If you mainly know David Graeber as the anarchist activist and intellectual guru of the Occupy movement, or as the swashbuckling author of 2018’s Bullshit Jobs: A Theory, you might be surprised to find that he had written a book about pirates and political thought in 18th-century Madagascar, the huge island (about two and a half times the size of Britain) that lies off the east coast of Africa.

In fact, Pirate Enlightenment marks a return to some of the celebrated anthropologist’s earliest interests. As a graduate student, Graeber, who died two years ago, spent several years doing fieldwork in Madagascar. When he hooked up with a local woman who identified herself as part of a distinct ethnic group whose members traced their ancestry back to Caribbean pirates, his interest was piqued.

Later, he discovered an unpublished early 19th-century manuscript in the British Library, in which a French adventurer had described some of Madagascar’s complex internal history. It centred on the legendary figure of Ratsimilaho, the young warrior son of an English pirate and a Malagasy woman, who at the start of the 18th century had apparently led the decade-long war that established the so-called Betsimisaraka confederation, a large, novel political entity on Madagascar’s north-eastern coast.

Only towards the end of his life, having lugged around a copy of this text for decades, did Graeber finally sit down to grapple with its contents. His slim, engaging volume, previously published in French and Italian, is the result.

Beginning in the 1690s, British pirates who had operated in the Caribbean began establishing bases in northern Madagascar. This was a great location for their exploits. In 1695, Henry Avery (also known as Every) and his crew attacked a Mughal convoy on its way to Mecca, captured two of its richly laden ships, and made off with a stupendous bounty worth the fabulous sum of £600,000 (about a £100m in today’s money).

Eventually, the British and French governments managed to crack down on the trade. Avery disappeared and hundreds of other pirates retired to Madagascar. The idea of such swaggering white outlaws setting up personal fiefdoms on an African island fascinated Europeans high and low. From London to Moscow, governments entered into treaty negotiations with supposed emissaries of Avery, who was rumoured to have become a Madagascan pirate king. Many popular accounts embroidered his exploits.

Among those who were captivated by these stories was Daniel Defoe. In 1724, a few years after he’d published Robinson Crusoe, a book entitled A General History of the Pyrates appeared in London, probably written by him. This included a description of an egalitarian Madagascan pirate republic called Libertalia – a utopian settlement of free-thinking, free-living freebooters, dedicated to equality and direct government by the people.

Graeber acknowledges that this account is widely agreed to be a fable, and that the surviving evidence for how exactly pirates and locals lived in Madagascar 300 years ago is vanishingly thin. But he doesn’t let that deter him. His overall project is to argue that what we think of as the European Enlightenment drew much of its inspiration from now forgotten non-European thinkers and social practices. He speculates that the Betsimisaraka confederation was a kind of Libertalia, “a creative synthesis of pirate governance and some of the more egalitarian elements in traditional Malagasy political culture”.

His exposition of this thesis involves lengthy parsing of the anthropological literature about Madagascan political, sexual and social history. Graeber’s approach unfortunately also exaggerates the extent to which piratical social relations were truly revolutionary. When Defoe describes pirate communities as choosing 12 men by lot to settle disputes, for example, Graeber excitedly trumpets this as evidence of their novel “democratic institutions”, supposedly based on the uniquely egalitarian environment of a pirate ship. He doesn’t notice, as Defoe’s original readers surely would have, that the passage simply riffs on the well-known principle of trial by jury.

As in his recent blockbuster, The Dawn of Everything: A New History of Humanity, co-authored with David Wengrow – which sets out related arguments on a much larger scale – the chief pleasure of Graeber’s writing is not that one always agrees with his arguments about the past. It is rather that, through a series of provocative thought experiments, he repeatedly forces us to reconsider our own ways of living in the present. Whatever happened in 18th-century Madagascar, Pirate Enlightenment implies, we could surely all do with a bit more free-thinking and egalitarianism in our own social, sexual and political arrangements.

• Pirate Enlightenment, or the Real Libertalia by David Graeber is published by Allen Lane (£16.99). To support the Guardian and the Observer buy a copy at guardianbookshop.com. Delivery charges may apply.