Why there is a crossing in the brain
Strange world: in vertebrates, i.e. in humans too, the left half of the brain controls the right side of the body, and the right half controls the left side. The optic nerves also cross on their way from the eyes to the brain in the optic chiasm. The reasons for this are unclear. One explanation is that the so-called contralateral arrangement in the forebrain developed because it improves sensory perceptions. Dr Marc de Lussanet, a biologist and neuroscientist at Münster University's Institute of Psychology, has his doubts about this explanation. Together with Prof. Jan Osse from Wageningen, in the Netherlands, he provides an alternative explanation, published in the Animal Biology journal, which says that about five hundred million years ago an early ancestor of the vertebrates turned on to its left side.
The early ancestor – a primeval fish – turned itself through 90 degrees at least 450 million years ago, say the German/Dutch researchers. The two scientists have developed various scenarios for this. For example, the twist may have enabled the animal to hide on the sea-bed, like a flatfish. However, a simple twist would have meant the loss of the bilaterally symmetrical arrangement of organs, whereby the eyes are positioned on the right and the left side of the head and the fins grow out of both sides of the body, left and right.
What the two researchers assume is that in the course of evolution individual parts of the body migrated – some clockwise, some anti-clockwise – in order to re-establish this symmetrical arrangement. Accordingly eyes, nostrils and the forebrain migrated in the direction of the original twist, while areas of the brain and the body positioned nearer the tail migrated in precisely the opposite direction. As a result, some crossings of nerve tracts developed between areas of the body, for example the optic chiasm – the crossing of the optic nerves. The scientists underpin their hypothesis with, among other things, observations gathered from the development of embryos in zebra fish and chickens. In the earliest embryonic states asymmetrical cellular movements take place for which there has been no explanation so far. This new work shows that these cell movements do in fact happen in a way that would be expected in the case of an adaptation to the twist.
A bilateral and symmetrical arrangement of sensory organs and extremities provides animals with an evolutionary advantage. It might, for example, be necessary for a fish to have pectoral fins on the right and the left side. For this reason this lateral arrangement was kept, despite the twist, say the team of researchers. The evolutionary advantage may not, they say, necessarily apply to the inner organs. So the heart and the gastrointestinal tract did not have to "twist back" – which would explain why the heart is still on the left-hand side.
"What we have done is to provide the first logical explanation for the multitude of crossed nerve connections in the forebrain, and for the fact that these crossings are so widespread in vertebrates," says Marc de Lussanet. "And – contrary to what the old theory says – without any improvement to sensory perception or motor control."
Dr. Marc de Lussanet is a member of the Otto Creutzfeldt Center for Cognitive and Behavioral Neuroscience (OCC). The OCC is a research centre at Münster University with scientists from the fields of medicine, biology and psychology. The researchers are engaged on interdisciplinary work on current topics from the area of behavioural neurosciences, using methods of modern imaging, molecular genetics and neurophysiology.
Original publication:
Marc H. E. de Lussanet and Jan W. M. Osse (2012): An ancestral axial twist explains the contralateral forebrain and the optic chiasm in vertebrates. Animal Biology 62 (2), 193-216; DOI: 10.1163/157075611X617102