Could be a bad tire. Check to see if the tire on the other side looks the same. If it is it is likely alignment, unless it's a front position and you drive aggressively (but then, it is typical, but not necessarily, that the outside ribs wear more in this case.)
You are justified to have the alignment checked.
Rotate the tires if you haven't already. (You didn't say how many miles were on them.)
You don't want the wear to reach the cords, as said above. Traction & handling will be degraded, but this happens even as tires wear "normally", so this "uneven" wear isn't the end of the world in normal driving.