Native Sign Language

Nothing without Fundamentals

The earliest humans invented fundamentals of communication as a primary means of survival, whereas the other mammals and even other primates relied on evolved instincts to avoid harm and embrace health and life.
The early Mesopotamians developed written maps and mathematics and astronomy required to become formidable sailors.
The early Phoenicians developed an alphabetic writing system useful to coordinate merchant finance, military might, and every other human endeavor.
Yet before them, in fact perhaps before even spoken language, humanity had to develop a person to person means of communication, a Sign Language.

Up from Zero

Sign Language probalby began with very animalistic notions. Is this person interested in killing me, mating with me? And am I interested in doing the same to them? Quickly the human empathy, attraction to one another, socialization, and general capacity to team up for mutual benefit brought the need for more complex ideas to be communicated and debated.
Sign languages can be used to convey important concepts silently from a long distance (in daylight, provided good weather conditions). Military uses abound among allies, and at a much longer range the concept of, "if I get ahold of you, I'll beat the crap out of you," can be communicated with near 100% success and retention, using only arm movements and facial expressions.
In the Americas, separate tribes constantly moving led to disparity of languages. Communication between tribes fell back to a more universal and fundamental Indigenous Sign Language for the same purposes of trading, military action and alliances, and communicating threats, as well as entertainment on occassion, one would imagine.

Still in use in Modern Times

Deaf people and those temporarily in conditions of spotty communication but reasonable visibility can benefit from knowing the basics of Sign Language.
Undersea divers can communicate emergencies in specifics of what is going wrong. They can also divulge their intentions and give orders to subordinates as well as pass along simple information such as, "I will explore over there one hour, then I will return to the surface. You meet me there."
Astronauts generally have equipment which allows spoken language to be transmitted by radio and translated into the native language of the receiver, but in space things go wrong just like anywhere else, so being able to communicate to a passing vessel, "We are low on oxygen and an alien parasite is eating our communication antennae array," could be very helpful to the survival of all parties.