  During the peak of their accretion phase, supermassive black holes in
galactic cores are known to emit very high levels of ionizing radiation,
becoming visible over intergalactic distances as quasars or active galactic
nuclei (AGN). Here, we quantify the extent to which the activity of the
supermassive black hole at the center of the Milky Way, known as Sagittarius A*
(Sgr A*), may have affected the habitability of Earth-like planets in our
Galaxy. We focus on the amount of atmospheric loss and on the possible
biological damage suffered by planets exposed to X-ray and extreme ultraviolet
(XUV) radiation produced during the peak of the active phase of Sgr A*. We find
that terrestrial planets could lose a total atmospheric mass comparable to that
of present day Earth even at large distances (~1 kiloparsec) from the galactic
center. Furthermore, we find that the direct biological damage caused by Sgr A*
to surface life on planets not properly screened by an atmosphere was probably
significant during the AGN phase, possibly hindering the development of complex
life within a few kiloparsecs from the galactic center.
