Dog diets often contain more starch than those of their carnivorous wolf ancestors, and an analysis of fossilised dog faeces helps explain how the animals made the dietary change. Long before their genomes adapted to their plant-rich chow, their gut microbiome gained a starch-digesting profile.
Due to their close association with humans, it is thought that dogs鈥� diets shifted to less meat and more carbohydrates when farming began 鈥� an idea that was supported by an archaeological analysis published earlier this year.
In Europe, this shift occurred between 8000 and 6000 years ago. However, evidence suggests that dogs鈥� genes couldn鈥檛 produce much amylase 鈥� an enzyme that turns starch, a complex carbohydrate, into sugars 鈥� until thousands of years later.
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Now, at the University of Bologna in Italy and his colleagues have discovered that gut microbes probably helped dogs digest starch before their genomes had caught up and they possessed more copies of the required amylase gene.
Rampelli鈥檚 team sequenced the DNA in 13 fossil dog faeces, or coprolites, from the site of a Bronze Age agricultural community in Solarolo, near Bologna. The fossils date from between 3450 and 3600 years ago.
The scientists found traces of sheep, wheat and grape DNA in the coprolites, suggesting the dogs were omnivores like modern domestic dogs. But dog DNA in the faeces shows the animals had fewer copies of the amylase gene needed to process this diet than are seen in modern dogs.
However, the coprolite analysis also identified 56 microbe species from the ancient dogs鈥� guts, some of which are common in dogs鈥� guts today. Importantly, these metabolise starch by producing their own amylase enzymes.
The microbe DNA in the coprolites had almost double the amylase signal as that seen in modern dogs鈥� gut microbiomes. This suggests that when dogs started eating more starchy food, their gut microbes adapted quickly, says Rampelli. The bacteria that were better at breaking down starch would have outcompeted other microbes and therefore proliferated.
鈥淭he Solarolo dogs were probably already fully domesticated, eating some plant-based foods that humans left to them, but also livestock carcasses when possible,鈥� says Rampelli. 鈥淭his constituted a new 鈥榞ut condition鈥� that selected different microbes.鈥�
This meant ancient dogs would have been able to harness the energy stored in starch even before their genomes adapted to the new diet. Without such microbes, starch in the ancient dogs鈥� food would have remained largely undigested and been excreted, says Rampelli.
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