44 The Oldest Map
How Comparative Neuroscience Reconstructs an Ancient Hippocampal System
44.1 Different brains, one historical problem
Chapter 43 introduced the hippocampal formation as part of an ancient system for learning relations among places, states, events, and transitions. That claim creates a historical problem. The mammalian hippocampus is a curled, laminated structure in the medial temporal lobe. The avian hippocampal formation lies as a differently organized sheet along the dorsomedial forebrain. Reptiles possess medial and dorsomedial cortical fields rather than a mammalian-looking hippocampal curl. Amphibians have a comparatively simple medial pallium. In teleost fishes, the candidate hippocampal territory occupies a dorsolateral position in an everted forebrain.
No single adult shape runs through that series. Even the apparent position changes. A method that searched for a small mammalian hippocampus inside every other vertebrate brain would therefore fail before the comparison began.
The problem is broader than the hippocampus. Evolution rarely preserves an adult organ by copying its final geometry into every descendant. Developmental tissue can expand, fold, migrate, subdivide, and enter new circuits. A structure inherited from a common ancestor can become difficult to recognize, while unrelated structures can acquire superficially similar shapes or functions. Comparative neuroscience must therefore distinguish homology, similarity caused by common descent, from analogy, similarity produced independently.
The answer cannot come from one privileged feature. Adult morphology matters, but it can be transformed. Developmental position matters, but boundaries can shift. Gene expression identifies cell populations and territories, but genes can be reused. Connectivity and physiology reveal circuit organization, but similar computations can evolve in different tissue. Lesions show what a region contributes to behavior, but similar functions alone do not establish common ancestry.
The strongest reconstruction comes from convergence.
Across the bony-vertebrate lineages examined here, an ancient medial-pallial system can be recognized through developmental topology, molecular specification, cell types, connectivity, physiology, and behavior. Evolution has rearranged and elaborated that system differently in different lineages. Its clearest conserved contribution is flexible spatial and relational memory: navigation is an ancient expression of that capacity, not evidence that every nonspatial function was added later.
That conclusion is firm at the level that matters for this unit. Mammalian, avian, reptilian, amphibian, and teleost forebrains contain related hippocampal or medial-pallial systems. The exact matching of every field is a different question. A teleost region can belong to an ancient hippocampal territory without being a miniature dentate gyrus, CA3, or subiculum. The chapter will keep those levels separate.
44.2 How homology is reconstructed
The embryonic vertebrate forebrain begins with a conserved set of axes and signaling territories. Within the dorsal telencephalon, the pallium is regionalized into broad developmental sectors. In tetrapods, the medial pallium lies beside the dorsal midline, the roof plate, and a signaling region called the cortical hem. In mammals, this territory contributes to the hippocampal formation and adjoining regions. The comparative question is whether tissue in another lineage occupies a corresponding developmental position and follows a related program, even when the adult result looks different.
Developmental topology is more informative than adult compass direction. Topology asks which tissues border one another and how those relations change during growth. A patch that begins beside a conserved organizer may be carried laterally by folding or expansion. Its adult location does not erase its developmental history. This distinction will become decisive in the teleost forebrain, where an everted pallium rearranges the relation between ventricular surface and adult position.
Gene-expression studies add another level. Regulatory genes help specify regional identity during development, and combinations of markers can distinguish one pallial territory from another. The relevant evidence is not a single gene acting as a barcode. It is a pattern interpreted within anatomical position. A gene expressed in a mammalian dentate granule cell does not make every cell expressing that gene a dentate granule cell. A stronger comparison asks whether several genes, developmental boundaries, cell types, and connections align.
Single-cell transcriptomics extends that logic. Instead of comparing only whole regions, investigators can compare the expression profiles of individual neuronal populations across species. This can reveal old cell classes that persist inside very different adult architectures. It can also expose divergence. A region thought to correspond to one mammalian field may contain a mixture of conserved, shifted, and lineage-specific populations. The newer method therefore sharpens homology rather than automatically settling it.
Connections and function provide additional tests. A proposed hippocampal territory should participate in circuits appropriate to learning relations among sensory cues, locations, events, and actions. Damage should remove some of those capacities without simply blinding, paralyzing, or demotivating the animal. Neural activity should change with behavior in ways consistent with the proposed operation. None of those findings proves ancestry by itself. Together with development and cell identity, they can make one historical reconstruction much stronger than its competitors.
This is why the original slogan “morphology lies; molecules tell the truth” must be replaced. Morphology does not lie. It records what development and evolution produced. Molecules do not speak without interpretation. They provide another record, sometimes preserving relationships that adult shape obscures and sometimes complicating an appealing anatomical story.
Adult appearance can conceal shared ancestry. Homology is reconstructed from converging evidence.
44.3 The medial pallium across tetrapods
The mammal–bird comparison shows how the method works. In a developing mouse, the medial pallium contributes to the dentate gyrus, the fields of Ammon’s horn, the subiculum, and adjoining medial temporal territories. In a chicken, the adult hippocampal formation is not folded into the same interlocking architecture. It occupies the dorsomedial pallium and has its own internal organization.
Antonio Abellán, Ester Desfilis, and Loreta Medina compared the developing mouse and chicken forebrains using topological position and the expression of seven regulatory genes. In both species, the candidate medial pallium lay beside the cortical hem and roof plate. Its ventricular zone expressed a combination that included Lef1, Lhx2, and Lhx9 but excluded Lhx5. Finer patterns involving Prox1, Lmo3, and Lmo4 supported comparisons among dentate/CA3-like, CA1/subicular-like, and medial-entorhinal-like sectors [@abellan2014hippocampal].
The important result is not that chicken fields can be renamed as mammalian fields without remainder. It is that a radically different adult architecture develops from a comparable pallial territory under related regulatory programs. The mammalian curl and the avian sheet are lineage-specific constructions of an inherited system.
The same strategy extends across the living branches of tetrapods. Amphibians are not primitive mammals waiting at an earlier rung of a ladder. They are the living sister lineage to amniotes and have followed their own evolutionary history. A feature shared by amphibians and amniotes nevertheless constrains what was probably present in their common ancestor.
In Xenopus and the turtle Trachemys, developmental marker studies identify a medial-pallial territory despite substantial differences in adult cytoarchitecture. Within that territory, a Prox1-positive subdivision and an Er81/Lmo4-positive subdivision have been compared with dentate-like and Ammon’s-horn-like components, respectively [@jimenez2022medialpallium]. The comparison supports an inherited regional program in the common ancestor of living tetrapods. The labels DG-like and CA-like remain important. They identify evidence for correspondence without pretending that the frog or turtle contains a folded mammalian hippocampus.
Reptiles add cell-type evidence. Single-cell transcriptomics in turtle and lizard pallium found both continuity and divergence. Several major inhibitory-neuron classes were already present in the common ancestor of amniotes, whereas excitatory pallial populations show substantial lineage-specific diversification [@tosches2018evolution]. The result replaces a simple choice between “the same cortex” and “a different cortex.” Evolution can preserve components of a cellular program while altering how those components are distributed, combined, and expanded.
A similar principle applies within the hippocampal system. Reptilian medial and dorsomedial cortices participate in a connected medial-pallial complex, but their relation to mammalian dentate, CA fields, and subiculum is not a perfect field-for-field translation. Shared ancestry does not require the ancestral tissue to retain one adult layout. It requires a coherent transformation from one lineage to the next.
The broad tetrapod conclusion is therefore strong. Mammals, birds, reptiles, and amphibians possess medial-pallial territories identified by their position near conserved developmental landmarks, related molecular programs, and hippocampal-like contributions to behavior. Their common ancestor did not possess a human medial temporal lobe. It possessed the tissue and developmental organization from which the different tetrapod hippocampal systems were built.
This conclusion depends on phylogenetic comparison, not on treating any living species as a surviving ancestor. A frog does not display the stage before a turtle, and a turtle does not display the stage before a bird or mammal. Each lineage contains inherited features mixed with specializations acquired during its own history. What constrains the reconstruction is the pattern of distribution. A medial-pallial territory found in both living branches of tetrapods is most economically attributed to their common ancestor unless there is strong evidence for independent origin. Differences among the descendants then reveal how that inherited territory was reorganized. Comparative neuroscience uses living brains to infer transformations along branches; it does not arrange those brains as steps on a staircase.
The inference also has a natural level of resolution. The evidence can support an ancestral medial-pallial system even when it cannot reconstruct the exact number of fields, their adult boundaries, or every connection in the ancestral brain. Broad homology is not weakened by refusing to invent details that the comparison cannot recover. It becomes clearer.
Comparative studies often use Lef1, Lhx2, Lhx9, Lmo3, Lmo4, and Prox1 as evidence about medial-pallial identity. Their value lies in combinations, position, and developmental timing. Prox1, for example, is central to mammalian dentate granule-cell identity and has helped identify DG-like territories in other vertebrates. It is not a universal stamp that independently converts any Prox1-positive field into dentate gyrus.
Experimental manipulation of Lhx2 shows why regulatory genes are more than passive labels. In mouse genetic mosaics and timed inactivations, Lhx2 acted during an early critical period to specify cortical identity and suppress alternative hem or antihem fates. Medial cells lacking Lhx2 adopted cortical-hem identity, and the resulting ectopic hem tissue could organize neighboring hippocampal fields [@mangale2008lhx2]. The experiment demonstrates a causal role in constructing the developmental boundaries used for comparative inference.
It does not follow that one gene creates “the hippocampus” by itself. The cortical hem, Wnt signaling, other transcription factors, cell interactions, migration, and later circuit development all contribute. The causal lesson is narrower and stronger: the molecular patterns used to identify pallial territories participate in making those territories.
44.4 Conserved territory does not mean identical circuitry
A homologous structure can change greatly while remaining part of the same historical lineage. Vertebrate limbs provide the familiar example. A human arm, a bat wing, and a whale flipper share an inherited skeletal plan, but the proportions, joints, muscles, and functions have diverged. Calling them homologous does not make them interchangeable.
The same rule applies to pallium. A broad hippocampal or medial-pallial homology does not require identical lamination, the same number of fields, the same cellular proportions, or one-to-one correspondence between every named subdivision. Mammalian dentate gyrus is a particularly elaborate structure with distinctive development, folding, and neurogenesis. Its absence in that form from another lineage does not mean the entire medial-pallial comparison has failed.
Function can also be conserved at one level and transformed at another. A reptile and a mammal may both require medial-pallial circuitry to use relations among environmental cues, while relying on differently organized local networks. Conversely, two unrelated circuits can converge on similar behavior. This is why a lesion result strengthens a developmental comparison but cannot replace it.
One example from the original chapter illustrates the danger of turning a developmental difference into an evolutionary story. In the mouse, Abellán and colleagues assigned medial entorhinal cortex to the medial pallium while proposing a different caudolateral pallial origin for lateral entorhinal cortex [@abellan2014hippocampal]. That is a developmental claim about tissue origin. It does not establish that a spatial “where” system evolved first and that an object “what” system later arrived as a tenant. Developmental contiguity is not a fossil sequence, and medial and lateral entorhinal cortices both carry mixtures of spatial, sensory, temporal, and task-related information.
The chapter therefore makes its evolutionary claim at the level the evidence supports. An ancient medial-pallial system has been retained and elaborated across vertebrates. Spatial navigation offers the clearest conserved behavioral expression of that system. Whether every nonspatial relational operation arose later must be tested rather than read from adult anatomy or embryonic adjacency.
44.5 Eversion rearranges the teleost pallium
Teleost fishes create the hardest case because their adult forebrain violates tetrapod expectations. The major dorsal telencephalic territory is called D, and its subdivisions are named largely by adult position: Dl is lateral, Dm medial, Dc central, and Dp posterior. Those labels describe a fish brain. They do not identify mammalian homologies.
In tetrapods, the paired telencephalic hemispheres develop through evagination. The walls of the rostral neural tube expand outward, leaving the ventricular surface on the inside of each hemisphere. The medial pallium remains associated with the dorsal midline and the ventricular system, although later growth can fold it deeply into the mammalian temporal lobe.
The teleost pallium has an everted organization. Traditional diagrams portray its walls as folding outward rather than inward, displacing tissue related to the medial pallium toward a dorsolateral adult position. That contrast captures the resulting topological problem, but it should not be mistaken for a literal movie of a completed pallial sheet turning inside out like a sock.
Fate mapping in developing zebrafish showed a more complex morphogenesis. Posterior telencephalic tissue is displaced laterally, the pallial ventricular zone expands strongly along the anteroposterior axis, and the posterior wall bulges dorsally as the thin tela choroidea spreads over the pallial surface. The adult everted form can therefore emerge through growth and rearrangement without a simple, uniform lateral outfolding of every pallial sector [@folgueira2012eversion].
This distinction matters because a perfect mirror reversal makes predictions that the adult brain does not consistently satisfy. Current developmental and transcriptomic evidence favors partial eversion. Candidate medial-pallial tissue is displaced toward Dl, but other pallial territories do not all fall into a complete reversed sequence [@hegarty2024teleostatlas]. The teleost pallium is transformed, not merely flipped.
Teleosts and tetrapods belong to lineages that diverged near the base of the bony-vertebrate radiation. Finding related pallial territories on both sides of that division supports the presence of a hippocampal or medial-pallial system in their common ancestor [@hegarty2024teleostatlas; @rodriguez2002conservation]. That is a deep evolutionary claim, but it is not unlimited. The present comparison does not by itself reconstruct the precise organization of the earliest vertebrate forebrain, nor does it establish that every feature of a mammalian hippocampus was already present before ray-finned and lobe-finned fishes diverged. The inherited element is the regional and circuit ancestry from which later systems were built.
44.5.1 A broad correspondence inside a heterogeneous Dl
Molecular studies in adult zebrafish divided the pallium into major territories using combinations of ascl1a, eomesa, emx1, emx2, emx3, and Prox1. That work proposed a hippocampal relationship for part of Dl and an amygdalar relationship for Dm. It also divided Dl into dorsal and ventral components rather than treating the entire lateral pallium as one field [@ganz2015zebrafishpallium].
The proposal converges with older lesion evidence, but the fine-grained assignments remain active. Teleost species have undergone extensive independent diversification, and their atlases do not use every label in exactly the same way. Dlv in goldfish, Dl-vv and Dl-g in cichlids, and Dld in zebrafish cannot be translated as though they were fields in one standardized brain.
A 2024 atlas combined single-nucleus RNA sequencing with spatial transcriptomics in a cichlid. Cell populations in the ventral-most part of Dl, Dl-vv, showed strong transcriptional similarity to mammalian CA3 populations and to hippocampal-like populations in other vertebrates. Nearby Dl-v populations also showed similarities to subicular and retrosplenial populations. The same analysis did not support the expected one-to-one match between the proposed granular Dl-g territory and mammalian dentate gyrus. At the regional level, Dl-g was most similar to mammalian visual cortex, although some of its cell populations showed other mesocortical similarities [@hegarty2024teleostatlas].
That result does not erase the teleost hippocampal pallium. It changes the resolution of the claim. The broad correspondence between ventral dorsolateral pallium and tetrapod hippocampal or medial-pallial systems is supported by development, cell types, physiology, and behavior. The proposition that one specific teleost patch is “the dentate gyrus” while another is “CA3” remains unresolved.
44.5.2 Circuits and activity provide independent tests
Connectional anatomy offers another line of evidence. In weakly electric gymnotiform fishes, subdivisions of dorsolateral and dorsodorsal pallium form recursive loops. Tract tracing identified a pathway from DL to the intermediate dorsal pallium, DDi, then to a magnocellular dorsal field, DDmg, and back to DL. Elliott and colleagues proposed that this organization could support operations resembling pattern separation and pattern completion in mammalian hippocampal circuitry [@elliott2017hippocampallike].
The comparison is useful when kept at the correct level. A recurrent fish circuit may solve a related computational problem without reproducing the mammalian trisynaptic circuit field for field. Calling DDi “CA3-like” identifies a testable correspondence. It does not rename the fish structure as mammalian CA3.
Physiology supplies a further test. Wireless recordings from DDi in freely swimming weakly electric fish found sparse activity linked to movement and active electrosensory sampling. Many cells were especially active during backward swimming, a behavior that brings nearby features toward the fish’s electrosensory fovea and increases sensory sampling [@fotowat2019navigation]. The candidate hippocampal circuit was therefore engaged while the animal actively sampled its surroundings during navigation. The experiment did not establish a mammalian-style place-cell code, but it connected the proposed circuit to the behavior for which that code would be useful.
A different preparation has now revealed a population code for position. Tracking microscopy allowed brain-wide calcium imaging while larval zebrafish explored an arena freely. Cells meeting the study’s criteria for place encoding were strongly enriched in the telencephalon. Their collective activity could be used to decode the fish’s location, and the population representation became more orderly with experience. When visual and self-motion cues were manipulated, the cells combined multiple sources of information and could remap between environments [@yang2024zebrafishplace].
The anatomical resolution of that experiment matters. The recordings established a telencephalic spatial code in larval zebrafish; they did not assign each place-encoding cell to an adult Dl subdivision or demonstrate that every such cell belongs to a hippocampal homologue. The result is nevertheless important. A fish telencephalon can construct a distributed, experience-dependent representation of location rather than supporting navigation only through a fixed sensory-response rule. The comparative question shifts from whether spatial population coding exists in teleosts to how that coding is distributed across their transformed pallial territories.
The evidence now converges from several directions. Development explains why the candidate territory appears dorsolaterally. Molecular and cell-type comparisons identify hippocampal-like populations within Dl. Connectional work reveals recurrent pallial loops. Neural activity relates those loops to active sensing, landmarks, and location. Lesions provide the most direct test of what the system makes possible.
44.6 What damage removes: place, time, and relation
The logic of a lesion experiment depends on the comparison condition. An animal that fails to reach a hidden goal may have lost spatial memory. It may also be unable to see, swim, remain motivated, or learn any new response. A useful experiment asks whether the lesion selectively disrupts one way of organizing behavior while sparing another that uses the same sensory and motor equipment.
44.6.1 Place versus cue
Fernando Rodríguez and colleagues trained turtles and goldfish in two navigation problems. In the place problem, the goal occupied a stable location defined by its relation to surrounding environmental cues. Starting position varied, so the animal could not solve the task by repeating one turn or one motor sequence. In the cue problem, a conspicuous local marker directly identified the goal.
Lesions of medial cortex impaired the turtles when they had to navigate from new starting positions to the stable place. The same animals learned the visible-cue problem. In goldfish, lesions of lateral pallium produced a severe place-learning deficit, whereas lesions of medial or dorsal pallium did not produce the same impairment. None of those pallial lesions abolished cue learning [@rodriguez2002conservation].
The spared cue condition is decisive. The animals could perceive a relevant stimulus, approach it, move through the apparatus, and acquire a rule. What they could no longer use normally was the relation among distributed environmental cues that defined a place independently of the current starting point. Reptilian medial cortex and teleost lateral pallium therefore contribute to a map-like form of spatial memory comparable at the behavioral level to hippocampal function in mammals and birds.
That experiment establishes an ancient spatial contribution. It does not establish a navigation-only organ.
44.6.2 Trace versus delay
A second comparison removes physical space from the center of the task. In delay conditioning, a conditioned stimulus remains present until an unconditioned event begins; the two overlap. In trace conditioning, the conditioned stimulus ends, an empty interval follows, and only then does the unconditioned event occur. The animal must preserve a relation between events that are separated in time.
Goldfish with lesions centered in the ventral dorsolateral pallium were severely impaired in trace conditioning but learned delay conditioning. The sensory events, motor response, reinforcement, and general ability to condition were therefore available. The lesion selectively disrupted association across the stimulus-free gap [@gomez2022trace].
The result does not demonstrate human episodic recollection in a fish. Trace conditioning can be acquired without a narrative self or conscious return to a past event. It does show that the teleost hippocampal pallium contributes when separated events must be related across time. A system described only as a spatial navigator is too narrow.
44.6.3 Trained pairs versus a novel relation
A third experiment asked whether goldfish could use overlapping learned relations to guide a choice they had never been taught directly. The fish learned four premise pairs: A was rewarded over B, B over C, C over D, and D over E. Each pair could be learned as a separate discrimination.
The critical test paired B with D. Those two items had never appeared together. A fish that organized the overlapping premises into an ordered relation should prefer B, because B had been rewarded over C and C over D. Sham-operated fish did so. Goldfish with hippocampal-pallium lesions learned the original premise pairs but failed the novel B-versus-D test [@soteloparrilla2025transitive].
Again, the spared learning defines the deficit. The lesioned fish could discriminate the stimuli and store the rewarded member of each trained pair. They were impaired when behavior required the flexible use of relations among those pairs.
The term transitive inference describes the behavioral structure of the test. It should not be inflated into conscious syllogistic deduction. The experiment shows that a fish can express an untrained ordered relation and that hippocampal pallium is required for that flexible expression.
Together, the three lesion contrasts remove different operations while preserving simpler routes to performance. Place learning requires a location defined by relations among environmental cues. Trace conditioning requires a relation across a temporal gap. Transitive inference requires a novel choice based on relations among separately learned pairs. In each case, the hippocampal or medial-pallial lesion leaves some sensory, motor, motivational, and learning capacities intact while removing flexible relational use.
These experiments also illustrate how functional evidence enters a homology argument. The lesions do not prove that turtle medial cortex, goldfish Dl, and mammalian hippocampus descend from the same tissue. A similar behavioral deficit could, in principle, result from damage to independently evolved circuits. The anatomical and developmental comparisons establish the historical candidate. The selective behavioral deficits then test whether the candidate retained a related contribution. Agreement between those independent lines of evidence is more informative than either one alone.
Nor do the three tasks have to depend on one undifferentiated mechanism. Place learning, trace conditioning, and transitive inference differ in sensory demands, time scale, training history, and required output. The important result is not that every relation is processed identically. It is that damage to candidate hippocampal pallium repeatedly impairs behavior when separately encountered elements must be organized and used beyond the immediate cue.
44.7 The oldest map was already relational
Spatial navigation remains the clearest ancient window onto hippocampal function. Space provides an unusually tractable geometry. The animal occupies one location, faces one direction, encounters measurable boundaries, moves through identifiable transitions, and arrives at visible or hidden outcomes. Investigators can vary start points, rotate landmarks, move goals, and ask whether the animal has learned a place or merely a response.
That experimental clarity can be mistaken for evolutionary exclusivity. The place-versus-cue experiments show that flexible spatial organization is old. They do not show that ancestral hippocampal pallium represented space and nothing else. Trace conditioning and transitive inference now make that stronger claim difficult to sustain. Teleost pallium also contributes when relations must be maintained across time or recombined to support an untrained choice.
The better synthesis begins with the operation rather than the textbook category. A hippocampal system differentiates elements while binding them into an organized context. It allows a cue to mean something because of where it occurred, what preceded it, what followed it, or how it relates to other learned items. It supports behavior when the answer is not supplied by one immediately available stimulus.
Navigation is one ecological expression of that operation. A route is a sequence of states connected by possible transitions. A place is defined by relations among boundaries, landmarks, directions, and goals. A shortcut requires the animal to use those relations in a combination it has not directly traversed. The spatial map is already a relational structure.
Temporal association and ordered inference expose the same architecture under different task demands. They do not prove that all hippocampal functions were present in identical form in the common ancestor of teleosts and tetrapods. Human autobiographical memory requires capacities far beyond trace conditioning, including rich perceptual reconstruction, semantic knowledge, language, social identity, and a culturally extended sense of time. The continuity lies in an inherited way of organizing relations, not in a claim that a goldfish remembers a childhood afternoon.
Calling the system relational should not create a new psychological box and then assign that box to one brain structure. Relations are descriptions of task structure: among landmarks, between stimuli separated in time, or across overlapping premise pairs. Different relations can recruit different sensory systems, pallial partners, neuromodulators, and local hippocampal computations. The comparative evidence identifies a recurring demand that exposes the lesion deficit. It does not establish a unitary faculty of “relational memory” stored in one place. This distinction preserves the behavioral construct without reifying it as a self-contained neural module.
This conclusion also changes how human memory should be introduced. Evolution did not build a pure navigation device and later bolt a diary onto it. Nor did it invent a new human organ for episodes. It retained and elaborated an ancient system in which places, cues, events, and transitions could be bound, distinguished, and used beyond the present stimulus. Mammalian and avian circuits added new fields, cell populations, connections, and behavioral capacities. Human cortical and cultural systems expanded what could be supplied to and reconstructed from that architecture.
The oldest map was spatial, but it was not merely a map of space. It was a system for organizing relations so that past structure could guide the next action.
The title of the chapter can therefore remain. Map names the preservation of relations, not a picture projected onto an inner screen. Oldest names the deep ancestry of the system, not a claim that the first vertebrate hippocampus has been recovered as an unchanged object. The modern hippocampal formation is an evolutionary transformation of ancient medial-pallial circuitry.
44.8 Coda: from ancestry to mechanism
This chapter has established the historical identity of the system. Adult shape alone could not do that. Developmental topology linked differently arranged pallial territories. Gene expression and cell types strengthened the correspondence while revising some exact subfield assignments. Connectivity and physiology showed that candidate teleost territories participate in recurrent circuits engaged by active sensing and navigation. Lesions revealed a conserved requirement for flexible relations in space, time, and learned order.
The next chapter, Drawing the Map, turns from ancestry to mechanism. Place, direction, boundary, grid, speed, and event-related activity will not appear as isolated labels pasted onto neurons. They will be treated as interacting population signals through which movement and sensation become organized structure. The question will no longer be how the hippocampal system can be recognized across vertebrates. It will be how that system constructs a world through which an animal can act.
Established findings. Developmental topology, combinatorial gene expression, cell-type comparisons, connectivity, physiology, and lesion effects support an ancient hippocampal or medial-pallial system across tetrapods and teleost fishes, placing that system deep in bony-vertebrate ancestry. Mammalian, avian, reptilian, and amphibian systems derive from comparable medial-pallial territories. In teleosts, partial eversion relocates candidate hippocampal tissue toward the dorsolateral pallium. Damage to reptilian medial cortex or teleost hippocampal pallium disrupts flexible place learning while sparing visible-cue learning. Teleost lesions also impair association across a temporal gap and novel use of overlapping learned relations.
The chapter’s organizing interpretation. Spatial navigation makes an ancient relational operation especially visible. Places, routes, events, and learned orders can all require elements to be bound according to context and transition structure, then used when one present cue does not specify the answer. Human episodic memory elaborates this architecture; it is not a separate faculty pasted onto a completed navigation module.
Open questions. The exact correspondence between teleost pallial subdivisions and mammalian dentate, CA, subicular, and retrosplenial fields remains unresolved. Developmental, transcriptomic, and connectional comparisons do not always produce one-to-one matches, and teleost species should not be forced into a single atlas. The available evidence also does not establish a strict evolutionary sequence in which spatial representation necessarily preceded every nonspatial relational operation. What is well established is the deep conservation of a system whose damage removes flexible use of relations across several domains.