Any text on a creature is an ability, even if that text isn't currently applicable (such as a cycling ability of a creature).
If a land with a basic land type becomes a creature, the intrinsic mana ability it has stops Muraganda Petroglyphs from applying to it.
If an effect says that an affected creature "has" or "gains" an ability, Muraganda Petroglyphs won't apply to that creature because it has gained an ability. If an effect simply says that something is true of an affected creature (such as "target creature can't be blocked this turn") then that effect isn't causing the creature to gain an ability.
Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to a creature with no abilities may become lethal if that creature gains an ability or Muraganda Petroglyphs leaves the battlefield during that turn.
