When a baby is startled, even if it’s asleep, it will often go like this:
I’m not sure how much scientific literature is out there about this particular reflex. Someone I was talking to idly theorized that its roots go back to humanity’s more simian ancestors, when the possibility of falling out of trees was high; the purpose was perhaps to allow the baby to catch onto a branch and break its fall.
This seems unlikely. New humans spend years developing the strength and coordination they need for purposes ranging from from athletic or gymnastic activities (running, jumping, climbing, etc.) to basic tasks and abilities like holding their heads up and looking around. The chances of a newborn managing the split-second timing necessary to catch something, much less possessing the grip strength to support its own weight and thus to stop or slow its fall, are essentially nil.
What other functions could the reflex serve, then? It could be to startle predators and drive them away. Unexpected motion will spook most animals, even hunting animals. It could be a signal to parents or other caretakers that the baby is in some sort of danger. I fear, though, that the reason really is related to falling.
I have read that when an adult falls a long way, they tend to wind up with foot and/or leg injuries. We instinctively try to land on our feet, because they’re simply more expendable than all the organs above the waist. When an infant falls, however, it has no ability to control its orientation… and babies, with their proportionally large heads, are very top-heavy. Babies who fall will tend to fall head-first.
Where are infants often injured after a fall? In their arms. The statistics suggest that the arms-up reflex is to protect the head. This may be from dangers in general – animal bites, loose objects that might strike it, and so on – rather than from falling damage specifically, but the basic idea is the same.
The sad part of all this is that sufficient numbers of human or pre-human babies must have been put in danger over the course of evolutionary time that the V pose became ingrained as instinct. We get the protective reflex as a legacy from the survivors… but first, somewhere in our past there must be a great number who did not survive, and second there must also be a great number who survived, but at the cost of some sort of injury to the arms. “Nature red in tooth and claw” indeed.