Lungfish | The lungfish first evolved almost 400 million years ago.

The modern lungfish is the product of an ancient lineage that dates back almost 400 million years. This “living fossil” is sometimes called primitive, but that does not mean its features are undeveloped, just that it has changed very little since first evolving. The most interesting characteristic of the lungfish is the presence of the internal lung, which serves multiple purposes. Like the swim bladder in other fish, the lung provides buoyancy while swimming, but it also absorbs oxygen and removes waste.
Despite the presence of reduced or vestigial gills, most species must breathe exclusively through their pair of lungs. Only the Australian lungfish has fully operational gills, but one of its two lung sacs is also reduced or atrophied.
This creature might be the closest living relative of all tetrapods (mammals, amphibians, reptiles, etc). Some of the similarities between then include four limbs, a similar arrangement of skull bones, and the presence of tooth enamel and a pulmonary system. Some 400 million years ago, a fish-like ancestor began evolving into a tetrapod, and it probably looked something like the lungfish.
Some species survive the dry season by becoming dormant. This process involves burying their bodies in the bottom of the river or lake and then encasing themselves in mucous. Because the metabolic rate falls dramatically, the fish can remain in this state for more than a year. The South American lungfish burrows in the mud but doesn’t enter a dormant state. The Australian lungfish relies on neither strategy.