I often tease my cats regarding their lack of an opposable thumb. They ignore my taunts for the most part. I say things like "bet you wish you had one of these, then you could use the can opener." I wave my thumb in their whiskered faces. I chide them with "if only you had a thumb you could put scotch tape on my feet."
That thumb though, in reality, is a tremendous responsibility. The thumb, unlike other fingers, as mentioned previously, is opposable in that it is the only digit on the human hand which is able to oppose or turn back against the other four fingers, and thus enables the hand to refine its grip to hold objects which it would be unable to do otherwise - this, it turns out, has allowed us humans to get into all sorts of mischief.

There are other animals that also have opposable thumbs: orangutans, apes, gorillas, heck even raccoons and opossums (hmmm, aptly named that opossum), and they've hardly caused any trouble at all. Now I know a room full of monkeys and raccoons would likely incite a ruckus but I don't see any real chance for fundamental madness or destruction, at least not the kind we homosapiens are capable of.

Apparently we humans have the ability to move our thumbs farther across our hand than any other primate. Perhaps this has something to do with the magnitude of our shenanigans.

The original source of the thumbs up gesture is not entirely clear. Anthropologist, Carleton S. Coon, after seeing Barbary apes in Gibraltar using the gesture, hypothesized in his classic work 'The Story of Man' that it is “a mutual celebration of having opposable thumbs”.

The use of the thumbs up was popularized in the west by US fighter pilots during WWII, letting the ground crew know they were good and  ready to go drop some bombs on unsuspecting targets.

Roman emporers, it has been documented, used the thumbs up and down to signal whether a gladiator should show mercy or not to his fallen opponent - in this case though, a thumbs up was actually mean as the signal to show no mercy (perhaps the emperor was in effect saying -  "hey, you've still got your thumb, use it").

Ok, so here's the funny thing – whenever I see one homosapien giving a thumbs up to another homosapien for whatever reason, it could be for something simple like “hey nice car” or for something much more ingenious like “hey way to manipulate that stock” or “a laser guided hellfire from two thousand miles...wow!” - whenever I see someone giving the thumbs up it occurs to me that what they are really saying is - “Way to go opposable thumb” or maybe even something like “Hey I have a thumb too, aren't these great.”

  Ok that was the funny thing, now here's the interesting thing. In some parts of the world the thumbs up has another meaning - precisely the opposite meaning - precisely the equivalent of one in the west raising their middle finger. The thing is that these areas of antipodal thumb meaning are almost exclusively in the least developed, most impoverished areas of the world. One of the reasons these areas continue to remain poor and in abject misery is because of their being subjugated by the more advantaged West. In other words, in their opinion, being under it, that thumb - is maybe not such a good thing.