Cephalization is an trend in that, over a sufficient number of generations, concentrates the special sense organs and nerve ganglia towards the front of the body where the mouth is located, often producing an enlarged head. This is associated with the animal's movement direction and bilateral symmetry. Cephalization of the nervous system has led to the formation of a brain with varying degrees of functional centralization in three phylum of bilaterian animals, namely the , cephalopod , and . organise aspects of cephalization in the bilaterians.
Bilateria
Cephalization is both a characteristic feature of any animal that habitually moves in one direction, thereby gaining a front end, and an
trend which created the head of these animals. In practice, this means the
, a large group containing the majority of animal phyla.
These have the ability to move, using muscles, and a
body plan with a front end that encounters stimuli first as the animal moves forwards, and accordingly has evolved to contain many of the body's sense organs, able to detect light, chemicals, and gravity. There is often a collection of nerve cells able to process the information from these sense organs, forming a
brain in several
phylum and one or more
Ganglion (clusters of nerve cells) in others.
Complex active bodies
The philosopher Michael Trestman noted that three bilaterian phyla, namely the arthropods, the molluscs in the shape of the cephalopods, and the chordates, were distinctive in having "complex active bodies", something that the acoels and flatworms did not have. Any such animal, whether predator or prey, has to be aware of its environment—to catch its prey, or to evade its predators. These groups are exactly those that are most highly cephalized.
[ Available on Trestman's website] These groups, however, are not closely related: in fact, they represent widely separated branches of the Bilateria, as shown on the phylogenetic tree; their lineages split hundreds of millions of years ago. Other (less cephalized) phyla are omitted for clarity.
Arthropods
In
, cephalization progressed with the gradual incorporation of trunk segments into the head region. This was advantageous because it allowed for the
evolution of more effective mouth-parts for capturing and processing food.
are strongly cephalized, their brain made of three fused
ganglia attached to the ventral nerve cord, which in turn has a pair of ganglia in each segment of the thorax and abdomen, the parts of the trunk behind the head. The
insect head is an elaborate structure made of several segments fused rigidly together, and equipped with both simple and
, and multiple
including sensory antennae and complex mouthparts (maxillae and mandibles).
Cephalopods
including the
octopus,
squid,
cuttlefish and
nautilus are the most intelligent of
.
They are highly cephalized,
with well-developed senses, including
cephalopod eye and large brains.
Vertebrates
Cephalization in
, the group that includes
mammals,
birds,
reptiles,
amphibians and
fishes, has been studied extensively.
The heads of vertebrates are complex structures, with distinct sense organs for sight, olfaction, and hearing,
and a large, multi-lobed brain protected by a skull of bone or
cartilage.
like the
lancelet (
Amphioxus), a small fishlike animal with very little cephalization, are closely related to vertebrates but do not have these structures.
In the 1980s, the new head hypothesis proposed that the vertebrate head is an evolutionary novelty resulting from the emergence of
neural crest and cranial
(thickened areas of the embryonic
ectoderm layer), which result in the formation of all sense organs outside the brain.
However, in 2014, a transient
larva tissue of the lancelet was found to be virtually indistinguishable from the
neural crest-derived cartilage (which becomes
bone in jawed animals) which forms the vertebrate
skull, suggesting that persistence of this tissue and expansion into the entire head space could be a viable evolutionary route to forming the vertebrate head.
[ For lay summary see: ] Advanced vertebrates have increasingly elaborate brains.
Anterior Hox genes
Bilaterians have many more
controlling the development, including of the front of the body than do the less cephalized Cnidaria (two Hox clusters) and the Acoelomorpha (three Hox clusters). In the vertebrates, duplication resulted in the four Hox clusters (
HoxA to
HoxD) of mammals and birds, while another duplication gave
teleost fishes eight Hox clusters. Some of these genes, those responsible for the front (anterior) of the body, helped to create the heads of both arthropods and vertebrates. However, the
Hox1-5 genes were already present in ancestral arthropods and vertebrates that did not have complex head structures. The Hox genes therefore most likely assisted in cephalization of these two bilaterian groups independently by convergent evolution, resulting in similar
.
Partly cephalized phyla
The
Acoela are basal bilaterians, part of the
Xenacoelomorpha. They are small and simple animals with flat bodies. They have slightly more nerve cells at the head end than elsewhere, not forming a distinct and compact brain. This represents an early stage in cephalization.
Also among the bilaterians, Platyhelminthes (flatworms) have a more complex nervous system than the Acoela, and are lightly cephalized, for instance having an eyespot above the brain, near the front end.
Among animals without bilateral symmetry, the Cnidaria, such as the radially symmetrical (roughly cylindrical) Hydrozoa, show some degree of cephalization. The Anthomedusae have a head end with their mouth, photoreceptor cells, and a concentration of nerve cells.
See also
