At Poverty Point in the US state of Louisiana, a remarkable monument overlooks the Mississippi river. Built around 3,500 years ago entirely from earth, it consists of six semi-circular ridges and five mounds. 'Mound A', as archaeologists refer to it, is the largest at 22 metres high. The earth mounds at Poverty Point are not just impressive, they are also intriguing. Ancient monuments have always been regarded as products of large, hierarchical societies, built as tributes to gods and kings. But the creators of the Poverty Point monument were hunter-gatherers, who functioned in a more democratic way. They may have looked to elders for guidance, but these would not have exerted a commanding influence over their small groups. So who, or what, motivated building on such a grand scale?
Archaeologists have been excavating Poverty Point for more than a century. However, the truly remarkable nature of Mound A only emerged a few years ago. This was when a team led by Tristram Kidder of Washington University drilled into the mound. They saw for the first time that it consisted of neat layers of differently coloured earth. It rains a lot around Poverty Point, and we know that fluctuations in temperature and increased flooding eventually led to its abandonment. But Kidder could see no sign that the layers had combined, as you might expect if it had rained during construction. Kidder reached a startling conclusion: Mound A must have been built in one short period, perhaps in as little as 30 days, and probably no more than 90.
Mound A contains nearly 240,000 cubic metres of earth; the equivalent of 32,000 truckloads. There were no trucks then, of course, nor any other heavy machinery, animals like mules to carry the earth, or wheelbarrows. Assuming it did take 90 days, Kidder's group calculated that around 3,000 basket-carrying individuals would have been needed to get the job done. Given that people probably travelled in family groups, as many as 9,000 people may have assembled at Poverty Point during construction. 'If that's true, it was an extraordinarily large gathering, says Kidder. Why would they have chosen to do this?
Another archaeologist, Carl Lipo, thinks he has the answer: the same reason that the people of Easter Island built their famous stone heads. When Lipo first when to Easter Island, the prevailing idea was that the enormous statues had been rolled into place using logs, and the resulting deforestation contributed to the human population's collapse. But Lipo and fellow archaeologist Terry Hunt showed the statues could have been walked' upright into place by cooperating bands of people using ropes, with no need for trees. They argue further that by making statues, people's energy was directed into peaceful interactions and information-sharing. They ceased crafting statues, Lipo claims, precisely because daily existence became less of a challenge, and it was no longer so important that they work together.
An ancient temple known as Göbekli Tepe in south-east Turkey is another site where a giant team-building project might have taken place. Since excavations started, archaeologists have uncovered nine enclosures formed of massive stone pillars. Given the vast size of these pillars, a considerable workforce would have been needed to move them.
But what archaeologists have also discovered is that every so often, the workers filled in the enclosures with broken rock and built new ones. The apparent disposability of these monuments makes sense if the main aim was building a team rather than a lasting structure. Indeed, the many bones from animals such as gazelle found in the filled-in enclosures suggest people held feasts to celebrate the end of a collaborative effort.
A number of researchers share Lipo's view that the need to cooperate is what drove monument makers. But as you might expect when a major shift in thinking is proposed, not everyone goes along with it. The sceptics include Tristram Kidder. For him, the interesting question is not 'Did cooperative building promote group survival' but 'What did the builders think they were doing?' All human behaviour comes down to a pursuit of food and self-preservation, he says. As for why people came to Poverty Point, he and his colleagues have suggested it was a pilgrimage site.
If Lipo is right, have we in any way inherited our ancestors' tendency to work together for the sake of social harmony? Evolutionary biologist David Sloan Wilson thinks we have. Wilson cites the Burning Man festival, promoted as an experiment in community and art, which draws thousands of people to Nevada's Black Rock Desert each summer. Among the ten principles laid down by co-founder Larry Harvey are 'inclusion' and 'communal effort'. Another is 'leaving no trace', meaning that whatever festival-goers create they destroy before departing. In this way, the desert landscape is only temporarily disturbed. Wilson says there is evidence that such cooperative ventures matter more today than ever because we are dependent on a wider range of people than our ancestors were. Food, education, security: all are provided by people beyond our family group. Recently, as part of his Neighbourhood Project in Binghamton, Wilson and his colleagues helped locals create their own parks. 'This brought people together and enabled them to cooperate in numerous other contexts,' he explains. This included helping with repairs after a series of floods in 2011. Social psychologist Susan Fiske of Princeton University also sees value in community projects. Her research shows, for example, that they can help break down the ill-informed views that people hold towards others they have observed but do not usually interact with. So if modern projects really help build better communities, that will surely be a monumental achievement.
