system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do humans tend to open their mouth when they are surprised?
gpt
Human gestures are dominated by two functions: Reflex and social cues.

The gasp is likely a reflex inherited from distant ancestors who out competed others by quickly breathing air as an automatic response to surprise. They were perhaps better able to survive a flood, or were better prepared to outrun a threat. Who knows?

For whatever reason, we are descended from gaspers who gasped when surprised.

But we are also born into a community of gaspers. We often want to tell others that our feeling is authentic surprise - which really can't happen if all we do is say, "I am surprised." Performing elements of our shared gasp reflex (dropping the jaw, opening eyes wide, inhaling, etc) is a way of telling others how you feel be cause both sender and receiver inherently understands what a gasp is.