Evolution of the brain

01The evolution of the brain refers to the progressive development and complexity of neural structures over millions of years, resulting in the diverse range of brain sizes and functions observed across different species today, particularly invertebrates.

02The evolution of the brain has exhibited diverging adaptations within taxonomic classes, such as mammalia, and even more diverse adaptations across other taxonomic classes.

03Brain-to-body size scales allometrically.

04This means that as body size changes, so do other physiological, anatomical, and biochemical connections between the brain and body.

05Small-bodied mammals tend to have relatively large brains compared to their bodies, while larger mammals, such as whales, have smaller brain-to-body ratios.

06When brain weight is plotted against body weight for primates, the regression line of the sample points can indicate the brain power of a species.

07For example, lemurs fall below this line, suggesting that for a primate of their size, a larger brain would be expected.

08In contrast, humans lie well above this line, indicating they are more encephalized than lemurs and, in fact, more encephalized than any other primate.

09This suggests that human brains have undergone a larger evolutionary increase in complexity relative to size.

10Some of these changes have been linked to multiple genetic factors, including proteins and other organelles.

11Early history.

12One approach to understanding overall brain evolution is to use a paleoarchaeological timeline to trace the necessity for ever-increasing complexity in structures that allow for chemical and electrical signaling.

13Because brains and other soft tissues do not fossilize as readily as mineralized tissues, scientists often look to other structures as evidence in the fossil record to get an understanding of brain evolution.

14This, however, leads to a dilemma as the emergence of organisms with more complex nervous systems with protective bone or other protective tissues that can then readily fossilize occur in the fossil record before evidence for chemical and electrical signaling.

15Evidence from 2008 showed that the ability to transmit electrical and chemical signals existed even before more complex multicellular life forms.

16Fossilization of brain tissue, as well as other soft tissue, is nonetheless possible and scientists can infer that the first brain structure appeared at least 521 million years ago, with fossil brain tissue present in sites of exceptional preservation.

17Another approach to understanding brain evolution is to look at extant organisms that do not possess complex nervous systems, comparing anatomical features that allow for chemical or electrical messaging.

18For example, conoflagellates are organisms that possess various membrane channels that are crucial to electrical signaling.

19The membrane channels of conoflagellates are homologous to the ones found in animal cells, and this is supported by the evolutionary connection between early conoflagellates and the ancestors of animals.

20Another example of extant organisms with the capacity to transmit electrical signals would be the glass sponge, a multicellular organism which is capable of propagating electrical impulses without the presence of a nervous system.

21Before the evolutionary development of the brain, nerve nets, the simplest form of a nervous system developed.

22These nerve nets were a sort of precursor for the more evolutionarily advanced brains.

23They were first observed in Nidaria and consist of a number of neurons spread apart that allow the organism to respond to physical contact.

24They are able to rudimentary Tenophores also demonstrate this crude precursor to a brain or centralized nervous system.

25However, they phylogenetically diverged before the phylum periphera, the sponges, and Nidaria.

26There are two current theories on the emergence of nerve nets.

27One theory is that nerve nets may have developed independently in tenophores and Cnidarians.

28The other theory states that a common ancestor may have developed nerve nets, but they were lost in periphera.

29While comparing the average neuron size and the packing density, the difference between primate and mammal brains is shown.

30A trend in brain evolution according to a study done with mice, chickens, monkeys, and apes concluded that more evolved species tend to preserve the structures responsible for basic behaviors.

31A long-term human study comparing the human brain to the primitive brain found that the modern human brain contains the primitive hindbrain region, what most neuroscientists call the protoreptilian brain.

32The purpose of this part of the brain is to sustain fundamental homeostatic functions, which are self-regulating processes organisms use to help their bodies adapt.

33The pons and medulla are major structures found there.

34A new region of the brain developed in mammals about 250 million years after the appearance of the hindbrain.

35This region is known as the paleomammalian brain, the major parts of which are the hippocampi and amygdalas, often referred to as the limbic system.

36The limbic system deals with more complex functions including emotional, sexual, and fighting behaviors.

37Of course, animals that are not vertebrates also have brains, and their brains have undergone separate evolutionary histories.

38The brainstem and limbic system are largely based on nuclei, which are essentially balled-up clusters of tightly packed neurons and the axon fibers that connect them to each other, as well as to neurons in other locations.

39The other two major brain areas, the cerebrum and cerebellum, are based on a cortical architecture.

40At the outer periphery of the cortex, the neurons are arranged into layers, the number of which vary according to species and function, a few millimeters thick.

41There are axons that travel between the layers, but the majority of axon mass is below the neurons themselves.

42Since cortical neurons and most of their axon fiber tracks do not have to compete for space, cortical structures can scale more easily than nuclear ones.

43A key feature of cortex is that because it scales with surface area, more of it can be fit inside a skull by introducing convolutions, in much the same way that a dinner napkin can be stuffed into a glass by wadding it up.

44The degree of convolution is generally greater in species with more complex behavior, which benefits from the increased surface area.

45The cerebellum, or little brain, is behind the brain stem and below the occipital lobe of the cerebrum in humans.

46Its purposes include the coordination of fine sensor motor tasks, and it may be involved in some cognitive functions, such as language and different motor skills that may involve hands and feet.

47The cerebellum helps keep equilibrium.

48Damage to the cerebellum would result in all physical roles in life to be affected.

49Human cerebellar cortex is finely convoluted, much more so than cerebral cortex.

50Its interior axon fiber tracks are called the arborvitae, or tree of life.

51The area of the brain with the greatest amount of recent evolutionary change is called the neocortex.

52In reptiles and fish, this area is called the pallium and is smaller and simpler relative to body mass than what is found in mammals.

53According to research, the cerebrum first developed about 200 million years ago.

54It is responsible for higher cognitive functions, for example, language, thinking, and related forms of information processing.

55It is also responsible for processing sensory input, together with the thalamus, a part of the limbic system that acts as an information router.

56The thalamus receives the different sensations before the information is then passed onto the cerebral cortex.

57Most of its function is subconscious, that is, not available for inspection or intervention by the conscious mind.

58The neocortex is an elaboration or outgrowth of structures in the limbic system, with which it is tightly integrated.

59The neocortex is the main part controlling many brain functions as it covers half of the whole brain in volume.

60The development of these recent evolutionary changes in the neocortex likely occurred as a result of new neural network formations and positive selections of certain genetic components.

61Role of Embryology In addition to studying the fossil record, evolutionary history can be investigated via embryology.

62An embryo is an unborn slash unhatched animal and evolutionary history can be studied by observing how processes in embryonic development are conserved, or not conserved, across species.

63Similarities between different species may indicate evolutionary connection.

64One way anthropologists study evolutionary connection between species is by observing orthologs.

65An ortholog is defined as two or more homologous genes between species that are evolutionarily related by linear descent.

66By using embryology, the evolution of the brain can be tracked between various species.

67Bone morphogenetic protein, BMP A growth factor that plays a significant role in embryonic neural development is highly conserved amongst vertebrates, as is sonic hedgehog, SHH.

68A morphogen that inhibits BMP to allow neural crest development.

69Tracking these growth factors with the use of embryology provides a deeper understanding of what areas of the brain diverged in their evolution.

70Varying levels of these growth factors lead to differing embryonic neural development, which then in turn affects the complexity of future neural systems.

71Studying the brain's development at various embryonic stages across differing species provides additional insight into what evolutionary changes may have historically occurred.

72This then allows scientists to look into what factors may have caused such changes, such as links to neural network diversity, growth factor production, protein coding selections, and other genetic factors.

73Randomizing access and increasing size.

74Some animal phyla have gone through major brain enlargement through evolution, e.g.

75Vertebrates and cephalopods both contain many lineages in which brains have grown through evolution.

76But most animal groups are composed only of species with extremely small brains.

77Some scientists argue that this difference is due to vertebrate and cephalopod neurons having evolved ways of communicating that overcome the scalability problem of neural networks, while most animal groups have not.

78They argue that the reason why traditional neural networks fail to improve their function when they scale up is because filtering based on previously known probabilities cause self-fulfilling prophecy-like biases that create false statistical evidence giving a completely false worldview and that randomized access can overcome this problem and allow brains to be scaled up to more discriminating conditioned reflexes at larger brains.

79That lead to new worldview forming abilities at certain thresholds.

80This means when neurons scale in a non-randomized fashion that their functionality becomes more limited due to their neural networks being unable to process more complex systems without the exposure to new formations.

81This is explained by randomization allowing the entire brain to eventually get access to all information over the course of many shifts even though instant privileged access is physically impossible.

82They cite that vertebrate neurons transmit virus-like capsules containing RNA that are sometimes read in the neuron to which it is transmitted and sometimes passed further on unread which creates randomized access.

83And that cephalopod neurons make different proteins from the same gene which suggests another mechanism for randomization of concentrated information in neurons, both making it evolutionarily worth scaling up brains.

84Brain reorganization.

85Brain reorganization.

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156Hominins.

157Show that.

158Immature.

159Hominins.

160Including.

161Australopithecines.

162And members.

163Of Homo.

164Have a quiescent period.

165Bown at all.

1661987.

167A quiescent period.

168Is a period.

169In which.

170There are no.

171Dental eruptions.

172Of adult teeth.

173At this time.

174The child.

175Becomes more.

176Accustomed.

177To social structure.

178And development.

179Of culture.

180During this time.

181The child.

182Is given.

183An extra advantage.

184Over other.

185Hominins.

186In the community.

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198Genes.

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378The archaic.

379Gene variant.

380Nova 1.

381Present in Neanderthals.

382And Denisovans.

383Via CRISPR.

384Cas9.

385Shows that it has a major impact.

386On neurodevelopment.

387And that such genetic mutations.

388During the evolution.

389Of the human brain.

390Underlie traits.

391That separate modern humans.

392From extinct.

393Homospecies.

394They found.

395That expression.

396Of the archaic.

397Nova 1.

398In cortical organoids.

399Leads to modified.

400Synaptic protein.

401Interactions.

402Affects.

403Glutametergic.

404Signaling.

405Underlies.

406Differences.

407In neuronal connectivity.

408And promotes.

409Higher heterogeneity.

410Of neurons.

411Regarding their.

412Electrophysiological.

413Profiles.

414This research.

415Suggests.

416Positive selection.

417Of the modern.

418Nova 1.

419Which may have.

420Promoted the randomization.

421Of neural scaling.

422A subsequent.

423Study.

424Failed to replicate.

425The differences.

426In organoid.

427Morphology.

428Between the modern.

429And the archaic.

430Nova 1.

431Variant.

432Consistent.

433With suspected.

434Unwanted.

435Side effects.

436Of CRISPR.

437Editing.

438In the original study.

439Sergap.

4402C.

441And neuronal maturation.

442Less is known.

443About neuronal maturation.

444Synaptic.

445And protein.

446Protracted.

447In line.

448With the protracted.

449Synaptic.

450Maturation.

451Of human.

452Cortical.

453Neurons.

454So-called.

455Neoteny.

456This probably.

457Relies.

458On the evolution.

459Of non-coding.

460Genomic regions.

461The consequence.

462Of the.

463Neoteny.

464Of the period.

465Of synaptic.

466Plasticity.

467And therefore.

468Of learning.

469A human.

470Specific.

471Duplicated.

472Gene.

473S.

474S.

475G.

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1048S.

1049S.

1050S.

1051S.

1052S.

1053S.

1054S.

1055S.

1056S.

1057S.

1058S.

1059S.

1060S.

1061S.

1062S.

1063S.

1064S.

1065S.

1066S.

1067S.

1068S.

1069S.

1070S.

1071S.

1072S.

1073S.

1074S.

1075S.

1076S.

1077S.

1078S.

1079S.

1080S.

1081S.

1082S.

1083S.

1084S.

1085S.

1086S.

1087S.

1088S.

1089S.

1090S.

1091S.

1092S.

1093S.

1094S.

1095S.

1096S.

1097S.

1098S.

1099S.

1100S.

1101S.

1102S.

1103S.

1104S.

1105S.

1106S.

1107S.

1108S.

1109S.

1110S.

1111S.

1112S.

1113S.

1114S.

1115S.

1116S.

1117S.

1118S.

1119S.

1120S.

1121S.

1122S.

1123S.

1124S.

1125S.

1126S.

1127S.

1128S.

1129S.

1130S.

1131S.

1132S.

1133S.

1134S.

1135S.

1136S.

1137S.

1138S.

1139S.

1140S.

1141S.

1142S.

1143S.

1144S.

1145S.

1146S.

1147S.

1148S.

1149S.

1150S.

1151S.

1152S.

1153S.

1154S.

1155S.

1156S.

1157S.

1158S.

1159S.

1160S.

1161S.

1162S.

1163S.

1164S.

1165S.

1166S.

1167S.

1168S.

1169S.

1170S.

1171S.

1172S.

1173S.

1174S.

1175S.

1176S.

1177S.

1178S.

1179S.

1180S.

1181S.

1182S.

1183S.

1184S.

1185S.

1186S.

1187S.

1188S.

1189S.

1190S.

1191S.

1192S.

1193S.

1194S.

1195S.

1196S.

1197S.

1198S.

1199S.

1200S.

1201S.

1202S.

1203S.

1204S.

1205S.

1206S.

1207S.

1208S.

1209S.

1210S.

1211S.

1212S.