The Essentials.

Omega-3 and omega-6 fats are called essential fatty acids because we cannot manufacture them ourselves, meaning they must come from the foods we eat. These fatty acids are named for their carbon-to-carbon double bond on a carbon chain, and only plants are able to make a double bond at the third or sixth carbon position. There are no known animals that create their own omega-3 or omega-6 fatty acids.
New England Journal of Medicine, June 2012