In our research we are interested in how jumping genes are different between human and chimpanzees (are our closest living relative). Chimpanzees and humans are, we think, both descended from an ape-like creature that lived 6-8 million years ago. That ape-like ancestor passed all the same jumping genes to the ancestors of chimpanzee and humans so any changes in the numbers and positions of jumping genes between humans and chimpanzeez must have happened since that time. This gives ua a way to work to how fast these changes happen.
I agree that dolphins and pigs are pehaps as smart as chimpanzees, but as we are researching jumping genes, rather than intelligence having a close relative species is more important. We could also work on Neanderthal DNA, as some has been extracted from bones recently. Unfortunately the DNA is too damaged (it is 10-20,000 years old) to be used in our current research.