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One of the fascinations of archaeology is the question of how people managed to build huge structures without modern technology. How did the builders of Stonehenge lug those stones all the way from Wales to Salisbury Plain, and how did they lift them up? How did the inhabitants of Rapa Nui (also known as Easter Island) move their Moai statues to the coast? How did the ancient Egyptians build the pyramids? As for making and sailing boats across the ocean tens of thousands of years ago, it seems impossible, but people did it.

Questions like these attract fevered speculation. Sometimes the result is absolute codswallop: in particular, people who speculate about lost advanced societies or alien visitations often use prehistoric megastructures as evidence, with an argument that they couldn鈥檛 have been built any other way.

Ancient structures are particularly intriguing to me because I鈥檓 not a very handy person. I can put up shelves and build flatpack furniture, but any more advanced carpentry is just not going to happen.

Hence my enduring love of practical archaeology: experiments that seek to recreate ancient technologies to see how people achieved these feats. These studies are a window into the immense creativity and ingenuity of our ancestors, who often achieved an enormous amount with very little.

Chaco Canyon

The latest example of practical archaeology relates to Chaco Canyon in New Mexico. Between about AD 850 and 1200, Chaco Canyon was a major political centre for Indigenous Americans. They left behind structures including 鈥済reat houses鈥� built over multiple storeys, circular structures called great kivas and roadways up to 9 metres wide.

There are very few burials at Chaco Canyon, so it may be that people didn鈥檛 live there, at least not year-round. 鈥淭here鈥檚 a lot of indication that this might have been a gathering place,鈥� says James Wilson at the University of Colorado School of Medicine. For instance, it may have been a place of trade or of religious pilgrimage. 鈥淚t鈥檚 quite controversial actually, and it鈥檚 easy to have a controversy when you don鈥檛 have hard evidence,鈥� says Rodger Kram, also at the University of Colorado Boulder.

What we know for sure about Chaco Canyon is that the people who built it used a lot of wood, in the form of more than 200,000 logs. These were brought to Chaco Canyon from mountains at least 75 kilometres away.

Wilson, Kram and their colleagues now think they have figured out how it was done.

On the head

The first step was to clarify how much the logs weighed. A study published in 1986 estimated that each log was about 5 metres long and weighed 275 kilograms. 鈥淚鈥檝e studied a lot of load-carrying in my career,鈥� says Kram. 鈥淭hat鈥檚 a tremendous amount of weight.鈥�

However, when he and his colleagues re-examined the study, they discovered mistakes. Wilson redid the calculations and concluded that a more likely weight for each log was 85 kilograms. 鈥淛ames showed [the authors] his calculations and those scientists, to their great credit, said, 鈥極h we鈥檝e made a mistake and it鈥檚 very important to correct this number鈥�,鈥� says Kram. The new estimate was published last year. 鈥淚t sets out the labour force and the amount of time that was needed to construct this.鈥�

The question then became, how did people carry an 85-kilogram log over 75 kilometres? Previous studies had suggested that teams of eight people walked on either side of the log, using horizontal crossmembers held at roughly waist height to support it between them. 鈥淭hat seemed like a terrible way to carry it,鈥� says Kram.

So instead, Wilson proposed that people used tumplines. A tumpline is a simple strap that fits over a person鈥檚 head and drapes down their back to carry a load. The person leans forward, keeping their spine straight, and the weight of the load is supported by the entire spine. Tumplines are common in many cultures, notably among professional porters in Nepal who use them to carry heavy loads up mountains. There is abundant archaeological evidence of tumplines being used at Chaco Canyon.

To find out if this was feasible, the researchers tried it for themselves. Wilson, Kram and a third author named Joseph Carzoli made tumplines and trained themselves to each carry a weight of 30 kilograms. Then Kram and Wilson paired up to carry a 60-kilogram log stretched horizontally between the two of them for 25 kilometres in a day. They suggest that a team of three could carry an 85-kilogram log with minimal difficulty. The study was published in February.

鈥淚 don鈥檛 feel pain or muscle exhaustion,鈥� says Wilson. 鈥淭hat鈥檚 the beauty of the tumpline being perfectly in line with your cervical spine.鈥� The only downside was irritation on the top of his scalp, where the tumpline compressed his skin.

While we don鈥檛 have direct evidence that the Chaco Canyon people used tumplines to carry logs in teams, they did use them to carry other loads. It鈥檚 a very neat explanation for what is otherwise an inexplicable feat of endurance. Kram adds that some of the logs at Chaco Canyon are 鈥渁pproaching the limit of what a human could carry if they used tumplines and they were shoulder to shoulder鈥�, but none are heavier.

For me, the study is a reminder that humans have been ingenious for many thousands of years. Past societies didn鈥檛 have as many technologies as we do today, but that鈥檚 because culture is cumulative: you can鈥檛 invent the printing press until you first invent writing. It鈥檚 not a reflection on earlier people鈥檚 intelligence or inventiveness. If you want proof, try your hand at making stone tools the way early hominins did, by knapping. You鈥檒l quickly find you need to get the technique just right, and unless you have a teacher you鈥檒l be there for ages figuring it out. Even seemingly crude tools take skill and planning.

Similarly, tumplines are very simple in one respect, but complex in others. Using them requires a leap of imagination to carry heavy loads using your head, and careful practice to do it without hurting yourself. A preserved tumpline doesn鈥檛 look like much, just a scrap of cloth 鈥� but it鈥檚 actually a record of creative thinking.