Is it normal for depression to kick in when you are alone for long periods of time?

Most definitely. Being social is part of human nature. I feel

yes, we were never meant to live totally isolated lives, apart from human contact. being alone for long periods of time changes your brain chemistry. that's why solitary confinement is rarely used anymore for long periods of time.

yes that would seem like a likely time for it to kick in

For some yes of course. For other's no. Some like the solitude.

yep the feeling of loneliness can make it worse

Yes.

Give me a campfire with supplies and I could wander a forest for a month on my own

Yep