From the evolutionary economics perspective, interest is a function of the biological replication rate.
What I mean is this: if an organism replicates at a rate of 1.1 a year (10%), it is able to, gladly pay you a sum, let's say 5% to 10% interest to borrow your fallow resource (e.g. if you're gaining less than that 5%.) In that way, both organisms do a win/win, so to speak.