"My Inner Fish"

One of the main themes in the two episodes "Your Inner Fish" and "Your Inner Reptile" is that humans and fish have a shared ancestry, and humans and reptiles are actually closely related. There is a lot of concrete evidence in fossils and embryos to support this.

In the initial stages, a human embryo and and fish embryo look very similar because of the common Hox genes, which suggest common ancestry. Other evidence for common ancestry includes some structures that humans and fish both have today: bony skeletons, backbones and skulls, and shared basic brain anatomy.
The Sonic Hedgehog gene sends out the organizing signal; it tells cells to do different things, and creates the array of digits on the hand. Hair, which is a defining characteristic of mammals today, may have first evolved in our small mammalian ancestors as a sensory organ or a tool to help keep warm.

Another example of humans' relation to reptiles is their hearing anatomy. Mammals have an acute sense of hearing because they have 3 bones in their middle ear which form a lever system, while reptiles only have 1 bone in their middle ear. Scientists think that two jaw bones in reptiles got smaller, moved further up the jaw, and evolved to have a new purpose in the ear in mammals.

Human and fish embryos look very similar initially because of Hox genes, indicating a common ancestry