Most human body parts have been doing the same thing for as long as they can remember. They muddle along from millennium to millennium, performing functions that were already perfected when our ancestors lived in caves. For other body parts, however, the world can be turned upside down overnight.
Consider the thumb. The first opposable thumb was one of the triumphs of evolution: allowing humans to pick fruit from trees, create tools, etc, and giving us a competitive edge over lesser animals. But that all happened a long time ago. And until 15 years ago this week, the thumb's role had been well defined in the scheme of things.
Any new functions it acquired were minor. At some point in modern history, humans learned to flex the thumb in such a way as to create an indentation at the junction of the hand and wrist - "the human snuff box" - to facilitate the nasal ingestion of tobacco.
By the 20th century, the same movement was used to hitch lifts from cars; and later as a gesture to indicate one's well-being to television news cameras when - for example - one was being winched into a rescue helicopter after a failed attempt to row the Atlantic.
For all its evolutionary importance, the thumb was not regarded as one of our more important digits. Significantly, when computer keyboards became central to office work, the thumb was allocated to the space bar: the least-skilled function on the PC. On no account was it ever allowed to touch the smaller buttons.
And then - in early December 1992 - all this changed, when an English engineer called Neil Papworth sent the world's first text message ("Merry Christmas") to a friend's phone. True, he used a computer keyboard and his fingers because mobiles hadn't been adapted yet to write messages. But the breakthrough had been made. And soon the humble thumb found itself thrust centre stage in a communications revolution.
Almost overnight, thumbs everywhere had to undergo retraining. Previously forbidden from touching the characters on a computer keyboard, because of their perceived clumsiness, they now had to hit buttons less than a quarter that size, at high speed, and often in pressure situations. Young, teenage thumbs were quickest to adapt, naturally. But even middle-aged and elderly thumbs were soon being re-educated.
The balance of power on the human hand had changed utterly. Once all-powerful, fingers were suddenly redundant, fit only for the unskilled work of holding the phone.
The implications are still unfolding. A decade later, as a generation of thumb-only typists reaches adulthood, the Japanese have even begun adapting computer keyboards in reverse to suit their needs.
With the number of text messages now measured in trillions per annum and growing, Papworth - still only 37 - jokes about a back-handed compliment he received recently from the daughter of a friend.
The friend had mentioned how he knew the man who sent the world's first phone-text. To which the daughter replied, apparently in all innocence: "He's still alive?"
Yes, it must be astonishing to someone under 18 that a world ever existed in which there was no texting and even - God help us - no mobile phone. I struggle myself to remember a time when we had to work out, via a combination of guesswork, intelligence-gathering, and knowledge of previous behaviour patterns, that a friend of ours was probably on a train.
Now that we can find this out at the touch of a button, it is difficult to imagine it was ever otherwise. But I dimly recall the late Note Age, when even teenagers still had to contact each other via handwritten messages, scrawled on paper, and often passed on through a network of trusted fellow humans. Sometimes we even exchanged love letters by this means. It all sounds so Dickensian now.
Then again, that Dickensian invention - the Christmas card - is still going strong, against all odds. No doubt this is partly because Victorian England patented much of what we consider the traditional Christmas, which is why you can still buy cards featuring people in top hats and bonnets and horse-drawn carriages, and ploughing through another thing you never see in the modern world: snow.
But even in the age of instant communication, we like to know that the Christmas cards we receive have spent at least one night in a dark post-box, or a sorting centre, or a dusty mailbag. And if it was more than one night, and several mailbags, so much the better.
Half the thrill of sending a Christmas card to America is the sense of mystery about why December 7th is the last day for guaranteed delivery. I have no idea why this is the case.
But I like to imagine mine crossing the Atlantic by storm-tossed steamship and then being relayed from town to town by pony express: through snow drifts, badlands, and Injun country, before it reaches friends in Seattle or Spokane.
You never know what inventions will take off and what will last. A few years ago, when the first "e-card" arrived on a computer screen, the people behind it must have thought it was the future. It promised to doom its three-dimensional rival as surely as the text message sank the love-note.
Instead, the old posted greeting remains as popular as ever. And even before it was hijacked by spammers, the e-card had died of shame.