THE FRONTIERS OF LIFE
BARRINGTON'S LECTURES ON PHYSIOLOGY AND DEVELOPMENT IN THE LIGHT OF
EVOLUTION
BCOR 12
David Barrington, 2008
Most of what I want to teach in this
fourth portion of BCOR 12 relates one way or another to activity at
cell membranes, especially as a frontier between the inside and outside
of cells, so we have both pursued course development with this
mindset.
I will use the frontier theme throughout, with the
frontiers including those 1) between cells of the same organism, 2)
between cells and the environment, and 3) between cells of different
individuals (mostly of the same species). In the lecture
sequence, this whole body of ideas is preceded by two introductory
segments, the first on the origin of the phsopholipid bilayer and the
second on the origin or multicellularity. The origin of the
bilayer relates directly to the origin of life, which I find compelling
in its relevance to thinking about origins in general. The origin
of multicellularity was required for interactions between cells of the
same organism to begin in the first place. These same
interactions relate to the contacts between cells of unrelated species
(self and non-self). The specialization that comes with
multicellularity in turn implies a developmental program, which
provides me an avenue to the very exciting field of development and
evolution. Because evolution is my own research discipline, I
will cast everything possible in the context of evolutionary
history. The role of the environment in shaping
evolutionary history allows me to bring ecological factors, which
completes the progress of this syllabus towards a fully integrated
approach to biology.
This version of this file was put
together for Dave Kerr and Don Stratton in September of 2008.
In this first part of my lecture, I review membrane structure in
function as a way to introduce the most critical frontier in
biology. The structural and functional ideas that we introduce
here return again and again as we consider the diversity of topics
related to the physiology and development of animals and
plants.
A. Membrane Structure
1. Amphiphilic phospholipids self assemble into
envelopes that are barriers to polar molecules.
2. The fatty acids give them their non-polar
character; the phosphate and R groups such as choline give them their
polar character.
3. The membrane envelope is fluid – laterally but not
flip-flop (tangentially but not radially).
B. Membrane Proteins and
their Functions
1. Proteins are positioned in the membrane by the
location of their non-polar amino acids.
2. The proteins are also fluid.
3. Membrane protein functions fall in six classes
(our examples in parentheses):
a. Transport
(channels and pumps)
b. enzymatic activity
c. signal transduction (tyrosine kinase)
d. cell-cell recognition
e. intercellular joining
f. attachment to the cytoskeleton and extracellular
matrix (integrin)
4. The channels escort polar molecules, especially
ions, across the non-polar interior in the direction of lower
concentration or favorable (unlike) net charge.
5. The pumps do the same, using ATP energy, when the
gradient is unfavorable (ATPase).
C. Membranes are
self-assembling; they predate the origin of life. (Deamer, 1999)
1. Phospholipids self-assemble into bilayer envelopes
in water.
2. So do fatty acids: the length of the fatty acid
matters – and suggests that phospholipids are of a set length that is
ideal for bilayer stability
3. Meteorite contents reveal an array of soluble
organic molecules: included in these are fatty acids that self-organize
into bilayer envelopes in water.
4. These envelopes are osmotically functional: they
limit the flow of polar molecules in and out.
II. Multicellularity
The overarching theme here is that cells living in colonies
evidence cell specializations and enzymes that are critical for the
organisms that evolved from them.
A. Cyanobacteria are
photosynthetic prokaryotes with a very long history in the fossil
record; evidence the stromatolites.
1. The earliest cyanobacteria are filaments of uniform cells.
2. However, with the advent of the oxygen-rich atmosphere,
cyanobacteria evolved specialized cells to cope with a problem: their
nitrogen-fixing enzymes are permanently renderd inactive by
oxygen. These cells are oxygen-free nitrogen-reduction
factories called heterocysts. The rest of the cells in the colony
do the photosynthesis.
B. Choanoflagellates
1. Choanoflagellates are colonial one-celled eukaryotes that secrete a
communal stalk.
2. Current opinion is that they are ancestral to sponges and thus to
metazoans.
3. The colonial cells have tyrosine kinase, a key signal receptor in
the functioning of metazoans.
4. Tyrosine kinase is a membrane-bround dimeric protein that functions
in the binding of small signal proteins and the transduction of this
binding event into action inside the cell.
5. Inside the cell, tyrosine residues are phosphorylated, leads to the
activation of proteins that carry out cell functions. [The
lin-3 gene we are studying in the C. elegans lab constructs a small
protein that interacts with a tyrosine kinase in the membrane of the
future vulva cells.]
C. Sponges
1. An introduction to sponges: filter feeders with flagellated cells
(choanocytes) that move water through porocytes into the large central
cavity, in the process filtering food from the water.
2. Sponges also have amoebocytes that can move about the animal, as
well as other specialized cells.
3. Sponges have recently been shown to have the integrin proteins that
are critical to integration of the metazoans, so sponge specialists
argue that sponges are true metazoans.
4. Integrins are cell-membrane proteins that sense the extracellular
environment and signal to the cell to control differentiation,
survival, and migration of cells.
5. The integrins are connected directly to fibronectin proteins, and
indirectly through these to the collagen and proteoglycan
proteins. (Note, these are the three major kinds of proteins in
the extracellular matrix.)
6. The extracellular matrix is as important as the cell in
understanding the animals as organisms.
7. Sponges are known from the Late Proterozoic (Precambrian): the
invention of integrins and of cells integrated into true organisms
happened long ago.
III. Distinguishing Self from
Non-self: the Immune System
The approach in this section of the lecture is to address two
difference human health problems, allergies and AIDS, as examples of
the function of the two fundamental aspects of the immune system, the
humoral and cell-mediated responses.
A. Allergies
1. Components of the blood and focus on the white blood cell types,
namely:
Monocytes: phagocyte
Neutrophils: phagocyte
Basophils: histamine producers (dilation and increased permeability of
capillaries)
Eosinophils: protection against large blood invaders like blood flukes
Lymphocytes ….
2. Focus on the lymphocytes, B (maturing in bone marrow) and T
(maturing in the thymus)
3. Introduction to receptor proteins and their soluble counterparts,
the antibodies. The key point here is the specificity of the
binding of the variable region of the immunoglobulin with the epitope a
specific peptide (or sometimes carbohydrate) sequence of the antigen,
called.
4. B-lymphocyte receptor proteins interact with antigens, yielding
clones of antigen-specific memory B cells and plasma B cells.
5. The plasma B cells produce the antibodies, which are soluble
receptors that bind to these antigens (humoral means in solution,
referring to these antibodies operating in solution, indpendent of
cells).
6. Thus memory (through setting aside some of the B cells as
memory cells) and specificity (through the precise match of antibody to
antigen) are developed.
7. Allergies are the result of pollen-secific antibodies binding with
mast cells, and pollen binding to these bound antibodies (now
receptors), leading to histamine release and allergy symptoms.
B. HIV and AIDS 1 – the
cell-mediated immune response
1. The interaction of cells via MHC proteins is central to the
cell-mediated immune response.
2. MHC-1 proteins, located on virtually all living animal cells,
present peptides from the cell interior, including antigens, for
inspection by cytotoxic T cells.
3. MHC-2 proteins present the same fragments, but only from the
interior of selected white blood cells called antrigen-presenting
cells, including T cells, macrophages, and dendritic cells.
4. Receptors (similar to those on B cells) bind to the antigens;
cytotox T cells kill the infected cells presenting the antigens.
5. Helper T cells react to a bound antigen by cloning itself, yielding
more helper T cells and memory T cells with these receptors as well as
cytotoxic T cells and plasma B cells with the same specificity.
6. Cytokines are the small messenger proteins that T cells use to
stimulate these cloning events.
6. The first encounter with an antigen in the organism is up to
dendritic cells, which are macrophages specialized for working the body
frontiers, hunting for invaders.
C. HIV and AIDS 2 – HIV’s
attack on antigen-presenting cells
1. HIV is a retrovirus (lacks RNA, incorporates its RNA into the DNA of
a host cell, where its RNA is replicated).
2. HIV infection yields a typical immune response, with rapid increase
in antibodies specific to the virus as well as T cell concentration.
3. However, the continued attack on the antigen-presenting cells those
with MHC-II proteins (using the CD-4 link-stabilizing protein and
cytokine receptors to gain entry to the cell) eventually depletes the
population of these cells, compromising the cell-mediated immune
system.
IV. Self and non-self involving Plants
Here, the theme is an exploration of various
departures from the rejection of non-self cells by self cells as
exemplified by the immune system. To keep things balanced, we
consider these departures mostly using plants, but end up with the
origin of eukaryotic cells – the most significant departure from
rejecting non-self in the history of life.
A. Mycorrhizae
1. Mycorrhizae are symbiotic relationships between fungi and plant
roots: there are two types, ectomycorrhizae and endomycorrhizae.
2. Mycorrhizae are ancient (they are known from the most primitive land
plants),widespread (most mushrooms are producing spores for mycorrhisal
fungi), and specific (a mush room species usually has a specific host
preference).
3. Mycorrhizae infect plant roots in the apoplast (which requires
understanding the difference between the apoplast and the symplast in
the plant body.)
4. The symplast is the realm within the common membrane-bound cytoplasm
of the plant, interconnected through all cells by the plasmodesmata.
5. Mycorrhizal fungi, though they appear to enter into the
interior or the cortex cells in roots, in fact are always contained in
the apoplast, though their hyphae (filaments) penetrate and fill the
inside of the apparent cell boundaries.
6. In contrast, a pathogenic fungus is immediately recognized as
non-self by an immune-like interaction of receptor proteins with fungus
antigens, yielding 1) cell-wall thickening at the site of contact, 2)
movement of the nucleus to the battle zone, and 3) in the event of
invasion, apoptosis (programmed cell death) of the cell.
B. Bacteria symbiotic with
plants: Root Nodules and Stem Greenhouses
1. Root nodules resemble mycorrhizae in being symbiotic relationships,
this time between nitrogen-fixing bacteria and the roots of
legume-family plants.
2. Interaction of the bacteria, originally in the soil, with the roots,
leads to invasion of the bacteria via root hairs and organization of a
bacterial colony in a prominent nodule with vascular tissue connecting
to the vascular cylinder of the root.
3. Another nitrogen-fixing bacterium, this time a cyanobacterium,
infects the stem cortex of Gunnera, a tropical plant that lives in
disturbed, nitrogen-poor habitats.
C. Parasitic Plants
1. Parasitic plants are common in the tropics – they provide no benefit
to their host, but they get both a physical niche and nutrition from
their host.
2. Parasite seeds are stimulated to germinate by compounds in the host
plant; establishment of the haustorial (plant-plant) connection also
requires sensing of host compounds by the parasite.
3. The self-nonself response is not triggered in this interaction.
4. In one case, a parasitic plant has incorporated genes from the host
through horizontal transfer of genetic material from the mitochondria.
D. Origin of Eukaryotic Cell
1. Both mitochondria and chloroplasts are interpreted
as engulfed but tolerated prokaryotes living inside a host prokaryote.
2. The evidence: plastid membrane proteins are
homologous to procaryote proteins, and chromosome structure (one
circle) and chemistry (no histones) are like prokaryotes [plus several
additional features]
3. Many plastid proteins are encoded in the nucleus,
suggesting that there has been horizontal transfer of genes from the
plastids to the nucleus. Primitive plants and animals have larger
plastid genomes, suggesting the same history.
4. The advantage for the chloroplast ancestor may
have been protection from the oxidizing atmosphere – reminiscent of the
logic for the evolution of heterocysts.
V. Development
Development is the differentiation in the individual of the specialized
features that have evolved in its lineage.
A. totipotency:
1. The classic experiment was with carrots: isolated root cells could
be cultured to produce whole new plants.
2. Cell differentiation during development depends on selective
expression of the whole genome present in every cell.
3. Stem cells reveal the same insights: they can be nurtured to produce
various specialized cells depending on the environment provided to them.
B. blastula to gastrula:
comparative analysis yields insights into the general nature of
development
1. general pattern: differential cell division at the vegetal pole
leads to invagination of the digestive tract: gastrulation ends with
the primitive gut (archenteron) in place.
2. overall differences relate to the distribution of yolk (food
resource) in the zygote and hence the early embryo
3. three germ layers are evident at the end of gastrulation
a. ectoderm – the set of cells on the exterior of the animal
b. endoderm – the set of cells lining the archenteron
c. mesoderm – a third layer, lying in the interior between the other two
4. The germ layers have specific targets for their cell descendants in
the mature organism.
C. the three fundamental
processes:
1. They are:
–cell division (differential rates of division are critical, programmed
cell death is significant)
–cell differentiation (changes in integration and shape are critical;
targeting cells with signals is a critical part of the process)
–morphogenesis of tissues and organs (includes defining the
individual’s polarities, dividing the organism into segments, and – in
animals -- migration of cells in tissue origin)
2. The targeting of cell descendants governs cell differentation, best
seen in C. elegans, where the fate of every cell of the embryo is known.
3. For instance, signaling leads to dedication of one of four early
embryo cells to yield all the intestines from one of its daughters.
4. Similarly, the targeting of vulval precursor cells by anchors cells
leads to formation of the vulva – as seen in the lab on the lin-3
mutant.
5. Apoptosis ( programmed cell death) is significant in shaping the
growing body of worm or vertebrate. [not covered in detail this year]
6. Morphogenesis begins with definition of polarity in the egg by the
mother – best exemplified by work in Drosophila, the fruit
fly.
7. Once polarity is established, a series of genes divide the
embryo into segments.
D. Homeotic genes I: the determination of appendage identity on
fruitfly segments
1. The antennipedia mutant captures the whole idea of homeotic genes: a
perfect leg is programmed to develop, but too early in development, so
it appears where an antenna normal belongs.
2. Homeotic genes have a characteristic, highly conserved region of 180
nucleotides in their DNA called a homeobox.
3. The homeobox codes for a homeodomain of 60 amino acids in the
protein, which assemble into three alpha helices.
4. These alpha helices are critical the gene function, which is
interacting with DNA as a transcription factor – that is they turn on
other genes.
5. Homeotic genes come in families: members of the families determine
events on different segments – the members are called paralogs (that is
they are paralogous with one another).
6. The copies of the genes that control functions in equivalent
segments can be compared between species – these are orthologs (that is
they are orthologous with one another.)
D. Homeotic genes II:
the evolution of form in segmented animals
1. Serial homology is the organism-level homology of the segments of an
individual animal; evolutionary homology is the lineage-level homology
of the same segment on different animals.
2. Comparing the molecular to organismal levels, paralogs and orthologs
relate to serial and evolutionary homology.
3. These organismal homologies are fundamental similarities that are
discernible in spite of transformations for different functions, as in
the case of crustacean appendages.
4. The origin of segmentation and the divergence in function on
different segments was a necessary prerequisite to the evolution of the
higher animals, as was sufficient oxygen to allow rapid movement and
the origin of predator-prey relationships.
5. The appearance of hard-shelled fossils marks these accomplishments,
as exemplified by the abrupt appearance of several animal phyla (in
such deposits as the Burgess shale) in the Cambrian period.
6. The transformation of appendages on different segments has
been determined by the evolution of homeotic genes.
7. For instance, the homeotic gene Ubx evolved a long group of alanines
(a poly-A tail) in the common ancestor of insects, limiting the
segments with pairs walking legs to three.
8. Homeotic genes change body plans through increase in paralogs and
change in the set of segments in which the paralogs (individual
homeobox genes) operate.
E. Homeotic Genes in Plants
1. Plants are also segmented organisms that develop distally.
2. However, in the case of plants, development yields new segments
throughout life, and there is no cell migration.
3. Just as in plants, change in body plan involves increase in paralogs
and change in the set of segments in which the paralogs (individual
homeobox genes) operate.
4. The best example is the fixation of separate whorls of sepals and
petals in flowers.
V.
The Nervous System
The grand
theme here is one of cell-to-cell communication within the organism,
with a general tendency to focus on the role of membrane proteins as
gatekeepers in the comunication within and between cells.
A.
The
Cells: neurons and glia
1.
Neurons
have dendrites and an axon; information
flow is from the dendrites along the axon. Branches
of the axon communicate with the
post-synaptic cell across a synapse.
2.
Neurons
are accompanied by glia
cells, whose functions are to…
1. Provide
Structural support
2.Regulate extracellular
concentrations of ions and neurotransmitters
3.Facilitate information
transfer of neurons
4.Enhance neuron
respiration
5.Neuron insulation
B.
Resting
potential of neurons: ion pumps and channels
The neuron’s resting
potential is maintained by the sodium-potassium pump, which is
countered by
numerous ungated potassium channels and a
few ungated sodium channels.
Together these proteins maintain a net flow of both
ions across the membrane – establishing a net negative charge inside
the
cell.
C.
Signal
conduction: action potentials
1.
The
action
potential depends on a depolarization of the neuron membrane.
2.
An
action
potential begins with a graded depolarization, which can lead to rapid repolarization with a negative
charge outside
the cell,
then a return to a negative charge inside.
3.
All
gated
channels are closed in the resting state; a series of gated-channel
openings
and closings reverse the membrane polarity, then
re-establish the original polarity.
4.
The
action
potential moves down the neuron because depolarization at one place
initiates
depolarization downstream, but not upstream because upstream the
membrane is
still recovering.
5.
The
myelin
sheath speeds up the action potential, since it jumps from cell
boundary to
cell boundary in the sheath.
D.
Synapses:
communication with other cells
1.
A presynaptic neuron communicates
across a
synaptic cleft to
a postsynaptic neuron.
2.
Arriving
action
potentials trigger gated calcium channels; calcium flow inward brings
about
the release of
neurotransmitters from vesicles into the synaptic cleft.
3.
Ligand-gated channels in
the postsynaptic neuron bind with a neurotransmitter and open, bringing
about
excitatory or inhibitory responses, which are both graded and summed.
E.
The
Vertebrate Nervous System
1.
The
vertebrate
nervous system has two parts, a central (brain and spinal cord) and
peripheral
(the rest); the peripheral is divided into a skeletal and autonomic. The autonomic includes both
sympathetic
and parasympathetic divisions.
2.
The sympathetic division deals
mostly
with dilation, inhibition, and relaxation though it quickens the heart;
the
parasympathetic division mostly with constriction and stimulation,
but slows the heart.
3.
The
key
neurotransmitters are acetylcholine and three amino-acid
derived molecules – norepinephrine,
serotonin,
and dopamine.
4.
Plant
neurotoxins affect the body by imitating these neurotransmitters.
5.
Specialist
herbivores have figured out how to avoid the effects of these toxins.
VI.
The Senses
This
section is an extension of the last, with increasing focus on the
interaction between animal cells and their environment in providing the
animal with sensory information.
A.
Sensory
receptors in general –
transduction
1.
All
stimuli represent forms of energy.
2.
Sensation
involves converting energy into
change in the membrane potential of sensory receptors.
3.
Functions
of sensory receptors: sensory
transduction, amplification, transmission, and integration.
4.
Crayfish
stretch receptors provide a basic
model for sensing, especially the graded response.
5.
Vertebrate
hairs cells are separate receptor
cells that communicate information about the liquid environment to
neurons as
action potentials.
B.
Sound
receptors - the cochlea and pitch
1.
The
cochlea is a
good example of a sense organ that uses hair cells.
2.
Sound
is
transmitted from air to bone, then from bone to the liquid in the
cochlea
canals, then transduced
by
hair cells into action potentials.
C.
Chemoreceptors -
insect pheromones
Insect
pheromones are sensed by chemoreceptors in
the moth
antenna that depend on precise binding in receptor proteins; remarkably
minute
concentrations can be amplified into action potentials.
D.
Electromagnetic
receptors – migration
1.
Many
animals
navigate as they migrate, and appear to do so without visual cues.
2.
For
instance,
beluga whales regularly relocate summer breeding grounds and winter
retreats
without apparent visual aids.
3.
In
trout,
compass-like magnetite needles have been isolated from uniquely shaped
cells
spread through tissue of the nose – but there is no organ as such.
4.
Bacteria
use
similar compasses, consisting of synthesized magnetite in
membrane-bound
vesicles. to orient to electromagnetic
fields applied
to their cultures.
VII.
Plant Reproduction
In giving
this set of lectures, I worked toward the ultimate message of the how
plants distinguish self from non-self during reproduction to bring this
set of topics in line with the other cell themes in the sequence.
A.
Review
of
Flower Function
1.
Typical
flowers
have four organs, sepals for protection and support, petals for animal
attraction, stamens for pollen production, and carpels/pistils
for seed production.
2.
The
anthers of
stamens are the location of pollen development; the filaments position
the
anthers in the flower.
3.
The
ovule develops
inside the ovary of the flower; the stigma, which is positioned in the
flower
by the style, provides a landing place for the pollen.
4.
Stigma
and style
also function in the sorting of pollen into self and non-self groups.
5.
There
is
variation in the number and fusion of flower parts, and some flowers
lack one
or more of the standard organs.
6.
Many
cells in
the anthers go through meiosis, then one mitosis,
to
yield the two-celled haploid male gametophyte that is released as
pollen.
7.
A
single cell in
each ovule goes through meiosis, then three mitotic divisions, to yield
a
female gametophyte with eight haploid nuclei in seven cells – including
an egg and two polar nuclei.
8.
Pollen
germinating on the stigma grows down the style to the ovule, during
which time a
final mitosis yields two sperm.
9.
Each
sperm
functions; one fuses with the egg to yield
the diploid
zygote and the other fuses with the two polar nuclei to yield the
triploid
endosperm nucleus.
B.
Pollination
Ecology
1.
Many
flowers are
designed to promote the efficient use of pollen; precise placement of
pollen on
animals and restricting the number of kinds of visitors are important.
2.
Outcrossing is promoted,
but not ensured, in flowers by separating the sexes in space or in time.
3.
Even
flowers
with both sexes are often outcrossed –
often
with complex combinations of features, as in the foxglove and primrose.
4.
Heterostyly, as in the
primrose, has both structural and genetic mechanisms for promoting outcrossing.
5.
Among
the
structural features separating the long-styled and short-styled forms
are style
length, stamen position, pollen size, and stigma-papilla size.
C.
The
Genetics
of Outcrossing
1.
Long
ago Darwin
demonstrated that when primrose flowers were selfed,
they yielded fewer fruits with fewer, lighter seeds.
2.
The
short-styled
and long-styled forms maintain about even numbers because the
short-styled is
genetically heterozygous and the long-styled is a homozygous recessive.
3.
Primroses
have sporophytic incompatibility: if both
alleles of the
pollen-parent genotype match the stiga-parent
genotype, the pollen is prevented fro germinating by lack of hydration
from the
style.
4.
Petunias
have gametophytic self-incompatibility: if
the allele in the
haploid pollen genotype matches either of the alleles in the diploid
style
genotype, the style signals the pollen tube to slow down or stop.
5.
However,
some
plants choose to self, usually because the pollinators are not
dependably
present. Selfing
flowers have little petals, or they never open, but still they set seed.
6.
Genetically,
these plants have sacrificed variability to go with a single-purpose
genotype. Lineages with
these selfing flowers do not see to give
rise to
major evolutionary
VIII.
Chordate Phylogeny
Here, in the most blatently evolutionary part of the course, I
review chordat e evolution with frequent adventures into the
evolution of physiological features, such as the lateral line.
A.
General features and earliest groups
dorsal hollow nerve tube, notochord, pharyngeal
slits, postanal tail
B.
The neural crest and the origin of the craniates
1. Cells of the neural crest lose
integration and
migrate to originate various tissues, especially in the lower part of
the skull.
2. This change was important in the origin
of the
craniates, which have a skull, elaboration of the brain, and paired
sensory
organs in the head.
3. However, these animals (the hagfish)
have no
vertebrae and no paired appendages.
4. Vertebrae were invented in the common
ancestor of
the lamprey and jawed vertebrates.
C.
With Gnathostomes come jaws and two
pairs of
appendages
The jaws
are transformed skeletal rods.
D.
Lateral Lines, Lungs, and Swim Bladders
1. In addition to the appendages, the
lateral line
was invented – with neuromasts that are
homologous to the hair cells of the cochlea.
2. With the ray-finned fishes, the swim
bladder
– homologous with the lung – is invented.
E.
Origin of Tetrapods, Amphibians
1. Lobe-fin fishes show first
transformations for
life near land, with thickened fins and lungs.
2. An array of fossil intermediates
between fish and
amphibians is known from the Devonian period.
3. These animals had gills and a swimming
tail, but
they also had true legs with digits (they were tetrapods).
4. Amphibians can be terrestrial, but
almost all are
dependent on water in the early stages of development.
F. The Amniotic Egg, the Dinosaur
Pelvis, and the Origin of Birds
1. The amniotic egg is a
self-contained structure with membrane envenlopes providing shock
absorption, waste storage, and a respiration barrier.
2. These eggs have the
advantage of being highly dessication-resistant, so they can be laid
and tended far from water.
3. Dinosaurs differ from
reptiles in a key anatomical feature, having a pelvis designed to hold
the hind limbs erect, allowing more agile movement.
4. In the dinosaurs, this
pelvis – with its characteristic femur hole, has been transformed in
the vegetarian lineage to accommodate what is assumed to have been a
large stomach for holding large quantities of vegetation.
5. The result is what’s
known as a posterior process to the pubis.
6. Dinosaurs also had
nesting behavior, and they may have been warm-blooded and migratory.
7. All birds have the
dinosaur pelvis; ancient birds retain teeth, wing-claws, and a true
tail.
8. An array of
intermediates connects the carnivorous dinosaurs to the true birds.
G. The Origin of Mammals: Minor bones
of the reptilian jaw were modified to add to the sound transmission
apparatus of the middle ear in mammals.
H. Marsupials
1. The marsupials, now largely
confined to Australia, include numerous lineages that converge on
placental mammals in form and ecological niche specialization.
2. The protection from
placentals afforded by isolation in Australia was also the case until
recently (3 million years ago) in South America. There has
been recent broad extinction of marsupials in South America.
I. Primates: evolution in the context
of climate change
1. The primates include
early-diverging groups like the Madagascar-endemic lemurs, the
New-World Monkeys, and our own lineage.
2. The earliest fossils
of Homo are from Africa, ca. 2.4 million years ago.
3. Two recent species,
Neanderthal and our own lineage, originated in Africa and migrated
north.
4. Neanderthals appeared
about 300,000 years ago during a glacial maximum and were widespread in
Europe until 30,000 years ago. Their bodies were adapted to cold
tolerance.
5. Homo sapiens
originated ca. 120,000 years ago and was widely dispersed by 50,000
years ago.
6. Though sapiens and
Neanderthal lived in the same terrain and are genetically 99.5%
identical, they apparently did not interbreed.