Chimpanzee alarm calls are spoken warnings rather than cries of fear, research suggests.
The apes are saying "watch out" to friends who may be in danger, not just shouting because they are scared, scientists believe.
The findings contradict the widespread assumption that ape alarm calls are not produced intentionally.
This has led some experts to suggest that human language evolved from primitive gestures instead of vocal sounds.
Researchers working in Uganda presented wild chimpanzees with a moving snake model and monitored their vocal and behavioural responses.
They found the apes were more likely to produce alarm calls when close friends arrived on the scene.
Threatened chimps watched and monitored group members both before and during the calls. Crucially, they continued to call until all the group members were a safe distance away.
Lead researcher Dr Katie Slocombe, from the University of York, said: "These behaviours indicate that these alarm calls were produced intentionally to warn others of danger and thus the study shows a key similarity in the mechanisms involved in the production of chimpanzee vocalisations and human language.
"Our results demonstrate that certain vocalisations of our closest living relatives qualify as intentional signals, in a directly comparable way to many great ape gestures, indicating that language may have originated from a multi-modal vocal-gestural communication system."
Colleague Dr Anne Schel said: "Observing the chimpanzees reacting to the snake model was intriguing. It was particularly striking when new individuals, who had not seen the snake yet, arrived in the area: if a chimpanzee who had actually seen the snake enjoyed a close friendship with this arriving individual, they would give alarm calls, warning their friend of the danger. It really seemed the chimpanzees directed their alarm calls at specific individuals."
The research, reported in the online journal Public Library of Science ONE, was conducted in the Budongo Forest Reserve in Uganda.