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Clovis point (made over 13,000 years ago). Stone spearhead found in Arizona

Imagine. You're in a green landscape studded with trees and bushes. You're working in a team of hunters quietly stalking a herd of mammoths. One of the mammoths, you hope, is going to be your supper. You're clutching a light spear with a sharp, pointed stone at the end of it. You get closer - you hurl your spear - and it misses. The mammoth you wanted to kill snaps the shaft under its foot. That spear is useless now. You take another one, and you move on. And you leave behind you on the ground something that's not just a killing tool that failed, but a thing that's going to become a message across time, because thousands of years after the mammoth trod on your spear, humans will find that pointed stone spearhead and know that their ancestors were in this place far earlier than anyone had imagined.

'It looks so tiny and then it's only sort of two or three inches in length.' (Michael Palin)

'These are people on the move - explorers, and I can really feel quite a bit of empathy, and I can really feel what it must've been like to enter a country that nobody had told you about, that nobody had actually been in before you.' (Professor Gary Haynes)

It's 13,000 years ago, and you're in America. Things that are thrown away or lost can tell us as much about the past as any objects carefully preserved for posterity. Broken things tell poignant stories - in fact, mundane everyday items discarded long ago as rubbish, are as much a defining characteristic of being human as great art, and these modest but essential things can tell us some of the most important stories of all in human history. In the case of this programme, how modern humans - the toolmakers and the artists we've been following this week - took over the world. How, after populating Africa, Asia, Australia and Europe, they finally got to America.

In the North American gallery of the British Museum, among the magnificent feather headdresses, and in a case beside the totem poles, is a very interesting bit of rubbish indeed. It is the business end of a deadly weapon; a spear - the shaft, of course, is long gone. It's made of stone and it was lost by a person like you or me in Arizona over 13,000 years ago. The spearhead is made of hard flint and it's about the size of a small, slim mobile phone, but it's in the shape of a long thin leaf. The point is still intact and still very sharp. The surface of both sides has beautiful ripples and, when you look closely, you can see that these are the scars from its making, where the flakes of the flint have been carefully chipped off. It's a lovely thing to touch and it's very well adapted to its lethal purpose - a thing of beauty and a kill forever!

This spearhead raises many questions. But perhaps the most surprising fact is that it was found in America. After all, for most of our history we humans have been a resolutely land-locked African, Asian and European species. So how did the people who made spears like this get to America, and who were they?

This stone spearhead is by no means unique; it is just one of thousands that have been found across North America, from Alaska to Mexico. They're known as Clovis points, after the small town in the US State of New Mexico where they were first discovered in 1936, alongside the bones of the animals they'd killed. And so the makers of these stone points, the people who hunted with them, are known as Clovis people.

The discovery at Clovis was one of the most dramatic leaps forward in our understanding of the history of the Americas. These spearheads are the firmest evidence yet found for the first human beings to inhabit America. Almost identical Clovis points have been found in clusters from Alaska to Mexico, and from California to Florida, and what they show is that these people were able to establish small communities right across this immense area as the last Ice Age was coming to an end, about 13,000 years ago.

Are the Clovis people really the first Americans? The leading expert in this period is Professor Gary Haynes:

'There's some scattered evidence that people were in North America maybe before these Clovis points were made (which would be before 13,000 years ago), but most of that evidence is arguable. The fact is that Clovis look like the first people. If you dig an archaeological site almost anywhere, the bottom levels are going to be about 13,000 years old, and if there are any artefacts, it will be Clovis or Clovis-related. So it looks like maybe these are the very first dispersers who filled up the continent and became the ancestors of modern Native Americans. The area that was populated by Clovis was just about all of North America, and they came from somewhere up north, because the studies of genetics seem to prove conclusively that the ancestry of Native Americans is north-east Asian.'

So archaeology, DNA, and the bulk of academic opinion, are telling us effectively that everybody in America arrived from north-east Asia less than 15,000 years ago. When history gets re-written like this, it can lead to head-on collision with deeply-held beliefs.

Historian Gabrielle Tayac is a Piscataway Indian. She works for the Smithsonian's National Museum of the American Indian, and she studies how Native Americans are reacting to this new narrative that science is giving them:

'This is an affront to their very specific beliefs ... If you look at creation stories, there are certainly people who have very strong beliefs that either they emerged from the earth, or fell from the sky or developed out of the back of a water beetle, depending on where they were ... Native American religions were repressed for a very long time and so people have become very protective. For some Native people, though not all, the insertion of scientific findings that Native people did not get created from the very site that they emerged from, or that there are findings that might be counter to a specific oral recitation, can be seen as a way of invalidating Native traditions.'

By about 40,000 years ago, humans like ourselves had spread from Africa all over Asia and Europe, even crossing seas to get to Australia. But no humans had yet set foot in the Americas, and it needed major changes in climate before they could. Firstly, 20,000 years ago, the Ice Age locked up the water in ice-sheets and glaciers, leading to a huge fall in sea level, and the sea between Russia and Alaska (what's now the Bering Straits) became a wide and easily passable land-bridge. Animals - mammals, bison and reindeer - moved across to the American side, and hunting humans followed. The way further south into the rest of America was through an ice-free corridor between the Rocky Mountains on the Pacific side, and the vast continental ice-sheet covering Canada on the other.

15,000 years ago, as the climate warmed up again, it was possible for large numbers of animals, followed again by their human hunters, to get through this corridor to the rich hunting grounds across what is now the United States. This is the new American world of the Clovis points. It was clearly a great environment for those go-getting humans from north Asia but, if you were a mammoth, the outlook wasn't quite so rosy. The ripples on the side of the Clovis point, which I find so beautiful, produce intense bleeding in any animal they hit, so you don't need to be a dead shot and strike a vital organ, you can hit your prey anywhere and the blood loss will gradually weaken it until you can easily finish it off. And by 10,000 BC, all the mammoths and a lot of other big mammals, had been finished off. How far it's the Clovis people that are responsible for these extinctions is a matter for debate, but Gary Haynes thinks they were:

'I think there's a direct connection between the first appearance of people and the last appearance of many of the large mammals - if not all of them - that disappeared in North America. You can actually trace this sort of connection across the world, wherever modern 'homo sapiens' turns up. There had never been a human population before this. It's almost invariable that large mammals disappeared - and not just some animals, it's a large proportion. In North America it's something like two-thirds to three-quarters.'

This was going to become a familiar story. By around 12,000 years ago, the Clovis people and their descendants had not only spread across North America, but had also reached the southernmost tip of South America. Not long after this, warming climate and melting ice raised sea levels sharply so that the land-bridge to Asia flooded once again. There was no way back. For the next nine thousand years, in fact until European contact in the sixteenth century AD, the civilisations of the Americas would develop on their own. So, 12,000 years ago, we had reached a key moment in human history. With the exception of the islands of the Pacific, human beings had settled the whole habitable world. We seem to be hard-wired to keep moving, to want more, to find out what's beyond the next hill. Broadcaster and traveller Michael Palin has covered a good deal of the globe - what does he think drives us on?

'In myself I've always been very restless and, from when I was very small, interested in where I wasn't, [in] what was over the horizon, [in] what was round the next corner. And the more you look at the history of 'homo sapiens', it's all about movement, right from the very first time they decided to leave Africa. It is this restlessness which seems a very significant factor in the way the planet was settled by humans. It does seem that we are not settled, we think we are, but we are still looking for somewhere else where something is better - where it's warmer, it's more pleasant. Maybe there is an element, a spiritual element, of hope in this whole thing. You know, that you are going to find somewhere that is going to be wonderful. It's the search for paradise, the search for the perfect land - maybe that's at the bottom of it all, all the time.'

Hope, as the defining human quality - wouldn't that be an encouraging note in which to end this first week of our history of the world? What's stood out for me in this week's long journey of nearly two million years, is the constant human striving to do things better; to make tools that are not only more efficient but also more beautiful, to explore not just environments but ideas, to struggle towards something not yet experienced. The objects I've looked at this week have tracked that move - from tools for survival not so different from what other animals might use, to a great work of art and the beginnings of religion.

Next week, I'm going to be looking at how, about ten thousand years ago, we began to transform the natural world by starting to farm. In the process, changing not just the landscape, but plants, animals and, above all, ourselves. And I'm going to be focussing on two favourite pastimes - food and sex.