Dirty dishes reveal what ancient civilizations ate. Food scraps on 8,000-year-old ceramic shards found in Turkey include barley, wheat, peas, and bitter vetch.

Do different types of plants have different types of proteins? Sorry if this is incredibly obvious, I'm not very knowledgeable with biology.

Yup, they do! For example, a particular plant might have a specific enzyme (a protein catalyst) that makes a toxin or pigment. But even when plants have the same general protein with the same function, there are slight differences due to evolution.

Or thinking about it another way, the way we often look at differences in plants and animals is by looking at their genetic code, their DNA. And the vast majority of genes in DNA are instructions for making proteins, so the differences we see between species in their DNA are then reflected in those proteins.

I was curious to see which proteins they used to distinguish them, now that you've asked, so I decided to take a look at the paper. Taking a peek at the supplemental material, the proteins they found varied. For example, Legumin K was traced to Pisum sativum (peas), while B1-hordein was traced to Hordeum vulgare (barley). (Basically once they'd determined protein sequences, they compared them to a large database of known sequences to figure out what plants they could have come from.)

A detail I thought was cool was that they weren't just able to determine the plants. In some cases they could also make a guess as to what the food itself was based on the proteins present:

Similarly, we can also use the tissue specificity of the proteins identified to comment further on plant processing. For the cereals, the proteins derived from barley (Hordeum vulgare) and wheat (Triticum sp.) are all expressed in the grain endosperm (Supplementary Data 2), indicating that this part of the plant was contained in these vessels, possibly as part of a porridge or soup.