Being the first domesticated species has created a strong bond between dogs and humans and entwined their histories. There is an extensive list of genes that showed signatures of parallel evolution in dogs and humans. A suite of 311 genes under positive selection in dogs are related to a large number of overlapping loci which show the same patterns in humans, and these play a role in digestion, neurological processes, and some being involved with cancers. This fact can be used to study the coevolution of gene function. Dogs accompanied humans when they first migrated into new environments. Both dogs and humans have adapted to different environmental conditions, with their genomes showing parallel evolution. These include adaptation to high altitude, low oxygen hypoxia conditions, and genes that play a role in digestion, metabolism, neurological processes, and some related to cancer. It can be inferred from those genes which act on the serotonin system in the brain that these have given rise to less aggressive behavior when living in a crowded environment. Dogs suffer from the same common diseases – such as cancer, diabetes, heart disease, and neurological disorders – as do humans. The underlying disease pathology is similar to humans, as is their responses and outcomes to treatment.