The bugs your primate ancestors loved to eat found a way into your genes
To answer this question, she used published genomes for some of the primates to look for copies of a gene challed CHIA, which codes for acidic mammalian chitinase – the stomach enzyme that breaks down chitin. She sequenced the CHIA gene for those primates without publicly available genomes. Janiak hypothesized that primates that consume more insects should have working versions of the CHIA gene, while primates that eat fewer insects or none at all should have nonfunctional versions. These results would suggest both that the idea that mammals can’t digest chitin was wrong and that the ability to break down chitin was an evolutionary advantage for primates that eat a lot of insects.
Janiak found that our early primate ancestors (which tended to be very small) probably had three working copies of the CHIA gene, while most living primates have only one. The species in her sample that had the most insect-heavy diets (like the tarsier and the galago) also had at least three working copies of the gene, like early primates did. (The tarsier has five copies!) These additional copies suggest that strong selection was at work in the tarsier lineage – those tarsier ancestors that were able to more effectively break down chitin had a nutritional advantage over those that couldn’t.
Mystery of the CHIA debt
The same might be true for humans. If eating insects was actually an important part of our evolutionary history as a species, this might explain why we still have a functional copy of the CHIA gene. The unsolved mystery here is whether that gene still causes the production of chitinase in the human stomach; researchers have found conflicting results about this in the past. However, those studies were done on people from populations that don’t often eat insects (for example, one was a side project on gastric juice samples from Italians obtained for other medical reasons).
The expression of the CHIA gene could be mediated by whether or not a person/population has a history of eating insects. This would mean that populations might vary in their ability to effectively digest insect exoskeletons. If a difference in chitinase production exists, it is likely occurring at the gene expression level - actually eating insects might turn the CHIA gene on. This would be a different genetic mechanism than what happens with lactose tolerance (where a mutation within the gene controls enzyme production) and to figure out whether or not it’s happening we will need better sampling of the digestive juices from non-Western populations, some of which feature insects in their diets.
This study is potentially important both for understanding primate origins and for thinking about our future as a species. It has been suggested that our grasping hands and acute vision were early adaptations to an insect-eating lifestyle; this research provides some genetic support for those ideas. In terms of our future, many cultures around the world still consume insects, and the United Nations has proposed edible insects as a viable source of protein for our ever-growing world population. Understanding the importance of this resource to our own evolution might open the door to greater acceptance of insects as food – call them part of the original paleo diet.