Paleontologists have long noted how mammal fossils seem to change dramatically in form from one era to the next. In fact, few things in the fossil record seem to contrast more than the mammals that lived alongside the dinosaurs and those that survived after most dinosaurs went extinct some 66 million years ago.
One of the most obvious changes is that the Mesozoic (dinosaur era) mammals were generally very small and had teeth with sharp, pointed ends, both of which are characteristic of modern insect-eating mammals. Post-Mesozoic mammals, however, attained much larger body sizes and had teeth that were clearly used for grinding up plant material or slicing meat.
Why did mammals change? The typical thought among scientists is that when dinosaurs were the dominant land animals, they occupied nearly all of the herbivore and carnivore niches, restricting mammals to insect-eating and therefore small body sizes. Then when these dinosaurs went extinct, mammals were able to evolve and adapt to occupy these newly vacant ‘jobs’ within their respective ecosystems.
Together, this implies that all modern herbivores and carnivores, ranging from rhinos and cows to tigers and dolphins, descended from tiny, insect-eating ancestors. At some point, these ancestors began foregoing their insect prey in favor of plants and/or meat, and their bodies changed in form and function along with these newfound diets. To help you imagine the scope of what this means, picture the small insectivorous Mesozoic mammal below (top left), being the greatest of grandmothers to the herbivorous mammals in the images alongside it.
My colleagues and I wondered if there might be any evidence of this pattern, implied by the fossil record, in the genomes of modern day mammals. We looked at a gene encoding an enzyme called acidic mammalian chitinase (CHIA), which is produced in the stomach and other digestive organs and can break down the chitin-rich exoskeletons of insects [1]. We reasoned that if modern day herbivores and carnivores evolved from insect-eating ancestors, then they might have remnants (pseudogenes) of this gene in their genome.
Interestingly enough, we found that there are actually five chitinase genes, not one, and our analyses suggest that all five are common to the largest group of mammals (i.e., placental mammals). This implies that there were five in the common ancestor of these mammals, which we think lived in the Mesozoic alongside dinosaurs. Interestingly, modern mammals that have all five chitinase genes are all strongly insectivorous, suggesting that this ancestor was primarily an insect-eater too, consistent with the fossil evidence.
Furthermore, we found that in modern-day meat-eaters and plant-eaters, every species we examined has pseudogene remnants of one or more chitinase genes. So herbivores, like sloths, elephants, manatees, fruit bats, horses, rhinos, camels and rabbits, and carnivores, such as tigers, polar bears, walruses and dolphins, have remnants of genes that were likely once used to digest the prey of their ancestors: insects!
What’s more is that many of these mammals that are thought to be related, based on DNA and anatomical similarity, share the exact same mutations in some or all of the chitinase genes. For example, horses and rhinos look quite different from one another, yet have been considered by anatomists, paleontologists and geneticists to be related for over a century and a half. They are both herbivores, which implies their diet was inherited from a common ancestor, and indeed the earliest fossils that resemble them had teeth and jaws that appeared to be particularly good at eating plants. Such adaptations would have rendered insect-digesting genes relatively useless. Indeed, horses and rhinos have pseudogene remnants for four of the five chitinases, and they share at least one disabling mutation in each gene, suggesting they inherited defunct copies from a plant-eating ancestor.
Humans also possess three chitinase pseudogenes, alongside a single functional chitinase gene. Two of the pseudogenes share the same inactivating mutations with monkeys and apes, and a third shares a mutation only with apes. Interestingly, many of the earliest primate fossils appear to have been insect-eaters, but as monkeys and apes appeared, plants, as well as meats, became more important for their diets, so insect-digesting genes were likely less useful.
Together, these data suggest that in our very own genomes, we retain ‘molecular fossils’, that hearken back to a time when our distant ancestors were not the top of the food chain, but rather scurried along amidst dinosaurs, eating insects.
Questions for Creationists
Why would the Creator design humans, rhinos, tigers and other mammals that never or almost never eat insects with remnants of insect-digesting genes? Is it just a coincidence that the earliest mammal fossils, found alongside dinosaurs, appear to have been insect eaters, and modern herbivores and carnivores have remnants of insect-digesting genes? For those that believe that all animals were plant-eaters in the Garden of Eden, why do so many herbivores have remnants of insect-eating genes? If mammals were created in the last 10,000–6,000 years, how could they evolve from insect-eaters to herbivores and carnivores so quickly, modifying their teeth, jaws, intestinal tracts, etc. to be optimized for their new diets?
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