Final answer:
Modern non-human primates can learn sign language to a certain extent, as proven by studies involving gorillas and bonobos. These studies have shown that primates can use learned signs to communicate and even create new signs, offering insights into the cognitive and linguistic abilities shared by humans and our closest relatives.
Step-by-step explanation:
Modern non-human primates such as chimpanzees, gorillas, bonobos, and orangutans are indeed capable of learning American Sign Language and other symbolic systems when in controlled conditions. The studies of primatologist Dr. Francine Patterson with Koko the gorilla, and those of Sue Savage-Rumbaugh with bonobos Kanzi and Panbanisha, have shown that these apes can learn a basic vocabulary, use signs to communicate, and even create new signs. Koko was reported to understand about a thousand signs and demonstrate complex cognitive behaviors such as joking and lying, although these claims are debated among researchers. On the other hand, Kanzi and Panbanisha were able to use a computer-based language program with lexigrams, showing comprehension of verbal commands higher than that of a human toddler in controlled testing environments. The study of non-human primates' learning capabilities contributes to our understanding of the cognitive linguistic skills that separate humans from animals but also illuminates the biological features that enable communication. Questions remain as to whether these primates display these abilities due to innate capacity or learning through teaching, and whether learning symbolic systems has altered their brains in unique ways. Nevertheless, these language studies in non-human primates provide insights into the evolution of human language and cognition, as well as the cultural capabilities of these animals.