Archaeologists in some post-apocalyptic future sifting through the remains of the 21st century may spot a puzzling disappearance of the pieces of plastic that had hitherto defined the Computer Age.
"Where," they might inquire, "did all the buttons go?" A period of head-scratching might lead them to identify the explosion of the touch screen.
Touch has transformed our mobiles in the past two years, but recent months have seen the spread of button-free screens into just about every corner of gadget land.
Last month, Sony launched a touch edition of its eBook Reader, allowing users to turn pages with a swipe of the screen. In the same week, LG unveiled a Dick Tracy-inspired watchphone (with a touch screen, naturally) and the past few days have seen the announcement of everything from new touch screen media players to a touch-controlled printer.
But what may be the biggest milestone in the history of touch comes next month with the launch of Windows 7. The latest edition of Microsoft's ubiquitous operating system will be fully touch-enabled - you'll be able rotate photos by, well, rotating them, and scroll by stroking a finger across a screen.
Not surprisingly, manufacturers are racing to launch laptops, desktop computers and tablets with compatible screens.
So if we are about to plunge into a new era of computer interaction, what does it mean? For sure we'll see a new wave of smug gadgeteers pinching and caressing their devices in front of not-always-impressed friends.
But more significantly, the touch screen's keenest advocates say the interface spells the demise of the keyboard and mouse. Others believe the spread of touch has a deeper significance.
"This is a dramatic evolution in how we interact with technology," says Chris Harrison, who is part of the Human-Computer Interaction Institute at America's Carnegie Mellon University.
"We have grown used to intermediate devices that martial our input into computers - they allow us to talk to them. Touch is moving us to direct manipulation. That's a big change."
But in technology as in nature, evolution doesn't happen in a year, or even a decade. Touch screen show-offs would be forgiven for thinking their new toys are cutting-edge.
But the act of controlling computers by means of fingers and a screen is nothing new. Bill Buxton is a principal researcher at Microsoft and has been working with touch screens for more than 25 years.
"The first multitouch screen I saw was in 1985," Buxton says. "Guys were using mini-computers the size of two fridges to explore the ways people could interact with computers."
It was at around the same time that Buxton developed his own touch screen - something that wouldn't look out of place on one of today's devices.
"I was trying to make a digital drum that I could hit with one hand while changing the tone with the other hand," he says.
The first touch screens predate even Buxton's work. During the Sixties a team at the University of Illinois developed a computerised educational tool called Plato. The fourth version, launched in schools in 1972, incorporated a plasma touch screen that used infra-red sensors around its periphery to sense the position of a finger. Children could navigate educational software, touching the answers to questions.
There were also rare examples of consumer touch screens that pre-date the iPhone by almost 30 years.
"I have a Casio watch that I bought in 1984," Buxton says. "It allows me to use my finger to operate a calculator. I was able to trace '1 + 1 =' on the screen and it would say '2'. That's how long this stuff has been available."
Buxton applies what he calls the "long nose theory" to the evolution of technology. He cites the mouse as a prime example. "It was invented in 1965, I first used one in 1971, but it wasn't until the launch of Windows 95, some 30 years after its invention, that the mouse became ubiquitous."
If 2009 is to the touch screen what 1995 was to the mouse - the tipping point that sends it mainstream - then the "nose" of its evolution has been even longer.
Early examples cropped up most commonly in information kiosks, cash machines and bar tills.
"We've been using touch screens for a decade or more without noticing it," Buxton says. "What's changed isn't the technology but who's using it, how it's designed and, of course, the fact that the technology is now smaller, faster and cheaper."
Cheaper but as Buxton says, principally the same. Capacitive screens such as those used in smartphones work like Buxton's digital drum. They combine the glass of the monitor with a transparent layer of conducting material that stores an electrical charge. The introduction of a finger takes some of that charge away. By measuring the reduction, circuits at each corner of the screen can identify exactly where the finger has touched.
Now used in devices as diverse as satnavs, ereaders and kitchen computers, touch screens have, in some ways, come full circle. Conceived to make interacting with technology intuitive before computer literacy existed, touch is now opening devices to new users who for whom computers still aren't second nature.
"Take my three-year-old son," says Russell Beale, an expert in human-computer interaction at the University of Birmingham. "He can unlock my iPhone, take photos and show them back to me. He couldn't do that if it had buttons."
But while touch screens lead us to a new world of opportunity, they are not necessarily the keypad's death knell. "If you're blind, your touch screen has no value," Buxton says. "And we're all blind sometimes - like when we're driving. What's more, unlike buttons, touch screens offer no tactile feedback. If you're a 12-year-old school pupil who likes texting under your desk you're screwed with a touch screen because you can't touch type."
Buxton isn't the first person to spot this flaw, which is why Chris Harrison at Carnegie Mellon is already devising the touch screens of the future. His remarkable prototype combines the near-infinite flexibility now transforming gadgets with the tactile responsiveness of buttons.
It features a layer of latex that works as a touch screen. Then, with the flick of a switch (or the swipe of a screen), air is pumped behind the latex, deforming it to create pop-up buttons.
In Harrison's mocked-up cash machine, a user can select options by touching the screen as normal. But when it's time to punch in the PIN, the screen deforms to offer a numerical keypad.
