Last updated May 17, 2018 at 1:54 pm
Conserved genes for breaking down exoskeletons point to a bug-munching common ancestor.
The first mammals – very likely small creatures that spent much of their time trying not to get crushed under the feet of reptilian megafauna – have left markers of their diet in the genomes of most, perhaps all, of their descendants.
Research conducted by scientists from the University of California, Berkeley, in the US, has found remnants of genes that enable insect exoskeletons to be digested in the genomes of 107 mammal species, including humans.
The discovery, says senior researcher Christopher Emerling, supports the hypothesis that the first mammals, arising around the end-Cretaceous extinction event roughly 65 million years ago, were small insectivores.
“In essence, we are looking at genomes and they are telling the same story as the fossils: that we think these animals were insectivorous and then dinosaurs went extinct,” he said. “After the demise of these large carnivorous and herbivorous reptiles, mammals started changing their diets.”
The remnants concerned are those of chitinase genes (CHIA). These code for enzymes that work to break down the tough outer structure of insects, which is made from a carbohydrate known as chitin.
A working CHIA gene
The ability to do this is essential for any species that includes insects in its diet. Humans and mice, for instance, both eat insects, and thus retain a working CHIA gene.
Emerling’s team, however, found three more in the human genome, all non-functional. Across the genomes of placental mammals ranging from shrews to whales, the researchers found a total of five – even in species such as tigers that come from a long line of non-insectivores.
The only mammals today that have five working chitinase genes are those with diets that comprise more than 80 per cent insects. But even this turned out not to be a hard-and-fast rule. Emerling discovered that pangolins (from the family Manidae) have only one working CHIA despite feeding exclusively on ants and termites.
The researcher suggests this may indicate the animals evolved from carnivores that had previously lost most of their functional CHIA quotient.
Functioning or not, though, chitinase genes represent in a sense the evolutionary signature of our mammalian common ancestor.
“One of the coolest things is, if you look at humans, at Fido your dog, Whiskers your cat, your horse, your cow; pick any animal, generally speaking, they have remnants in their genomes of a time when mammals were small, probably insectivorous and running around when dinosaurs were still roaming Earth,” Emerling observed.
“It is a signature in your genome that says, once upon a time you were not the dominant group of organisms on Earth. By looking at our genomes, we are looking at this ancestral past and a lifestyle that we don’t even live with any more.”
The research is published in the journal Science Advances.