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Evolution and the brain
Our Evolutionary Inheritance

Help Lien : Compare brains Lien : Compare Brain and Body Sizes Lien : Evolution of complexity?: "Behavioral complexity", brain size, and the relation between the two
Lien : Brain Size and Evolution - Where we are ... so as to see where we might go next Lien : Size and shape matter Lien : Discovering the Things that Make Us Human: Evolution of the Brain Lien : Evolution of Complexity in Paleozoic Ammonoid Sutures
Lien : Co-evolution of Neocortex size, group size and language in humans Lien: The evolution of the human brain Lien: Transition from amphibians to amniotes (first reptiles) Lien : The gradual evolution of mammals.
Lien : The Evolution of Mammals Lien : The Evolution of Mammals Lien : The Evolution of Mammals Lien : The Evolution of Mammals
Lien : Livre : Evolving Brains, John Morgan Allman, Scientific American Library Series, No. 68., January 1999 Lien : The Emergence of Neocortex in Mammals Lien : The Evolution of Reentrance in the Vertebrate Brain Lien : Comparative Mammalian Brain Collections
Lien : Comparative Neuroanatomy Lien : The Cerebral Cortex Lien : Regulation of Cerebral Cortical Size by Control of Cell Cycle Exit in Neural Precursors Lien : Primates on Facebook
Chercheur : Dr Leah A. Krubitzer receives a MacArthur Fellowship
Original modules
History Module: Hominization, or The History of the Human Lineage Hominization, or The History of the Human Lineage
  Tool Module: What Is Evolution?   What Is Evolution?
Tool Module: Primatology   Primatology

The cortex is the thin layer of neurons, about 3 mm thick, that covers the surface of each cerebral hemisphere. In the human brain, the cortex is generally divided into the following three parts that emerged successively in the course of evolution:
- the archicortex (or archaeocortex) developed in association with the olfactory system; it corresponds to the dentate gyrus and hippocampus in mammals and does not yet have the six layers of the neocortex;
- the paleocortex is also associated with the olfactory system and is not stratified into layers; it corresponds to the piriform cortex and the parahippocampal gyrus;
- the neocortex (or isocortex) is the part of the cortex that appeared most recently and constitutes by far the largest portion of the cortex in primates; it is stratified into six distinct layers of neurons.

A larger cortex obviously contains a larger number of neurons. But what increases the most in mammals is the number of connections between these neurons. As a corollary, the proportion of white matter in the brain (and hence of the axons that make the connections) grows steadily as one proceeds from rats to human beings.

Over the course of evolution, the cerebral cortex has grown considerably in surface area but very little in thickness. Thus, in humans, the cortex is only 15% thicker than in macaque monkeys, but has at least 10 times more surface area. The difference between the human cortex and the mouse cortex is even larger: the human cortex is only twice as thick, but has about 1 000 times the surface area!

The brain does not have just one single function on which evolution might operate. The brain is rather a collection of systems (or, as some would call them, modules), each controlling different functions. In the realm of emotions, for example, we know a fair amount about the various systems where feelings of fear, anger and disgust originate.

Consequently, evolution tends to act on these systems individually rather than on the brain as a whole. For example, evolution may have favoured growth in the overall size of the primate brain, but its influence is seen mainly in specific systems.

In vertebrates, cephalization generally increases as one proceeds from the oldest taxonomic groups (fish) to the most recent ones (mammals). Among invertebrates, however, the process appears to be associated more with general body shape, diversity of sensory organs, habits, and variety of behaviours than with the group's place in the phylogenetic tree.

Lien : Invertebrate nervous system

The model of the triune brain proposed by MacLean in 1970 is a useful piece of shorthand for the complex evolutionary history of the human brain. But the brain's combination of reptilian, paleomammalian and neomammalian structures is far more intricate than a mere set of nested Russian dolls.

Ever since the first mammals appeared more than 200 million years ago, the cerebral cortex has assumed greater and greater importance compared with the brain's other, older structures. Because these structures had proven their effectiveness for meeting certain fundamental needs, there was no reason for them to disappear. Instead, evolution favoured a process of building expansions and additions, rather than rebuilding everything from the bottom up.

The brains of various species of mammals
(Left: all on the same scale; Right: enlarged, on various scales)

This expansion of the surface of the neocortex (also known as the isocortex) is more apparent in predatory mammals than in herbivorous ones. Catching prey may be difficult, but a successful hunt provides a far more nutritious meal than any plant. To hunt successfully requires a highly evolved sensorimotor system. Mammals with a large neocortex thus have an advantage, because that is where the sensory and motor regions are located.

Scientists have also observed that the size of the neocortex has increased tremendously in primates, from the smallest monkeys, such as lemurs, on to the great apes and human beings. Many scientists believe that this growth in the primates’ neocortex reflects the growing complexity of their social lives. Indeed, the ability to predict the behaviour of other individuals within a group seems to have conferred a large evolutionary advantage. Thus evolution would have favoured the growth of certain parts of the cortex that are responsible for social skills such as language, because they improved this ability.

Another point on which everyone agrees is that the increase in the folds of the cortex has been a major factor in the evolution of the brain. These folds, by enabling a larger cortical surface area to fit inside the cranial vault, allow for a better organization of complex behaviours.

It is still common to hear other animals discussed as if they were some inferior form of human beings — as if there were some kind of natural ladder on which human beings obviously occupied the top rung.

But that is not what we actually see in nature. Every evolutionary line has developed independently, and rats, for example, are perfectly adapted to their environment. They are not in the process of extinction, and they live in perfect harmony with their surroundings. The same could be said for most of the species that populate the Earth, even though their brains are far smaller than ours.

Growth in size and importance of the associative areas of the brain, from rats to cats to humans. Green = sensorimotor area, red = visual area, blue = auditory area.

Consequently, success, from an evolutionary standpoint, does not depend on brain size. We also know that larger brains have not replaced smaller ones, but have simply added to them, thus expanding the existing distribution of brain sizes.

Countless perfectly adapted small brains can still be found in animals today. The larger brains that developed later on are simply the result of evolution's intrinsic tendency to try out all the possibilities. Thus the growing complexity observed everywhere in nature is not the “objective” of evolution, but rather a side effect of its endless experiments over thousands and thousands of years.

Is there a relationship between the size of an animal's brain and some kind of “behavioural complexity”? Not really. Elephants and whales have brains 4 to 5 times the size of a human being's, yet their behaviour is generally agreed to be less complex than ours.

To identify a meaningful relationship between brain size and behavioural complexity, we must consider an animal's brain size in proportion to its body size, because, to cite just one example, an animal with a larger body surface will automatically need larger sensory areas in its brain. Once we weight brain size for body size, we do in fact observe a relationship between the size of the brain and the complexity of behaviour.

But the differences in the evolutionary development of the various parts of the brain have an even greater effect on behaviour than brain size does. For instance, the cerebellum is a part of the brain involved in co-ordinating muscle movements, and its weight as a percentage of the brain's total weight remains remarkably constant in all mammals. In contrast, the percentage for the neocortex varies greatly from one species to another. In fish and amphibian brains, there is no neocortex. In shrews, the neocortex accounts for 20% of the brain's weight, and in humans, it accounts for 80%!

It was during the transition from primates to human beings that the neocortex underwent the greatest development. And among all the regions of the neocortex, it was definitely the prefrontal cortex that expanded the most in human beings.

Evolutionary biologists make a distinction between homologous and analogous anatomical structures.

Two structures are said to be homologous if they have a common embryonic and phylogenetic origin. For example, a human being's forearm, a bird's wing, and a dolphin's pectoral fin are all composed of the same bones, inherited from a common distant ancestor, and are therefore considered homologous structures. The very existence of homologous structures in the animal kingdom is regarded as one of the strongest proofs of evolution.

In contrast, analogous structures are structures that fulfil the same functions in different species but that are derived from two completely different evolutionary lines. An insect's wings and a bird's wings offer a good example. Both are used to fly, but taxonomically, they have nothing in common with each other. Analogous structures are the product of “"convergent evolution"; they represent differing solutions that various organisms developed independently to cope with the same constraints in their environments.

Lien : Vous avez dit « Evolution convergente » ? Lien : INTRODUCTION TO PHYLOGENY AND CLASSIFICATION

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