Physio Section 3 Content

Take Home Message for the Comparison of Sex Determination Mechanisms

Mammals are incredibly boring, only have one version of determination, genetic, which limits our diversification. Other groups, however, have been able to become diverse in part due to their varied sex determination strategies

Names of the Parity and Nutritional Modes

Parity Modes:

• Oviparity

• Viviparity

Nutritional Modes:

• Lecithotrophy

• Matrotrophy

Oviparity

Eggs are laid and develop outside of the mother; passive interaction with the environment

ex. frog, turtles, crocs

Viviparity

Eggs develop inside the mother and young are born live; active interaction with the environment

ex. boas, anacondas, viviparous lizard, humans

Lecithotrophy

Nutrition from stored yolk (originally comes from mother)

Matrotrophy

Nutrition from current maternal physiology (often placenta or milk)

What parity and nutritional modes are likely ancestral in vertebrates?

Lecithotrophic Oviparity

How are the developing vertebrate’s needs met in Lecithotrophic Oviparity?

• oxygen: from environment

• food: from the yolk

• calcium: from the yolk

NOTE: a physiological danger of LO is risk of predation during development

Lecithotrophic Oviparity Evolutionary Trade Off Strategies

• balance the amount of yolk v. the number of eggs

• cast and blast strategy: produce many eggs with a small amount of yolk in each

  • okay if some get eaten because you have so many potential offspring

• produce a few eggs with lots of yolk, invest more into each egg

  • Enhancing reproductive success with this strategy

    • Maiden’s purse: encase embryo in a hard keratin structure

    • Mouth brooding: hold eggs in mouth when faced with danger

      • Note: spots on male’s tail often look like eggs, female tries to pick them up and eats his sperm instead

Greatest Threat to the Ancestral Egg on Land (Care Options seen in Lecithotrophic Oviparous Amphibians like Frogs)

Threat: on land, eggs can dry up! 

• Marsupial frog: male carries embryos on his back in a pouch

• Surinam toad: male rolls onto eggs and they are carried around by being embedded in his back skin

• Gastric-brooding frog: males ingest embryos and hold hem in their stomach. levels of stomach acid production decrease during this incubation to avoid eggs being digested

Amniotic Lecithotrophic Oviparity On Phylogenetic Tree and Problem(s) It Solves/Introduces

• Solves water problem of eggs drying up on land, but now oxygen is harder to get from the environment due to the shell and amnion

  • No longer tied to water for reproduction

• Develops in amniota group

Components of the Amniotic Egg

• Membranes

  • Four extraembryonic membranes (yolk sac, amnion, chorion, allantois) that protect, nourish, and support the embryo.

• Yolk Sac

  • Surrounds the yolk.

  • Provides nutrients to the embryo via blood vessels.

• Amnion

  • Fluid-filled membrane around the embryo.

  • Cushions, hydrates, and protects from mechanical shock.

• Chorion

  • Outermost membrane.

  • Main site of gas exchange (oxygen in, CO₂ out).

• Allantois

  • Stores nitrogenous waste (e.g., uric acid).

  • Assists in gas exchange.

  • Contributes to umbilical structures in mammals.

• Shell

  • Protective outer covering (hard or leathery).

  • Prevents desiccation and offers structural support.

  • Porous, allowing gas exchange.

Amniotic Lecithotrophic Oviparity: Egg Size Trade-Offs

Larger eggs face issues due to the SA/V relationship and the intake of oxygen from the environment

• as egg gets larger, the volume needs more oxygen than the SA can allow in

• Solutions:

  • Larger eggs have bigger pores to let oxygen in

  • As eggs get larger, shell thickness is negatively allometric (does not scale up as much)

Amniotic Lecithotrophic Oviparity: Role and Source of Calcium During Development

• Calcium stores, which are necessary for muscle and bone development, are mobilized for the developing embryo

• Eggshel is the storage device for calcium

• Across development, the total amount of calcium in the system of yolk and body increase as calcium is pulled from the shell, as opposed to a non amniotic egg ancestor where total calcium is static and gets pulled from yolk to the body only

Anatomical Adaptation for the Evolution of Licithotrophic Viviparity

Retention of large, yolk-filled sacs within the maternal reproductive tract

Monotremes: What Reproductive Strategy did Mammals Begin With?

• Monotremes represent the ancestors of mammals

• Have Matrotrophic Oviparity Strategy, think of platapi

• Monotremes are mammals with…

  • Eggs

  • Minimal yolk

  • “incubatorium”: where eggs are laid and the site of…

  • lactation: milk delivered

Examples of Matrotrophic Oviparity

• Monotremes like platapi

• some caecilian worms: develop outside the body and feed on the skin of their mother

Matrotrophic Viviparity in Non-Amniotes

• trophonemata: ex. southern stingray

  • glandular filaments that develop from the inner uterine surface and secrete a nutritive fluid for the embryo

• oophagy: ex. sand tiger shark

  • no direct connection to mother but mom provides nutritional support by producing extra eggs which the large embryos consume

• trophotaenia: ex. goodeid teleosts

  • branching, ribbon-like structure that extends from the perianal region of the embryo

  • hindgut-derived pseudoplacenta, which contributes to absorbing maternal nutrients

• oviducalphagy: ex. scolecomorphid caecilians

  • embryos eat the internal organs (oviduct) of the mother

• yolk sac placentation: ex. sharpnose shark

Matrotrophic Viviparity in Mammals

• Physiological needs of oxygen, food, waste, and calcium, are all managed by the placental connection

• Two types of placental mammals:

  • Choriovitelline Placenta: yolk sack placenta

    • ex. marsupials (closest ancestor to eutherians)

    • marsupials have transient yolk sac placentation: develop for a short time via placenta, then are born and live in pouch of the mother (matrotrophic support switches to milk)

  • Chorioallatoic Placenta: placenta comes from a different placental layer, the allantois

    • ex. eutherians, the true placental mammals

How did the placenta evolve? 

• ERVs: endogenous retrovirus penetrates the cell and takes over its DNA

  • ERV docks to the cell and syncytins (an envelope protein) allow it to open up the cell and enter

• In each case of placentation, in the germ line (egg) an ERV got into the egg and gave the genome the envelope proteins

• When envelope proteins, syncytins, are expressed in both the mother and the fetus, they can form a syncytial trophoblast

  • embryo attaches to endometrium (uterine lining)

  • chorioalantoic (fetal) layer gives forth cells that then merge with maternal cells and give rise to the syncytial trophoblast

    • syncytial trophoblast: merging of cells between embryo and endometrium where nuclei are contributed from the mother and the fetus

    • as this layer grows, it develops into the placenta

  • signals to the mother that the embryo and her and one and the same

Where/What is the interface between mother and fetus and what is it’s key component?

• placenta is the interface between mother and fetus

• key component is the chorionic villus: finger-like projections that extend from the chorion, the outermost membrane surrounding the developing fetus

  • covered by syncytiotrophoblast, which facilitates the exchange of nutrients, oxygen, and waste products between the mother and fetus

  • They contain blood vessels that carry fetal blood, allowing for the exchange of substances.

  • Chorionic villi enable the placenta to provide the fetus with oxygen, nutrients, and antibodies and remove waste products and carbon dioxide from the fetal blood.

Role of HPG Axis in Ovulation Occuring

1. Hypothalamus induced (by what we don’t know exactly) to release GnRH

2. The GnRH affects the pituitary to release Luteinizing Hormone and Follicle-Stimulating Hormone

3. LH and FSH stimulate the maturation of a few eggs in the ovary as well as follicle development

4. This developed egg is then released, and the developed follicle becomes the corpus luteum and produces progesterone (which drives the endometrium to thicken in preparation for embryo implantation)

5. The egg travels down the fallopian tube (+ may get fertilized)

   1. If fertilized, the embryo will dock on the endometrium, and development begins

Role of HPG Axis on Ovulation when Low-Dose Estradiol is taken

• the additional hormone taken in sets off the negative feedback loop of the HPG axis

  • little to no FSH and LH will be released, and thus no ovulation will occur

  • this is the basis of oral contraceptives

Role of hCG in Development

• human chorionic gonadotropin is produced by the syncytial trophoblasts after implantation and serves to thicken the endometrium

  • most important during the first trimester, where progesterone is only being produced by ovaries

• in the 2nd trimester: the placenta, the old syncytial trophoblast, then takes over the production of hCG, progesterone, and estrogen

• OTC pregnancy tests sample the amount of hCG, indicate implantation and initial placental formation

• If no implantation, menstruation occurs and the endometrium sloughs off

Evolution Tradeoffs

• Definition: the process through which a trait increases in fitness at the expense of decreased fitness in another trait

• Causes:

  • Resource limitations (energy, habitat/space, time)

  • Physical constraints

• Results: Simultaneous optimization of two traits cannot be achieved

Results of a Tradeoff: Length of Human Gestation: Basics

• human gestation length is shorter than expected for our mass when compared to primates

• Why?

Length of Gestation Tradeoff: Obstetric Dilemma

Basis:

• Childbirth is difficult and risky

• Humans born with brains 30% adults brain size

Phenotype 1: Large Fetus:

• Neoteny: evolutionary process where juvenile traits are retained into adulthood (how we have and keep large brains)

• Being large at birth is generally advantageous

• Humans are very large at birth, especially that we have a really large brain

Phenotype 2: Hip Width and Locomotion

• humans are bipedal, not brachiators

• the wider we make the hips, the more de-stabilized walking becomes (negatively impacts the pendulum movement)

• Larger brained babies require wider maternal hips but hips can only be so wide for us to still be able to walk properly

  • baby must come out before its too big to fit through the birth canal

Human bipedalism constrains brain size, limiting gestation time

Length of Gestation Tradeoff: EGG Theory

Energetic and metabolic constraint on fetal Growth and Gestation

Phenotype 1: Large Fetus:

• Neoteny: evolutionary process where juvenile traits are retained into adulthood (how we have and keep large brains)

• Being large at birth is generally advantageous

• Humans are very large at birth, especially that we have a really large brain

Phenotype 2: Maternal Metabolic Scope

• metabolic scope definition: baseline metabolism compared to the highest metabolism we can reach

• during gestation, a woman’s metabolic scope increases, but it isn’t infinite

• if gestation got any longer, the fetus’ metabolic needs exceed the mother’s metabolic range

• more accepted theory: also is supported by our data from multiples (twins, triplets, etc)

  • multiple fetuses require more energy, which. is why we see gestational time shorten

  • if obstetric theory was correct, 2 fetuses would not impact gestation length

Summary of Obstetric Dilemma and EGG Hypothesis

Obstetric: Out before the hips are too small for an enormous brain to be birthed

EGG: Out before the hungry brain kills the mother

• more accepted theory

Divisions of the Brain: Noting Developmental and Functional Term Differences

Evolutionary Patterns of Brain Region Size in Mammals

• Hindbrain: relatively smaller

• Midbrain: relatively smaller

• Forebrain: relatively larger

  • ESPECIALLY the cerebrum aka telencephalon

  • telencephalon expansion is also seen in birds

Brain Changes Across Phylogenies

• Starting with vertebrates

  • Cephalic expansion (new head)

  • Four-partite brain

  • LGE and MGE

  • Basal ganglia and dopamine system

  • Medium spiny neurons

  • Migration of GABAergic interneurons (telencephalon)

  • Three-layered pallium

  • amgdala (pallial + subpallial)

  • habenula

  • reticulospinal

• Starting with jawed vertebrates

  • Cerebellum

  • myelinating glia

    • the neuroglia oligodendricytes and astrocytes

Cerebellum (“little brain”)

• developed first in gnathastomes

• as many neurons as rest of brain

• monitors sensory input from body and coordinates with outgoing motor commands from cerebrum

• gross motor function

  • posture/balance

  • muscle tone

  • coordination

• damage leads to loss of control on contralateral side

limbic system

• functional compartment of the brain

• cortical/subcortical structures around brainstem; generation/regulating emotions/learning

• key regions

  • hypothalamus

  • hyppocampus

  • amygdala

  • cingulate gryus

  • septal area

What links the two hemispheres of the brain?

• corpus callosum: a thick bundle of myelinated fibers

• damages to one side of the brain can be recovered by the other side of the brain through the corpus callosum’s connection

Phineas Gage

• used as an example of damage to the left frontal lobe (site of the limbic system and damage to this allegedly led to his emotional dysregulation)

• today this idea of localization of function has been disproved, and instead accounts of Gage’s good personality after the accident demonstrate the concept of brain plasticity

How does brain mass scale with body mass across different species?

• Mammals: brain and body size increase proportionally (slope of 1) and have an isometric relationship

• Other fishes: body weight v brain weight is negatively allometric (slope 0.67), as body weight increases, brain weight does not increase by the same amount

Encephalization Quotient (EQ)

Accounts for brain mass in determination of brain size; removes the effect of body size on comparing brain sizes between species

• calculated based on normalized residual (how much value differs from the trend) of general trend

• EQ= Ea(actual value)/ Ee (predicted value)

• Humans have a positive EQ

Alternative Ways to Estimate Brain Size and Complex Cognition

Estimate the size of a brain subunit: the neocortex of mammals (not used much today)

• Rhinal fissure placement: serving as a landmark, particularly in comparative neuroanatomy and paleontology. It marks a crucial boundary between the ancient olfactory cortex (paleocortex) and the newer neocortex (neocortex). By using the rhinal fissure and other sulci, researchers can estimate the relative sizes of different brain regions, such as the neocortex and olfactory lobe, in both living and fossil specimens to understand brain evolution and size.

• Cerebral sulcation: how many crevices (sulci) there are. More sulci, more brain volume and better cognition

Are birds really “bird-brained?”

• birds have smaller brains compared to mammals and less sulci

• BUT

  • very densely packed neurons

  • expanded telencephalon

• pigeons

  • telencephalic fraction (proportion of brain covered by telencephalon) of 0.552

  • loose social interactions (covey), so lower cognitive ability

  • BUT even with telecephalic fraction of 0.5 can be trained to decipher art

• corvids (crows, ravens, jackdaws, jays)

  • some of the smartest birds

  • can handle complex cognitive tasks such as droping walnuts in road to be opened by cars

  • non-mammals with brain expansion also have high level cognition

General v. Special Senses

General sensation: enters the CNS via the sensory component of the general cranial nerves (afferent and efferent fibers): V, VII, IX, X as well as spinal nerves from the rest of the body

• Chemical: taste

• Electromagnetic: thermo-reception

• Mechanical: touch

Special sensation: enters the CNS via special sensory nerves, CN I, II, and VIII. They lack motor components and are located only in the head (afferent, to CNS, NOT efferent)

• Chemical: smell, CN I

• Electromagnetic: vision, CN II

• Mechanical: hearing and balance, CN VIII

Brain Areas for Special Senses

The same basic brain areas that:

1. are stimulated by a particular type of environmental energy

2. transduced in a specialized sense organ

3. reaches the brain via a “dedicated” special sensory cranial nerve

Can be recognized in ALL vertebrate brains

• Smell, CN I: Forebrain

• Vision, CN II: Midbrain

• Hearing and Balance, CN VIII: Hindbrain

Note: for mammals, our enlarged cortex can often intercept these APs going to their intended location

How can you identify the dominant sensory modality of non-mammals?

Perform a size comparison of the special sense referral areas

• Expanded Forebrain: Smell important

• Expanded Midbrain: Vision important

• Expanded Hindbrain: Hearing and Balance important

Note: In mammals, secondary integration in our enlarged cerebrum obscures this pattern

Principle of Labeled Lines for Special Sensation

• each sense has its own pathway back to the brain

• all info goes back to the brain as APs, so which pathway the AP arrives on allows the brain to interpret what sense the AP is coming from

Does an incoming sensory AP reflect the environmental modality that generated it? The APs don’t look any different, but where they go is different

Signal Cascade of Olfaction in Mammals

1. We have nasal turbinates: wafer-like bones that have olfactory epithelial layer (skin) which contains olfactory receptors

   1. Each olfactory receptor is specific to a particular odorant

2. When an odorant binds to olfactory receptor, the receptor changes shape and kicks off a g-protein

3. G-protein triggers a graded potential, and these graded potentials can lead to APs

4. APs generated by olfactory neurons are integrated in glomeruli of the olfactory bulb (get here via CN I)

   1. Each glomerulus receives input from one particular olfactory receptor cell

5. Olfactory info is relayed from olfactory bulb to the olfactory cortex and the hypothalamus and ultimately many other parts of the brain after integration in olfactory bulb

Uses for Olfaction

• near sensing: able to detect what is near you

  • ex. pig sniffing for truffles, kiwi sticking nose in soil to sense insects

• remote sensing: sensing things that are far away

  • ex. pacific salmon smell way back to natal stream and birds smell way back to same island they breed at yearly

• signal territories or sexual receptivity

  • leave odorants and detect them for territory marking

How Well do Humans Smell?

• Evidence that we might not smell well

  • relatively small olfactory bulbs

  • fewer functioning olfactory receptors (ORs)

  • fewer olfactory neurons (ONs)

• Humans don’t rely on olfaction

  • Bipedal: 2 meters away from olfactory environment on the group bc we stand up

  • Don’t sniff deeply very often; we rely on vision and touch to interact with others more

• How well do humans smell when we try?

  • when deprived of other senses and follow 10 m long scent trail…students able to detect and complete trail with 67% accuracy

    • deprived of touch (gloves), vision (blindfold), and hearing (headphones)

    • also able to detect direction (is smell detected in left v. right nose)

Vomeronasal: A sixth sense

• System is keen to/responds only to specific odorants called pheromones

• APs of the vomeronasal organ (VNO) are relayed to the accessory olfactory bulb

  • found in certain mammals, like rats, and very important to reptiles

• Flehmen response: curling of the lips in response to pheromones (typically related to reproductive behavior)

• VNO in humans?

  • thought for a while that humans must have lost VNO

  • Nowadays modern imaging shows most people have VNO but it is very small

  • we do use pheromones to communicate in some sense

  • potentially seen in McClinktock effect where women’s menstrual cycles align in close quarters due to detection of pheromones by VNO

How does the retina generate receptor potentials? 1878 experiments by Wilhelm Kuhne

• Rods and Cones in the retina use pigments to transduce electromagnetic radiation into APs

• Kuhne’s experiment

  • Kuhne ground up rabbit retinas and made a solution with it

  • started in dark, then opened window: solution changed color where light was most intense: retinas use pigments

Role of Rhodopsin in Vision

• Rhodopsin (a pigment) composed of 

  • Retinal (Vitamin A)

  • Opsin (protein)

• When exposed to light, rhodopsin dissociates into retinal + opsin and changes color (“bleaches”)

• This color change allows transduction to occur

Two Vision Pathways: What Happens in the Dark v. What Happens in the Light

In the Dark: “Off Pathway”

• Na+ channels are open (cGMP is bound)

• Na+ entry and K+ exit is countered by Na+/K+ pump

• constant glutamate release

  • up-regulates and excites dark pathway

In the Light: “On Pathway”

• cGMP is enzymatically degraded

• Na+ channels close, and receptor cells hyperpolarize

• reduction of glutamate release

  • glutamate leaking stops: and since glutamate is inhibitory for the on pathway (down-regulates it), the on pathway can now act

Vision: Graded Hyperpolarization to Light

• hyperpolarization response to light is graded

  • light causes hyperpolarization which shuts down glutamate release

Vision: Scheme of Visual Integration for Dark and Light Pathways

1. Receptor potential generated in receptor cells (rods and cones)

   1. no APs, only graded potentials

In the Dark: Glutamate leaks into synapses

• hyperpolarization of ON BPC (bipolar cell)

• depolarization of OFF BPC

• depolarization of OFF ganglion cell (beginning of optic nerve)

In the Light: Glutamate reduced in synapses

• depolarization of ON BPC

• hyperpolarization of OFF BPC

• depolarization of ON ganglion cell (beginning of optic nerve)

Note: each pathway has a dedicated line back to the brain

Integrative Cell Layers in the Retina, Eye Structure, and Blind Spot

1. Surface: Rods (low light) and Cones (high light)

2. Horizontal cell

3. Bipolar Cell BPC arranged under rods and cones

4. Amacrine cell: helps integrate info to ganglia

5. Ganglion cell → Axons of ganglion cell exit as optic nerve, creating a blind spot

   1. For each ganglion, there are hundreds of photoreceptors it gathers info from

Pathways of Visual Information from the Eye

• optic nerve carries light and dark signals to the brain

• non-mammals and non-birds have visual info go straight to the midbrain

• mammals (and birds) have the telencephalon intercept visual information

Vision: Accommodation

• Accommodation (focusing light on the retina) requires:

  • materials of different refractive index (changes the path of light → concentrates protons on retina)

  • oblique orientation of light and refractor

• Refractice Indices

  • air 1.0

  • water 1.3

  • cornea 1.3

  • lens of eye 1.4

  • glass 1.5

  • diamond 2.4

• Terrestrial vertebrates have two refractors: one fixed (cornea) and one variable (lens)

• Aquatic vertebrates only have one refractor, a variable lens

  • bc water and cornea have same refractive index so it is ineffective underwater

Vision: Tuning

• spectral tuning: process of modifying the wavelengths of light that a system absorbs or emits to meet a specific need

• spectral tuning in coho salmon ontogeny: wavelength of max absorbance in cones of different ontogenetic stages/environments

  • alevins freshwater: opsins shift to tune to red wavelengths

  • smolts oceanic: life change of visual environment leads to shft toward blue wavelength bc ocean has less red wavelength

• NOTES

  • close to surface of the water there is full range of light visible

  • humans have red, green, and blue cones

Special Mechanical Sense CN VIII: Transduction Mechanism

1. External structure (hair cells) deflected/deformed

   1. hair cell tissue comes from placodes

2. Deformation opens K+ cation channels

3. Changes membrane potential

Special Mechanical Sense CN VIII: Where are the Hair Cell Placodes Found in Diffferent Species?

• Placodes (form from neural crest cells and result in hair cells) “sink” into the dermis (lateral line and ampullae of Lorenzini) or dermal bone (inner ear) during development

• Lateral Line: mechanical sense

  • ability to detect mechanical disturbances in the water

  • sense aquatic environment and determine velocity in water

• Ampullae of Lorenzini: electroreception

  • sense change in electromagnetic field in water

• Vestibular and Acoustic System of the Inner Ear: mechanical sense

Special Mechanical Sense CN VIII: Hair Cells of the Lateral Line System

• located superficially in frogs

• located linear canals beneath the scales of many “fishes”

  • hair cells contained in pores right under the skin which are open to the aquatic environment

• able to detect mechanical disturbances in the water

• blinded fish can still school if their lateral line system is intact

Special Mechanical Sense CN VIII: Ampullae of Lorenzini

• ability to have passive electroreception

  • can detect muscles contracting: helps find prey hiding or buried in sand

• found in sharks, skates, rays, and chimera

• pits on the noses of sharks are ampullae: pores beneath the skin contain hair cells embedding in conductive jelly

  • conductive gel changes shape, which then causes hair cells to be deformed and release APs

Special Mechanical Sense CN VIII: Acoustic (hearing) and Vestibular (balance) Systems Intro

• located in the INNER EAR: water-based system

  • still water-based in mammals that are terrestrial

• based on fluid movement and the deflection of hair cell cilia

• inner ear structure has not changed much over evolution

  • all based upon the movement of water deflecting hair cells since mechanical energy has to move through water

Special Mechanical Sense CN VIII: Vestibular System

• hair cells in the semicircular canals (anterior, posterior, horizontal) detect angular movements of the head

• hair cells in the maculae of utricle (x-plane) and sacculus (y-plane) detect linear acceleration

Special Mechanical Sense CN VIII: Acoustic System

• mechanical disturbances of the environment, transferred to the endolymph of the cochlea, are detected by hair cells

  • deflection of stereocilia (hair cells) during basilar membrane vibration)

• steps:

  • mechanical energy (sound waves) moves stapes

  • last inner ear bone, stapes, taps on the oval window (a membrane)

  • oval window shakes water and the basilar membrane (where hair cells are mounted)

  • hair cells move and rub against the cochlea (tectoral membrane)

Evolution of Ear from Water to Land: Acoustic Impedance

• acoustic impedance: measure of pressure generated by sound waves that varies with frequency and acoustic medium

• at the air:water interface, less that 1% of sound energy passes to the second medium due to impedance mismatch

  • reflection and refraction of sound energy occurs, energy lost as move from one medium to the next

Evolution of Ear from Water to Land: Ancestral (aquatic) Ears

• fish have inner ears: basically no impedance mismatch because sound goes from water through the water-based inner ear system

  • don’t need an outer ear: use suspensorium (bones in the cheek of fish)

    • hyomandibular = stapes: shakes and sends sound energy sirectly to skull/inner ear

    • articular

    • quadrate

Evolution of Ear from Water to Land: Terrestrial Ears for Amphibians, Reptiles, and Birds

• amphibians, reptiles, and birds have inner and middle ears

  • have to address the impedance mismatch of mechanical energy from the air environment to the water-based inner ear

• surface between environment and middle ear: eardrum (tympanum)

  • concentrates/distills sound energy onto the hyomandibular bone (stapes) of the middle ear

• middle ear: contains hyomandibular bone (stapes) which directly connects to the oval window to address the impedance mismatch

  • middle ear acts as an amplification device

Evolution of Ear from Water to Land: Terrestrial Ears for Mammals

• mammals have inner, middle, and outer ears

• middle ear bones (incus, malleus, and stapes) act as a lever, amplifying force/energy

  • 3-ossicle mammalian ear converts tympanum displacement into smaller oval window displacements of higher force

What is a hominid?

Great Apes: orangutans, bonobos, gorillas, chimps, and humans

Old Idea of hominid evolution vs true evolutionary tree

• Old idea: based on visuals, suggests humans are most separated from other great apes

• True evolutionary tree: humans are most closely related to chimps, and chimps are more closely related to us than other great apes

  • we share a MRCA and 99.9% of genome

What is a hominin?

• lineage that led to modern humans: split from chimps

• modern humans are the only extant hominins, all others are extinct

• homo neanderthalensis very recently extinct: northern europeans may have neanderthal mitochondria because they at some point had neanderthal mother

What makes a human (hominin) different from a non-human hominid?

1. Bipedalism

2. Big brains

   1. Large Energy Budgets

   2. HQ (high quality) diet

   3. Being fat

   4. Shorter gestation

   5. Delayed development

Major Events in Hominin Evolution

• Bipedalism: when split from chimps 7 mya

• 3 mya split from paranthropus to homo species

  • brain expansion

  • childhood (protected development)

  • adiposity (fatness)

  • HQ diets

What was the selective force for bipedalism?

Savannah Hypothesis

• humans all came from Africa, north of equatorial Africa, east Africa

• 10 mil/6mya African ecosystem changed from dense forests to savannah: spreading out resources

• other great apes don’t have to move much to find food, but now we have to move around to find resources

  • fruit main diet for other great apes, very plentiful

• bipedalism has lower COT than quadrupedal locomotion, is more efficient

  • helps us expend less energy now that we have to travel longer distances for food

What was the selective force for brain expansion in hominins?

• brain expansion occurred another 4-5 mya after bipedalism

• originally, we walked around to find the starchiest/most sugary food

  • fruit rarely found

  • transitioned too starchy root organs, but they weren’t incredibly nutrient dense

• 12-15 species all looking for root organs: competition led to a Red Queen Event

  • arms race for increased brain size to have the biggest brain (best cognitive abilities) to be the best at finding food

    • better communication, can find resources more effectively

  • exponential increase in cranial volume

Social Brain Hypothesis: Pros of Having a Larger Brain

Cognitive improvements

• avoid predation

• allomaternal care: care for others’ children

• food and water sharing

• stabilized resources during ecological fluctuation: stash resources for tough times

Social Brain Hypothesis: Trade-Offs of Having a Larger Brain

Brain is very expensive!

• increased energetic costs

• broader diet

• increased adiposity

• shorter gestation (obstetric dilemma)

• delayed development

Social Brain Hypothesis: Trade-Offs of Having a Larger Brain: Increased Energetic Costs

• homo has highest energy expenditure in a day per mass compared to great apes

• because our large brain is very hungry (and also a little due to our increased locomotion)

Social Brain Hypothesis: Trade-Offs of Having a Larger Brain: Broader Diet

• roots organs we used to eat weren’t high enough quality to support our big brain so…

• hominins are the only ones who eat meat: we need a much more HQ diet

Social Brain Hypothesis: Trade-Offs of Having a Larger Brain: Increased Adiposity

Hominins are much fatter than other great apes

• Fat needed to store energy for when food resources are patchy/scarce

• Fat storage provides stability for hungry brain when food resources are low

• High energy expenditure from brain requires us to be much fatter

• Crafty Genotype Hypothesis of adiposity: no matter your environment, you have a gene that will turn on to increase adiposity. Adiposity is encoded for ALL throughout your genome: no single “fat gene”

• Leanest Human = Fattest Primate

  • Male: 3-8% human v. 0.005% bonobo

  • Female: 10-12% human v. 3.6 % bonobo

Social Brain Hypothesis: Trade-Offs of Having a Larger Brain: Shorter Gestation

• longer gestation is understood as a good thing, but our large brain prohibits long gestation time

  • direct trade off

• obstetric dilemma and EEG

Social Brain Hypothesis: Trade-Offs of Having a Larger Brain: Delayed Development

• since we have less gestation time, we are less developed when we are born 

• we need more developmental time when we are young before becoming adults

• in youth, we require a lot of care to support our costly brain while our body doesn’t develop as quickly

• we spend more time developing before reaching adulthood than other species

What have hominins become? In what context did we become this?

We’ve Become…

• clever

• big-brained

• bipedal

• fat

• slow-maturing

Context:

• evolved over course of 8 million years and is now 4 myo

• evolved in this evolutionary context that we no longer live in

  • savannah subsistence hunting

  • resources were scarce

  • activity was required

  • small social units were essential

Hominin Evolution: Recent Changes → Agriculture

10k years ago shifted from hunter-gatherer to agriculture (very recent considering took 8 million years to get to this point)

• evolved to stabilize food resources in changing/fluctuating climates

  • domesticated plants to grow them

• allowed us to live in more urban environments: concentrate living next to food production

  • large social groups introduce disease

• diminished diet variety

Left Africa 100k years ago: Wherever humans went, we established agriculture, shifting diet and way of life

Hominin Evolution: Recent Changes → Industrialization

• all of our production due to machines: agriculture, labor, products we buy/sell

• type of industrialization has changed in past few decades

• completely changed the way we live and communicate

  • mismatches to our ancient environment: affects health

Hominin Evolution: Summary of What Has Changed

Through 8 million years of:

• savannah subsistence hunting

• resources were scarce

• activity was required

• small social units were essential

In last 10k years or even few hundred years:

• Agriculture

• Food on demand (ad libidum)

• Low activity levels

• Huge, complex social units (cities and towns)

Difference between Aging and Senescence

Aging: change in phenotype with age (neutral)

• note: from here on our when we say aging we really mean senescence

Senescence: functional deterioration with age in absence of disease

• deleterious effects of aging without disease

• as we senesce, function starts to decline

• decrease function and reproductive success

• increase morbidity and mortality

Universality of Aging

• no evidence that prokaryotes undergo senescence

• populations of single-celled eukaryotic organisms are immortal

• in multicellular organisms, senescence occurs in those that undergo somatic (body) cell differentiation

Primary Senescence Process

• deterioration of function over time in the relative absence of disease

• influence maximum life span: senescence limits lifespan bc function will deteriorate so much so that we die

• underlying causes of senescence across species

Pace of Senescence

1. Rapid: occurs abruptly after maturation (ex. nematodes, flies) or soon after reproduction (ex. annual plants, Pacific salmon)

2. Gradual: slow but persistent deterioration after maturation (all placental mammals, birds)

3. Negligible: no clear evidence for post-maturation increases in mortality (ex. clams, trees, most “fishes”, reptiles)

Who Lives the Longest (Avoids the Terminal Effects of Senescence)?

Blue Zones: exceptional number of 100 year olds

1. Okinawa (Diet)

2. Sardinia (Genetics/Diet)

   1. genetics: founder effect → founded 2k years ago by settlers with really good genes

3. Loma Linda, CA (Social)

   1. same US healthcare system but have social system of families caring for older members in their nuclear families

Japanese Women

• Have been adding 3 months of expected life each year since WWII

  • (those born in 2025 expected to live 3 mo longer than those born in 2024)

• But: Does living longer compress morbidity or do we just live longer with function loss?

  • Morbidity: total loss of some important function (that occurs as result of senescence)

Theories of Aging: Cumulative Oxidative Stress

• Proximate Theory: How? of Aging

• when we metabolize and burn fuel (oxidize) this produces free radicals which are dangerous and compromise the functioning of cellular machinery

• longer we live, the accumulation of oxidative stress on tissues affects aging

Theories of Aging: Genetic Mechanisms

• Proximate Theory: How? of Aging

• Support for genetic mechanisms being an important proximate factor

  • High conservation of maximum life span between species

  • similarity of attained age between monozygotic twins compared to dizygotic twins

  • examples of exceptional longevity within families

  • aging features in human progeria genetic symptoms (see chart)

Theories of Aging: Somatic Division and Telomerase

• Proximate Theory: How? of Aging

• When we engage in mitosis, issues can arise with telomerase (which repairs damage at the end of the genome, protecting it from shortening)

  • low levels of telomerase result in telomere shortening and effects on cellular function

  • when less telomerase is present, certain diseases associated with aging can occur (see chart)

  

• Replication Potential of Human Cells

Theories of Aging: Evolutionary Mechanisms

• Ultimate Theory: Why of Aging

• Antagonistic pleiotropy: genes with early benefits are deleterious later

  • ex. genes producing testosterone are later in life implicated in prostate cancet

• Mutation accumulation: decline of selection against deleterious mutations after reproduction

  • if deleterious mutations develop after reproduction, there is no way for natural selection to act on it because it has already been passed on to future generations

• Disposable body: body is useless after reproduction

  • in an evolutionary sense, you’ve done what you need to do (especially after you’re done raising children in people)

  • 2 main periods of senescence (40s and 70s): 40s matches theory bc that’s after you have children

Interventions for Senescence: Diet

• caloric restriction extends average and maximum lifespan by 30-40% if initiated in early adulthood and by 20% in early middle age

  • effect seen in a variety of texa including rodents, fish, flies, and worms

• Primate Study: one given calorie restricted diet and other given SAD (Standard American Diet) 

  • senesce faster on SAD, body loses ability to fight disease so more instances of cancer and diabetes seen 

How does a CR (calorie restricted diet) reduce age-related declines in function?

• reduced temperature

• lower insulin and increased repair

• reduced ROS (free radicals: reactive oxidative species) and reduced DNA damage

• cellular hormesis (low-dose hunger stress is beneficial)

  • hormesis: process in a cell or organism that exhibits biphasic response to exposure to increasing amounts of a substance or condition

Mismatch Hypothesis

many illnesses arise because we are adapted to very different conditions, so a trait in our current environment is mismatches

• when a mismatch occurs, either the environment needs to change again, organism needs to evolve, or organism declines/goes extinct

• the way we now work, live, and feed have evolved fairly recently, and we are not adapted to these circumstances

  • only 600 generations ago we were pre-agricultural and pre-industrial

Example of Pre-Agricultural Human Existence Today: Hadza

• modern link to human existence and survival abandoned by most of humanity

• hunter-gatherer society

• no domesticated livestock, o large-scale agriculture (they grow and store their own food)

• does not imply simplicity or unsophistication: they choose to maintain the ancestral lifestyle

Other Influences on Obesity Besides Diet

• socioeconomic status

• built environment

• physical inactivity/sedentary activity

• genetic/antenatal factors

Diet has a profound impact on our health BUT diet is just one factor that leads to poor health outcomes: too complex to shame or judge those who experience those outcomes