Summery:
French philosopher René Descartes believed that the pineal gland, a tiny button of neurons located in the depth of our brain, was the seat of the soul.
Today, thanks to palaeontology, genetic and developmental studies, we know that it is actually the evolutionary relic of a long-vanished organ, the third eye. This is also known as the pineal eye and is a receptor located on the top of the head. Many existing reptiles such as monitor lizards, some iguanas and the tuatara still have a pineal eye.
Link for more: https://theconversation.com/what-our-ancestors-third-eye-reveals-about-the-evolution-of-mammals-to-warm-blood-68454
