Image copyright Anti-Slavery International

Warning: This article contains an image which readers may find distressing.

The black-and-white photograph shows a man, perched on the edge of a wooden deck, looking down at two objects. At first, you can't take in what they are.

In the background are palm trees. Two other men stare grimly at their friend or perhaps the photographer, it's hard to tell.

The photo was taken in 1904 at a missionary outpost in Baringa, in what was then called the Congo Free State. The man's name was Nsala and his wife and children had just been killed.

Alice Seeley Harris's photograph of Nsala, looking at his five-year-old daughter Boali's severed hand and foot caused an uproar back in Europe.

Image copyright Anti-Slavery International Image caption Alice Seeley Harris and her husband arrived in the Congo in August 1898

Printed in pamphlets and displayed at public meetings, Alice's harrowing images formed the world's first photographic human rights campaign.

The resulting public pressure eventually forced Belgium's King Leopold II - Queen Victoria's cousin - to loosen his grip on the colony famously depicted in the novel Heart of Darkness.

But why was Leopold's Congo so horrific? It was down to rubber.

50 Things That Made the Modern Economy highlights the inventions, ideas and innovations that helped create the economic world.

It is broadcast on the BBC World Service. You can find more information about the programme's sources and listen to all the episodes online or subscribe to the programme podcast.

Rewind 70 years to New York, 1834. An impoverished, unwell, but optimistic young man knocked on the door of the Roxbury India Rubber Company.

Charles Goodyear had landed in debtors' prison when his family's hardware business went bankrupt, but he planned to invent his way out of financial trouble. His latest idea was an improved kind of air valve for inflatable rubber life preservers.

Image copyright Getty Images

Unfortunately for Goodyear, the manager loved his valve - but confessed that his company was on the verge of ruin.

He was not alone. All over the US, investors had sunk money into this miraculous new substance - stretchy, pliable, airtight and waterproof - but it was all going horribly wrong.

Rubber was not exactly new. It had long been known to South Americans, and Europeans first reported in the 1490s that natives made "a kind of wax" from trees that "give milk when cut". That "milk" was latex - it comes from between the inner and outer bark.

Some rubber made its way to Europe, but mostly as a curiosity. In the 1700s, a French explorer brought the name "caoutchouc" from a local language: it meant "weeping wood". The scientist Joseph Priestley bestowed its common name when he noticed it rubbed pencil marks off paper.

Image copyright Getty Images Image caption A collectable card from a 1910 set showing the extraction of "caoutchouc"

By the 1820s, an increasing amount of rubber was being shipped around the world from Brazil, and made into coats, hats, shoes, and those inflatable life preservers. Then came a really hot summer, and entrepreneurs watched aghast as their inventories melted into foul-smelling goo.

Goodyear saw his chance.

A fortune awaited whoever could invent a way to make rubber cope with heat - and cold, which made it brittle. True, Goodyear had no background in chemistry, and no money, but why should that stop him?

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For years he dragged his wife Clarissa and their growing brood from town to town, renting ever more insalubrious houses, pawning their dwindling stock of possessions, and running up debts.

When Clarissa was not trying to feed the children, Charles commandeered her saucepans to mix rubber with anything he could think of: magnesium, lime, carbon black.

Eventually he found the answer: heat the rubber with sulphur. It is a process we now call vulcanisation.

Image copyright Getty Images Image caption Charles Goodyear demonstrating the vulcanisation process

Sadly for the long-suffering Clarissa, it led to her husband borrowing yet more money for lawsuits to try to protect his patents. He died owing $200,000 (£161,000).

But Charles's doggedness had put rubber at the very heart of the industrial economy. It was in belts and hoses and gaskets, used to seal, insulate and absorb shocks.

In the late 1880s, Scottish inventor John Dunlop supplied the missing part of the puzzle by reinventing the pneumatic tyre, which had been developed few decades previously, but which had failed to take off.

Dunlop was a vet. He had been tinkering with his son's tricycle, trying to find a way to cushion the ride. Bike manufacturers quickly saw the advantages, as did the nascent car industry.

Image copyright Getty Images Image caption John Dunlop's son pictured on the first bicycle to have pneumatic tyres

Demand for rubber boomed. Europe's colonial powers set about clearing vast areas of Asia to plant Hevea brasiliensis - more widely known as the "rubber tree".

But those new rubber tree plantations would take time to grow, and hundreds of other plants also produce latex, in varying quantities - even humble dandelions.

In the Congo's rainforest were vines that could be tapped to meet demand immediately.

How to get that rubber, as much and as quickly as possible?

In the absence of scruples, the answer was distressingly simple. Send armed men to a village, kidnap the women and children, and if their menfolk did not bring back enough rubber, chop off a hand - or kill a family.

Image copyright Anti-Slavery International Image caption Alice Seeley Harris's 1904 photograph of Nsala, looking at his five-year-old daughter's severed hand and foot

Some things have changed since Nsala met Seeley Harris in Baringa. More than half the world's rubber now comes not from weeping wood but gushing oil.

Attempts to make synthetic rubber began as the natural stuff grew popular, and took off during World War Two. With supply lines from Asia disrupted, America's government pushed industry to develop substitutes. Synthetic rubber is often cheaper, and sometimes better - for example, for bicycle tyres.

But for some uses, you still can't beat a bit of Hevea brasiliensis. About three-quarters of the global rubber harvest goes into tyres for heavier vehicles.

And as we make more cars, trucks and planes, we need ever more rubber to clothe their wheels, and that is difficult.

The rubber tree is thirsty, so environmentalists worry about water shortages, and biodiversity, as South East Asia's tropical rainforest increasingly gives way to large-scale plantations.

Image copyright Getty Images Image caption Workers collecting sap from rubber trees

It is happening in Africa, too.

Travel 1,000 km through the rainforest from Baringa, where Seeley Harris met Nsala, bearing west and slightly north, and you will come to Meyomessala in Cameroon, part of the Dja Faunal Reserve, a Unesco World Heritage Site.

Nearby, the world's largest rubber processing company Halcyon Agri is clearing thousands of hectares for rubber trees on its Sudcam plantation.

Environmental groups including Greenpeace, WWF and the Center for International Forestry Research have all raised concerns about the impact of deforestation in the area. Some villagers allege they have not been properly compensated for loss of their lands.

In response, in November 2018, Halcyon Agri announced a new "sustainable" supply chain policy, and launched a Sustainability Commission covering its work in Cameroon.

Promising to address working conditions, responsible land acquisition and use, ecosystem protection, and ethical practices, the company acknowledged that it "understands its role and obligation to minimise impact on the environment while continuing to meet the growing demand for a raw material vital to modern life".

Halcyon Agri is a subsidiary of Sinochem, a Chinese state-owned enterprise. So demand for rubber from a great foreign power is still causing controversy in Africa. But now that controversy is about cutting down trees, not hands.

It is progress, of a sort.

The author writes the Financial Times's Undercover Economist column. 50 Things That Made the Modern Economy is broadcast on the BBC World Service. You can find more information about the programme's sources and listen to all the episodes online or subscribe to the programme podcast.