About 95 million years ago Mesonix, the ancestor of today's dolphin, appeared on Earth. It was a terrestrial animal that went into the water to feed. Mesonix had front and hind limbs containing bones to support its weight. It was covred with hair. Over the next 30 million years Mesonix began to become more adapted to life in the water. Perhaps changing environmental conditions on the earth at that time prompted this adjustment in life style. Forelegs became pectoral flippers. (The bone structure in the pectoral fins of today's dolphin is a remnant of its life on land.)  Mesonix's hind legs began to disappear. (Today, the dolphin has vestigial hind limbs in the form of rod-shaped pelvic bones, perhaps remnants of hind limbs used for land life.)  Its hair disappeared because hairless skin is more hydrodynamic and is not needed to warm an animal living in the water.

The Tokyo Institute of Technology recently reported developing a new way to look at an animal’s genetic history using DNA tracking. The DNA sequences they studied were called short interspersed elements, or SINEs, and long interspersed elements, or LINEs. These elements, once found in an animal’s genome do not disappear and will point to its evolutionary origins. The genetic evidence from this technology indicates that dolphins are closely related to cows, antelopes, giraffes, and that pigs may be their closest relatives, as they all have the same SINEs and LINEs. The dolphin’s 3-chambered stomach functions in the same way as the 4-chambered stomach of a cow, perhaps providing additional evidence of this evolutionary link. Additional work in genetics along this line is being carried out by Dr. David Busbee and his team at Texas A & M University. Through Dr. Busbee's team's work it has been discovered that dolphins and humans share many chromosomal similarities and that dolphin and human genomes are basically the same. Every chromosome in the dolphin has a comparative chromosome in the human.