Why is dog poop often mixed in with archaeological sites?

If you have the opportunity to become a student majoring in archaeology, you may have a "special feeling" for feces. But don't get me wrong, this is not Lao Ba's possession, but in archaeological research, a piece of ancient feces is very precious. They can be preserved for thousands of years, and even their original shape and color can be preserved. Archaeologists can use these feces to restore people's eating habits, parasite evolution, and microbiome ecology at that time.

Dog feces from the Xiaosungang site in Anhui, China

However, there is one embarrassing thing that makes it difficult for archaeologists to distinguish, that is, who is the source of the feces? For example, human feces can easily be mixed with dog feces.

In 1981, Melinda Zeder, then a graduate student, was sorting fossils in a Paleolithic cave in southern Iran when she came across an unidentifiable fragment. She then remembered a common saying in archaeology: "When you are presented with an indistinguishable fossil, you press the tip of your tongue against it. If it is a bone fragment, it will stick to your tongue."

Zeder did this, but the fragment did not stick to his tongue. Instead, it dissolved a little as the saliva soaked it. Zeder, who had never encountered such a situation before, looked at his more experienced colleague with a puzzled look and asked him what this was.

His colleague smiled and said, “Oh, that’s bullshit.”

Domestication of dogs

That’s right, as one of the earliest animals domesticated by humans, some dog “items” often appear in human relics.

Although there is still controversy about the earliest domestication time and place of dogs, there is no doubt that the first domesticated dogs came from the "grey wolf" (gray wolf). As we all know, wolves are pure carnivores, while dogs are omnivores. This difference between the two is the key to the domestication of some wolves into dogs.

In the Neolithic Age, the main source of human food was agricultural planting, so starch became a very important part of the human diet at that time. However, humans are omnivores, and in addition to being vegetarians, meat intake is also very necessary, so for early ancestors, they could only obtain it through "hunting in the wild".

But to be honest, the efficiency of junglers at that time was very low, and the "wild monsters" were generally better developed than humans, and it was not uncommon to return empty-handed. This was when the advantages of human wisdom evolution came into play. Since I couldn't beat the wild monsters, I might as well call for a few more assistants.

So humans began to use the crops they grew to lure some animals for their own use. Among them, they were more optimistic about wolves. But the problem is that these starch crops grown by humans are simply indigestible for wolves because they lack the amylase that promotes starch hydrolysis.

But being unable to digest does not mean they won’t eat. If they fail to catch prey, the wolves will still succumb to reality, which at least ensures that they will not starve to death. Eating starchy foods for a long time has also caused some wolves to mutate their genes, with some mutations in genes related to amylase .

According to a study published in the journal Nature in 2013, by sequencing the whole genomes of dogs and wolves, it was found that ten genes related to starch digestion and fat metabolism were expressed in dogs but not in wolves .

The results of this study also show that the wolves with genetic mutations at that time had stronger environmental adaptability than those pure carnivorous wolves. After all, they could accept starch-rich food grown by humans and were not limited to eating meat to survive. Because survival became easier, this type of mutant wolf would actively approach humans.

However, since there were not many mutations at the beginning, some wolves may have died due to indigestion, allergies and other reasons caused by human food. Only those individuals with more mutations could survive. After generations of "semi-artificial selection", wolves were gradually domesticated into early dogs.

Because of the existence of dogs, humans' "hunting" efficiency has also been improved. As a result, dogs have become more and more present in our lives. But this is not a good thing. For early humans with scarce living resources, it is inevitable that some people will want to "have a good meal" when there are a group of "dogs" around them.

Since some people eat dogs, human feces may contain canine genetic material, and dogs may eat human feces, so human genetic information may appear in dog excrement . This makes the archaeological record "full of dog shit", which has caused a lot of trouble for archaeologists.

AI poop test

To solve this problem, Christina Warinner, a molecular archaeologist at Harvard University, and her colleagues developed a program called coproID that can identify ancient feces. (Zeder really thinks it's great)

The researchers fed the program a large amount of modern human and dog feces data, which included not only human and dog DNA information, but also genetic sequences of intestinal microorganisms, plants, and other substances present in the intestine. Then, after a lot of learning, the machine identified 13 ancient feces samples from all over the world, ranging from 7,000 to 400 years ago.

(a) From the Xiaosungang site in Anhui, China, Neolithic period; (b) From Durango, Mexico, 1,300 years ago; (c) Durango, Mexico, 1,300 years ago.

In addition, they tested seven control samples that were not feces. CoproID correctly identified seven control samples that were not feces; and for the 13 experimental samples, AI identified five as human feces and two as dog feces.

One of the samples, which archaeologists originally thought was human feces, turned out that coproID reported that the feces did not come from humans but from a dog.

However, this program is still being improved, and the data "fed" to it basically comes from modern dogs, and more ancient feces data may be needed to learn.