A recent study found ancestors of whales and dolphins might of lost some genes when they made the transition from land to sea animals. It was found that cetaceans(dolphins and whales) lost 85 genes since living in the water. By comparing these genes to hippopotamuses it was concluded these genes were lost when cetaceans transitioned to water creatures around fifty million years ago. Losing these genes helped cetaceans adapt to living in the water. POLM was a gene that helps repair DNA, and regulate blood clotting. The loss of this gene helped increase the fidelity of the repair of DNA. Other genes that dealt with lung function was also lost. This helped avoid damage to the lungs that could occur while deep diving. They also lost the saliva producing gene(SLC4A9). Since cetaceans live under water, they don't need saliva to break down food. Producing less saliva also helps keep fresh water in their bodies. Cetaceans lack melatonin which is a sleep inducing hormone. Researchers believe they loss this gene, since its no longer necessary.