Many things can't be explained clearly with your mouth, so you need to sign at this time. But many people don't know how to write their names at all, which is very troublesome. Later, it suddenly occurred to people that everyone's fingerprints are different, and they can confirm themselves by pressing a mark with their own fingers, so it became so popular to use chess pieces instead of signatures.
Many people may wonder why there was no so-called fingerprint recognition in ancient times. What I want to say is that fingerprints are everyone's own attributes. Many people's fingerprints are very different and it is not easy to imitate. Now we can extract fingerprints, but it was very difficult at that time. Moreover, it is easy to identify fingerprints in front of those professionals. Even now, some people can distinguish the fingerprints of ordinary people that look exactly the same with the naked eye. Therefore, in ancient times, signing vows was a very safe way, which was difficult to imitate, convenient and quick, and saved a lot of unnecessary trouble.
Moreover, the ancients at that time thought that fingers had souls. Once they recognized this, they would be cursed if they violated what they had said. So many people generally don't take fingerprints seriously, they will take them seriously. Therefore, it is convenient and safe, avoiding the embarrassment of not signing. This is really a great invention.