  Understanding and control of the spin relaxation time $T_1$ is among the key
challenges for spin based qubits. A larger $T_1$ is generally favored, setting
the fundamental upper limit to the qubit coherence and spin readout fidelity.
In GaAs quantum dots at low temperatures and high in-plane magnetic fields $B$,
the spin relaxation relies on phonon emission and spin-orbit coupling. The
characteristic dependence $T_1 \propto B^{-5}$ and pronounced $B$-field
anisotropy were already confirmed experimentally. However, it has also been
predicted 15 years ago that at low enough fields, the spin-orbit interaction is
replaced by the coupling to the nuclear spins, where the relaxation becomes
isotropic, and the scaling changes to $T_1 \propto B^{-3}$. We establish these
predictions experimentally, by measuring $T_1$ over an unprecedented range of
magnetic fields -- made possible by lower temperature -- and report a maximum
$T_1 = 57\pm15$ s at the lowest fields, setting a new record for the electron
spin lifetime in a nanostructure.
