http://advances.sciencemag.org/content/4/5/eaar6478.full
We have the remnants of genes inherited from our arthropod eating
ancestors. Eutherian mammals have a common ancestor that had 5 of the
genes that could digest chitin (arthropod exoskeletons). That ancestor
was likely an insectavore that was running around the feet of the dinos.
So all plancental mammals excluding montremes (egg laying mammals) and
marsupials had the same common ancestor that had these 5 genes.
Our primate ancestors were still insectavores as the primate lineage
evolved, but we started changing our diet so that by the time of the
common ancestor we share with monkeys evolved we were mostly fruitivores
and vegans, but humans evolved into the omnivores that we are today and
a lot of humans eat arthropods (crabs, shrimp, insects). So it is a
good thing that we still retain some of the old chitinase genes that we
inherited from that common ancestor of all eutherian mammals.
The fact is that we have lost some of these genes, but we still retain
their remains in our genomes even though the genes are defective. The
paper linked to above can trace our evolution by the genes that we have
lost over time, but we still have defective copies.
Since agriculture was invented humans have been selected to increase our
amylase genes (digest starch) by gene duplication and the humans that
have relied on agriculture for the last few thousand years have more
amylase genes than those humans like the San that did not take up
agriculture.
This just means that as our diets changed the number of genes that we
needed to digest our food changed. We may be increasing the amylase
genes, but most of us aren't selecting for increasing the chitinase
genes any longer.
Ron Okimoto