The man was talking garbage. Willy Fowler knew this because he was an experimental nuclear physicist and nobody could do what the man, Fred Hoyle, was claiming: predict the energy of a complex atomic nucleus. An atomic nucleus was a "many-body system", with protons and neutrons buzzing about inside. Physicists could not calculate the precise properties of anything bigger than a "two-body system" such as the moon going round the Earth.

Yet here, sitting in Fowler's office at Caltech's Kellogg Radiation Lab, was a Limey astronomer claiming he could do what no nuclear physicist could do. And what was more outrageous was that the prediction was based not on considerations of nuclear physics but on an argument the likes of which Fowler had never heard.

"I exist," he was sure Hoyle said, "therefore the carbon-12 nucleus must possess an energy level at 7.65 megaelectronvolts (MeV)." Hoyle was convinced that the nuclei of the atoms in our bodies - the iron in our blood, the calcium in our bones, the oxygen that fills our lungs each time we take a breath - were assembled from the simplest nucleus - hydrogen - in the furnaces of stars.

In the first step, four hydrogen nuclei would collide and stick together to make a nucleus of helium, the second-lightest atom. In the second step, two helium nuclei would stick to make a nucleus of beryllium-8. Only there was a problem: beryllium-8 was unstable, splitting apart one billion billionth of a second after forming. The route to building the heavier atomic nuclei essential for life was blocked.

The year before, in 1952, Ed Salpeter, a researcher in New York, had pointed out that the beryllium barrier might be leapfrogged if, in the heart of "red giant stars", three helium nuclei collided almost simultaneously, gluing together to make carbon-12. It was the nuclear physics equivalent of three shopping trolleys colliding simultaneously in a car park. Unfortunately, this process was fantastically unlikely.

Enter Hoyle. His argument, as as far as Fowler could make out, was that the process would be speeded up if, by a bizarre coincidence, carbon-12 had an energy state exactly equal to the energy of three colliding helium nuclei at the 100 million-degree temperature inside a red giant. That energy was 7.65 MeV. The state had to exist, reasoned Hoyle, because life existed and life was based on carbon.

Although Fowler would later tell people that his first impression of Hoyle was of someone who had gone off his mental compass bearings, he heard him out. Working in Pasadena, in the shadow of the Mount Wilson 100-inch telescope, with which Edwin Hubble had discovered the expanding universe, had made Fowler tolerant of ideas astronomical.

Not showing Hoyle the door that day half a century ago would prove to be the smartest move Fowler ever made. It was highly probable that Hoyle was wrong. On the other hand, Fowler adhered to the experimenter's maxim: never close your mind to the unexpected. He rounded up his small research group and Hoyle repeated his argument. Was there any possibility, Hoyle asked, that experiments could have missed the 7.65 MeV state of carbon-12? Much of the technical discussion went over Hoyle's head.

Eventually, however, Fowler's colleagues concluded that, if the state of carbon-12 had some special properties, yes, it could have been missed. Hoyle had piqued the interest of Kellogg's nuclear physicists, who undertook an experiment. For 10 days, Hoyle was on tenterhooks. Each afternoon, he crept into the bowels of the Kellogg Lab, where Fowler's colleague Ward Whaling and his team beavered away amid a jungle of power cables, transformers and diving bell-like chambers in which atomic nuclei were fired at each other.

On the 10th day, Hoyle was met by Whaling. He pumped Hoyle's hand and gushed his congratulations. Hoyle's pre diction had been borne out. Unbelievably, there was an energy state of carbon-12 within a whisker of 7.65 MeV. It was the most amazing result Fowler had witnessed.

Hoyle's outrageous prediction had been proved right - quite spectacularly. Hoyle had peered into the heart of nature and spied something that mere mortals or, at least, mere nuclear physicists - had been unable to see.

But what compounded Fowler's amazement was the manner of Hoyle's prediction. He had predicted the 7.65 MeV energy state of carbon-12 using an anthropic argument: it had to exist because, if it didn't, neither could human beings.

To Fowler, such flaky logic smacked of religion rather than science. To this day, Hoyle is the only person to have made a successful prediction from an anthropic argument in advance of an experiment.

The discovery of the 7.65 MeV state of carbon-12 was just the start. Hoyle, together with Fowler and Geoffrey and Margaret Burbidge, went on to figure out the origin of all the elements in our bodies.

Fowler won the 1983 Nobel Prize for Physics. Hoyle did not share that prize, although, as Fowler later remarked, he would probably have remained a run-of-the mill nuclear physicist had it not been for the visit of Fred Hoyle that fateful day in the spring of 1953.

· The story of the cosmic origin of the elements is told in Marcus Chown's book The Magic Furnace