Researchers from the US and Australia report in Science that all three marine mammal lineages — cetacean, pinniped, and sirenian — amassed genome lesions within their PON1 genes, while terrestrial mammal lineages still have intact PON1 genes. Since PON1 is involved in lipid metabolism, the researchers suspect its loss in marine mammals could be due to differences in diet between them and terrestrial mammals. But, as National Geographic notes, it's not yet fully clear why it was lost.
However, it adds, a functioning PON1 gene would now be useful for marine mammals as it also helps break down organophosphates, pesticides that wash into the ocean. Terrestrial animals exposed to them with normal PON1 level can deal with exposure, but without it, marine mammals could be at risk, though National Geographic says more research is needed. It adds that University of Pittsburgh's Nathan Clark, the senior author of the Science study, and his colleagues are now examining the blood of manatees in Florida where there is pesticide runoff for organophosphate toxicity.