Figure 2. Distribution of Blue Whale Subspecies. NOAA Fisheries.

It's hard to believe that about 55 million years ago, whales evolved from land-based animals similar to cow and deer. Mesonychidae was a family of hoofed mammals that lived in Asia, Europe, and North America. These ranged from small dog-sized mammals to mammals as large as modern bears.

Natural selection selects genetic mutations that make an organism most suited to its environment and therefore more likely to survive and reproduce. Random mutation resulted in at least one whale whose genetic information placed its "nose" farther back on its head.

Over centuries of adapting to life in the oceans, whales lost their hind-limbs, grew a flat tail, developed flippers, and streamlined their bodies. One major homologous structure is the fin of a whale. If you look at the skeleton of a whale's fin, notice that all of the bones match up to comparative bones in other mammals. This is evidence that whales share a common ancestor with other mammals. In modern whales, they in fact possess vestigial structures, such as remnants of hind limbs in their pelvic region that were used to walk on land by their ancestors.