  Recent seminal works on human mobility have shown that individuals constantly
exploit a small set of repeatedly visited locations. A concurrent literature
has emphasized the explorative nature of human behavior, showing that the
number of visited places grows steadily over time. How to reconcile these
seemingly contradicting facts remains an open question. Here, we analyze
high-resolution multi-year traces of $\sim$40,000 individuals from 4 datasets
and show that this tension vanishes when the long-term evolution of mobility
patterns is considered. We reveal that mobility patterns evolve significantly
yet smoothly, and that the number of familiar locations an individual visits at
any point is a conserved quantity with a typical size of $\sim$25 locations. We
use this finding to improve state-of-the-art modeling of human mobility.
Furthermore, shifting the attention from aggregated quantities to individual
behavior, we show that the size of an individual's set of preferred locations
correlates with the number of her social interactions. This result suggests a
connection between the conserved quantity we identify, which as we show can not
be understood purely on the basis of time constraints, and the `Dunbar number'
describing a cognitive upper limit to an individual's number of social
relations. We anticipate that our work will spark further research linking the
study of Human Mobility and the Cognitive and Behavioral Sciences.
