Transcript Development ch. 42

Animal Development
Chapter 42
Principles of Animal Development?

Development – where multicellular organisms grow
and increase in organization and complexity
◦ Development begins with a fertilized egg and ends with a
sexually mature adult

Three principle mechanisms:
◦ Individual cells multiply
◦ Some of their daughter cells differentiate, or specialize in both
structure and function
◦ As they differentiate, groups of cells move and become
organized into multicellular structures
From identical to differentiated

All of the cells of an individual animal’s body are
genetically identical in an animal’s body,

How can they differentiate into different structures with
distinct functions?

The solution is that different genes in different places
and at different times in an animal’s body, are active.
Indirect and Direct Development

Baby mammals and reptiles are miniature versions of
the adults of their species, undergoing direct
development

The majority of animals species undergo indirect
development - the newborn has a very different body
structure than the adult
Indirect Development

Animals undergo a radical change in body form

Amphibians - frogs and toads, most invertebrates

Females produce huge numbers of eggs, each containing a
small amount of food reserve called yolk

The yolk nourishes the developing embryo until it hatches
into a small, sexually immature feeding stage called a larva

Parents provide these vulnerable offspring with neither food
or protection from predators, most die in their larval stage

After feeding for weeks to years, a handful of survivors
undergo a revolution in body form - metamorphosis, and
become sexually mature adults
Butterflies and Larvae

Most larvae look very
different from adults, but
also play different roles in
their ecosystems

Most adult butterflies sip
nectar from flowers and
unintentionally pollinate
the flower in return

Their caterpillar larvae
munch on leaves, often of
specific host plants
Cicadas

We regard the adult form
as the “real animal” and
the larval stage as
“preparatory”

Most of the life span of
some animals – insects is spent in the larval form

North American periodical
cicadas spend 12- 16 years
as underground larvae,
sucking juices from plant
roots, and only 4-6 weeks
as adults, mating and laying
eggs
Direct Development

Newborn animals
resemble miniature
adults

Snails and fish, all
mammals, reptiles
(including birds),
undergo direct
development

As the young animal
matures, it may grow
bigger, but does not
fundamentally change its
body form
Two Direct Development Strategies
 Juveniles are typically much larger, so they need more
nourishment before emerging into the world

Two strategies have evolved that meet the embryo’s
food requirement:
◦ Birds, most reptiles, and a few fish produce eggs that
contain large amounts of yolk
◦ Mammals, snakes, and a few fish have little yolk in
their eggs, embryos are nourished within the
mother’s body
Thank your mom

Providing food for directly developing embryos places
great demands on the mother
◦ Many offspring, - of birds and mammals - require additional care
and feeding after birth, placing additional demands on one or
both parents
◦ Relatively few offspring are produced, but a higher proportion
reach adulthood, because the parents devote more resources to
each individual

Most of the mechanisms of development—the control
of gene expression to allow differentiation of individual
cells and entire body parts—are fundamentally similar in
vertebrates and invertebrates, and in animals with
indirect or direct development
Cleavage of the Zygote

Cleavage of the zygote
begins development

The formation of an
embryo begins with
cleavage – a series of
mitotic cell divisions of the
fertilized egg or zygote

The zygote is a large cell
Morula

During cleavage, there is
little or no cell growth
between cell divisions

As cleavage progresses,
the available cytoplasm
is split up into ever
smaller cells that
gradually approaches
the size of cells in the
adult

Eventually a solid ball of
small cells, the morula,
is formed
Blastula

A cavity opens within the
morula and the cells become
the outer covering of a hollow
structure, the blastula

The details of cleavage differ
by species and are partly
determined by the amount of
yolk, which hinders cytokinesis
◦ Eggs with large yolks (hen’s
egg) don’t divide all the way
through; nevertheless
◦ In birds and other reptiles,
the blastula is flattened on
top of the yolk
Gastrula

The location of cells on the
surface of the blastula
forecasts their developmental
fate in the adult

Gastrulation begins when a
dimple - the blastopore forms on one side of the
blastula
◦ Surface cells migrate
through the blastopore in a
continuous sheet
◦ The resulting indentation
enlarges to form a cavity
that will become the
digestive tract
Gastrulation

The migrating cells form three tissue layers in
the embryo
◦ Cells that move through the blastopore to line the
future digestive tract are called endoderm, it also
forms the respiratory tract lining, liver and pancreas
◦ The cells remaining on the outside form the
ectoderm and form surface structures such as skin,
hair, nails, and nervous system
◦ Cells that migrate between the endoderm and
ectoderm form the third layer, - mesoderm - which
forms structures between these two, the muscles,
skeleton, and circulatory systems
Gastrulation in the Frog
Animation: Overview of Animal
Development
Organogenesis

Adult structures develop during organogenesis
Organogenesis - development of adult
structures from embryonic tissue layers
 Two major processes:

1. A “master switch” genes turn on and off, each
controlling the transcription and translation of the
genes involved in producing each structure
2. Organogenesis prunes away excess cells
Organogenesis

This sculpting requires death of excess cells

Embryonic vertebrates have more motor neurons than
adults
◦ Embryonic motor neurons are programmed to die unless they
form a synapse with a skeletal muscle, which releases a chemical
that prevents the death of the neuron

All amphibians, reptiles, and mammals pass through
embryonic stages in which they have tails and webbed
fingers and toes
◦ In humans, these stages appear during the fourth to seventh weeks
of development
◦ A few weeks later, the cells of the tail and webbing die; the tail
disappears, and the hands and feet have separated fingers and toes
Extraembryonic Membranes

Development in reptiles and mammals depends on
extraembryonic membranes
◦ Fish live and reproduce in water, by spawning
◦ Amphibians spend adult lives on land, lay their eggs in water
◦ In both, the embryo obtains nutrients from the yolk of the egg,
oxygen from water, and releases its wastes into water

Terrestrial vertebrate life was not possible until the
evolution of the amniotic egg
◦ First in reptiles, persists today in reptiles, birds and their
descendents, the mammals
◦ It allows these groups to complete their development in their
own “private pond”
The Amniotic Egg

The amniotic egg is characterized by four
extraembryonic membranes –
◦ The chorion, amnion, allantois, yolk sac
◦ In reptiles ◦ The chorion lines the shell and exchanges O2 and CO2
between the embryo and the air
◦ The amnion encloses the embryo in a watery environment
◦ The allantois stores and isolates wastes
◦ The yolk sac contains the yolk
Placental Mammals

In placental mammals—except marsupials—the
embryo develops within the mother’s body until
birth
◦ Marsupials - Kangaroos and monotremes (platypuses)
◦ All four extraembryonic membranes are essential for
development
Differentiation of Cells

A zygote contains all genes needed to produce
an entire animal
◦ Every cell of the body contains all of these genes—
that makes cloning possible
◦ In any given cell some genes are expressed, others are
not
 The differentiation of cells during development
happens because of differences in gene expression
Controlling Gene Expression

Cells have ways of controlling gene expression, ie
regulating which genes are transcribed into mRNA
◦ Transcription factors bind to DNA near the
promotor regions, where gene transcription begins
◦ Different transcription factors bind to different genes
and turn their transcription on or off
◦ Which genes are transcribed determines the
structure and function of the cell
◦ This leads to one of the central questions about
development: What causes different cells to
transcribe different genes?
Animal Development

Molecules positioned in the egg and produced by
nearby cells control gene expression during embryonic
development
◦ In animal embryos, the differentiation of individual cells and the
development of entire structures are driven by one or both of
two processes:
 The actions of gene-regulating substances inherited from the
mother in her egg
 Chemical communication between the cells of the embryo
Maternal Molecules in the Egg

May direct early embryonic differentiation

All of the cytoplasm in a zygote was in the egg before
fertilization, sperm only contributed a nucleus

In most invertebrates and some vertebrates, specific
mRNA and protein molecules are concentrated in
different places in the egg’s cytoplasm during oogenesis

Some of these proteins are transcription factors that
regulate which genes are turned on and off
How cells divide is important

During the first cleavage divisions after fertilization, the zygote and
daughter cells divide at specific places and orientations

As a result, these cells receive different maternal mRNAs and
transcription factors

So, different cells transcribe different genes, start differentiating into
distinct cell types and ultimately give rise to specific structures

The positioning of maternal molecules in some eggs so strongly
controls development, that the egg can be mapped according to
the major structures that will be produced by daughter cells
inheriting each section of cytoplasm
A “Fate Map” of the Sea Squirt Egg
Mammal Embryos

In mammal embryos, current evidence indicates that all
cells formed during cleavage are functionally equivalent
◦ Which cells give rise to which parts of the embryo appears to
be a matter of chance, depending where the cells happen to be
located during the transition from the morula to the blastocyst
Chemical Communication
Regulates embryonic development
 Induction - in animal embryos, the developmental fate
of each cell is determined by chemical interactions
between cells
◦ Cells release chemical messengers that alter the
development of other, nearby cells
◦ Specific groups of genes are selectively activated in
the recipient cells, causing them to differentiate

 In amphibian embryos, a cluster of cells near the blastopore,
called the organizer, determines whether nearby cells will
become ectoderm or mesoderm and where the head and
nervous system will form
Organizer Proteins
◦ Organizer proteins interact with other messengers to
stimulate or repress the expression of genes in
nearby cells, often genes that encode transcription
factors and that therefore exert widespread effects
on gene expression
◦ Which genes are expressed determines the
structures and functions of the cells
◦ As these cells differentiate, they release other
chemicals that alter the fate of still other cells, in a
cascade that culminates in the development of the
tissues and organs of the adult body
Homeobox Genes

Regulate development of entire segments of the body
◦ Although their functions differ in different animals, homeobox
genes code for transcription factors that regulate the
transcription of many other genes
◦ Each homeobox gene has major responsibility for the
development of a particular region in the body
◦ Homeobox genes were discovered in fruit flies, where specific
mutations cause entire parts of the body to be duplicated,
replaced, or omitted
 For example, one mutant homeobox gene causes the
development of an extra body segment, complete with an
extra set of wings
◦ Homeobox genes are arranged in a head-to-tail order
Fruit Fly Homeobox Genes
◦ Early in development,
homeobox genes are
transcribed in a specific
sequence within the animal
body
◦ “head” homeobox genes
are transcribed in the
anterior part of the embryo,
“tail” homeobox genes are
transcribed in the tail
How Do Humans Develop?

Controlled by the same mechanisms as in the
development of other animals

Differentiation and growth are rapid for the first 2 mo.
◦ A human egg is usually fertilized in the uterine tube and
undergoes a few cleavage divisions there, becoming a morula on
its way to the uterus
◦ By the 5th day after fertilization, the zygote has developed into a
hollow ball of cells, known as a blastocyst (mammalian version
of a blastula)
 A blastocyst is an outer layer of cells surrounding a cluster of
cells called the inner cell mass; the outer cell layer becomes
the chorion
 The outer cell layer attaches to, then burrows into the
endometrium of the uterus,= implantation
The Journey of the Egg
day 3
(b) An egg within the uterine
tube
day 2
day 4
day 1
4 cells
2 cells
morula
day 7
blastocyst
inner cell
mass of
blastocyst
zygote
Fertilization occurs
within the uterine
tube
embryo
sperm
day 0
The blastocyst
implants in the
uterus
ovary
ovulated egg
muscle layer
endometrium
(a) The first week of development
uterine wall
Animation: Fertilization
Week 1

The chorion and endometrium form the placenta
◦ The chorion secretes chorionic gonadotropin (CG), which
prevents the death of the corpus luteum
◦ The corpus luteum sustains pregnancy by secreting
progesterone and estrogen for the first couple of months, until
the placenta takes over

All the cells of the inner cell mass have the potential to
develop into any type of tissue
◦ This allows the inner cell mass to produce the entire embryo
and the three remaining extraembryonic membranes
◦ The inner cell mass is also the source of human embryonic stem
cells, may be used to replace damaged adult tissues
A Blastocyst Implants
outer cell layer
(future chorion)
cavity
inner cell
mass
endometrium
(uterine lining)
(a) Early blastocyst
yolk sac
chorion
embryonic disk
(future embryo)
endometrium
amniotic cavity
amnion
(b) Late blastocyst
Week 2

After implantation, two cavities form and
gastrulation occurs
◦ During the 2nd week, the inner cell mass grows and
splits, forming two fluid-filled sacs that are separated
by a double layer of cells called the embryonic disk
 One layer of cells is continuous with the yolk sac, in
placental mammals it contains no yolk
 The second layer of cells is continuous with the
amnion
Week 3
◦
Gastrulation begins near the end of the 2nd after
fertilization, and is usually complete by the end
of the 3rd week
 The two layers of the embryonic disk separate
slightly,and a slit forms in the cell layer on the
“amnion side” of the disk
 Cells migrate through the slit into the interior of the
disk, forming mesoderm, endoderm, and the fourth
extraembryonic membrane, the allantois
 The cells remaining on the surface become ectoderm
3-4 weeks
 3rd week, the embryo begins to form the spinal cord and brain





The heart expands and becomes muscular, starts to beat at the
beginning of the 4th week
During the 4th week, embryo bulges into the uterine cavity, bathed
in fluid contained within the amnion
The umbilical cord forms from the fusion of the yolk sac to the
embryonic digestive tract
The body stalk contains the allantois, which contributes the blood
vessels that will become the umbilical arteries and vein
The umbilical cord now connects the embryo to the placenta,
which has formed from the merger of the chorion of the embryo
and the lining of the uterus
Human Development in the 4th Week
location of
the developing
embryo in
the uterus
chorion
embryo
placenta
chorionic villi
body stalk
yolk stalk
yolk sac
amnion
form the umbilical cord
4-7 weeks
◦
During the 4th and 5th weeks, the embryo
develops a tail and pharyngeal (gill) grooves
 These structures are reminders that we share
ancestry with other vertebrates that retain their gills
in adulthood
 In humans they disappear as development continues
◦ By the 7th week, the embryo has rudimentary eyes, a
rapidly developing brain; the webbing between fingers
and toes is disappearing
A 5-Week-Old Human Embryo
pharyngeal grooves
8 mm
arm bud
heart
eye
leg
bud tail
umbilical
cord
brain
2 months

At the end of the 2nd month, nearly all major organs
have begun to develop
◦ Structures like the arms and legs are recognizably human
◦ The gonads appear and develop into testes or ovaries

The first two months of pregnancy are a time of rapid
differentiation and growth for the embryo and a time of
danger
◦ Although vulnerable throughout development, the rapidly
developing organs are extra sensitive to toxic substances

Sex hormones are secreted. These hormones affect
future development of the embryo - the reproductive
organs, brain and other parts of the body
An 8-Week-Old Human Embryo
30 mm
umbilical
cord
placenta
amniotic sac
The last 7 months

Growth and development continues
◦ The brain continues to develop rapidly and the head remains
disproportionately large
◦ The lungs, stomach, intestine, and kidneys enlarge and become
functional, although they will not be used until after birth

As the brain and spinal cord grow, they begin to
generate behaviors
◦ As early as the 3rd month of pregnancy, the fetus can move and
respond to stimuli
◦ Some instinctive behaviors appear, such as sucking
Premature Births


Nearly all fetuses 32 weeks or older can survive outside
the womb with some medical assistance
Heroic measures can often save infants born as early as
26 weeks, although more mature fetuses have a much
greater chance of healthy survival
A Calender of Development from
Zygote to Birth
week 1
week 2
week 3
week 4
zygote to late blastocyst
week 5
week 6
0.32–0.43 inch
(8–11 mm)
embryo
blastocyst
zygote
morula
Cleavage of zygote
forms the morula and
then the blastocyst,
which implants in the
uterus.
late blastocyst
The blastocyst burrows
into the endometrium;
forms the yolk sac,
amnion, and embryonic
disk.
0.06–0.1 inch
(1.5–2.5 mm)
0.12–0.20 inch
(3–5 mm)
0.28–0.35 inch
(7–9 mm)
Gastrulation occurs;
the notochord and
beginning of the
neural tube form.
The neural tube closes;
arm buds, tail, and
pharyngeal (gill)
grooves form; the heart
beats.
The eyes begin to
form; leg buds form;
the brain enlarges.
External ears and
webbed fingers
form; the
pharyngeal (gill)
grooves and tail are
disappearing.
A Calender of Development from
Zygote to Birth
week 7
week 8
week 10
embryo
week 12
week 16
fetus
0.67–0.79 inch
(1.7–2.0 cm)
0.90–1.10 inches
(2.3–2.8 cm)
Webbed toes form;
bones begin to stiffen;
the back straightens;
the eyelids begin to
form.
All the major organs
begin to form; the arms
can bend; fingers are
distinct. Facial features
and outer ears take
shape.
1.25–1.75 inches
(3.2–4.4 cm)
After 8 weeks, the embryo
is called a fetus. Red blood
cells form; toes separate;
eyelids have developed;
major brain parts are present;
the hands can form fists.
2–3 inches (5–7.6 cm)
The neck is well defined; all
organs are present; male or
female genitals are present;
arms and legs move; teeth
begin to form; a heartbeat can
be detected electronically.
4–5 inches (10.2–12.7 cm)
Sucking and swallowing movements
occur; the liver and pancreas begin
functioning. The body has grown
relative to the head; major organs
continue developing. The mother may
feel movement; weight is about 5 oz.
A Calender of Development from
Zygote to Birth
week 20
week 24
week 30
week 36
fetus
6–7 inches (15.2–17.8 cm)
The fetus may suck its thumb;
arms and legs can punch and
kick; the body can change
position. Fingernails are formed;
fat is deposited under the skin;
eyebrows and eyelashes appear.
8–9 inches (20.3–22.9 cm)
Brain development continues,
hearing develops, and the eyes can
move. The fetus can hiccup, squint,
smile, and frown. The fetus may
have hair on its head. Unique footand fingerprints appear. Weight is
about 1–1.5 pounds.
15–16 inches (38.1–40.6 cm)
Brain development continues; the
eyes open and close and see light;
the fetus kicks and stretches.
Breathing movements occur but the
lungs are not mature. Bones are
present but flexible. The baby may
survive if born.
16–19 inches (40.6–48.3 cm)
Eyes open and close corresponding to
wake and sleep cycles; body fat
increases; lungs and other organs are
functional. The child can grasp and orient
toward light. Weight is about 5–6 pounds,
and the child is no longer considered
premature if born. Full term is 38 weeks.
The Placenta and Nutrient Exchange

The placenta exchanges materials between mother and embryo
◦ During the first days after implantation, the embryo obtains
nutrients directly from the endometrium
◦ During the following week, the placenta begins to develop from
the interlocking structures produced by the embryo and the
endometrium
◦ The outer layer of the blastocyst forms the chorion, which
grows fingerlike chorionic villi that extend into the
endometrium
Blood Flow of the Placenta

Blood vessels of the umbilical cord connect the embryo’s
circulatory system with a dense network of capillaries in the villi

Implantation erodes some of the blood vessels of the
endometrium, producing pools of maternal blood that bathe the
chorionic villi

The embryo’s blood and the mother’s blood remain separated by
the walls of the villi and their capillaries, so the two blood supplies
do not mix

This permits many small molecules to move between the mother’s
and embryo’s blood
What moves between embryo and mother
◦ Oxygen diffuses from the mother to the embryo
◦ Nutrients, aided by active transport, travel from the mother to
the embryo
◦ CO2 and wastes diffuse from the embryo to the mother blood
◦ The membranes of the capillaries and chorionic villi act as
barriers to some substances, including large proteins and cells
◦ Some disease-causing organisms and harmful chemicals, like
alcohol, can penetrate the placenta
The Placenta
placenta
maternal
venule
fetal
umbilical
vein
chorionic
fetal villi
capillaries
fetal umbilical cord
umbilical
arteries
endometrium
maternal
arteriole
(amniotic fluid)
(pool of
maternal
blood)
amnion
fetal
chorion
uterine
muscle
The Placenta Secretes Hormones

By the end of the first trimester, the placenta
secretes estrogen and progesterone, enough to
sustain its own growth and development

At this time, the corpus luteum degenerates
◦ These hormones stimulate development of the
mammary glands
◦ Progesterone also inhibits premature contractions
of the uterine muscles
Labor and Delivery



The fetus positions head down in the uterus, with the
crown of the skull resting against the cervix
Childbirth begins around the end of the 9th month
Birth results from a complex interplay between:
◦ Uterine stretching caused by the growing fetus
◦ Maternal and fetal hormones trigger labor, contractions of the
uterus result in delivery
◦ Unlike skeletal muscles, smooth muscle of the uterus can
contract spontaneously, stretching enhances their tendency to
contract
 The uterine muscles occasionally contract before delivery
◦ No one knows what triggers labor in humans, but chemical
signals from the placenta and maturing fetus may be involved
◦
Whatever the initial stimulus, the placenta releases
prostaglandins, which make the uterine muscles more
likely to contract
 As the uterus contracts, it pushes the fetus’ head
against the cervix, stretching it
 This has two effects:
 The cervix expands so the fetus’ head can fit
 Second, stretching the cervix sends signals to the
mother’s brain, causing release of oxytocin
Its positive feedback
◦
Oxytocin stimulates contractions of the uterine
muscles, pushing the baby harder against the cervix,
which stretches further, causing still more oxytocin
to be released
◦
This positive feedback cycle continues until the
baby emerges
◦
After a brief rest following childbirth, the uterus
resumes its contractions and shrinks
◦
◦
During these contractions, the placenta is separated
from the uterine wall and expelled
Human Childbirth
1 The baby orients
head downward,
facing the mother’s
side; the cervix
begins to thin and
expand in diameter
(dilate)
2 The cervix dilates
completely to 10
centimeters (almost
4 inches wide), and
the baby’s head
enters the vagina, or
birth canal; the baby
rotates to face the
mother’s back
3 The baby’s
head emerges
4 The baby rotates
to the side once
again as the
shoulders emerge
Cutting the Cord

The umbilical cord releases prostaglandins that cause
the muscles surrounding fetal blood vessels in the
umbilical cord to contract and shut off blood flow

Although tying off the umbilical cord is standard
practice, it is not necessary; if it were, other mammals
would not survive birth
Milk Secretion

Stimulated by the hormones of pregnancy
◦ Large quantities of estrogen and progesterone are
secreted by the placenta
 Acting together with other hormones, they
stimulate the milk-producing mammary glands in
the breasts to grow, branch, and develop the
capacity to secrete milk
◦ The mammary glands are arranged in a circle around
the nipple, each with a milk duct leading to the nipple
The Structure of the Mammary Gland
muscle
fat
suspensory
ligaments
mammary glands
milk duct
nipple
Lactation
◦ Prolactin, anterior pituitary gland, promotes both
mammary gland development and the actual secretion
of milk, lactation
◦ Prolactin release is stimulated by high levels of
estrogen produced by the placenta, so you might
think that milk secretion would begin even before
childbirth
◦ However, lactation is inhibited by progesterone, also
secreted by the placenta
 During childbirth the placenta is ejected
 Progesterone levels plummet, allowing prolactin to
cause lactation
Milk Production

The infant’s suckling stimulates nerve endings in the
nipples, which signal the hypothalamus to cause the
pituitary gland to release an extra surge of prolactin and
oxytocin and milk is released
◦ Oxytocin causes muscles surrounding the mammary glands to
contract, ejecting the milk into the ducts that lead to the nipples
◦ The prolactin surge stimulates rapid milk production for the
next feeding

The first few days after birth, the mammary glands
secrete a yellowish fluid called colostrum
 High in protein and contains antibodies
 Is gradually replaced by mature milk, which is higher in fat and
milk sugar (lactose) and lower in protein
Aging, the Final Stage of Human
Development

Aging - accumulation of damage to biological
molecules, particularly DNA the nucleus and
mitochondria, results in defects in cell functioning,
declining health, and death
◦ This damage from natural errors in DNA replication, radiation,
chemicals in food, cigarettes, and industrial products—begins at
fertilization
◦ Many biological molecules are harmed by free radicals, some
produced by environmental pollutants, but most by energygenerating reactions in our cells, especially in mitochondria
◦ Some molecular injury can be tolerated
 Young bodies may be able to repair damage or compensate
 As an individual ages, its repair abilities diminish; eventually the
body’s tolerance for damage is exceeded
◦
Aging is manifested in many ways
Muscle and bone mass is lost
 Skin elasticity decreases


Reaction time slows

Senses, such as vision and hearing, become less acute

A less-robust immune response renders the aging
individual more vulnerable to disease

Eventually, the individual can no longer fight off natural
assaults, and dies
Youth Meets Age
How old is OLD?

For thousands of years, people have attempted
to delay aging and extend life span
◦ Modern care can prevent or cure many diseases and
can fix or replace some damaged organs
◦
◦ Some dietary changes, particularly not eating very
much food can prolong life, at least in animal
experiments

However the maximum human life span, about
130 years, has not changed
Evolutionary Hypotheses

Evolutionary hypotheses suggest that aging is unavoidable
 Natural selection favors organisms that leave the largest number of
healthy, successful offspring
 Even a hypothetically immortal animal that wouldn’t die from
“inside” so to speak, will eventually succumb to predation, accident,
or disease
◦
Perhaps natural selection favors devoting more of the body’s
resources to reproduction than to the continuous bodily repair
required for immortality
◦
The fact that humans can live so long after they stop reproducing is
probably evidence of the selective advantage conferred by the care and
teaching given to the young by their elders