The environment where human groups live not only influences the type of economy that individuals adopt, but their social structures and culture. These elements contribute to create the groups’ distinctive signs. But, what happens when a drastic change in that environment appear? At what level these elements get adapted to the new environment? Do they change at the same rate? Do humans have enough tools to deal properly with these sudden changes? We try to find out how these elements change. We hypothesized that economic, social and cultural practices change more quickly than social psychology of personal trust, as a more basic evolutionary trait. To test this prediction, we studied how personal trust networks of Pygmies have changed, after being forced to migrate from rainforests to the settlement of Vyegwa-Gika in the savanna of Northern Burundi. First, we analyzed the social, economic and cultural changes adopted by Pygmies with this migration, and then compared them with their current networks of personal trust and patterns of altruistic behavior using an experimental trust game with some variations. We found (i) small networks of personal trust, similar to those in rainforests that continue to drive altruistic exchanges among Pygmies, despite cohabitating in larger groups; and (ii) a very strong altruistic behavior towards trustees, despite their situation of poverty. The results support the view that personal trust networks are very robust and change more slowly than cultural practices.