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Hoa Hakananai'a statue (made between 1000 and 1200). Stone; from Easter Island (Rapa Nui)

Today we're on an island far out in the Pacific Ocean, it's about half the size of the Isle of Wight, it's 1,200 miles (1,900 km) from the nearest inhabited island, and it's 2,000 (3,200 km) miles from the nearest land-mass. It's Rapa Nui - Easter Island - the most remote inhabited island, not just in the Pacific, but in the world. Not surprisingly, it took human beings a long time to get there. The people of the southern Pacific Ocean, the Polynesians, were without question the greatest open ocean voyagers in the history of the world, and their ability to move in double-hulled canoes over the vast expanses of the Pacific is truly one of the greatest achievements of humanity. They settled both Hawaii and New Zealand, and between 700 and 900, they got to Rapa Nui - and so brought to an end one immense chapter of human history, for Easter Island was possibly the last place on earth to be permanently settled.

It was another thousand years before European sailors matched the Polynesian feats of navigation, and when they reached Rapa Nui on Easter Day 1722, they were astonished to find a large population already established. Even more astonishing were the objects which the inhabitants had made - the great monoliths of Easter Island are like nothing else in the Pacific, or indeed anywhere else, and they've become some of the most famous sculptures in the world. This programme is about one of them. He's called Hoa Hakananai'a - the name has been roughly translated as 'hidden friend'. He came to London in 1869 and he's been one of the most admired inhabitants of the British Museum ever since.

"This marvellous mass, and strength, and power, these are all things that every sculptor wants to get, but to get naturally without having to strain for it, but to have - and this sculpture has it." (Anthony Caro)

This week we've glimpsed some of the variety of gods that people around the world were engaging with about seven hundred years ago, and the objects that they made to get close to them. It's a constant of human history that societies devote huge amounts of time and resource to ensuring that the gods are on their side, but few societies have ever done it on such a heroic scale as Rapa Nui. The population was probably never more than about 15,000, but in a few hundred years the inhabitants of this tiny island quarried, carved and erected over a thousand massive stone sculptures. Hoa Hakananai'a was one of them. He was probably made around the year 1200, and was almost certainly made to house an ancestral spirit. He's a stone being, which an ancestor may from time to time visit and inhabit.

Standing below him, you're immediately conscious of the solid basalt rock that he's made out of. Although we see him only from the waist up, he's almost nine feet (2.7m) high, and he dominates whatever gallery he's in. When you're working hard stone like this, and have only got stone tools to chip away with, you can't do detail, so everything about this giant had to be big. And bold. The heavy rectangular head is huge, almost as wide as the torso below. The overhanging brow is one straight line, running across the whole width of the head. Below it are cavernous eye sockets, and a straight nose with flaring nostrils. The square jaw juts assertively forward, and the lips are closed in a strong frowning pout. In comparison to the head, the torso is only sketched in. The arms are barely modelled at all, and the hands disappear into the stone block of a swelling paunch. The only details on the body are the prominent nipples.

Hoa Hakananai'a is a rare combination of physical mass and evocative potency. For the sculptor Sir Anthony Caro, this is the essence of sculpture:

"I see sculpture, the setting up of a stone - it's a basic human activity. You're investing that stone with some sort of emotive power, some sort of presence. That thing, that way of making a sculpture, is a religious activity. What actually the Easter Island sculpture does, it gives just the essence of a person. Every sculptor since Rodin has looked to primitive sculpture, because all the unnecessary elements are removed. Anything that is left in, is what stresses the power of the stone. We are down to the essence - its size, its simplicity, its monumentality, and its placement - those are all things that matter."

The statues were placed on specially built platforms ranged along the coastline, a sacred geography reflecting the tribal divisions of Rapa Nui. Moving these statues would have taken days, and a large workforce. Hoa Hakananai'a would have stood on his platform with his giant stone companions in a formidable line, their backs to the sea, keeping watch over the island. These uncompromising ancestor figures must have made a haunting, and daunting, vision to any potential invaders, and a suitably imposing welcoming party to any visiting dignitaries. And they've been credited with a whole range of miracle-working powers. Here's the anthropologist and art historian Steve Hooper:

"It was a way of human beings who were alive relating to, and exchanging with, their ancestors, who have very great influence on human life. Ancestors can affect fertility, prosperity, abundance. As you can imagine, they are colossal - this one in the British Museum is small! There is one unfinished in a quarry in Easter Island which is over 70 feet (20m) tall - how they ever would have erected it goodness only knows. And it does put me in mind of medieval cathedral building in Europe or in Britain, where you have these extraordinary constructions involving enormous amounts of time and labour and skill... and it's almost as if these sculptures scattered around the slopes of Easter Island, large sculptures, are almost equivalent to these medieval churches. You don't actually need them all, and they are sending messages not only about piety, but also about social and political competition."

So, a populous island, effectively organised, practising religion in a carefully structured, competitive way, and then, quite suddenly, around 1600, the monolith-making stopped. No-one has a very clear idea why. Certainly all islands like this are fragile ecosystems, and this one was being pushed beyond what was comfortably sustainable. The islanders had gradually cut down most of the trees, and had hunted land birds almost to extinction. The sea birds moved away, to nest on safer offshore rocks and islands. It must have seemed as if the favour of the gods was being withdrawn.

The islanders responded by changing their religious practices, turning to a ritual that, not surprisingly, was all about the scarce resources. The Birdman cult focused on an annual competition to collect the first egg of the migrating sooty tern from a neighboring islet. The man who pulled off the feat of bringing an egg back, unbroken, through the sea and over the cliffs, would for a year become the Birdman. Invested with sacred power, he would live in isolation, grow his nails like bird talons, and wield a ceremonial paddle as a symbol of prestige. And surprisingly, we can tell this story too, through our sculpture. Rather than being abandoned along with the other monoliths, Hoa Hakananai'a was moved, placed in a hut, and entered a new phase of his life, now part of the Birdman cult.

All the key elements of this later ritual are present in our statue. They're carved on the back. They must have been added several hundred years after the statue was first made, and the carving style here could hardly be more different from the front. It's in low relief, the scale is small, and the sculptor has tried to accommodate a large range of disparate detail. Each shoulder blade has been turned into a symbol of the Birdman - two frigate birds with human arms and feet face each other, their beaks touching at the back of the statue's neck. On the back of the statue's head are two stylised canoe paddles, each with what looks like a miniature version of our statue's face at the upper end, and between the paddles is a standing bird. It's thought to be a sooty tern, whose eggs were so central to the Birdman ritual. This carving could never have been very legible as sculpture. We know it was originally painted in bright colours, so that this cluster of very potent symbols could be easily recognised and understood. Now, without its colour, the carving looks to my eyes feeble, fussy, diminished - a confused and timid postscript to the confident vigour of the front.

It's not often that you see ecological change recorded in stone. There is, I think, something very poignant in this dialogue between the two sides of Hoa Hakananai'a, a sculpted lesson that no way of living or thinking can endure forever. His face speaks of the hope that we all have of unchanging certainty, his back of the shifting expedients that have always been the reality of life. He is Everyman.

And Everyman is usually a survivor. The Easter Islanders seem to have adapted reasonably well to their changing ecological circumstances, as Polynesians have always had to. But in the nineteenth century, there were challenges of a completely different order. From across the sea came slavery, disease and Christianity. When the British ship, HMS Topaze, pulled in to shore in 1868, there were only a few hundred people left on the island. The chiefs, by now baptised, presented Hoa Hakananai'a to the officers of the Topaze. We don't know why they wanted him to leave the island, but perhaps the old ancestral sculpture was seen as a threat to the new Christian faith. A troop of islanders moved him to the ship, and he was taken to England to be presented to Queen Victoria, and then sent to be housed here at the British Museum. He stands facing south, looking towards Rapa Nui, over eight and a half thousand miles away.

Rapa Nui is perhaps a good place to end this second part of our history of the world. It was one of the last places on earth to be settled by humans - a final destination in our great journey out of Africa. Hoa Hakananai'a now stands in the gallery devoted to living and dying, surrounded by objects that show how other societies in the Pacific and the Americas have addressed the predicaments that confront humanity everywhere. He is a supremely powerful statement of the fact that all societies keep looking for new ways to make sense of their changing world, and to ensure that they survive in it. In 1400, when this part of our story ends, none of these cultures was known to Europeans. But this of course was about to change.

In the final part of the series, we will be looking at the way in which these many different worlds - even islands as remote as Rapa Nui - became integral parts of one global system. It's a history that's in many ways very familiar, but as always objects have the power to engage, to surprise and to enlighten. I hope that you'll join me on that last part of our story.