Why does my left brain control my right side?

Will - When we describe someone as having a twisted mind, we probably don't mean it literally, but this is the case for humans and other vertebrates. Some bundles of neural pathways crossover in the central nervous system. So the right side of the brain controls the left side of the body. In evolution, most useful stuff sticks around. And if these twists are in so many species of vertebrate, that means either every vertebrate evolved separately, convergent evolution, or more likely it developed right at the start of the existence of vertebrates. So that answers the octopus question straight away. They aren't vertebrates and so don't have the twist. In fact, most of their neurons are in their tentacles. So when and why did this twisting of our nerves take place? Well, for a long time, the most widely accepted explanation came from Spanish neuroscientist Ramon y Cajal. And here to explain it is the University of Münster's Marc de Lussanet.

Marc - In 1899, the famous Spanish neuroscientist Ramon y Cajal proposed that the crossed brain has a direct evolutionary advantage for vision through the pupil. The light projects an inverted image on the retina of each eye.

Will - So since the retina takes a flipped image, the twist is necessary in order to flip it back. Makes sense. However, this theory isn't without its critics, including Marc himself.

Marc - Unfortunately, though, Cajal's explanation doesn't work. Since the optic radiation inside the brain follows a loop, thus turning the image around.

Will - So no need to flip an image twice then. So what could be the reason for the twist? Well, perhaps the answer lies in the evolution of vertebrates themselves. When backbones first started evolving, they were on the ventral side of the body, which is to say the tummy side. Now imagine your spine going down your front. Actually don't do that. But as vertebrates evolved, the spine moved towards the back, and in doing so, may have twisted, almost wrapping the nerves around with it. This is the somatic twist hypothesis. Mark has another idea too, called the axial twist hypothesis.

Marc - According to this alternative line of explanation, the forebrain that is cerebrum and thalamus, along with the anterior part of the head, is turned around entirely. So not only left and right, but also front and back are reversed. In the early embryo just three to four weeks after gestation, this occurs by opposite asymmetric growth and leads to a complete twist. This involves not only the nervous system, but the entire body. Accordingly, the facial region, eyes, nose, and mouth grows from the left side to the midline and the ears, spine and limbs grow in the opposite direction around your body axis. Only the heart and bowels retain their original orientation and thus end up really asymmetrically.

Will - So an embryo's brain develops asymmetrically and causes the twist by being slightly lopsided.

Marc - In fact, careful observers can see the remnant of asymmetry in other people's faces. The right side of the face is larger than the left side, especially in babies and young children.

Will - The biological rationale of these two hypotheses would therefore be the spine is better off at the back and the twisting of the nerves don't seem to do any harm. The conversation is still ongoing though, so don't put your back out waiting for an answer. Thanks to Donald for the question, and Marc de Lussanet for the help with the answer. If you have a scientific quandary of your own, send it in to chris@thenakedscientists.com.