You're no doubt familiar with Stonehenge, the moai stone heads of Easter Island, and the Nazca Lines of southern Peru. But one example of prehistoric monumental architecture that’s remained remarkably unknown is the colossal rectangle shapes found in the Arabian Peninsula.

A new study has radiocarbon dated a number of these monuments and found some date back to around 7,000 years ago, making them among the oldest known large-scale monuments associated with rituals on Earth. However, older examples of stone constructions do exist, for example "kites" in the Middle East.

Reported in the journal Holocene, the researchers from the Max Planck Institute for the Science of Human History in Germany suggest the structures were likely used in ancient rituals that emerged as cultures in northern Arabia started to deal with radical environmental change and the emergence of pastoralism in the region. 

A total of 104 of these vast stone structures have been documented in the Nefud Desert of northwest Saudi Arabia. Called “mustatil” after the Arabic word for “rectangle,” the structures consist of long, thin walls less than half a meter high that stretch along the rocky landscape. Typically, the rectangle-shaped structures are between 100 ?meters and 200?meters (328 to 656 feet) in length, although they can vary in size from 26 meters to 616?meters (85 to 2,021 feet). 

Between 8,000 BCE and 4,000 BCE, the Arabian Peninsula was a rainy grassland covered in lakes and rivers. This big environment change appears to have led to a reduction in nomadic lifestyles and edged people towards the adoption of pastoralism.

Considering the mustatils were built around this time, the researchers suggest they had some relation to the development of pastoralism and a more sedentary lifestyle. However, the walls do not seem tall enough to serve as a defensive structure. The absence of objects, such as fireplace hearths, inside the structure also suggest they were only temporarily used and not permanently settled within. 

While they might denote some territorial boundary, the researchers say they also likely held a ritualistic value. 

“The lack of obvious utilitarian functions for mustatils suggests a ritual interpretation. In fact, mustatils seemingly represent one of the earliest examples known anywhere of large-scale ritual behaviors encoded in the practice of monumental construction and use,” the study authors write. 

The nature of this ritual remains unclear, especially since the structures remain so unique. But the researchers believe it might have something to do with ritual slaughter and "conspicuous consumption," such as a feast, in response to the rise of pastoralism in the area. 

"Our interpretation of mustatils is that they are ritual sites, where groups of people met to perform some kind of currently unknown social activities," Dr Huw Groucutt, lead study author and group leader of the Extreme Events Research Group at Max Planck, said in a statement. "Perhaps they were sites of animal sacrifices or feasts."