It's not going to happen very often but an unarmed person can beat a tank. The possibility may be less than .01% but it exists.
The tank driver could be outside... y'know. Call of nature, yeah that's the ticket!
No, really, Mirc makes a good point. Also, I'd like to repeat something posted in a thread like this awhile back: the unit animation is there for looks only, the computer only "sees" the 1.2.1 stats. This is where civ could have gone just one step farther, you know how the industrial workers look compared to the ancient ones, if the ancient units had a similar make over it would make more sense.
Think of it this way
Ancient age: Spearman with no iron armor or weapons
Middle age : Spearman maybe with iron, but not enough to qualify as pikes.
Industrial: Now uniformed, has no helmet or good gear, cannon fodder, probably armed with the left over muskets from the late mideaval.
Modern: Think of a paramilitary force, again no helmets, but probably very old rifles or handguns.
My point being is, these are men but don't think of only "spears" but any weapon that could be had.
I'd like to see a warrior modern replacement as a punk in a jacket with a knife or a baseball bat.