The mapping class group of a surface $S$ with a marked point can be identified with the group $\mathrm{Aut}(\pi_1(S))$ of automorphisms of the fundamental group of the surface. I will explain a new theorem with M. Wolff that shows that any nontrivial action of $Aut(\pi_1(S))$ on the circle is semi-conjugate to its natural action on the Gromov boundary of $\pi_1(S)$. This answers a well-known question of Farb. As a consequence, we can also quickly recover and extend some results of Farb-Franks and Parwani on the nonexistence of $C^1$ and $C^2$ actions of certain mapping class groups on the circle.