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.
All reptiles that still have the pineal eye today are “cold blooded”; they have what’s known as an ectotherm metabolism. Modern mammals – which of course have “warm blood” or an endotherm metabolism – don’t have a pineal eye.
Our group of researchers at the University of the Witwatersrand wondered whether being able to pinpoint when pre-mammalian species lost their pineal eye might unlock the secret of when “warm bloodedness” become a mammalian hallmark. That’s what drove an ambitious study using fossil remains from South Africa’s Karoo region.
We were proved right: our research revealed that mammalian ancestors likely shifted from “cold” to “warm” blood 246 million years ago. This was 10 million years before the first dinosaur even appeared.
Why have a third eye?
As with a regular eye, the pineal eye is made up of a cornea, a lens and a retina. Our paired eyes and the reptilian pineal eye are also very similar in terms of embryological development and the genes expressed during this. The pineal eye differs from a regular eye, though, in that it’s usually covered by a thick and large scale and can differentiate between light and dark only.
Our regular eyes can also see variations between light and dark, day and night – so what’s the point of having an organ as redundant as the pineal eye? Research shows that in reptiles the pineal eye acts as a calendar. It can see days getting longer and nights getting shorter, and the reverse, and so tells the brain how seasons are changing. As a consequence, it monitors most life cycles such as sleep and reproduction rhythms.