Blimey...
It's VERY common and yes, it is a normal part of human development - the first thing they do when they can move bits of their own body is stick fingers and toes in the mouth - they want to put things in their mouth, its how they get fed for fucks sake!
As they are more able to move around, again a valuable way of exploring their environment involves putting things in the mouth.
For a significant part of a childs first 18 months(and beyond), there is a HUGE element of 'things in the mouth' going on, there is also a fair degree of frustration due to their lack of ability to communicate, and their curiosity running far ahead of their motor skills.
And with all that reinforcement for putting things in the mouth it is not remotely abnormal for biting to occur when a very small child is frustrated, or simply curious to see what happens.
Normal in this context means exactly that, it is the norm.
It has not changed its meaning, it does not mean its ok, its fun, lets just sit and watch little Quentin chew his sisters arm off.
It is not acceptable to allow this normal part of one childs development, cause another child pain and distress.
But it absolutely bloody is normal. We should normalize it. Because it is normal. We shouldn't accept it and allow children to do it unchecked, or not bother to examine why they're doing it, that would be a problem.
But yes, its completely normal and doesn't mean your child is a savage monster and you don't need to be embarrassed about a bitey kid.
Now my sister carried on doing it til she was losing her fecking baby teeth and that WASN'T bloody normal and it hurt... but we didn't do much DX of ASD in girls in the 80s.