Summary: New lamprey brain research pushes the origin of the cerebral cortex back to more than 500 million years ago, offering important insights into how the human brain evolved.
Source: Karolinska Institute
The study and its significance
Researchers at Karolinska Institutet in Sweden report that detailed studies of the lamprey brain indicate the fundamental blueprint for the cortex, basal ganglia and dopamine systems existed far earlier than previously thought. Published in Nature Ecology & Evolution, the study argues that key elements of integrative cerebral function were present in the common ancestor of vertebrates more than 500 million years ago.
The human brain is among evolution’s most complex achievements. For decades, many neuroscientists believed that advanced forebrain structures, particularly a functional cortex, arose primarily in the lineage leading to mammals. Non-mammalian groups such as fish, amphibians and jawless vertebrates were thought to lack distinct cortical sensory and motor representations. The cortex—the outer, layered region of the brain—supports complex processing related to vision, movement and higher cognitive functions including memory and emotion.
Methods and main findings
“We’ve spent a long time studying brain evolution using the lamprey, one of the oldest extant vertebrate groups,” says Sten Grillner, last author of the study and professor of neurophysiology at the Department of Neuroscience, Karolinska Institutet. The team combined neurophysiological recordings with histochemical mapping and anatomical tracing to examine the lamprey pallium (the cortical-like structure) and its connections.
Their results show that the lamprey pallium contains a separate visual area with retinotopic organization—different parts of visual space are mapped across this region. Adjacent to that visual field representation, the team identified a somatosensory area representing inputs from the head and trunk, and a non-primary motor area between the sensory zones. Both visual and somatosensory inputs are routed through the thalamus, establishing a thalamocortical relay analogous to that seen in mammals.
In short, the lamprey brain exhibits a sensory topography and thalamic relay that mirror the basic sensorimotor organization long associated with the mammalian neocortex. While each structure contains far fewer neurons than comparable mammalian regions, the conserved layout and connectivity suggest a shared evolutionary origin.
Implications for brain evolution
“This evidence forces us to push the birth of the cortex back roughly 300 million years,” Professor Grillner explains. If the basic cortical blueprint existed before the lamprey split from the lineage that led to jawed vertebrates and mammals, then the essential plan for the human forebrain was already in place over 500 million years ago. That conclusion reframes our understanding of how complex sensory and motor circuits evolved across vertebrates.
The discovery that the lamprey possesses conserved, orderly arrangements of cortical-like sensory and motor zones, along with basal ganglia and dopaminergic systems, provides a new comparative perspective. It supports a model in which the core circuit motifs supporting sensorimotor integration were established early in vertebrate evolution and later expanded or specialized in different lineages rather than arising de novo in mammals.
Funding and publication details
The research received funding from the Swedish Research Council, the European Union’s Seventh Framework Programme for Research and Technological Development, the Parkinson Foundation and Karolinska Institutet.
Source:
Karolinska Institute
Media contacts:
Press Office – Karolinska Institute
Image source:
The image is in the public domain.
Original research (closed access):
“The evolutionary origin of visual and somatosensory representation in the vertebrate pallium.” Shreyas M. Suryanarayana, Juan Pérez-Fernández, Brita Robertson & Sten Grillner. Nature Ecology & Evolution. doi: 10.1038/s41559-020-1137-2.
Abstract (condensed)
Amniotes, including mammals and reptiles, show visual and other sensory representations in the pallium, whereas anamniotes (amphibians, fish and cyclostomes such as lampreys) were historically thought to process mainly olfactory information in the pallium. This study demonstrates a distinct visual area with retinotopic mapping and an adjacent somatosensory representation in the lamprey lateral pallium. These sensory areas flank a motor-related region, and both visual and somatosensory inputs are relayed through the thalamus. The findings imply that the basic sensorimotor layout of the mammalian neocortex and the thalamocortical relay mechanisms were already present in the last common ancestor of cyclostomes and jawed vertebrates about 560 million years ago.