Coprolites and Paleofeces: The Importance of Knowing Your Shit

As research on microbial diversity – or as it’s now been rebranded, microbiomics – continues, microbiologists are studying more environments, often overlapping with other fields. One interesting tendril of this new expansion extends into archaeology, where biologists armed with the latest DNA sequencing tools are exploring the microbiomes of lost tribes and cultures. It’s fascinating work.

One of the most abundant and informative sources of new data is ancient feces. For most of human history, people crapped wherever it was convenient, dumping generous samples of their microbiomes in privvies, trash heaps, or communal piles. In many cases, these deposits got buried and ignored until archaeologists unearthed them centuries later. The result is a globally distributed trove of paleofeces (or paleofaeces if you’re writing for Nature), encoding the microbial, dietary, and genetic history of past cultures. For scientists, paleofeces are more informative and likely better preserved than pottery shards, building foundations, or even corpses. All glory is fleeting, but shit remembers.

Unfortunately, the microbiologists now flocking to study ancient feces seem to have missed a crucial lesson on terminology: many of their papers now refer to these samples as coprolites, rather than paleofeces. Stay with me, this isn’t as pedantic a distinction as it sounds.

Coprolite cufflinks. A coprolite is a piece of fossilized feces. Fossilization is a chemical process that replaces most of the original sample with minerals, generally over a period of millions of years. It’s exceedingly unlikely that anyone could recover useful DNA samples from a coprolite. That’s why we don’t have the Apatosaurus microbiome already done, despite the abundant coprolites left by that enormous herbivore. Conversely, we don’t have many (if any) human coprolites around. We’re too new as a species.

Paleofeces are not fossils. They’re just old poop. As feces dry, sugars form a protective coating on their surface through the Maillard reaction. That’s the same chemical process that browns a steak, but it takes a bit longer for feces because they’re (hopefully) not being grilled. Thus protected, the fecal material inside the paleofeces can remain essentially unchanged for centuries. There’s no mineralization.

This matters for two reasons. First, scientists entering a new field should try to respect the terminology that field has already established. Archaeologists and paleontologists have good reasons for distinguishing paleofeces from coprolites – the two terms aren’t interchangeable, and they describe samples of radically different ages. Second, if we start conflating fossilization with other, faster forms of preservation, we’ll inevitably muddy an already confused discussion with the general public. We’re saying “fossil feces” formed in just a few centuries, but then turning around and telling young-earth creationists that dinosaur fossils must be millions of years old, because fossilization takes so long. Which is it?

So, fellow microbiologists: please get your shit straight. Thank you.