What we inherited from our bug-eating ancestors berkeley news gas urban dictionary

A spectral tarsier (Tarsius tarsier) feeding on a grasshopper in Tangkoko National Park, Northern Sulawesi, Indonesia. Tarsiers have five chitinase genes to digest the high amount of chitin in their insectivorous diet, which likely represents the ancestral condition of all placental animals, including humans. (Photo copyright Quentin Martinez)

Based on an analysis of the genomes of 107 different species of mammals, University of California, Berkeley, scientists conclude that our distant ancestors – the small, furry creatures that scurried around the feet of the dinosaurs 66 million years ago – were mostly insect eaters.

The scientists inferred this because the genes for the enzymes that allowed these early ancestors of all mammals to digest insects are still hanging around in nearly all mammal genomes today. Even animals like tigers and seals that would never touch an insect have non-functional pieces of these genes sitting in their chromosomes, betraying their ancient ancestors’ diet.

“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,” said postdoctoral fellow Christopher Emerling. “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 anymore.”

“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. After the demise of these large carnivorous and herbivorous reptiles, mammals started changing their diets,” he said.

But humans actually have remnants of three other chitinase genes in their genome, though none of them are functional. Emerling showed that these gene remnants in humans aren’t unique to humans or primates, but instead can be traced to the ancestral placental mammals.

In all, he and his colleagues found five different chitinase enzyme genes by looking through the genomes of the largest group of mammals, those that have placentas that allow longer development in the womb, which excludes marsupials like opossums and egg-laying monotremes like the platypus. These placental mammals ranged from shrews and mice to elephants and whales.

“The only species that have five chitinases today are highly insectivorous, that is, 80 to 100 percent of their diet consists of insects. Since the earliest placental mammals likely had five chitinases, we think that this makes for a strong argument that they were highly insectivorous,” Emerling said.

As you would expect, ant and termite specialists such as aardvarks and certain armadillos have five functioning chitinase genes. But so do the insect-loving primates called tarsiers. They appear to be the only primates that have so many functional chitinase genes, Emerling said.

The story told by these chitinase genes is one of early mammals hunkering down eating insects while the big guys, the huge herbivorous dinosaurs like the brontosaurus and the big meat-eaters like T. rex gobbled up the most abundant food resources. Only 66 million years ago at the end of the Cretaceous Period, when all non-bird dinosaurs died out, were mammals able to expand into other niches, which they quickly did. The first carnivorous and herbivorous mammals, as indicated by their teeth, arose within 10 million years of the dinosaurs’ demise.

He focuses primarily on weird animals that eat insects, including anteaters and armadillos, the unrelated aardvark and the distantly related pangolin. In exploring how these animals are able to digest insects, he decided to look at chitinases, whose roles in mammals are still poorly understood. It’s not known, for example, whether the enzymes allow animals to break down chitin into its component sugars and use them for energy, or if chitinases’ sole function is to break up the exoskeleton to allow access to the soft interiors of insects.

Using databases of animal genomes, plus newly sequenced genomes of armadillos and a lesser anteater (tamandua) obtained by colleagues at the Broad Institute at MIT and Harvard, he searched for genes similar to the known chitinase gene and dredged up four new varieties.

Among the surprises was that the insect-eating-specialist pangolin has only one functional chitinase gene, in contrast to the five in the aardvark and four in the lesser anteater. All eat ants and termites exclusively, but pangolins may have possibly evolved from carnivores that lost their chitinase genes shortly after taking over the ecological niche opened up when meat-eating dinosaurs died out.

“This is suggesting that there are a lot of these enzymes that might be helping organisms digest their food. This goes from being a simple curiosity – humans have a chitinase, how cool! – to being something that can help us understand how different animals are adapted to their specialized diets.”