The thumb has its place, no doubt. It is good for hitch-hiking after all. You may have a green thumb or you may be all thumbs. You could thumb your nose at something (or someone) you dislike or you could give a thumbs up to show your approval. I'm sure you've twiddled your thumbs at one time or have stuck out like a sore thumb. Perhaps you've been under someone else's thumb, not a very pleasant place to be. On the other hand, you may follow the rule of thumb and never step out of line.
We have created so many expressions to capture the thumb's purpose and yet we have failed to explain how it came to be. Was it evolution? If so, why hasn't every mammal evolved such a useful device? Was it divine intervention? That could explain why humans can't come up with an explanation on their own, only God knows for sure.
I'd like to think I have a green thumb. I do have many plants, flowering and not, and I can keep them alive for extended periods of time. I garden with gloves on so my thumb has never actually turned green from its ministrations in the yard but I venture to guess that it would if left uncovered. The older I get, the more I need to use the phrase 'all thumbs'. Nothing explains the increased frequency of the dropsy's except to say that as I age, my brain is gradually losing its connection with my thumb. This could be because the thumb was likely the latest in our evolutionary history and therefore the first to disconnect.
I have pondered the anatomy of the thumb and the way in which it joins to the body. It is, to say the least, unique, making people unique. In point of fact, I have seen chimpanzees handling items in their hands with as much ease as if they had their own opposable thumb, leading me to conclude that the opposability of one's thumb is not necessarily due to anatomy alone but perhaps may encompass ingenuity and necessity. I have even seen a koala bear strip down a eucalyptus plant with as much dexterity, indeed more, than I think I would have with my own valuable appendage.
Such a long and storied history the thumb has and yet, I have only two words to say about it that puts it to the best use possible: