It has been claimed that as many as two out of five humans on the planet today owe their existence to the discoveries made by one brilliant German chemist.

Image caption Fritz Haber's inventions both saved millions, and were eventually used to kill millions

Yet this is the same chemist denounced by young German students today as a "murderer".

No-one personifies better than Fritz Haber the debate over science's capacity for good and evil.

And there is more to his dramatic life even than this. For Haber personifies too the tragedy of a Jew desperate to be a patriotic German, whose life was destroyed after the Nazis came to power.

And in the cruellest of all the ironies, his work was developed under the Nazis to create the gas used to murder millions in the Holocaust - including his relatives.

Fritz Haber was born in 1868 in Breslau, in what is now Poland.

As a young man he was bursting with ambition. "We only want one limit, the limit of our own ability," he wrote.

He went to study chemistry in Berlin - the ideal formula, he hoped, for transforming a provincial Jewish boy into a successful German.

Historian Fritz Stern, whose parents were close friends of Haber, says he was "ambitious but also vulnerable".

It was an exhilarating time, as Germany, newly unified under the Kaiser, powered ahead with scientific research at the forefront.

But anti-Semitism also grew as the century drew to a close, which preyed on Haber's mind despite his decision to convert to Christianity.

'Bread from air'

The breakthrough that made his name answered one of the great challenges of the time - feeding growing populations.

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Crops needed better supplies of nitrogen to produce more food. Previously this had been supplied in a limited and laborious way by ships full of bird droppings or nitrates mined in South America.

But in 1909 Haber found a way of synthesising ammonia for fertiliser from nitrogen and hydrogen.

Working with Carl Bosch, an engineer from the chemical company BASF, the Haber-Bosch process was born, making it possible to create huge amounts of fertiliser.

It seemed miraculous, described as creating "bread from air".

The fertiliser went on to be used on a large scale, bringing about a huge increase in crop yields, and practically banishing the fear of famine in large parts of the world.

One observer describes it as "the most important technological invention of the 20th Century".

Poison gas

But the process was also highly useful for the military in making explosives.

Image caption Haber's invention of poison gas for World War I led to his wife's suicide

And when World War I broke out soon afterwards, Haber - now working for the Kaiser's research institute in Berlin - was desperate to prove his patriotism.

He began experimenting with chlorine gas which, he said, would shorten the war.

The first attack using his methods was at Ypres in 1915. Haber was promoted to captain in the German army - but on the night he celebrated promotion in his villa in Berlin, his wife committed suicide.

Clara Immerwahr, a trained chemist, had become increasingly frustrated with her life at home looking after their son, and with the military direction of her husband's research.

Haber's life was the tragedy of the German Jew - the tragedy of unrequited love Albert Einstein

Haber rushed back to the front, apparently unmoved. But in a letter soon afterwards he wrote: "I hear in my heart the words that the poor woman once said… I see her head emerging from between orders and telegrams, and I suffer."

By the end of the war he had re-married, but his reputation was as uncertain as ever. Awarded the Nobel Prize for his work on ammonia, he also feared arrest as a war criminal for his poison gas research.

In the new Germany of the Weimar Republic, Haber continued to strive patriotically, with characteristic self-confidence.

The country faced huge reparations payments. Haber claimed he could extract gold from seawater to pay off the debts - but this time there was no miraculous breakthrough.

'Jews not allowed'

By the early 1930s he could see vicious anti-Semitism spreading around him, and his claim to be a German patriot was no protection.

"In early 1933", his daughter Eva told me, "he went to his institute. There was the porter, who said: 'The Jew Haber is not allowed in here.'"

Haber resigned, devastated, went briefly into exile, and died of a heart attack in 1934.

Despite the significance of his discoveries he remains much less well known than his friend and colleague Albert Einstein - perhaps because his reputation is so disputed.

It was not just the poison gas. There was one other area of research in the 1920s in which Haber and his colleagues were successful: developing pesticide gases.

Of Haber's legacies, this was the bitterest. For this research was later developed into the Zyklon process, used by the Nazis to murder millions in their death camps, including his own extended family.

His godson, historian Fritz Stern, says we must remember Haber "in all his complexity". He was a man of "scientific greatness, deeply cultivated".

But in an "excess of patriotism" he invented gas warfare, which "has come to define… the unspeakable horror of the First World War".

And as for his tortured relationship with Germany, Einstein concluded: "Haber's life was the tragedy of the German Jew - the tragedy of unrequited love."

You can hear Chris Bowlby's The Chemist of Life and Death on BBC Radio 4 on Tuesday 12 April at 2100 BST and again on Wednesday 13 April at 1630. You can also listen on the iPlayer .