Category: Brains & Intelligence
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The animal central nervous system is believed to have a single origin, i.e. to be monophyletic. Some of the more basal animal groups completely lack a nervous system (e.g. sponges) or possess a decentralised nervous system, i.e. a loose network of nerves without a brain (e.g. jellyfish). Most animals, however, belong to the Bilateria, which refers to a diverse grouping of bilaterally symmetric organisms that first appeared in adundance in the Cambrian, from around 545 Ma. Genetic information shared by all living bilaterians suggests that their common ancestor had elongated, segmented bodies, and also a nervous system showing segmental organisation, with a ganglion (a conglomerate of nerve cells) in each body segment and an enlarged one at the front end. During the evolution of the distinct bilaterian groups (or 'phyla'), these segmental nervous inputs aggregated and became highly integrated at the anterior, forming a specialised brain. Among the invertebrates, complex brains can be found in the arthropods (including insects and crustaceans) and the cephalopods (octopuses, squids and similar molluscs), the latter having the largest brains of any invertebrates. All vertebrate brains are located in the head and share a common basic structure. During vertebrate evolution, some regions of the brain have undergone dramatic structural changes (e.g. the forebrain), whereas others have preserved their identity across the whole range of vertebrates (e.g. the hypothalamus, which controls many critical biological functions).
Despite its monophyletic origin, the specialisation of the nervous system into enlarged centres for sensory processing, leading ultimately to sentience and complex intelligence, is rampantly convergent. Brains, as such, have evolved independently many times. At this level, however, the convergence is relatively superficial, but there are much more striking convergences that are important in the evolution of such features as memory and learning. Linked to this is the convergent evolution of such structures as the cerebellum in vertebrates and mushroom bodies in insects. Particularly instructive examples exist between vertebrates and cephalopods, e.g. with respect to brain structures, certain aspects of intelligence or personality.
Cognition, i.e. the mental process of gaining knowledge and comprehension, is one of the most familiar of attributes, but one of the most elusive as well. Cognition is associated closely with the concept of intelligence, which, simply put, is the ability to learn and includes such aspects as the adaptation to changing or new environments. There is considerable and controversial divergence of opinion as to the cognitive abilities and capacities of animals. Evidence suggests that complex cognitive abilities are far more widespread among animals than is sometimes thought, but also that, despite very different neural systems, there are a number of surprising similarities between different taxa displaying forms of intelligence. To find cognitive abilities in mammals other than humans is not so surprising, especially in our closest relatives amongst the great apes, but high degrees of intelligence are also found in such groups as the cetaceans (including the dolphins) and elephants. Outside the mammals, certain bird groups (especially the corvids and parrots) are known to possess advanced cognitive abilities, and among the invertebrates remarkable information on cognition has emerged for the octopus (cephalopods) and also some species of bee and wasp (insects). These convergences are not only intrinsically fascinating, but also touch on one of the greatest scientific mysteries - what is the nature of consciousness?
