I’m a sucker for stories that imagine alternate histories. Philip K. Dick wrote a classic, 1962’s “The Man in the High Castle,” that supposed Japan and Germany won World War II, and annexed the United States between them. Another came to mind last week; “The Difference Engine” (1990) by William Gibson and Bruce Sterling.

The difference engine (and another machine, called the analytical engine) were steam-powered calculators — mechanical computers designed and built by the Victorian mathematician and engineer Charles Babbage. He was funded to the tune of £17,000 (about ¥3 million) by the British government, which was a gigantic sum in the 19th century.

Despite his genius, and the lavish funding, Babbage couldn’t get the machines to work properly. His ideas were ahead of their time and Victorian technology apparently wasn’t ready.

It wasn’t until 1991 (the bicentenary of Babbage’s birth) that a working difference engine was built from Babbage’s plans by the Science Museum in London. If only Babbage had succeeded! If he had, there is the tantalizing prospect that the information age we currently live in would have begun some 100 years earlier.

Gibson and Sterling’s book imagines just such a scenario and creates a Victorian society where computers are ubiquitous, just like today. It’s a mouth-watering idea if, like me, you are frustrated sometimes by the length of time it has taken us (human civilization,I mean) to progress.

But the reason I was reminded of “The Difference Engine” was because I heard about another mechanical computer that could have heralded an earlier arrival of the computer age. Only this one was built more than 2,000 years ago.

I was in the crowded upstairs room of a pub in London, talking with Michael Wright, a former engineer at London’s Science Museum. For 30 years, Wright has been fascinated by a scandalously underrated artifact of ancient Greece, a fused and corroded lump of bronze recovered from a shipwreck off the tiny Greek island of Antikythera in the Mediterranean.

When the wreck was discovered and partially salvaged in 1901, it yielded a treasure trove of statues from ancient Greece. The ship was Roman and had been heading back to the capital of the Empire around 70 B.C. when it sank. It carried beautiful statues plundered by the Romans, and the wreck was celebrated as yielding one of the most important hauls of statues ever found.

But in a wooden crate fished from the wreck was something better, arguably the most important artifact ever recovered from ancient Greece: a bronze mechanical computer of incredible complexity and technological sophistication.

No one realized at first. The mechanism was corroded, and no one expected such an instrument.

Suppose that when Neil Armstrong opened the door of the lunar lander and looked out on the surface of the moon, he had seen an old steam engine. That’s the kind of shock the Antikythera mechanism, as it became known, gave to scientists and historians. No one had any idea the ancient Greeks had such technological skill.

The Antikythera mechanism consists of more than 30 bronze gear wheels, precision-cut, connecting with dials and pointers and covered in inscriptions. The function of the device, until very recently, was unknown. (It was a bit like finding a real-life alethiometer, the mysterious golden compass in Philip Pullman’s “His Dark Materials” trilogy.) Gear wheels of such complexity were not seen again in historical instruments for at least another 1,000 years.

Wright is one of the people who have helped solve the mystery of what the machine — humanity’s oldest by far — was built for. The symbols represent the sun, the moon and the planets, and the Antikythera mechanism accurately modeled their movements. It could predict lunar and solar eclipses, and showed the date in the lunar calendar and even the location of the Greek games, including the Olympics, to be shown that year.

Science writer Jo Marchant has just had published a thrilling book describing the history of the Antikythera mechanism and the modern characters involved in discovering what it was built for. The crowd in the pub had gathered for the launch of her book, “Decoding the Heavens” (William Heinemann).

For me, the thrill of the story comes from the “what if” factor. The ancient Greeks were capable of building computers. What if this technical skill hadn’t been lost? This is not even the possibility of the computer age starting 100 years before it did, this technology — the sophisticated astronomical measurements and precision engineering — could’ve triggered the Industrial Revolution 1,000 years early!

Marchant said that when Nobel Prize-winning physicist Richard Feynman visited the museum in Athens where the Antikythera mechanism is displayed, he was dismayed when the staff could give him no more information about it. Surrounded by ancient statues and artifacts, the staff couldn’t understand why the American professor could be interested in a rusting chunk of bronze.

That attitude has changed now, but still the Antikythera mechanism is not so famous. Think of the Rosetta Stone, for example, or Tutankhamun’s tomb. Every school child has heard of these things. But the world’s first computer? It’s not as well known as it should be. Perhaps there are more out there.

For more about the Antikythera mechanism, see www.decodingtheheavens.com The second volume of Natural Selections columns translated into Japanese is published by Shinchosha. The title is “Hito wa Ima mo Shinka Shiteru” (“The Evolving Human: How New Biology Explains Your Journey Through Life”); ¥1,500.