To better understand human health in earlier centuries, scientists can turn to many different methods, including pottery analysis, radiocarbon dating or simply perusing old records. But answers can also be found in unlikely places — such as ancient latrine sediment, which is the collective biological waste generated by past communities.

(Yes, I am talking about ancient, fossilized human poop.)

In a new study published by The Royal Society, researchers investigated the parasites and human gut microbes found in ancient human feces and latrine sediment at two archaeological sites found in Jerusalem and Riga, Latvia. 

During the 15th century, the Jerusalem latrine was used by multiple different users from more than one household. Eggs in that fossilized feces belonged to various helminths, such as the roundworm and tapeworm. Similarly, the Riga latrine was dated to 1536 CE, and was reported to be used by the general town population. Researchers extracted DNA from sediment samples in the two latrine sites and sequenced that DNA to build a picture of the organisms present in ancient human gut microbiomes.

The researchers identified a range of bacteria, archaea, parasitic worms, and other organisms present in the ancient human gut, though it should be noted that findings are representative of the community as a whole, rather than a single person. One example is the Bifidobacterium species, which are generally enriched in populations with industrialized agriculture.

Overall, this study showed that DNA is still preserved in ancient latrines, and that ancient poop from toilets that have long been out of order can still reveal information about past human health.