Editing Brain (section)

=== Vertebrates ===
[[Image:Shark brain.png|thumb|upright|alt=A T-shaped object is made up of the cord at the bottom which feeds into a lower central mass. This is topped by a larger central mass with an arm extending from either side. |The brain of a [[shark]]]]
The first [[vertebrate]]s appeared over 500&nbsp;million years ago ([[mya (unit)|Mya]]) during the [[Cambrian period]], and may have resembled the modern [[jawless fish]] ([[hagfish]] and [[lamprey]]) in form.<ref>{{cite journal |year=2003 |title=Head and backbone of the Early Cambrian vertebrate ''Haikouichthys'' |journal=Nature |volume=421 |pages=526–529 |doi=10.1038/nature01264 |pmid=12556891 |issue=6922 |first1=D.-G. |last1=Shu |first2=S. |last2=Conway Morris |first3=J. |last3=Han |first4=Z.-F. |last4=Zhang |first5=K. |last5=Yasui |first6=P. |last6=Janvier |first7=L. |last7=Chen |first8=X.-L. |last8=Zhang |first9=J.-N. |last9=Liu | first10=Y. | last10=Li |first11=H.-Q. |last11=Liu |display-authors=9 |bibcode=2003Natur.421..526S|s2cid=4401274 }}</ref> [[Jawed vertebrate]]s appeared by 445&nbsp;Mya, [[tetrapod]]s by 350&nbsp;Mya, [[amniote]]s by 310&nbsp;Mya and [[mammaliaform]]s by 200&nbsp;Mya (approximately). Each vertebrate [[clade]] has an equally long [[evolution]]ary history, but the brains of modern [[fish]], [[amphibian]]s, [[reptile]]s, [[bird]]s and [[mammal]]s show a gradient of size and complexity that roughly follows the evolutionary sequence. All of these brains contain the same set of basic anatomical structures, but many are rudimentary in the hagfish, whereas in mammals the foremost part ([[forebrain]], especially the [[telencephalon]]) is greatly developed and expanded.<ref>{{cite book |last=Striedter |first=GF |year=2005 |title=Principles of Brain Evolution |publisher=Sinauer Associates |isbn=978-0-87893-820-9 |chapter=Ch. 3: Conservation in vertebrate brains}}</ref>

Brains are most commonly compared in terms of their [[mass]]. The [[encephalization quotient|relationship]] between [[brain size]], body size and other variables has been studied across a wide range of vertebrate species. As a [[rule of thumb]], brain size increases with body size, but not in a simple linear proportion. In general, smaller animals tend to have proportionally larger brains, measured as a fraction of body size. For mammals, the relationship between brain volume and body mass essentially follows a [[power law]] with an [[exponentiation|exponent]] of about 0.75.<ref>{{cite journal |last=Armstrong |first=E |title=Relative brain size and metabolism in mammals |journal=Science |year=1983 |volume=220 |pages=1302–1304 |doi=10.1126/science.6407108 |pmid=6407108 |issue=4603 |bibcode=1983Sci...220.1302A}}</ref> This formula describes the central tendency, but every family of mammals departs from it to some degree, in a way that reflects in part the complexity of their behavior. For example, [[primate]]s have brains 5 to 10 times larger than the formula predicts. [[Predator]]s, who have to implement various [[hunting strategies]] against the [[evolutionary arms race|ever changing]] [[anti-predator adaptation]]s, tend to have larger brains relative to body size than their prey.<ref>{{cite book|last=Jerison|first=Harry J.|url=https://books.google.com/books?id=XFUXAQAAIAAJ|title=Evolution of the Brain and Intelligence|publisher=Academic Press|year=1973|isbn=978-0-12-385250-2 |pages=55–74}}</ref>

[[Image:EmbryonicBrain.svg|thumb|250px|left|alt=The nervous system is shown as a rod with protrusions along its length. The spinal cord at the bottom connects to the hindbrain which widens out before narrowing again. This is connected to the midbrain, which again bulges, and which finally connects to the forebrain which has two large protrusions.|The main subdivisions of the [[embryogenesis|embryonic]] vertebrate brain (left), which later differentiate into structures of the adult brain (right)]]
All vertebrate brains share a common underlying form, which appears most clearly during early stages of [[embryonic development]]. In its earliest form, the brain appears as three [[brain vesicle|vesicular swelling]]s at the front end of the [[neural tube]]; these swellings eventually become the [[forebrain]] ([[prosencephalon]]), [[midbrain]] ([[mesencephalon]]) and hindbrain ([[rhombencephalon]]), respectively. At the earliest stages of brain development, the three areas are roughly equal in size. In many [[aquatic animal|aquatic]]/[[semiaquatic]] vertebrates such as fish and amphibians, the three parts remain similar in size in [[adult]]s, but in [[terrestrial animal|terrestrial]] [[tetrapod]]s such as mammals, the forebrain becomes much larger than the other parts, the hindbrain develops a bulky [[dorsal (anatomy)|dorsal]] extension known as the [[cerebellum]], and the midbrain becomes very small as a result.<ref name="Kandel 2000"/><!--p. 1019-->

The brains of vertebrates are made of very soft tissue.<ref name="Kandel 2000"/><!--Ch.17--> Living brain tissue is pinkish on the outside and mostly white on the inside, with subtle variations in color. Vertebrate brains are surrounded by a system of [[connective tissue]] [[biological membrane|membranes]] called [[meninges]], which separate the [[skull]] from the brain. [[Cerebral arteries]] pierce the outer two layers of the meninges, the [[dura mater|dura]] and [[arachnoid mater]], into the [[subarachnoid space]] and [[perfusion|perfuse]] the [[brain parenchyma]] via [[arteriole]]s perforating into the innermost layer of the meninges, the [[pia mater]]. The [[endothelial cell]]s in the cerebral blood vessel walls are joined tightly to one another, forming the [[blood–brain barrier]], which blocks the passage of many [[toxin]]s and [[pathogen]]s<ref name="CarpenterCh1">{{cite book|last1=Parent|first1=A|title=Carpenter's Human Neuroanatomy|last2=Carpenter|first2=MB|publisher=Williams & Wilkins|year=1996|isbn=978-0-683-06752-1 |chapter=Ch. 1}}</ref> (though at the same time blocking [[antibody|antibodies]] and some drugs, thereby presenting special challenges in treatment of diseases of the brain).<ref>{{cite journal |last=Pardridge |first=W |year=2005 |title=The Blood-Brain Barrier: Bottleneck in Brain Drug Development |journal=NeuroRx |volume=2 |pages=3–14 |pmid=15717053 |doi=10.1602/neurorx.2.1.3 |pmc=539316 |issue=1}}</ref> As a result of the [[osmotic]] restriction by the blood-brain barrier, the [[metabolite]]s within the brain are cleared mostly by [[bulk flow]] of the [[cerebrospinal fluid]] within the [[glymphatic system]] instead of via [[venule]]s like other parts of the body.

[[Neuroanatomy|Neuroanatomists]] usually divide the vertebrate brain into six main subregions: the [[telencephalon]] (the [[cerebral hemispheres]]), [[diencephalon]] ([[thalamus]] and [[hypothalamus]]), [[mesencephalon]] (midbrain), [[cerebellum]], [[pons]] and [[medulla oblongata]], with the midbrain, pons and medulla often collectively called the [[brainstem]]. Each of these areas has a complex internal structure. Some parts, such as the [[cerebral cortex]] and the cerebellar cortex, are folded into convoluted [[gyri]] and [[sulcus (neuroanatomy)|sulci]] in order to maximize [[surface area]] within the available [[intracranial space]]. Other parts, such as the thalamus and hypothalamus, consist of many small clusters of nuclei known as "ganglia". Thousands of distinguishable areas can be identified within the vertebrate brain based on fine distinctions of neural structure, chemistry, and connectivity.<ref name="Kandel 2000"/><!--Ch. 17 -->

[[Image:Vertebrate-brain-regions small.png|thumb|right|alt=Corresponding regions of human and shark brain are shown. The shark brain is splayed out, while the human brain is more compact. The shark brain starts with the medulla, which is surrounded by various structures, and ends with the telencephalon. The cross-section of the human brain shows the medulla at the bottom surrounded by the same structures, with the telencephalon thickly coating the top of the brain. |The main anatomical regions of the vertebrate brain, shown for shark and human. The same parts are present, but they differ greatly in size and shape.]]

Although the same basic components are present in all vertebrate brains, some branches of vertebrate evolution have led to substantial distortions of brain geometry, especially in the forebrain area. The brain of a shark shows the basic components in a straightforward way, but in [[teleostei|teleost]] fishes (the great majority of existing fish species), the forebrain has become "everted", like a sock turned inside out. In birds, there are also major changes in forebrain structure.<ref>{{cite journal |last=Northcutt |first=RG |year=2008 |title=Forebrain evolution in bony fishes |journal=Brain Research Bulletin |volume=75 |pages=191–205 |pmid=18331871 |doi=10.1016/j.brainresbull.2007.10.058 |issue=2–4|s2cid=44619179 }}</ref> These distortions can make it difficult to match brain components from one species with those of another species.<ref>{{cite journal |year=2005 |title=Organization and evolution of the avian forebrain |journal=The Anatomical Record Part A: Discoveries in Molecular, Cellular, and Evolutionary Biology |volume=287 |pages=1080–1102 |pmid=16206213 |doi=10.1002/ar.a.20253 |last1=Reiner |first1=A |last2=Yamamoto |first2=K |last3=Karten |first3=HJ |issue=1|doi-access=free }}</ref>

Here is a list of some of the most important vertebrate brain components, along with a brief description of their functions as currently understood:
{{See also|List of regions in the human brain}}
* The [[medulla oblongata|medulla]], along with the spinal cord, contains many small nuclei involved in a wide variety of sensory and involuntary motor functions such as vomiting, heart rate and digestive processes.<ref name="Kandel 2000"/><!--Ch.44 and 45-->
* The [[pons]] lies in the brainstem directly above the medulla. Among other things, it contains nuclei that control often voluntary but simple acts such as sleep, respiration, swallowing, bladder function, equilibrium, eye movement, facial expressions, and posture.<ref>{{cite book |title=Essential Neuroscience |url=https://archive.org/details/essentialneurosc00sieg |url-access=limited |publisher=Lippincott Williams & Wilkins |year=2010 |isbn=978-0-7817-8383-5 |pages=[https://archive.org/details/essentialneurosc00sieg/page/n197 184]–189 |last1=Siegel |first1=A |last2=Sapru |first2=HN}}</ref>
* The [[hypothalamus]] is a small region at the base of the forebrain, whose complexity and importance belies its size. It is composed of numerous small nuclei, each with distinct connections and neurochemistry. The hypothalamus is engaged in additional involuntary or partially voluntary acts such as sleep and wake cycles, eating and drinking, and the release of some hormones.<ref>{{cite book|last1=Swaab|first1=Dick F.|url=https://books.google.com/books?id=dOSzzQEACAAJ&q=9780444514905|title=The Human Hypothalamus – Basic and Clinical Aspects: Nuclei of the human hypothalamus. Part I|publisher=Elsevier|year=2003|isbn=9780444514905 |access-date=2021-01-22}}</ref>
* The [[thalamus]] is a collection of nuclei with diverse functions: some are involved in relaying information to and from the cerebral hemispheres, while others are involved in motivation. The subthalamic area ([[zona incerta]]) seems to contain action-generating systems for several types of "consummatory" behaviors such as eating, drinking, defecation, and copulation.<ref>{{cite book|last=Jones|first=Edward G.|url=https://books.google.com/books?id=WMxqAAAAMAAJ|title=The Thalamus|publisher=Plenum Press|year=1985|isbn=9780306418563|location=University of Michigan}}</ref>
* The [[cerebellum]] modulates the outputs of other brain systems, whether motor-related or thought related, to make them certain and precise. Removal of the cerebellum does not prevent an animal from doing anything in particular, but it makes actions hesitant and clumsy. This precision is not built-in but learned by trial and error. The muscle coordination learned while riding a bicycle is an example of a type of [[neuroplasticity|neural plasticity]] that may take place largely within the cerebellum.<ref name="Kandel 2000"/><!--Ch. 42--> 10% of the brain's total volume consists of the cerebellum and 50% of all neurons are held within its structure.<ref>{{cite web|last=Knierim|first=James |title=Cerebellum (Section 3, Chapter 5)|url=http://neuroscience.uth.tmc.edu/s3/chapter05.html|url-status=dead|archive-url=https://web.archive.org/web/20171118210533/http://neuroscience.uth.tmc.edu/s3/chapter05.html|archive-date=2017-11-18|access-date=22 January 2021|website=Neuroscience Online |location=Department of Neurobiology and Anatomy at The University of Texas Health Science Center at Houston, McGovern Medical School}}</ref>
* The [[superior colliculus|optic tectum]] allows actions to be directed toward points in space, most commonly in response to visual input. In mammals, it is usually referred to as the [[superior colliculus]], and its best-studied function is to direct eye movements. It also directs reaching movements and other object-directed actions. It receives strong visual inputs, but also inputs from other senses that are useful in directing actions, such as auditory input in owls and input from the thermosensitive [[pit organ]]s in snakes. In some primitive fishes, such as [[lamprey]]s, this region is the largest part of the brain.<ref>{{cite journal |year=2007 |title=Tectal control of locomotion, steering, and eye movements in lamprey |journal=Journal of Neurophysiology |volume=97 |pages=3093–3108 |pmid=17303814 |doi=10.1152/jn.00639.2006 |last1=Saitoh |first1=K |last2=Ménard |first2=A |last3=Grillner |first3=S |issue=4}}</ref> The superior colliculus is part of the midbrain.
* The [[Pallium (neuroanatomy)|pallium]] is a layer of grey matter that lies on the surface of the forebrain and is the most complex and most recent evolutionary development of the brain as an organ.<ref name="LullFerris1922">{{cite book |author1=Richard Swann Lull |author2=Harry Burr Ferris |author3=George Howard Parker |author4=James Rowland Angell |author5=Albert Galloway Keller |author6=Edwin Grant Conklin |title=The evolution of man: a series of lectures delivered before the Yale chapter of the Sigma xi during the academic year 1921–1922 |url=https://archive.org/details/evolutionofman01bait |year=1922 |publisher=Yale University Press |page=[https://archive.org/details/evolutionofman01bait/page/50 50]}}</ref> In reptiles and mammals, it is called the ''cerebral cortex''. Multiple functions involve the pallium, including [[olfaction|smell]] and [[spatial memory]]. In mammals, where it becomes so large as to dominate the brain, it takes over functions from many other brain areas. In many mammals, the cerebral cortex consists of folded bulges called [[gyrus|gyri]] that create deep furrows or fissures called [[Sulcus (neuroanatomy)|sulci]]. The folds increase the surface area of the cortex and therefore increase the amount of gray matter and the amount of information that can be stored and processed.<ref>{{cite journal |last=Puelles |first=L |year=2001 |title=Thoughts on the development, structure and evolution of the mammalian and avian telencephalic pallium |journal=[[Philosophical Transactions of the Royal Society B]] |volume=356 |pages=1583–1598 |pmid=11604125 |doi=10.1098/rstb.2001.0973 |pmc=1088538 |issue=1414}}</ref>
* The [[hippocampus]], strictly speaking, is found only in mammals. However, the area it derives from, the medial pallium, has counterparts in all vertebrates. There is evidence that this part of the brain is involved in complex events such as spatial memory and navigation in fishes, birds, reptiles, and mammals.<ref>{{cite journal |year=2003 |title=Evolution of forebrain and spatial cognition in vertebrates: conservation across diversity |journal=Brain, Behavior and Evolution |volume=62 |pages=72–82 |doi=10.1159/000072438 |pmid=12937346 |last1=Salas |first1=C |last2=Broglio |first2=C |last3=Rodríguez |first3=F |issue=2|s2cid=23055468 }}</ref>
* The [[basal ganglia]] are a group of interconnected structures in the forebrain. The primary function of the basal ganglia appears to be [[action selection]]: they send inhibitory signals to all parts of the brain that can generate motor behaviors, and in the right circumstances can release the inhibition, so that the action-generating systems are able to execute their actions. Reward and punishment exert their most important neural effects by altering connections within the basal ganglia.<ref name=Grillner2005>{{cite journal |year=2005 |title=Mechanisms for selection of basic motor programs—roles for the striatum and pallidum |journal=Trends in Neurosciences |volume=28 |pages=364–370 |pmid=15935487 |doi=10.1016/j.tins.2005.05.004 |last1=Grillner |first1=S |issue=7 |last2=Hellgren |first2=J |last3=Ménard |first3=A |last4=Saitoh |first4=K |last5=Wikström |first5=M|s2cid=12927634 | display-authors=1}}</ref>
* The [[olfactory bulb]] is a special structure that processes olfactory sensory signals and sends its output to the olfactory part of the pallium. It is a major brain component in many vertebrates, but is greatly reduced in humans and other primates (whose senses are dominated by information acquired by sight rather than smell).<ref>{{cite journal |last=Northcutt |first=RG |year=1981 |title=Evolution of the telencephalon in nonmammals |journal=Annual Review of Neuroscience |volume=4 |pages=301–350 |pmid=7013637 |doi=10.1146/annurev.ne.04.030181.001505}}</ref>

==== Reptiles ====
[[File:Comparative_zoology,_structural_and_systematic_-_for_use_in_schools_and_colleges_(1883)_(20482913440).jpg|thumb|394x394px|Anatomical comparison between the brain of a lizard (A and C) and the brain of a turkey (B and D). Abbreviations: ''Olf, olfactory lobes; Hmp, cerebral hemispheres; Pn, pineal gland ; Mb, optic lobes of the middle brain ; Cb, cerebellum; MO, medulla oblongata; ii, optic nerves; iv and vi, nerves for the muscles of the eye; Py, pituitary body.''[[File:Origin_of_Vertebrates_Fig_019.png|center|thumb|384x384px]]Comparison of Vertebrate Brains: Mammalian, Reptilian, Amphibian, Teleost, and Ammocoetes. ''CB., cerebellum; PT., pituitary body; PN., pineal body; C. STR., corpus striatum; G.H.R., right ganglion habenulæ. I., olfactory; II., optic nerves.'']]
Modern [[reptile]]s and [[mammal]]s diverged from a common ancestor around 320 million years ago.<ref name=":1">{{Cite journal |last1=Reiter |first1=Sam |last2=Liaw |first2=Hua-Peng |last3=Yamawaki |first3=Tracy M. |last4=Naumann |first4=Robert K. |last5=Laurent |first5=Gilles |date=2017 |title=On the Value of Reptilian Brains to Map the Evolution of the Hippocampal Formation |url=https://www.karger.com/Article/FullText/478693 |journal=Brain, Behavior and Evolution |language=english |volume=90 |issue=1 |pages=41–52 |doi=10.1159/000478693 |issn=0006-8977 |pmid=28866680}}</ref> The number of extant reptiles far exceeds the number of mammalian species, with 11,733 recognized species of reptiles<ref>{{Cite web |title=Species Statistics Aug 2019 |url=http://www.reptile-database.org/db-info/SpeciesStat.html |access-date=2022-12-06 |website=www.reptile-database.org}}</ref> compared to 5,884 extant mammals.<ref>{{Cite web |year=2022 |title=The IUCN Red List of Threatened Species. Version 2022-1 - Summary Statistics |url=https://www.iucnredlist.org/resources/summary-statistics |access-date=December 6, 2022 |website=IUCN Red List |issn=2307-8235}}</ref> Along with the species diversity, reptiles have diverged in terms of external morphology, from [[Slender glass lizard|limbless]] to [[Draco (lizard)|tetrapod gliders]] to [[Turtle|armored chelonians]], reflecting adaptive radiation to a diverse array of environments.<ref name=":9">{{Cite journal |last1=Nomura |first1=Tadashi |last2=Kawaguchi |first2=Masahumi |last3=Ono |first3=Katsuhiko |last4=Murakami |first4=Yasunori |date=March 2013 |title=Reptiles: A New Model for Brain Evo-Devo Research: REPTILES FOR EVO-DEVO RESEARCH |url=https://onlinelibrary.wiley.com/doi/10.1002/jez.b.22484 |journal=Journal of Experimental Zoology Part B: Molecular and Developmental Evolution |language=en |volume=320 |issue=2 |pages=57–73 |doi=10.1002/jez.b.22484|pmid=23319423 }}</ref><ref name=":0">{{Cite journal |last1=Salas |first1=Cosme |last2=Broglio |first2=Cristina |last3=Rodríguez |first3=Fernando |date=2003 |title=Evolution of Forebrain and Spatial Cognition in Vertebrates: Conservation across Diversity |url=https://www.karger.com/Article/FullText/72438 |journal=Brain, Behavior and Evolution |language=english |volume=62 |issue=2 |pages=72–82 |doi=10.1159/000072438 |issn=0006-8977 |pmid=12937346}}</ref>

Morphological differences are reflected in the nervous system [[phenotype]], such as: absence of lateral motor column neurons in snakes, which innervate limb muscles controlling limb movements; absence of motor neurons that innervate trunk muscles in tortoises; presence of innervation from the trigeminal nerve to [[Infrared sensing in snakes|pit organs]] responsible to infrared detection in snakes.<ref name=":9" /> Variation in size, weight, and shape of the brain can be found within reptiles.<ref name=":6">{{Cite journal |last=Northcutt |first=R. Glenn |date=2013 |title=Variation in Reptilian Brains and Cognition |url=https://www.karger.com/Article/FullText/351996 |journal=Brain, Behavior and Evolution |language=english |volume=82 |issue=1 |pages=45–54 |doi=10.1159/000351996 |issn=0006-8977 |pmid=23979455}}</ref> For instance, crocodilians have the largest brain volume to body weight proportion, followed by turtles, lizards, and snakes. Reptiles vary in the investment in different brain sections. Crocodilians have the largest telencephalon, while snakes have the smallest. Turtles have the largest diencephalon per body weight whereas crocodilians have the smallest. On the other hand, lizards have the largest mesencephalon.<ref name=":6" />

Yet their brains share several characteristics revealed by recent anatomical, molecular, and [[Ontogeny|ontogenetic]] studies.<ref name=":3">{{Cite journal |last1=Naumann |first1=Robert K. |last2=Ondracek |first2=Janie M. |last3=Reiter |first3=Samuel |last4=Shein-Idelson |first4=Mark |last5=Tosches |first5=Maria Antonietta |last6=Yamawaki |first6=Tracy M. |last7=Laurent |first7=Gilles |date=2015-04-20 |title=The reptilian brain |journal=Current Biology |language=English |volume=25 |issue=8 |pages=R317–R321 |doi=10.1016/j.cub.2015.02.049 |issn=0960-9822 |pmc=4406946 |pmid=25898097|bibcode=2015CBio...25.R317N }}</ref><ref name=":4">{{Cite journal |last1=Hain |first1=David |last2=Gallego-Flores |first2=Tatiana |last3=Klinkmann |first3=Michaela |last4=Macias |first4=Angeles |last5=Ciirdaeva |first5=Elena |last6=Arends |first6=Anja |last7=Thum |first7=Christina |last8=Tushev |first8=Georgi |last9=Kretschmer |first9=Friedrich |last10=Tosches |first10=Maria Antonietta |last11=Laurent |first11=Gilles |date=2022-09-02 |title=Molecular diversity and evolution of neuron types in the amniote brain |url=https://www.science.org/doi/10.1126/science.abp8202 |journal=Science |language=en |volume=377 |issue=6610 |pages=eabp8202 |doi=10.1126/science.abp8202 |pmid=36048944 |issn=0036-8075}}</ref><ref>{{Cite journal |last1=Tosches |first1=Maria Antonietta |last2=Yamawaki |first2=Tracy M. |last3=Naumann |first3=Robert K. |last4=Jacobi |first4=Ariel A. |last5=Tushev |first5=Georgi |last6=Laurent |first6=Gilles |date=2018-05-25 |title=Evolution of pallium, hippocampus, and cortical cell types revealed by single-cell transcriptomics in reptiles |url=https://www.science.org/doi/10.1126/science.aar4237 |journal=Science |language=en |volume=360 |issue=6391 |pages=881–888 |doi=10.1126/science.aar4237 |pmid=29724907 |bibcode=2018Sci...360..881T |issn=0036-8075}}</ref> Vertebrates share the highest levels of similarities during [[Embryology|embryological]] development, controlled by [[Conserved sequence|conserved]] [[transcription factor]]s and [[Developmental signaling center|signaling centers]], including gene expression, morphological and cell type differentiation.<ref name=":3" /><ref name=":9" /><ref>{{Cite journal |last1=Blanton |first1=Mark G. |last2=Kriegstein |first2=Arnold R. |date=1991-08-22 |title=Morphological differentiation of distinct neuronal classes in embryonic turtle cerebral cortex |url=https://onlinelibrary.wiley.com/doi/10.1002/cne.903100405 |journal=The Journal of Comparative Neurology |language=en |volume=310 |issue=4 |pages=550–570 |doi=10.1002/cne.903100405 |pmid=1719040 |issn=0021-9967}}</ref> In fact, high levels of transcriptional factors can be found in all areas of the brain in reptiles and mammals, with shared neuronal clusters enlightening brain evolution.<ref name=":4" /> Conserved transcription factors elucidate that evolution acted in different areas of the brain by either retaining similar morphology and function, or diversifying it.<ref name=":3" /><ref name=":4" />

Anatomically, the reptilian brain has less subdivisions than the mammalian brain, however it has numerous conserved aspects including the organization of the spinal cord and cranial nerve, as well as elaborated brain pattern of organization.<ref name=":5">{{Cite book |last1=William |first1=Butler |first2=Ann B. |last2=Hodos |url=http://worldcat.org/oclc/489018202 |title=Comparative vertebrate neuroanatomy : evolution and adaptation |date=2005 |publisher=Wiley-Liss |isbn=0-471-21005-6 |oclc=489018202}}</ref> Elaborated brains are characterized by migrated neuronal cell bodies away from the periventricular matrix, region of neuronal development, forming organized nuclear groups.<ref name=":5" /> Aside from [[reptile]]s and [[mammal]]s, other vertebrates with elaborated brains include [[hagfish]], [[Shark|galeomorph sharks]], [[Skate (fish)|skates]], [[Batoidea|rays]], [[teleost]]s, and [[bird]]s.<ref name=":5" /> Overall elaborated brains are subdivided in forebrain, midbrain, and hindbrain.

The hindbrain coordinates and integrates sensory and motor inputs and outputs responsible for, but not limited to, walking, swimming, or flying. It contains input and output axons interconnecting the spinal cord, midbrain and forebrain transmitting information from the external and internal environments.<ref name=":5" /> The midbrain links sensory, motor, and integrative components received from the hindbrain, connecting it to the forebrain. The tectum, which includes the optic tectum and torus semicircularis, receives auditory, visual, and somatosensory inputs, forming integrated maps of the sensory and visual space around the animal.<ref name=":5" /> The tegmentum receives incoming sensory information and forwards motor responses to and from the forebrain. The isthmus connects the hindbrain with midbrain. The forebrain region is particularly well developed, is further divided into diencephalon and telencephalon. Diencephalon is related to regulation of eye and body movement in response to visual stimuli, sensory information, [[circadian rhythm]]s, olfactory input, and [[autonomic nervous system]].Telencephalon is related to control of movements, neurotransmitters and neuromodulators responsible for integrating inputs and transmitting outputs are present, sensory systems, and cognitive functions.<ref name=":5" />

==== Birds ====
{{main|Avian brain}}{{Excerpt|Avian brain}}

==== Mammals ====
The most obvious difference between the brains of mammals and other vertebrates is their size. On average, a mammal has a brain roughly twice as large as that of a bird of the same body size, and ten times as large as that of a reptile of the same body size.<ref name=Northcutt2002>{{cite journal |last=Northcutt |first=RG |title=Understanding vertebrate brain evolution |journal=Integrative and Comparative Biology |volume=42 |pages=743–756 |pmid=21708771 |doi=10.1093/icb/42.4.743 |year=2002 |issue=4|doi-access=free }}</ref>

Size, however, is not the only difference: there are also substantial differences in shape. The hindbrain and midbrain of mammals are generally similar to those of other vertebrates, but dramatic differences appear in the forebrain, which is greatly enlarged and also altered in structure.<ref name=Barton>{{cite journal |last1=Barton |first1=RA |title=Mosaic evolution of brain structure in mammals |year=2000 |journal=Nature |volume=405 |pages=1055–1058 |pmid=10890446 |doi=10.1038/35016580 |last2=Harvey |first2=PH |issue=6790|bibcode=2000Natur.405.1055B |s2cid=52854758 }}</ref> The cerebral cortex is the part of the brain that most strongly distinguishes mammals. In non-mammalian vertebrates, the surface of the [[cerebrum]] is lined with a comparatively simple three-layered structure called the [[pallium (neuroanatomy)|pallium]]. In mammals, the pallium evolves into a complex six-layered structure called [[neocortex]] or ''isocortex''.<ref name=Aboitiz>{{cite journal |title=The evolutionary origin of the mammalian isocortex: Towards an integrated developmental and functional approach |journal=Behavioral and Brain Sciences |year=2003 |volume=26 |pages=535–552 |pmid=15179935 |doi=10.1017/S0140525X03000128 |last1=Aboitiz |first1=F |last2=Morales |first2=D |last3=Montiel |first3=J |issue=5 |s2cid=6599761 }}</ref> Several areas at the edge of the neocortex, including the hippocampus and [[amygdala]], are also much more extensively developed in mammals than in other vertebrates.<ref name=Barton/>

The elaboration of the cerebral cortex carries with it changes to other brain areas. The [[superior colliculus]], which plays a major role in visual control of behavior in most vertebrates, shrinks to a small size in mammals, and many of its functions are taken over by visual areas of the cerebral cortex.<ref name=Northcutt2002/> The cerebellum of mammals contains a large portion (the [[neocerebellum]]) dedicated to supporting the cerebral cortex, which has no counterpart in other vertebrates.<ref>{{cite book |year=1977 |title=The Vertebrate Body |publisher=Holt-Saunders International |page=531 |isbn=978-0-03-910284-5 |last1=Romer |first1=AS |last2=Parsons |first2=TS}}</ref>

In [[placental]]s, there is a wide nerve tract connecting the cerebral hemispheres called the [[corpus callosum]].

===== Primates =====
{{See also|Human brain}}

{| class="wikitable" align="right" style="margin-left: 10px;"
|+Encephalization Quotient
!Species
!EQ<ref name=Roth2005>{{cite journal |title=Evolution of the brain and Intelligence |last1=Roth |first1=G |last2=Dicke |first2=U |journal=Trends in Cognitive Sciences |volume=9 |issue=5 |pages=250–257 |year=2005 |doi=10.1016/j.tics.2005.03.005 |pmid=15866152|s2cid=14758763 }}</ref>
|-
|Human||7.4–7.8
|-
|[[Common chimpanzee]]||2.2–2.5
|-
|[[Rhesus macaque|Rhesus monkey]]||2.1
|-
|[[Bottlenose dolphin]]||4.14<ref name=Marino>{{cite journal |last=Marino |first=Lori |title=Cetacean Brain Evolution: Multiplication Generates Complexity |journal=International Society for Comparative Psychology |issue=17 |pages=1–16 |year=2004 |url=http://www.cogs.indiana.edu/spackled/2005readings/CetaceanBrainEvolution.pdf |access-date=2010-08-29 |archive-url=https://web.archive.org/web/20180916132752/http://www.cogs.indiana.edu/spackled/2005readings/CetaceanBrainEvolution.pdf |archive-date=2018-09-16 |url-status=dead }}</ref>
|-
|[[Elephant]]||1.13–2.36<ref>{{Cite journal |doi=10.1016/j.brainresbull.2006.03.016 |last1=Shoshani |first1=J |last2=Kupsky |first2=WJ |last3=Marchant |first3=GH |title=Elephant brain Part I: Gross morphology, functions, comparative anatomy, and evolution |journal=Brain Research Bulletin |volume=70 |issue=2 |pages=124–157 |year=2006 |pmid=16782503|s2cid=14339772 }}</ref>
|-
|[[Dog]]||1.2
|-
|[[Horse]]||0.9
|-
|[[Rat]]||0.4
<!--|-
|colspan="2" style="text-align: left;" |EQ relative to the cat as standard species: EQ(cat)=1-->
|-
|}

The [[human brain|brains of humans]] and other [[primate]]s contain the same structures as the brains of other mammals, but are generally larger in proportion to body size.<ref name=Finlay>{{cite journal |year=2001 |title=Developmental structure in brain evolution |journal=Behavioral and Brain Sciences |volume=24 |pages=263–308 |pmid=11530543 |last1=Finlay |first1=BL |last2=Darlington |first2=RB |last3=Nicastro |first3=N |issue=2 |doi=10.1017/S0140525X01003958|s2cid=20978251 }}</ref> The [[encephalization quotient]] (EQ) is used to compare brain sizes across species. It takes into account the nonlinearity of the brain-to-body relationship.<ref name=Roth2005/> Humans have an average EQ in the 7-to-8 range, while most other primates have an EQ in the 2-to-3 range. Dolphins have values higher than those of primates other than humans,<ref name=Marino/> but nearly all other mammals have EQ values that are substantially lower.

Most of the enlargement of the primate brain comes from a massive expansion of the cerebral cortex, especially the [[prefrontal cortex]] and the parts of the cortex involved in [[Visual perception|vision]].<ref>{{cite book|last=Calvin|first=William H.|url=https://archive.org/details/howbrainsthinkev00calv|title=How Brains Think|publisher=BasicBooks|year=1996|isbn=978-0-465-07278-1|edition=1st|location=New York, NY |url-access=registration}}</ref> The visual processing network of primates includes at least 30 distinguishable brain areas, with a complex web of interconnections. It has been estimated that visual processing areas occupy more than half of the total surface of the primate neocortex.<ref name = Sereno1995>{{cite journal |doi=10.1126/science.7754376 |last1=Sereno |first1=MI |last2=Dale |first2=AM |last3=Reppas |first3=AM |last4=Kwong |first4=KK |last5=Belliveau |first5=JW |last6=Brady |first6=TJ |last7=Rosen |first7=BR |last8=Tootell |first8=RBH |year=1995 |title=Borders of multiple visual areas in human revealed by functional magnetic resonance imaging |journal=Science |volume=268 |issue=5212 |pages=889–893 |url=http://www.cogsci.ucsd.edu/~sereno/papers/HumanRetin95.pdf |archive-url=https://web.archive.org/web/20060523153637/http://cogsci.ucsd.edu/~sereno/papers/HumanRetin95.pdf |archive-date=2006-05-23 |url-status=live |pmid=7754376 |bibcode=1995Sci...268..889S}}</ref> The [[prefrontal cortex]] carries out functions that include [[foresight (psychology)|planning]], [[working memory]], [[motivation]], [[attention]], and [[executive functions|executive control]]. It takes up a much larger proportion of the brain for primates than for other species, and an especially large fraction of the human brain.<ref>{{cite book|last=Fuster|first=Joaquín M.|url=https://archive.org/details/prefrontalcortex00fust_846|title=The Prefrontal Cortex|publisher=Elsevier|year=2008|isbn=978-0-12-373644-4|edition=4th |pages=[https://archive.org/details/prefrontalcortex00fust_846/page/n15 1]–7|url-access=limited}}</ref>