The cerebrum composes three quarters of the human brain.  It is the site of consciousness, personality, artistic ability, insight, conscience, and all of the higher order functions which separate humans from other animals in a way that no other anatomical structure does.  Here we store the memories of our grandmother’s face, our best friend’s voice, and our fourth grade field trip.  Although the capabilities of the human cerebrum make humans unique, our cerebrums are not qualitatively different from those of organisms.  There were some several debates in the late 1800s over which regions of the brain were supposedly unique humans.  The most famous of these was the controversy between Huxley and Owen in which Owen argued that only human brains possessed a hippocampus minor.  This controversy ended with the public dissection of an ape brain to demonstrate that apes also possess this structure.  It is now known that although the regions of the human cerebrum are often modified compared to homologous regions in apes, there is no region of the cerebrum which is unique to humans. 

     Humans do not have a well developed sense of smell.  Not only is the sense of smell not as developed in humans as it is in most other vertebrates, our lineage has actually been regressing from the ancestral state.  In the primitive chordates (such as lancelets), the telencephalon became the site for the processing of olfactory stimuli (Ariens).   In jawless fish, the olfactory bulbs comprise the majority of the cerebrum.   Bipolar neurons connect the olfactory bulbs to the olfactory epithelia in jawless fish and the olfactory pathway is structured similarly to that of higher vertebrates (Kardong).  In gnathostomes, the olfactory lobe of the cerebrum includes a bulb which is attached to the rest of the brain by a stalk (Weichert, 1970, p.627).  The pallial region of the cerebrum is primarily responsible for dealing with smell in both actinopterygian and sarcopterygian fish. (Weichert, 1970, p.627).  In primitive mammals, including some insectivores, the olfactory bulb is visible from a dorsal view of the brain (Shoshani, 2006).

     In higher primates in general, and in humans in particular, the olfactory bulb has become greatly reduced in size.  Humans also lack the accessory olfactory bulb and (apparently) a functional vomeronasal system.  Although the cerebrum was primarily an olfactory processing center at its outset, our lineage has greatly enhanced the non-olfactory processing and reduced the importance of olfaction.

The anatomy, neurochemistry, development, and gene expression in the olfactory bulb is conserved in tetrapods ( Moreno, 2008).

     Even though the cerebrum of jawless fish is primarily a site for the processing of olfaction, it includes the two major regions of the cerebrum present in higher vertebrates: the pallium which forms the cerebral cortex in mammals and the subpallium which forms the basal nuclei in mammals.  The pallium of jawless fish includes both the medial/hippocampal region and the dorsal and lateral/cortex region.  The subpallium of jawless fish includes both the striatum and septum (Kardong, p. 646).  Regions of the hagfish telencephalon correspond to the hippocampus, corpus striatum, part of the pallidum, preoptic nucleus, and cerebral hemispheres of vertebrates.  In hagfish, olfactory pathways project to the hippocampus, as in vertebrates   (Hardisty, p. 316; Ariens).   Lampreys and all higher vertebrates share a number of features such as common striatal regions, a medial olfactory nucleus, a lobus subhippocampus (Ariens, p.1247), and medial, dorsal, and lateral subdivisions of the pallium (Butler, 1996, p. 263).





     Vertebrates appear to share the expression of specific genes in the development of regions of the telencephalon.  All vertebrates share the expression of Distal-less in the subpallium ( amygdala and basal ganglia of tetrapods),  Pax6 and Tbr genes in the pallium, Emx in the ventral pallium (LIM homeodomain genes are shared in the ventral pallium of tetrapods) (Medina, 2005).

    In gnathostomes, there is an increase in the non-olfactory regions of the cerebrum (Romer p. 588) and the cerebral cortex is divided by a sulcus into two hemispheres. (Weichert, 1970, p.627).  Olfactory  fibers no longer project to all pallial areas (Butler, 1994b).  Gnathostomes possess additional cerebral characteristics including medial and lateral septal nuclei (Butler, 1996, p. 440), an increase in the interconnections between regions of the telencephalon, anterior and posterior olfacto-habenularis tracts, connections between the dorsal thalamus and striatum (Ariens, p.1262-6), distinct nuclei in striatum (Butler, 1996, p. 349),  an anterior commissure which connects the two cerebral hemispheres (Romer, p. 594), the projection of visual information to the dorsal and medial pallium (Butler, 1996, p. 370), and common connections of the basal ganglia (Medina, 1995).   In cartilaginous fish, the area periventricularis ventrolateralis and nucleus superficialis basalis may be equivalent to the dorsal and ventral striatal regions in mammals.  (Butler, 1996, p. 272).

     In bony fish, the corpus striatum is enlarged so that it bulges into the lateral ventricles (Weichert, 1970).   The cerebrum of bony fish and tetrapods is involved in reproductive behavior, aggression, sex drive, parental behavior, (Prosser, 1973, p. 691), color discrimination, and learning (Hoar,VOl. IV, 1970, p. 19-20).  The lateral subpallium of lungfish may be homologous to all the regions of the mammalian striatum (Butler, 1996, p. 351). The basal ganglia of lungfish have 2 major groups of projection neurons which release substance P and enkalphins (Medina, 1995).  The types of learning which occur in the lateral and medial pallia in bony fish are similar to those of the equivalent hippocampal pallium and amygdalar pallium in tetrapods (Broglio, 2005). 






Homologous regions between the pallial regions of lungfish and amphibians are depicted below.


     In amphibians, neuron migrations form an archpallium dorsal and medial to the ancestral palaeopallium which is still dedicated to processing olfaction.  The size of the cerebral hemispheres is larger than that of fish, although the cerebrum still seems to function primarily in olfaction (Weichert, 1970, p.627)  The amount of the cerebrum which performs non-olfactory tasks is increased over that observed in fish (Romer, Ariens).   A number of characteristics are typical of tetrapod cerebrums including the accessory olfactory bulb, amygdaloid complex, caudate nucleus, nucleus accumbens, vomeronasal organ (VNO), a vomeronasal nerve, set of connections involving the stria medullaris, amygdalo-habenularis tract, median forebrain bundle, fornix, bed nucleus of anterior commissure, striohypothalamic tract, primordium pallii dorsalis (Ariens, p.1293-1311), dorsal and ventral striatopallidal systems (Greenberg, 2002), and the entopeduncular nucleus (Butler, 1996, p. 272).  While non-mammalian vertebrates evolved the ability for more complex processing, higher order functions were not necessarily centered in the cerebrum the way they are in mammals.  In fish and amphibians, the tectum of the midbrain is the primary association center of the brain.  In reptiles and birds, both the cerebrum and tectum are centers for higher processing.  In mammals the tectum was reduced as the cerebrum became the main association center (Romer p. 585).

Comparisons of the expression patterns of regulatory genes expressed in the forebrain indicate that differences exist between tetrapods and jawless fish but that comparable subdivisions exist among tetrapods. In tetrapods, the subpallium is defined by the expression of genes of the Dlx family, Sonic Hedgehog and Gsh 1/2 while the pallium is defined by the expression of Pax6, Emx 1/2, Neurogenin2, T-brain family members and LIM homeodomain proteins. Bony fish also express Dlx genes in the subpallium and Pax6 and Emx in the pallium. Bony fish lack Sonic Hedgehog expression in the subpallium which defines the anterior entopeduncular region and jawless fish may lack a pallium since Nkx expression in not known ( Medina, 2005).






         Although at first glance the mammalian cerebrum appears to have a completely different organization than that of other amniotes (birds and reptiles), in reality early reptiles evolved many of the regions of the amniote cerebrum which were subsequently modified in mammals.  In amniotes, the cerebral cortex is much larger than that of amphibians and at least partially covers the diencephalon.  In reptiles, the cerebral surface is still smooth.  Some reptiles form the first neopallium at the anterior dorsal end of the cerebrum which performs higher association functions (Weichert, 1970, p.627).   Amniote cerebrums share a number of derived characteristics.  These include a dorsal pallium composed of two divisions (medial lemnopallial division and a lateral collopallial division; Butler, 1996, p. 377),  the division of amygdale in to several regions (Butler, 1996, p. 455), an increase in the size and complexity of the limbic system (Butler, 1996, p. 456), an increase in size of the telencephalon, greater separation of the olfactory bulbs, the lentiform nucleus,  putamen, claustrum, nucleus epibasalis, nucleus centralis, nucleus basalis accumbens, bed nucleus of the stria terminalis, globus pallidus, piriform lobe cortex (Ariens, p.1310-36), medial and lateral septal nuclei (Ariens, p.1408), homologs of the mammalian ventral nuclear group and posterior nuclear group, lemnothalamic and collothalamic visual pathways (Butler, 1996, p. 391), and a more medial position of the corpus striatum (Weichert, 1970).  Amphibians possess regions of the amygdala which seem to represent the ancestral regions to the olfactory and vomeronasal regions in the amygdala regions of amniotes ( Moreno, 2005).

     Amniotes evolved a medial division of dorsal pallium and a lateral division of dorsal pallium, ventricular and subventricular divisions of the lateral division of dorsal pallium, and an end to the overlap between lemnothalamic and collothalamic projections to the dorsal pallium.   Lemnothalamic fibers project through the lateral forebrain bundle to the dorsal pallium and striatum, at least some lemnothalamic tracts are bilateral, and the lateral part of the medial division of the dorsal pallium alone receives visual projections from the lemnothalamus.   The medial part of the medial division of the dorsal pallium alone receives somatosensory and limbic input from lemnothalamus, the lemnothalamus projects to both divisions of the dorsal pallium.  Amniotes also share dorsal pallium projections to dorsal thalamus, dopaminergic fibers to dorsal pallium, GABA-containing neurons in both divisions of dorsal pallium, and projections of dorsal pallium to striatum (Butler, 1994b).  Amniotes share the regions of pallium which form the extrastriate, auditory, secondary somatosensory, and other corticies in mammals (Butler, 1994b).  Amniotes share a number of pathways such as the homologs of mammalian auditory pathway from the inferior colliculus to the medial geniculate body to the auditory cortex and the pathway from the retina to the dorsal lateral geniculate nucleus to the striate cortex (Butler, 1994b).

     All amniotes seem to have similar cerebral circuitry.  The organization of the mammalian cerebral cortex is similar to that of the dorsal ventricular ridge in reptiles and birds (Karten, 1997).  The anterior dorsal ventricular ridge is a singular structure which seems contain the cells that, after migrations which occur during the development of the mammalian brain, are homologous to the basolateral amygdala complex, the lateral neocortex, and the claustrum-endopiriform nucleus.  In monotremes the first two structures, but not the claustrum-endopiriform nucleus, are present.  In mammals, the formation of these regions requires the Pax-6 gene and these regions in Pax-6 mutant mice are very similar to those observed in reptiles and birds (Butler, 2002).Cannabinoid receptors are GPCRs which are found in presynaptic regions of neurons. While the expression patterns of CB1 receptors in the basal forebrain were established early in the evolution of placental mammals, novel expression patterns in the neocortex occurred in early primates (Harkany, 2005).


Evidence supports the conclusion that large brains in birds increases the likelihood of adaptation to a novel type of environment (Sol, 2005). Despite the fact that the 27g echidna brain is three times the size of the 9 g platypus brain and the two are adapted for very distinct lifestyles, the regional subdivision of their brains are comparable and neither possesses a subdivision which is absent in the other. Monotreme brain organization possesses subdivisions which are lacking in therian brains and lack certain subdivisions found in therian brains. The electroreception of platypuses has not resulted in novel brain regions nor has the loss of this ancestral system resulted in a brain reorganization in echidnas (Manger, 2005). A number of animal lineages, such as crows, parrots, owls, anthropoid primates, and whales have evolved higher intelligence (Roth, 2005). Among whales, brain size can vary from 2.6 kg to 9 kg (Roth, 2005).Gibbon brains vary from 88 to 105 g and their encephalization quotient varies from 1.9 to 2.7. Capuchin monkey brains vary from 26 to 80 grams and their encephalization quotients vary from 2.4 to 4.8. Chimp brains range from 330 to 430 g with an EQ of 2.2 to 2.5. Normal human brains vary from 1250-1450 g with an EQ of 7.4-7.8, although microcephalic humans may possess brain sizes within chimp range (Roth, 2005).Humans posses about 11.5 billion cortical neurons, African elephants possess 11 billion, false killer whales possess 10.5 billion, bottlenose dolphins possess 5.8, chimps possess 6.2, gorillas 4.3, and dogs possess 0.6 billion cortical neurons (Roth, 2005). The brain has evolved as a mosaic in which the expansion of one region has led to the expansion of those specific regions connected to it. Correlated expansion of connected areas of the neocortex, cerebellar cortex, and vestibular complex is evident in recent human evolution (Whiting, 2003).

Increased brain size does not necessarily result in an increased complexity in the subdivision of the brain. For example, mice possess a brain weighing 3 g which includes one more region of the locus coeruleus complex than that of the dolphin whose brain weighs 1500 g. The subdivision of brain regions is often conserved within a mammalian order. Cats and dogs possess the same nuclear subdivisions of the midbrain despite a 3 fold difference in brain size. The human brain possesses the same subdivisions of the midbrain as macaques despite a 15 fold increase in brain size. The nuclear subdivision of the thalamus seems to be conserved across primates despite brain sizes which vary from 2 to 1300 grams (Manger, 2005). Cats and ferrets possess the same subdivisions of the visual cortex despite a six fold difference in brain size (Manger, 2005).A number of mammals have evolved reduced brain sizes. An extinct island species of bovid genus Myotragus reduced its brain size by half compared to other living and fossil bovids. Reduction of brain size has occurred in multiple species in at least 8 families of bats (Niven, 2005).

In general, mammals of the same order typically possess equivalent complexity in the subdivision of brain regions (Manger, 2005).

The brain of human infants is only a quarter of its adult size which is much lower than the fraction of adult brain size in infant chimps (40%) and macaques (70%) (McCollum, 2006).