What industrialized guts lost

A Paleolithic gut cannot be sampled directly, but two useful comparisons are available, and they point the same way.

The first comparison looks sideways, at people living today outside industrialized settings. Their gut microbiomes are measurably more diverse than those of people in industrialized ones, and deep sequencing of hunter-gatherer populations has recovered organisms that appear to have largely vanished from industrialized guts.C-I-03

The second looks back in time, at what people left behind. Researchers reconstructed microbial genomes from preserved human feces 1,000 to 2,000 years old. Those ancient gut communities resembled non-industrialized microbiomes far more closely than industrialized ones, and they carried markedly fewer antibiotic-resistance genes.C-I-05

The two comparisons are independent of one another, one spatial and one temporal, and they agree.

It is reasonable to be skeptical that anything can be recovered from remains that old, but the record does survive.

The Tyrolean Iceman, the 5,300-year-old body recovered from an Alpine glacier, carried Helicobacter pylori in his stomach, and the bacterial genome was sequenced from his gut contents. His strain was of nearly pure Asian origin, which established something that could not otherwise be dated: today's European strains are a hybrid, and the African contribution arrived more recently than anyone could previously say.C-I-06

That finding concerns ancestry and migration rather than diversity; it does not tell us that his microbiome was richer than ours. What it demonstrates is that the question can be answered at all. These organisms traveled with us, their history is written alongside ours, and it can still be read.

In mice carrying human gut microbiota, a low-fiber diet caused a loss of microbial diversity that compounded across generations. Within a single generation the loss was largely reversible: when fiber was restored, the community recovered. Across several generations it was not. The missing organisms did not return when fiber was restored, and they had to be reintroduced directly.C-I-07

In practical terms, the study describes a kind of loss that diet alone cannot correct, because what is missing is not a nutrient but an organism, and an organism that is no longer present cannot respond when fiber returns.

Antibiotic treatment disrupts the gut microbiota, and whether it recovers fully afterward is not settled.C-I-08

A systematic review asking whether probiotics prevent or reverse antibiotic-induced disruption concluded that their effectiveness remains uncertain. There is no supplement recommendation to be drawn from that.C-I-08

Human evidence does not yet establish whether these microbial differences contribute to obesity, result from it, or both, so nobody can currently tell you that correcting them is the route out. What is clear is that the microbial part of you changed faster and more completely than the rest, which makes it a real and underappreciated piece of the mismatch, and that what follows from it is still being worked out.

Human biology was shaped for one world and now operates in another, and the gap opened faster than biology could close it. The microbiome is the clearest available illustration, because it is the one component that could in principle have kept pace, with communities that turn over in days rather than generations.