Ancestor (left) and descendant (right). The image shows a reconstruction of the common ancestor of all living mammals (Hadrocodium wui) from the Early Jurassic, which has the size of a paper clip. Right, a model of a human brain. In terms of brain organisation, the mouse (centre) is probably a “living fossil”. The diagrams to the right show a mixed (right) and a modular ordered structure of nerve cells in the cortex. Credit: MPIDS

In
the course of its evolution, the architecture of the mouse brain may
have barely changed. Similar to the tiny ancestors of modern mammals
that lived about 80 million years ago, nerve cells in the mouse visual
cortex are densely packed in a small area of ??the brain. However,
during the subsequent evolution of larger brains the architecture of the
cerebral cortex was radically restructured. This is the conclusion of
an international team of researchers led by scientists at the Max Planck
Institute for Dynamics and Self-Organization, the University of
Göttingen and the Bernstein Center Göttingen. The brains of larger
mammals, such as humans, however, have a completely different structure
to those of mice. Processes of self-organization led to the emergence of
modules in which neurons conjointly are responsible for specific tasks.

Humans
are considerably larger than almost all of their ancestors. Our
great-great-great-grandparents were on average about 10 cm shorter than
us. Going further back in time, the difference increases impressively.
The ancestors of humans, and modern mammals in general, that lived 80
million years ago all weighed less than 100 g and were usually only a
few centimeters in size. Ecological niches that would have allowed a
larger body were occupied by dinosaurs. Only the great extinction that
wiped out the dinosaurs 65 million years ago allowed our ancestors a
“growth spurt” of historical dimensions. Within just a few million years
mammals evolved that were more than 100 times as large as their
Mesozoic ancestors.

A well-known international team of scientists led by Max Planck researchers reports in the journal Science
that this growth spurt probably led to a fundamental reshaping of
neural circuits in the brain. Scientists from the Goethe University in
Frankfurt and their international partners hwere also involved in the
study. As the researchers discovered, neural circuits in the visual
cortex of the brain, corresponding to the smallest details, developed
independently in different lineages. Computer simulations and
mathematical calculations show that this correspondence reflects basic
laws of self-organization of large-scale neuronal networks. The
researchers point towards the existence of “living fossils of brain
development”. This refers to species which preserved our ancestors’
neuronal circuits’ architecture until today. Among them, amazingly, is
also one of the closest relatives of primates: the mouse.

An
essential aspect of human evolution was the enlargement of the brain
and especially of the cerebral cortex, whose tasks include conscious
perception, decision making, and many memory processes. This brain area
in humans—as in many other mammals—is divided into modules in which
groups of neurons are interconnected in dense networks and contribute to
 common tasks, such as the perception of a certain hue. The paper,
which has been published in Science, analyses the evolution of what is
known as orientation columns, modules of the visual cortex that build
the basis of the perception of form.

Hundreds
of these modules, which typically have a size of about one millimeter,
are located side by side within the visual cortex. The new study shows
that this spatial orientation precisely follows geometric rules.
Surprisingly, the same laws have evolved independently in separate
lineages that led to the development of big brains and even in animals
that differ greatly from each other in brain size. The new results thus
refute a competing hypothesis that assumes strong dependencies of
geometrical properties and brain size. It suggests that during a
substantial period of ??the evolutionary enlargement of the brain only
the number of modules increased. The laws of their arrangement, however,
remained unchanged.

The
authors point out that these laws cannot apply for the entire
phylogeny. Wolfgang Keil, first author of the study explains: “In our
Mesozoic ancestors, these rules of brain architecture must have reached
their limits. Their brains were so tiny that not even a single module
would have fitted in the cerebral cortex.” Thus, the researchers
consider it to be likely that our ancestors had a fundamentally
different architecture of their visual cortex.

In
fact, all living mammals that are lighter than 100 g seem to lack
orientation columns completely. In mice, for example, nerve cells that
process different tasks in the visual cortex are seemingly randomly
mixed. Whether our brain architecture originates from a mixed or an even
stranger brain organisation can only be deduced after further
investigations, the researchers argue. An important task for future
studies will be an investigation of laws that govern small brains. “In
fact, there are many dark continents in terms of the architecture of the
visual cortex in the different lineages of mammals,” says Fred Wolf,
head of the study at the Max Planck Institute for Dynamics and
Self-Organization and the Bernstein Center for Computational
Neuroscience. The scientists hope that their work will encourage
colleagues around the world to help resolve this fundamental mystery of
our origins.

Source: Max Planck Institute