The evolution of nervous systems dates back to the first development
of nervous systems in animals (or metazoans). Neurons developed as
specialized electrical signaling cells in multicellular animals,
adapting the mechanism of action potentials present in motile
single-celled and colonial eukaryotes. Simple nerve nets seen in
animals like cnidaria evolved first, followed by nerve cords in
bilateral animals – ventral nerve cords in invertebrates and dorsal
nerve cords supported by a notochord in chordates. Bilateralization
led to the evolution of brains, a process called cephalization.
1 Neural precursors
3 Nerve nets
4 Nerve cords
4.3 In amphibians
Evolution of innate behaviors
Evolution of central nervous systems
Evolution of the human brain
6 See also
Action potential § Taxonomic distribution and
Action potentials, which are necessary for neural activity, evolved in
single-celled eukaryotes. These use calcium rather than sodium action
potentials, but the mechanism was probably adapted into neural
electrical signaling in multicellular animals. In some colonial
eukaryotes such as
Obelia electrical signals do propagate not only
through neural nets, but also through epithelial cells in the shared
digestive system of the colony.
Sponges have no cells connected to each other by synaptic junctions,
that is, no neurons, and therefore no nervous system. They do,
however, have homologs of many genes that play key roles in synaptic
function. Recent studies have shown that sponge cells express a group
of proteins that cluster together to form a structure resembling a
postsynaptic density (the signal-receiving part of a synapse).
However, the function of this structure is currently unclear. Although
sponge cells do not show synaptic transmission, they do communicate
with each other via calcium waves and other impulses, which mediate
some simple actions such as whole-body contraction.
Main article: Nerve net
Jellyfish, comb jellies, and related animals have diffuse nerve nets
rather than a central nervous system. In most jellyfish the nerve net
is spread more or less evenly across the body; in comb jellies it is
concentrated near the mouth. The nerve nets consist of sensory neurons
that pick up chemical, tactile, and visual signals, motor neurons that
can activate contractions of the body wall, and intermediate neurons
that detect patterns of activity in the sensory neurons and send
signals to groups of motor neurons as a result. In some cases groups
of intermediate neurons are clustered into discrete ganglia.
The development of the nervous system in radiata is relatively
unstructured. Unlike bilaterians, radiata only have two primordial
cell layers, endoderm and ectoderm. Neurons are generated from a
special set of ectodermal precursor cells, which also serve as
precursors for every other ectodermal cell type.
Nervous system of a bilaterian animal, in the form of a nerve cord
with a "brain" at the front
The vast majority of existing animals are bilaterians, meaning animals
with left and right sides that are approximate mirror images of each
other. All bilateria are thought to have descended from a common
wormlike ancestor that appeared in the
Ediacaran period, 550–600
million years ago. The fundamental bilaterian body form is a tube
with a hollow gut cavity running from mouth to anus, and a nerve cord
with an especially large ganglion at the front, called the "brain".
Area of the human body surface innervated by each spinal nerve
Even mammals, including humans, show the segmented bilaterian body
plan at the level of the nervous system. The spinal cord contains a
series of segmental ganglia, each giving rise to motor and sensory
nerves that innervate a portion of the body surface and underlying
musculature. On the limbs, the layout of the innervation pattern is
complex, but on the trunk it gives rise to a series of narrow bands.
The top three segments belong to the brain, giving rise to the
forebrain, midbrain, and hindbrain.
Bilaterians can be divided, based on events that occur very early in
embryonic development, into two groups (superphyla) called protostomes
and deuterostomes. Deuterostomes include vertebrates as well as
echinoderms and hemichordates (mainly acorn worms). Protostomes, the
more diverse group, include arthropods, molluscs, and numerous types
of worms. There is a basic difference between the two groups in the
placement of the nervous system within the body: protostomes possess a
nerve cord on the ventral (usually bottom) side of the body, whereas
in deuterostomes the nerve cord is on the dorsal (usually top) side.
In fact, numerous aspects of the body are inverted between the two
groups, including the expression patterns of several genes that show
dorsal-to-ventral gradients. Some anatomists now consider that the
bodies of protostomes and deuterostomes are "flipped over" with
respect to each other, a hypothesis that was first proposed by
Geoffroy Saint-Hilaire for insects in comparison to vertebrates. Thus
insects, for example, have nerve cords that run along the ventral
midline of the body, while all vertebrates have spinal cords that run
along the dorsal midline. But recent molecular data from different
protostomes and deuterostomes reject this scenario.
Earthworm nervous system. Top: side view of the front of the worm.
Bottom: nervous system in isolation, viewed from above
Worms are the simplest bilaterian animals, and reveal the basic
structure of the bilaterian nervous system in the most straightforward
way.  As an example, earthworms have dual nerve cords running
along the length of the body and merging at the tail and the mouth.
These nerve cords are connected by transverse nerves like the rungs of
a ladder. These transverse nerves help coordinate the two sides of the
animal. Two ganglia at the head end function similar to a simple
brain. Photoreceptors on the animal's eyespots provide sensory
information on light and dark.
The nervous system of one very small worm, the roundworm
Caenorhabditis elegans, has been mapped out down to the synaptic
level. Every neuron and its cellular lineage has been recorded and
most, if not all, of the neural connections are known. In this
species, the nervous system is sexually dimorphic; the nervous systems
of the two sexes, males and hermaphrodites, have different numbers of
neurons and groups of neurons that perform sex-specific functions. In
C. elegans, males have exactly 383 neurons, while hermaphrodites have
exactly 302 neurons.
Internal anatomy of a spider, showing the nervous system in blue
Arthropods, such as insects and crustaceans, have a nervous system
made up of a series of ganglia, connected by a ventral nerve cord made
up of two parallel connectives running along the length of the
belly. Typically, each body segment has one ganglion on each side,
though some ganglia are fused to form the brain and other large
ganglia. The head segment contains the brain, also known as the
supraesophageal ganglion. In the insect nervous system, the brain is
anatomically divided into the protocerebrum, deutocerebrum, and
tritocerebrum. Immediately behind the brain is the subesophageal
ganglion, which is composed of three pairs of fused ganglia. It
controls the mouthparts, the salivary glands and certain muscles. Many
arthropods have well-developed sensory organs, including compound eyes
for vision and antennae for olfaction and pheromone sensation. The
sensory information from these organs is processed by the brain.
In insects, many neurons have cell bodies that are positioned at the
edge of the brain and are electrically passive—the cell bodies serve
only to provide metabolic support and do not participate in
signalling. A protoplasmic fiber runs from the cell body and branches
profusely, with some parts transmitting signals and other parts
receiving signals. Thus, most parts of the insect brain have passive
cell bodies arranged around the periphery, while the neural signal
processing takes place in a tangle of protoplasmic fibers called
neuropil, in the interior.
Iodine and T4 stimulate the spectacular apoptosis (programmed cell
death) of the cells of the larval gills, tail and fins, and also
stimulate the evolution of the nervous system transforming the
aquatic, vegetarian tadpole into the terrestrial, carnivorous frog
with better neurological, visuospatial, olfactory and cognitive
abilities for hunting. Contrary to amphibian metamorphosis,
thyroidectomy and hypothyroidism in mammals may be considered a sort
of phylogenetic, metabolic and neurologic regression to a former stage
of reptilian life. Indeed, many disorders that seem to afflict
hypothyroid humans have reptilian-like features, such as a general
slowdown of nervous reflexes with lethargic cerebration, metabolism,
digestion, heart rate, hypothermia and a dry, hairless, scaly, cold
Evolution of innate behaviors
Caridoid escape reaction
Caridoid escape reaction §
Evolution of the tail
flip escape mechanism
Behaviors such as the "tail-flip" escape reaction in crustacea such as
crayfish and lobsters are fixed action patterns that may have evolved
from earlier ancestral patterns.
Evolution of central nervous systems
Evolution of the brain, Cephalization, and
Evolution of the eye
Evolution of the human brain
Evolution of human intelligence and Human brain
§ Comparative anatomy
There has been a gradual increase in brain volume as the ancestors of
modern humans progressed along the human timeline of evolution (see
Homininae), starting from about 600 cm3 in
Homo habilis up to
1736 cm3 in Homo neanderthalensis. Thus, in general there is a
correlation between brain volume and intelligence. However, modern
Homo sapiens have a smaller brain volume (brain size 1250 cm3)
than neanderthals; women have a brain volume slightly smaller than
men, and the Flores hominids (Homo floresiensis), nicknamed "hobbits",
had a cranial capacity of about 380 cm3, about a third of the
Homo erectus average and considered small for a chimpanzee. It is
proposed that they evolved from H. erectus as a case of insular
dwarfism. In spite of their threefold smaller brain there is evidence
that H. floresiensis used fire and made stone tools as sophisticated
as those of their proposed ancestor, H. erectus. Iain Davidson
summarizes the opposite evolutionary constraints on human brain size
as "As large as you need and as small as you can".
Brain evolution can be studied using endocasts, a branch of neurology
and paleontology called paleoneurology.
Evolutionary developmental biology
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Evolutionary history of life
Index of evolutionary biology articles
Outline of evolution
Timeline of evolution
Earliest known life forms
Evidence of common descent
Last universal common ancestor
Origin of life
Evolutionary developmental biology
dolphins and whales
Programmed cell death
Life cycles/nuclear phases
Tempo and modes
Renaissance and Enlightenment
Transmutation of species
On the Origin of Species
History of paleontology
The eclipse of Darwinism
History of molecular evolution
Extended evolutionary synthesis
Teleology in biology
Central nervous system
Peripheral nervous system