Chapter 34 Vertebrates
Lecture Outline
Overview: Half a Billion
Years of Backbones
·
Vertebrates are named for vertebrae,
the series of bones that make up the vertebral column or backbone.
·
There
are about 52,000 species of vertebrates, far fewer than the 1 million insect
species on Earth.
°
Plant-eating
dinosaurs, at 40,000 kg, were the heaviest animals to walk on land.
°
The
biggest animal that ever existed is the blue whale, at 100,000 kg.
°
Humans
and our closest relatives are vertebrates.
·
This
group includes other mammals, birds, lizards, snakes, turtles, amphibians, and
the various classes of fishes.
Concept 34.1 Chordates have a notochord and a dorsal, hollow nerve
cord
·
The
vertebrates belong to one of the two major phyla in the Deuterostomia, the chordates.
°
Chordates
are bilaterian animals, belonging to the Deuterostomia.
·
The
phylum Chordata includes three subphyla, the vertebrates and two phyla of
invertebrates—the urochordates and the cephalochordates.
Four derived characters define the phylum
Chordata.
·
Although
chordates vary widely in appearance, all share the presence of four anatomical
structures at some point in their lifetime.
·
These
chordate characteristics are a notochord; a dorsal, hollow nerve cord;
pharyngeal slits; and a muscular, post-anal tail.
1. The notochord, present in all chordate embryos, is a longitudinal,
flexible rod located between the digestive tube and the nerve cord.
°
It
is composed of large, fluid-filled cells encased in fairly stiff, fibrous
tissue.
°
It
provides skeletal support throughout most of the length of the animal.
°
While
the notochord persists in the adult stage of some invertebrate chordates and
primitive vertebrates, it remains only as a remnant in vertebrates with a more
complex, jointed skeleton.
°
For
example, it is the gelatinous material of the disks between vertebrae in
humans.
2. The dorsal, hollow nerve
cord of a chordate embryo develops from a plate of ectoderm that rolls into a
tube dorsal to the notochord.
°
Other
animal phyla have solid nerve cords, usually located ventrally.
°
The
nerve cord of the chordate embryo develops into the central nervous system: the
brain and spinal cord.
3. The digestive tube of
chordates extends from the mouth to the anus.
°
The
region posterior to the mouth is the pharynx.
°
In
all chordate embryos, a series of pouches separated by grooves forms along the
sides of the pharynx.
°
In
most chordates, these grooves (known as pharyngeal
clefts) develop into pharyngeal gill
slits that allow water that enters the mouth to exit without continuing
through the entire digestive tract.
°
In
many invertebrate chordates, the pharyngeal gill slits function as
suspension-feeding devices.
°
The
slits and the structures that support them have become modified for gas
exchange (in aquatic vertebrates), jaw support, hearing, and other functions
during vertebrate evolution.
4. Most chordates have a
muscular tail extending posterior to the anus.
°
In
contrast, nonchordates have a digestive tract that extends nearly the whole
length of the body.
°
The
chordate tail contains skeletal elements and muscles.
°
It
provides much of the propulsive force in many aquatic species.
Invertebrate chordates provide clues to the
origin of vertebrates.
·
Members
of the subphylum Urochordata, commonly called tunicates, belong to the deepest-branching lineage of chordates.
°
They
most resemble chordates during their larval stage, which may be brief.
·
The
tunicate larva uses its tail muscles and notochord to swim through the water in
search of a suitable substrate on which it can settle, guided by cues from
light- and gravity-sensitive cells.
·
Tunicates
undergo a radical metamorphosis to form a sessile adult with few chordate
characteristics.
°
Its
tail and notochord are resorbed, its nervous system degenerates, and its organs
rotate 90 degrees.
·
Tunicates
are suspension feeders.
°
Seawater
passes inside the animal via an incurrent siphon, through the pharyngeal gill
slits, and into a ciliated chamber, the atrium.
°
Food
filtered from the water is trapped by a mucous net that is passed by cilia into
the intestine.
°
Filtered
water and feces exit through an anus that empties into an excurrent siphon.
·
Lancelets (members of the subphylum
Cephalochordata) are blade-like in shape.
°
The
notochord; dorsal, hollow nerve cord; numerous gill slits; and post-anal tail
all persist in the adult stage.
°
Lancelets
are up to 5 cm long.
°
They
live with their posterior end buried in the sand and the anterior end exposed
for feeding.
·
Adult
lancelets retain key chordate characteristics.
·
Lancelets
are suspension feeders, feeding by trapping tiny particles on mucous nets
secreted across the pharyngeal slits.
°
Ciliary
pumping creates a flow of water with suspended food particles into the mouth
and out the gill slits.
°
In
lancelets, the pharynx and gill slits are feeding structures and play only a
minor role in respiration, which primarily occurs across the external body
surface.
·
A
lancelet frequently leaves its burrow to swim to a new location.
·
Though
feeble swimmers, their swimming mechanism resembles that of fishes through the
coordinated contraction of serial muscle blocks.
°
Contraction
of chevron-shaped muscles flexes the notochord and produces lateral undulations
that thrust the body forward.
°
The
muscle segments develop from blocks of mesoderm, called somites, arranged serially along each side of the notochord of the
embryo.
·
Tunicates
and lancelets may provide clues about the evolutionary origin of the vertebrate
body plan.
·
Tunicates
display a number of chordate characteristics only as larvae, while lancelets
retain those characters as adults.
°
Thus,
an adult lancelet looks more like a larval tunicate than like an adult
tunicate.
·
In
the 1920s, biologist William Garstang suggested that tunicates represent an
early stage in chordate evolution.
°
This
stage may have occurred through paedogenesis,
the precocious development of sexual maturity in a larva.
°
Garstang
proposed that ancestral chordates became sexually mature while still in the
larval stage.
·
The
paedogenetic hypothesis is deduced from comparing modern forms, but the weight
of evidence is against it.
·
The
degenerate adult stage of tunicates appears to be a derived trait that evolved
only after the tunicate lineage branched off from other chordates.
°
Even
the tunicate larva appears to be highly derived.
°
Studies
of Hox gene expression suggest that
the tunicate larva does not develop the posterior part of its body axis.
§
Rather,
the anterior region is elongated and contains a heart and digestive system.
·
Research
on lancelets has revealed important clues about the evolution of the chordate
brain.
°
Rather
than a full-fledged brain, lancelets have only a slightly swollen tip on the
anterior end of the dorsal nerve cord.
°
The
same genes that organize major regions of the forebrain, midbrain, and
hindbrain of vertebrates express themselves in a corresponding pattern in this
small cluster of cells in the lancelet’s nerve cord.
°
The
vertebrate brain apparently is an elaboration of an ancestral structure similar
to the lancelet’s simple nerve cord tip.
Concept 34.2 Craniates are chordates that have a head
·
After
the evolution of the basic chordate body plan, the next major transition was
the appearance of a head.
·
Chordates
with a head are known as craniates.
·
The
origin of a head—with a brain at the anterior end of the dorsal nerve cord,
eyes and other sensory organs, and a skull—opened up a new way of feeding for
chordates: active predation.
Living craniates have a set of derived
characters.
·
Living
craniates share a set of derived characters that distinguishes them from other
chordates.
·
On
the genetic level, they possess two clusters of Hox genes, while lancelets and chordates have only one.
°
Other
important families of genes that produce signaling molecules and transcription
factors are also duplicated in craniates.
°
This
additional genetic complexity made a more complex morphology possible.
·
In
craniates, a group of embryonic cells called the neural crest forms near the dorsal margins of the closing neural
tube.
°
Neural
crest cells disperse through the body and contribute to the formation of
various structures, such as teeth, some of the bones and cartilages of the
skull, the dermis of the face, several types of neurons, and the sensory
capsules of the eyes and other sense organs.
°
The
vertebrate cranium and brain (the enlarged anterior end of the dorsal, hollow
nerve cord) and the anterior sensory organs are evidence of a high degree of
cephalization, the concentration of sensory and neural equipment in the head.
·
In
craniates, the pharyngeal clefts evolved into gill slits.
°
Unlike
the pharyngeal slits of lancelets, which are used primarily for suspension
feeding, gill slits are associated with muscles and nerves that allow water to
be pumped through the slits.
°
This
pumping sucks in food and facilitates gas exchange.
Cambrian fossils provide clues to craniate
origins.
·
Several
recent fossil finds in
°
They
appear to be “missing links” that straddle the transition to craniates.
°
The
most primitive of these fossils is a 3-cm-long animal called Haikouella.
§
This
animal resembles a lancelet and was probably a suspension feeder.
§
Haikouella also had a small but
well-formed brain, eyes, and muscular segments.
§
It
also had hardened structures (“denticles”) in the pharynx that may have
functioned somewhat like teeth.
§
However,
Haikouella did not have a skull.
°
In
other Cambrian rocks, paleontologists have found fossils of more advanced
chordates, such as Haikouichthys.
§
Haikouichthys had a skull composed of
cartilage and is the oldest known true craniate.
°
These
fossils push craniate origins back to the Cambrian explosion.
Class
Myxini: Hagfishes are the least derived craniate lineage.
·
Hagfishes
have a skull of cartilage but lack jaws and vertebrae.
°
They
swim in a snakelike fashion by using their segmental muscles to exert force
against their notochord, which they retain in adulthood as a strong, flexible
rod of cartilage.
·
Hagfishes
have a small brain, eyes, ears, and a nasal opening that connects with the
pharynx.
°
They
have toothlike formations made of keratin.
·
All
of the 30 or so species of hagfishes are marine scavengers, feeding on worms
and sick or dead fish.
°
Rows
of slime glands along a hagfish’s body produce small amounts of slime perhaps
to repulse other scavengers or larger amounts to deter a potential predator.
·
Vertebrate
systematists do not consider hagfishes to be fish.
°
The
taxonomic term fish refers only to a
specific clade of vertebrates, the actinopterygians.
Concept 34.3 Vertebrates are craniates that have a backbone
·
During
the Cambrian period, a lineage of craniates evolved into vertebrates.
·
With
a more complex nervous system and a more elaborate skeleton, vertebrates became
active predators.
·
After
vertebrates branched off from other craniates, they underwent another genetic
duplication, this one involving a group of transcription factor genes called
the Dlx family.
·
This
additional genetic complexity was associated with innovations in vertebrate
nervous systems and skeletons, including a more extensive skull and a backbone
composed of vertebrae.
·
In
the majority of vertebrates, the vertebrae enclose the spinal cord and have
taken over the biomechanical roles of the notochord.
·
Aquatic
vertebrates also have a number of adaptations associated with faster swimming,
including fins stiffened by fin rays and a more efficient gas exchange system in the gills.
Class Cephalaspidomorphi: Lampreys are the
oldest living lineage of vertebrates.
·
Like
hagfishes, lampreys offer clues to early chordate evolution but also have
acquired unique characters.
·
There
are about 35 species of lampreys inhabiting both marine and freshwater
environments.
°
Most
lampreys are parasites that feed by clamping a round, jawless mouth onto a
fish.
°
They
use their rasping tongues to penetrate the skin of their fish prey and to
ingest the prey’s blood.
·
Lampreys
live as suspension-feeding larvae in streams for years before migrating to the
sea or lakes as adults.
°
These
larvae resemble lancelets and live partially buried in sediment.
·
Some
species of lampreys feed only as larvae.
°
After
metamorphosis, these lampreys attain sexual maturity, reproduce, and die within
a few days.
·
The
skeletons of lampreys are made of cartilage.
°
Unlike
most vertebrate cartilage, lamprey cartilage contains no collagen. Instead, it
is a stiff protein matrix.
·
The
notochord persists as the main axial skeleton in adult lampreys.
°
Lampreys
also have a cartilaginous pipe surrounding the rodlike notochord.
°
Pairs
of cartilaginous projections extend dorsally, partially enclosing the nerve
cord with what might be a vestige of an early-stage vertebral column.
Many vertebrate lineages emerged early.
·
Conodonts were slender, soft-bodied
vertebrates with prominent eyes.
°
At
the anterior end of their mouth, they had a set of barbed hooks made of
mineralized dental tissue.
·
Conodonts
ranged in length from 3 to 30 cm.
°
They
probably hunted with their large eyes and impaled their prey on hooks.
°
The
food then passed to the pharynx, where a different set of dental elements
crushed and sliced it.
·
Conodonts
were very abundant for more than 300 million years.
·
Other
vertebrates emerged during the Ordovician and Silurian periods.
°
These
vertebrates had paired fins and an inner ear with two semicircular canals that
provided a sense of balance.
·
Although
they lacked jaws, they had a muscular pharynx that may have sucked in detritus
or bottom-dwelling organisms.
·
They
were armored with mineralized bone that offered protection from predators.
·
The
vertebrate skeleton evolved initially as a structure of unmineralized
cartilage.
°
Its
mineralization began only after lampreys diverged from other vertebrates.
·
What
initiated the process of mineralization in vertebrates?
°
Mineralization
may have been associated with the transition to new feeding mechanisms.
·
The
earliest known mineralized structures in vertebrates were conodont dental elements.
·
The
armor seen in later jawless vertebrates was derived from dental mineralization.
°
Only
in more derived vertebrates did the endoskeleton begin to mineralize, starting
with the skull.
Concept 34.4 Gnathostomes are vertebrates that have jaws
·
The
gnathostomes have true jaws, hinged
structures that enable vertebrates to grasp food firmly.
°
According
to one hypothesis, gnathostome jaws evolved by modification of the skeletal
rods that had previously supported the anterior pharyngeal gill slits.
°
The
remaining gill slits were no longer required for suspension feeding and
remained as the major sites of respiratory gas exchange.
Gnathostomes have a number of shared, derived
characters.
·
Gnathostomes
share other derived characters besides jaws.
·
The
common ancestors of all gnathostomes underwent an additional duplication of the
Hox genes, so that the single cluster
present in early chordates became four.
°
Other
gene clusters also duplicated, allowing further complexity in the development
of gnathostome embryos.
·
The
gnathostome forebrain is enlarged, in association with enhanced senses of
vision and smell.
·
The
lateral line system evolved as a row
of microscopic organs sensitive to vibrations in the surrounding water.
·
The
common ancestor of living gnathostomes had a mineralized axial skeleton,
shoulder girdle, and two sets of paired appendages.
·
Gnathostomes
appeared in the fossil record in the mid-Ordovician period, about 470 million
years ago, and steadily diversified.
·
Gnathostome
jaws and paired fins were major evolutionary breakthroughs.
°
Jaws,
with the help of teeth, enable the animal to grip food items firmly and slice
them up.
°
Paired
fins, along with the tail, enable fishes to maneuver accurately while swimming.
·
With
these adaptations, many fish species were active predators, allowing for the
diversification of both lifestyles and nutrient sources.
·
The
earliest gnathostomes in the fossil record are an extinct lineage of armored
vertebrates called placoderms.
°
Most
placoderms were less than a meter long, although some giants were more than 10
m long.
·
Another
group of jawed vertebrates called acanthodians
radiated in the Devonian.
·
Acanthodians
were closely related to the ancestors of osteichthyans (ray-finned and
lobe-finned fishes).
·
Both
placoderms and acanthodians disappeared by the beginning of the Carboniferous
period, 360 million years ago.
Class Chondrichthyes: Sharks and rays have
cartilaginous skeletons.
·
The
class Chondrichthyes, sharks and
their relatives, includes some of the biggest and most successful vertebrate
predators in the oceans.
·
Chondrichthyes
have relatively flexible endoskeletons of cartilage rather than bone.
°
In
most species, parts of the skeleton are impregnated by calcium.
·
Conodonts
and armored, jawless fishes show that mineralization of the vertebrate skeleton
had begun before the chondrichthyan lineage branched off from other
vertebrates.
°
Traces
of bone can be found in living chondrichthyes, in their scales, at the base of
their teeth and (in some sharks) in a thin layer on the surface of their
vertebrae.
°
The
loss of bone in chondrichthyes is a derived condition, which emerged after they
diverged from other gnathostomes.
·
There
are about 750 extant species, almost all in the subclass of sharks and rays,
with a few dozen species in a second subclass of chimaeras or ratfishes.
°
All
have well-developed jaws and paired fins.
·
The
streamlined bodies of most sharks enable them to be swift, but not
maneuverable, swimmers.
°
Powerful
axial muscles power undulations of the body and caudal fin to drive the fish
forward.
°
The
dorsal fins provide stabilization.
°
While
some buoyancy is provided by low-density oils in the large liver, the flow of
water over the pectoral and pelvic fins also provides lift to keep the animal
suspended in the water column.
·
Continual
swimming also ensures that water flows into the mouth and out through the
gills.
°
Some
sharks and many skates and rays spend much time resting on the seafloor, using
the muscles of their jaws and pharynx to pump water over the gills.
·
Most
sharks are carnivores that swallow their prey whole or use their powerful jaws
and sharp teeth to tear flesh from animals too large to swallow.
°
In
contrast, the largest sharks and rays are suspension feeders that consume
plankton.
°
Sharks
have several rows of teeth that gradually move to the front of the mouth as old
teeth are lost.
°
Within
the intestine of a shark is a spiral
valve, a corkscrew-shaped ridge that increases surface area and prolongs
the passage of food along the short digestive tract.
·
Acute
senses are adaptations that go along with the active, carnivorous lifestyle of
sharks.
°
Sharks
have sharp vision but cannot distinguish colors.
°
Their
acute olfactory sense (smelling) occurs in a pair of nostrils that do not
function in breathing.
°
Sharks
can detect electrical fields, including those generated by the muscle
contractions of nearby prey, through patches of specialized skin pores.
°
The
lateral line system, a row of microscopic organs sensitive to pressure changes,
can detect low-frequency vibrations.
°
In
sharks, the whole body transmits sound to the hearing organs of the inner ear.
·
Shark
eggs are fertilized internally.
°
Males
transfer sperm via claspers on their pelvic fins to the reproductive tract of
the female.
°
Oviparous sharks encase their eggs
in protective cases and lay them outside the mother’s body.
§
These
hatch months later as juveniles.
°
Ovoviviparous sharks retain fertilized
eggs in the oviduct.
§
The
embryo completes development in the uterus, nourished by the egg yolk.
°
A
few sharks are viviparous, providing
nutrients through a placenta to the developing offspring.
·
Rays
are closely related to sharks, but they have adopted a very different
lifestyle.
°
Most
rays are flattened bottom dwellers that crush molluscs and crustaceans in their
jaws.
°
The
enlarged pectoral fins of rays are used like wings to propel the animal through
the water.
°
The
tail of many rays is whiplike and may bear venomous barbs for defense against
threats.
·
Chondrichthyans
have changed little in more than 300 million years.
°
They
are severely threatened by overfishing.
°
In
2003, researchers reported that shark stocks in the northwest
Osteichthyes: The extant classes of bony
fishes are the ray-finned fishes, the lobe-finned fishes, and the lungfishes.
·
The
vast majority of bony fishes belong to a clade of gnathostomes called the
Osteichthyes (meaning “bony fish”).
·
Systematists
today include tetrapods with bony fish in Osteichthyes, which otherwise would
be paraphyletic.
·
Nearly
all bony fishes have an ossified endoskeleton with a hard matrix of calcium
phosphate.
°
It
is not clear when the shift to a bony skeleton took place during gnathostome
evolution.
·
Bony
fishes breathe by drawing water over four or five pairs of gills located in
chambers covered by a protective flap, the operculum.
°
Water
is drawn into the mouth, through the pharynx, and out between the gills by
movements of the operculum and muscles surrounding the gill chambers.
·
Most
fishes have an internal, air-filled sac, the swim bladder.
°
The
positive buoyancy provided by air counters the negative buoyancy of the
tissues, enabling many fishes to be neutrally buoyant and remain suspended in
the water.
°
The
swim bladder evolved from balloonlike lungs that may have been used to breathe
air when dissolved oxygen levels were low in stagnant shallow waters.
·
The
skin of bony fishes is often covered with thin, flattened bony scales that
differ in structure from the toothlike scales of sharks.
·
Glands
in the skin secrete mucus that reduces drag in swimming.
·
Like
sharks, aquatic osteichthyes have a lateral line system, which is evident as a
row of tiny pits in the skin on either side of the body.
·
The
reproduction of aquatic osteichthyes varies.
°
Most
species are oviparous, reproducing by external fertilization after the female
sheds large numbers of small eggs.
°
Internal
fertilization and birthing characterize other species.
·
The
most familiar families of fishes belong to the ray-finned fishes, members of class Actinopterygii.
°
This
class includes bass, trout, perch, tuna, and herring.
°
In
this group, the fins are supported by long, flexible rays.
°
The
fins may be modified for maneuvering, defense, and other functions.
·
Bony
fishes, including the ray-finned fishes, probably evolved in fresh water and
then spread to the seas during their long history.
°
Many
species of ray-finned fishes returned to fresh water at some point in their
evolution.
°
Some
ray-finned fishes, such as salmon, make a round-trip from fresh water to
seawater and back to fresh water during their life cycle.
·
Ray-finned
fishes evolved during the Devonian period, along with the lobe-finned fishes (Sarcopterygii).
·
The
key derived character in lobe-fins is the presence of muscular pectoral and
pelvic fins supported by extensions of the bony skeleton.
°
Many
Devonian lobe-fins were large, bottom dwellers that may have used their paired,
muscular fins to “walk” along the bottom.
°
By
the end of the Devonian period, lobe-fin diversity was dwindling.
·
Today,
only three lineages survive.
°
One
lineage, the coelacanths (class Actinistia) probably originated as freshwater
animals with lungs, but others shifted to the ocean, including the only living
genus, Latimeria.
°
The
second lineage of living lobe-fins is represented by three genera of lungfishes
(class Dipnoi), which live today in the Southern Hemisphere.
§
They
generally inhabit stagnant ponds and swamps.
§
They
can gulp air into lungs connected to the pharynx of the digestive tract to
provide oxygen for metabolism.
§
Lungfishes
also have gills, which are the main organs for gas exchange in Australian
lungfishes.
§
When
ponds shrink during the dry season, some lungfishes can burrow into the mud and
estivate.
°
The
third lineage of lobe-fins that survives today is far more diverse than
coelacanths or lungfishes.
·
During
the mid-Devonian, the tetrapods adapted to life on land and gave rise to
terrestrial vertebrates, including humans.
Concept 34.5 Tetrapods are gnathostomes that have limbs and feet
·
One
of the most significant events in vertebrate history took place 360 million
years ago, when the fins of some lobe-fins evolved into tetrapod limbs and
feet.
·
The
most significant character of tetrapods
is the four limbs, which allow them to support their weight on land.
°
The
feet of tetrapods have digits that allow them to transmit muscle-generated
forces to the ground when they walk.
·
With
the move to land, the bones of the pelvic girdle (to which the hind legs are
attached) became fused to the backbone, permitting forces generated by the hind
legs against the ground to be transferred to the rest of the body.
·
Living
tetrapods do not have pharyngeal gill slits.
°
The
ears are adapted to the detection of airborne sounds.
·
The
Devonian coastal wetlands were home to a wide range of lobe-fins. Those that
entered shallow, oxygen-poor water could use their lungs to breathe air.
·
Some
species likely used their stout fins to move across the muddy bottom.
°
At
the water’s edge, leglike appendages were probably better equipment than fins
for paddling and crawling through the dense vegetation in shallow water.
°
The
tetrapod body plan was thus a modification of a preexisting body plan.
·
In
one lineage of lobe-fins, the fins became progressively more limb-like, while
the rest of the body retained adaptations for aquatic life.
°
For
example, fossils of Acanthostega from
365 million years ago had bony gill supports and rays in its tail to support
propulsion in water, but it also had fully formed legs, ankles, and digits.
°
Acanthostega is representative of a
period of vertebrate evolution when adaptations for shallow water allowed
certain fishes to make a gradual transition to the terrestrial side of the
water’s edge.
·
A
great diversity of tetrapods emerged during the Carboniferous period.
°
Judging
from the morphology and location of the fossils, most of these early tetrapods
remained tied to water.
Class Amphibia: Salamanders, frogs, and
caecilians are the three extant amphibian orders.
·
Today
the amphibians (class Amphibia) are
represented by about 4,800 species of salamanders (order Urodela, “tailed
ones”), frogs (order Anura, “tail-less ones”), and caecilians (order Apoda,
“legless ones”).
·
Some
of the 500 species of urodeles are entirely aquatic, but others live on land as
adults or throughout life.
°
On
land, most salamanders walk with a side-to-side bending of the body that may
resemble the swagger of the early terrestrial tetrapods.
·
The
4,200 species of anurans are more specialized than urodeles for moving on land.
°
Adult
frogs use powerful legs to hop along the terrain.
°
Frogs
nab insects by flicking out their sticky tongues, which are attached to the
front of the mouth.
·
Anurans
may be camouflaged or secrete a distasteful, even poisonous, mucus from skin
glands.
°
Many
poisonous species are brightly colored, perhaps to warn predators who associate
the coloration with danger.
·
Apodans,
the caecilians (about 150 species), are legless and nearly blind.
°
The
reduction of legs evolved secondarily from a legged ancestor.
·
Superficially
resembling earthworms, most species burrow in moist forest soil in the tropics.
°
A
few South American species live in freshwater ponds and streams.
·
Amphibian means “two lives,” a
reference to the metamorphosis of many frogs from an aquatic stage, the
tadpole, to the terrestrial adult.
°
Tadpoles
are usually aquatic herbivores with gills and a lateral line system, and they
swim by undulating their tails.
°
During
metamorphosis, the tadpole develops legs, the lateral line disappears, and
lungs replace gills.
°
Adult
frogs are carnivorous hunters.
·
Many
amphibians do not live a dualistic—aquatic and terrestrial—life.
°
There
are some strictly aquatic, and some strictly terrestrial frogs, salamanders,
and caecilians.
°
The
larvae of salamanders and caecilians look like adults and are also carnivorous.
·
Paedomorphosis,
the retention of some larval features in a sexually mature adult, is common
among some groups of salamanders.
°
For
example, the mudpuppy (Necturus)
retains gills and other larval features when sexually mature.
·
Most
amphibians retain close ties with water and are most abundant in damp habitats.
°
Those
adapted to drier habitats spend much of their time in burrows or under moist
leaves where the humidity is higher.
°
Most
amphibians rely heavily on their moist skin to carry out gas exchange with the
environment.
§
Some
terrestrial species lack lungs entirely and breathe exclusively through their skin
and oral cavity.
·
Amphibian
eggs lack a shell and dehydrate quickly in dry air.
°
Most
species have external fertilization, with eggs shed in ponds or swamps or at
least in moist environments.
°
Some
species lay vast numbers of eggs in temporary pools where mortality is high.
°
Others
display various types of parental care and lay relatively few eggs.
§
In
some species, males or females may house eggs on the back, in the mouth, or
even in the stomach.
§
Some
species are ovoviviparous or viviparous, retaining the developing eggs in the
female reproductive tract until released as juveniles.
·
Many
amphibians display complex and diverse social behavior, especially during the
breeding season.
°
Then
many male frogs fill the air with their mating calls as they defend breeding
territories or attract females.
°
In
some terrestrial species, migrations to specific breeding sites may involve
vocal communication, celestial navigation, or chemical signaling.
·
For
the past 25 years, zoologists have been documenting a rapid and alarming
decline in amphibian populations throughout the world.
·
Several
causes that have been proposed include habitat degradation, the spread of a
pathogen (a chytrid fungus), and acid precipitation.
°
Acid
precipitation is damaging to amphibians because of their dependence on wet
places for completion of their life cycles.
Concept 34.6 Amniotes are tetrapods that have a terrestrially
adapted egg
·
The
amniote clade consists of the
mammals and reptiles (including birds).
·
The
evolution of amniotes from an amphibian ancestor involved many adaptations for
terrestrial living.
·
The
amniotic egg is the major derived character of the clade.
·
Inside
the shell of the amniotic egg are several extraembryonic
membranes that function in gas exchange, waste storage, and the transfer of
stored nutrients to the embryo.
°
The
amniotic egg is named for one of these membranes, the amnion, which encloses a
fluid-filled “private pond” that bathes the embryo and acts as a hydraulic
shock absorber.
·
The
amniotic eggs enabled terrestrial vertebrates to complete their life cycles
entirely on land.
°
In
contrast to the shell-less eggs of amphibians, the amniotic eggs of most
amniotes have a shell that retains water and can be laid in a dry place.
°
The
calcareous shells of bird eggs are inflexible, while the leathery eggs of many
reptiles are flexible.
°
Most
mammals have dispensed with the shell.
§
The
embryo implants in the wall of the uterus and obtains its nutrition from the
mother.
·
Amniotes
acquired other adaptations to terrestrial life, including less-permeable skin
and the increasing use of the rib cage to ventilate the lungs.
·
Amniotes
adopt a more elevated stance than earlier tetrapods and living amphibians.
·
The
most recent common ancestor of living amphibians and amniotes lived about 340
million years ago, in the early Carboniferous period.
°
No
fossils of amniotic eggs have been found from that time.
·
Early
amniotes lived in drier environments than did earlier tetrapods.
·
Some
were herbivores, with grinding teeth. Others were large and predatory.
The reptile clade includes birds.
·
The
reptile clade includes tuatara,
lizards, snakes, turtles, crocodilians, and birds, as well as extinct groups
such as dinosaurs.
·
Reptiles
have several adaptations for terrestrial life not generally found in
amphibians.
°
Scales
containing the protein keratin waterproof the skin, preventing dehydration in
dry air.
§
Crocodiles,
which are adapted to water, have evolved more permeable scales called scutes.
°
Reptiles
obtain all their oxygen with lungs, not through their dry skin.
§
As
an exception, many turtles can use the moist surfaces of their cloaca for gas
exchange.
·
Most
reptiles lay shelled amniotic eggs on land.
°
Fertilization
occurs internally, before the shell is secreted as the egg passes through the
female’s reproductive tract.
°
Some
species of lizards and snakes are viviparous, with their extraembryonic
membranes forming a placenta that enables the embryo to obtain nutrients from
its mother.
·
Nonbird
reptiles are sometimes labeled “cold-blooded” because they do not use their
metabolism extensively to control body temperature.
°
However,
many nonbird reptiles regulate their body temperature behaviorally by basking
in the sun when cool and seeking shade when hot.
·
Because
they absorb external heat rather than generating much of their own, nonbird
reptiles are more appropriately called ectotherms.
°
One
advantage of this strategy is that an ectothermic reptile can survive on less
than 10% of the calories required by a mammal of equivalent size.
·
The
reptile clade is not entirely ectothermic.
°
Birds
are endothermic, capable of keeping
the body warm through metabolism.
·
The
oldest reptilian fossils date back to the Carboniferous period, about 300
million years ago.
·
The
first major group of reptiles to emerge was the parareptiles, large, stocky, quadrupedal herbivores.
°
Some
parareptiles had dermal plates on their skin, which may have provided defense
against predators.
·
Parareptiles
died out 200 million years ago, at the end of the Triassic period.
·
As
parareptiles were dwindling, an equally ancient clade of reptiles, the diapsids, was diversifying.
°
The
most obvious derived character of diapsids is a pair of holes on each side of
the skull, behind the eye socket.
·
The
diapsids are composed of two main lineages.
°
One,
the lepidosaurs, includes lizards, snakes,
and tuataras.
§
This
lineage also produced a number of marine reptiles including plesiosaurs and
ichthyosaurs.
°
The
archosaurs include crocodilians, and
the extinct pterosaurs and dinosaurs.
·
Pterosaurs, which originated in the
late Triassic, were the first flying tetrapods.
°
The
pterosaur wing is formed from a bristle-covered membrane of skin that stretched
between the hind leg and the tip of an elongated finger.
°
Well-preserved
fossils show the presence of muscles, blood vessels, and nerves in the wing membrane,
suggesting that pterosaurs could dynamically adjust their membranes to assist
their flight.
·
Dinosaurs were an extremely diverse
group varying in body shape, size, and habitat.
°
There
were two main dinosaur lineages: the ornithischians, which were mostly
herbivorous, and the saurischians, which included both long-necked giant
herbivores and bipedal carnivorous theropods.
§
Theropods
included the famous Tyrannosaurus rex
as well as the ancestors of birds.
·
There
is increasing evidence that many dinosaurs were agile; fast moving; and, in
some species, social.
°
Paleontologists
have discovered signs of parental care among dinosaurs.
·
There
is continuing debate about whether dinosaurs were endothermic, capable of keeping their body warm through metabolism.
°
Some
experts are skeptical.
°
In
the warm, consistent Mesozoic climate, behavioral adaptations may have been
sufficient for maintaining a suitable body temperature for terrestrial
dinosaurs.
°
Also,
the low surface-to-volume ratios would have reduced the effects of daily
fluctuations in air temperature on the animal’s internal temperature.
°
Some
anatomical evidence supports the hypothesis that at least some dinosaurs were
endotherms.
§
Paleontologists
have found fossils of dinosaurs in both Antarctica and the
°
The
dinosaur that gave rise to birds was certainly endothermic, as are all birds.
·
By
the end of the Cretaceous, all dinosaurs (except birds) became extinct.
°
It
is uncertain whether dinosaurs were declining before they were finished off by
an asteroid or comet impact.
·
Lepidosaurs
are represented by two living lineages.
·
One
lineage includes the tuatara, two species of lizard-like reptiles found only on
30 islands off the coast of
°
Tuatara
relatives lived at least 220 million years ago, when they thrived on every
continent well into the Cretaceous period.
·
The
other major living lineage of lepidosaurs are the squamates (lizards and
snakes).
·
Lizards
are the most numerous and diverse reptiles alive today.
°
Most
are relatively small, but they range in length from 16 mm to 3 m.
·
Snakes
are legless lepidosaurs that evolved from lizards closely related to the Komodo
dragon.
·
It
was once thought that snakes were descendents of lizards that adapted to a
burrowing lifestyle through the loss of limbs.
°
However,
recently discovered fossils of aquatic snakes with complete hind legs suggest
that snakes likely evolved in water and then recolonized land.
°
Some
species of snakes retain vestigial pelvic and limb bones, providing evidence of
their ancestry.
·
Snakes
are carnivorous, and a number of adaptations aid them in hunting and eating
prey.
°
Snakes
have acute chemical sensors and are sensitive to ground vibrations.
§
The
flicking tongue fans odors toward olfactory organs on the roof of the mouth.
°
Heat-detecting
organs of pit vipers, including rattlesnakes, enable these night hunters to
locate warm animals.
°
Some
poisonous snakes inject their venom through a pair of sharp, hollow or grooved
teeth.
°
Loosely
articulated jaws enable most snakes to swallow prey larger than the diameter of
the snake itself.
·
Turtles
are the most distinctive group of reptiles alive today.
·
All
turtles have a boxlike shell made up of upper and lower shields that are fused
to the vertebrae, clavicles, and ribs.
·
The
earliest fossils of turtles are 220 million years old, with fully developed
shells.
°
The
origin of the turtle shell remains a puzzle.
§
Some
paleontologists suggest that turtle shells evolved from the dermal shells of parareptiles.
°
Other
studies link turtles to archosaurs or lepidosaurs.
°
There
are two separate branches of turtles that have independently evolved mechanisms
to retract their heads.
°
Turtles
live in a variety of environments, from deserts to ponds to the sea.
·
Crocodiles
and alligators (crocodilians) are among the largest living reptiles.
°
They
spend most of their time in water, breathing air through upturned nostrils.
°
Crocodilians
are confined to the tropics and subtropics.
Birds evolved as feathered dinosaurs.
·
Like
crocodilians, birds are archosaurs, but highly specialized for flight.
°
In
addition to amniotic eggs and scales, modern birds have feathers and other
distinctive flight equipment.
·
Almost
every part of a typical bird’s anatomy is modified in some way to reduce weight
and enhance flight.
°
One
adaptation to reduce weight is the absence of some organs.
§
For
instance, females have only one ovary.
°
Modern
birds are toothless and grind their food in a muscular gizzard near the
stomach.
·
The
skeletons of birds have several adaptations that make them light and flexible,
but strong.
°
The
bones are air-filled and honeycombed to reduce weight without sacrificing much
strength.
·
A
bird’s feathers have a hollow, air-filled shaft that is light and strong.
°
Feathers
are made of beta-keratin, a protein similar to the keratin of reptile scales.
·
The
shape and arrangement of feathers forms wings into airfoils.
·
Power
for flapping the wings comes from contractions of the pectoral muscles,
anchored to a keel on the sternum.
·
The
evolution of flight required radical alteration in body form but provides many
benefits.
°
Flight
enhances hunting and scavenging.
§
It
enables many birds to exploit flying insects, an abundant, highly nutritious
food resource.
°
Flight
provides a ready escape from earthbound predators.
°
It
enables many birds to migrate great distances to exploit different food
resources and seasonal breeding areas.
·
Flying
requires a great expenditure of energy with an active metabolism.
°
Birds
are endothermic, using their own metabolic heat to maintain a constant body
temperature.
§
Feathers
and, in some species, a layer of fat provide insulation.
°
Efficient
respiratory and circulatory systems with a four-chambered heart keep tissues
well supplied with oxygen and nutrients.
§
The
lungs have tiny tubes leading to and from elastic air sacs that help dissipate
heat and reduce body density.
°
Birds
have excellent vision and coordination, supported by well-developed areas of
the brain.
§
The
large brains of birds (proportionately larger than those of reptiles or
amphibians) support very complex behavior.
°
During
the breeding season, birds engage in elaborate courtship rituals.
§
This
culminates in copulation, contact between the mates’ vents, the openings to
their cloacae.
§
After
eggs are laid, the avian embryo is kept warm through brooding by the mother,
father, or both, depending on the species.
·
Cladistic
analyses of fossilized skeletons support the hypothesis that the closest
reptilian ancestors of birds were theropods.
·
In
the late 1990s, Chinese paleontologists unearthed a treasure trove of feathered
theropods that are shedding light on bird origins.
°
These
fossils suggest that feathers evolved long before feathered flight, possibly
for insulation or courtship.
·
Theropods
may have evolved powered flight by one of two possible routes.
1. Small ground-running
dinosaurs chasing prey or evading predation may have used feathers to gain
extra lift as they jumped into the air.
2. Dinosaurs could have
glided from trees, aided by feathers.
·
The
most famous Mesozoic bird is Archaeopteryx,
known from fossils from a German limestone quarry.
°
This
ancient bird lived about 150 million years ago, during the late Jurassic
period.
°
Archaeopteryx had clawed forelimbs,
teeth, and a long tail containing vertebrae.
§
Without
its feathers, Archaeopteryx would
probably be classified as a theropod dinosaur.
§
Its
skeletal anatomy indicates that it was a weak flyer, perhaps a tree-dwelling
glider.
·
Neornithes,
the clade that includes 28 orders of living birds, arose after the
Cretaceous-Tertiary boundary, 65 million years ago.
·
Most
birds can fly, but Neornithes includes a few flightless birds, the ratites, which lack both a breastbone
and large pectoral muscles.
°
The
ratites include the ostrich, kiwi, and emu.
·
The
penguins make up the flightless order Sphenisciformes.
°
They
have powerful pectoral muscles, which they use in swimming.
·
The
demands of flight have rendered the general form of all flying birds similar to
one another.
°
The
beak of birds is very adaptable, taking on a great variety of shapes for
different diets.
Concept 34.7 Mammals are amniotes that
have hair and produce milk
Mammals diversified extensively in the wake of
the Cretaceous extinctions.
·
Mammals have a number of derived
traits.
°
All
mammalian mothers use mammary glands to nourish their babies with milk, a
balanced diet rich in fats, sugars, proteins, minerals, and vitamins.
°
All
mammals also have hair, made of keratin.
§
Hair
and a layer of fat under the skin retain metabolic heat, contributing to
endothermy in mammals.
°
Endothermy
is supported by an active metabolism, made possible by efficient respiration
and circulation.
§
Adaptations
include a muscular diaphragm and a four-chambered heart.
·
Mammals
generally have larger brains than other vertebrates of equivalent size.
°
Many
species are capable of learning.
°
The
relatively long period of parental care extends the time for offspring to learn
important survival skills by observing their parents.
·
Feeding
adaptations of the jaws and teeth are other important mammalian traits.
°
Unlike
the uniform conical teeth of most reptiles, the teeth of mammals come in a
variety of shapes and sizes adapted for processing many kinds of foods.
°
During
the evolution of mammals from reptiles, two bones formerly in the jaw joint
were incorporated into the mammalian ear and the jaw joint was remodeled.
·
Mammals
belong to a group of amniotes known as synapsids.
°
Synapsids
have a temporal fenestra behind the eye socket on each side of the skull.
·
Synapsids
evolved into large herbivores and carnivores during the Permian period.
·
Mammal-like
synapsids emerged by the end of the Triassic, 200 million years ago.
°
These
animals were not mammals, but they were small and likely hairy, fed on insects
at night, and had a higher metabolism that other synapsids.
°
They
likely laid eggs.
·
The
first true mammals arose in the Jurassic periods.
°
Early
mammals diversified into a number of lineages, all about the size of a shrew.
·
During
the Mesozoic, mammals coexisted with dinosaurs and underwent a great adaptive
radiation in the Cenozoic in the wake of the Cretaceous extinctions.
°
Modern
mammals are split into three groups: monotremes (egg-laying mammals),
marsupials (mammals with pouches), and eutherian (placental) mammals.
·
Monotremes—the platypuses and the
echidnas—are the only living mammals that lay eggs.
°
The
reptile-like egg contains enough yolk to nourish the developing embryo.
·
Monotremes
have hair, and females produce milk in specialized glands.
°
After
hatching, the baby sucks milk from the mother’s fur because she lacks nipples.
·
Marsupials include opossums,
kangaroos, bandicoots, and koalas.
·
In
contrast to monotremes, marsupials have a higher metabolic rate, have nipples
that produce milk, and give birth to live young.
·
A
marsupial is born very early in development and, in most species, completes its
embryonic development while nursing within a maternal pouch, the marsupium.
°
In
most species, the tiny offspring climbs from the exit of the female’s
reproductive tract to the mother’s pouch.
·
Marsupials
existed worldwide throughout the Mesozoic area but now are restricted to
°
In
§
Through
convergent evolution, these diverse marsupials resemble eutherian mammals that
occupy similar ecological roles.
·
While
marsupial mammals diversified throughout the Tertiary in South America and
°
°
Invasions
of placental mammals from North America impacted the marsupial fauna of South
America about 12 million years ago and then again about 3 million years ago
when the continents were connected by the
§
This
mammalian biogeography is an example of the interplay between biological and
geological evolution.
·
Compared
to marsupials, eutherian mammals
(placentals) have a longer period of pregnancy.
°
Young
eutherians complete their embryonic development within the uterus, joined to
the mother by the placenta.
°
Eutherians
are commonly called placental mammals because their placentas are more complex
than those of marsupials and provide a more intimate and long-lasting
association between mother and young.
Concept 34.8 Humans are bipedal hominoids with a large brain
Primate evolution provides a context for
understanding human origins.
·
Primates
include lemurs, monkeys, and apes.
·
Primates
have large brains and short jaws.
·
Their
eyes are forward-looking.
·
Most
primates have hands and feet adapted for grasping.
·
Relative
to other mammals, they have large brains and short jaws.
·
They
have flat nails on their digits, rather than narrow claws.
·
Primates
also have relatively well-developed parental care and relatively complex social
behavior.
·
The
earliest primates were probably tree dwellers, shaped by natural selection for
arboreal life.
°
The
grasping hands and feet of primates are adaptations for hanging on to tree
branches.
§
All
modern primates, except Homo, have a
big toe that is widely separated from the other toes.
§
The
thumb is relatively mobile and separate from the fingers in all primates, but a fully opposable thumb is
found only in anthropoid primates.
§
The
unique dexterity of humans, aided by distinctive bone structure at the thumb
base, represents descent with modification from ancestral hands adapted for
life in the trees.
·
Other
primate features also originated as adaptations for tree dwelling.
°
The
overlapping fields of vision of the two eyes enhance depth perception, an
obvious advantage when brachiating.
°
Excellent
hand-eye coordination is also important for arboreal maneuvering.
·
Primates
are divided into two subgroups.
°
The
Prosimii (prosimians) probably
resemble early arboreal primates and include the lemurs of
°
The
Anthropoidea (anthropoids) include
monkeys, apes, and humans.
·
The
oldest known anthropoid fossils, from about 45 million years ago, support the
hypothesis that tarsiers are the prosimians most closely related to
anthropoids.
·
By
the Oligocene, monkeys were established in Africa, Asia, and
°
The
Old World and
°
All
New World monkeys are arboreal, but
°
Most
monkeys in both groups are diurnal, and usually live in bands held together by
social behavior.
·
In
addition to monkeys, the anthropoid suborder also includes four genera of apes:
Hylobates (gibbons), Pongo (orangutans), Gorilla (gorillas), and Pan
(chimpanzees and bonobos).
°
Modern
apes are confined exclusively to the tropical regions of the
°
They
evolved from
·
With
the exception of gibbons, modern apes are larger than monkeys, with relatively
long arms and short legs and no tails.
°
Only
gibbons and orangutans are primarily arboreal.
·
Social
organization varies among the genera, with gorillas and chimpanzees being
highly social.
°
Apes
have relatively larger brains than monkeys, and their behavior is more
flexible.
Humans are bipedal hominoids.
·
In
the continuity of life spanning more than 3.5 billion years, humans and apes
have shared ancestry for all but the past few million years.
·
Human
evolution is marked by the evolution of several major features.
°
Humans
stand upright and walk on two legs.
°
Humans
have a much larger brain than other hominoids and are capable of language,
symbolic thought, and tool use.
°
Humans
have reduced jawbones and muscles and a shorter digestive tract.
°
Human
and chimpanzee genomes are 99% identical.
§
Scientists
are comparing the genomes of humans and chimpanzees to investigate the 1%
difference.
·
Paleoanthropology is the study of human
origins and evolution.
·
Paleoanthropologists
have found fossils of 20 species of extinct hominoids that are more closely
related to humans than to chimpanzees.
°
These
species are known as hominids.
·
The
oldest hominid is Sahelanthropus
tchandensis, which lived 7 million years ago.
°
Sahelanthropus and other early hominids
shared some of the derived characters of humans.
°
They
had reduced canine teeth and relatively flat faces.
°
They
were more upright and bipedal than other hominoids.
·
While
early hominids were becoming bipedal, their brains remained small—about 400 to
450 cm3 in volume.
°
Early
hominids were small in stature, with relatively large teeth and a protruding
lower jaw.
·
Avoid
three common sources of confusion:
1. First, our ancestors were
not chimpanzees or any other modern apes.
§
Chimpanzees
and humans represent two divergent branches of the hominoid tree that evolved
from a common ancestor that was neither a chimpanzee nor a human.
2. Second, human evolution
did not occur as a ladder with a series of steps leading directly from an
ancestral hominoid to Homo sapiens.
§
If
human evolution is a parade, then many splinter groups traveled down dead ends,
and several different human species coexisted.
°
Human
phylogeny is more like a multibranched bush with our species as the tip of the
only surviving twig.
3. Third, the various human
characteristics, such as upright posture and an enlarged brain, did not evolve
in unison.
§
Different
features evolved at different rates, called mosaic evolution.
§
Our
pedigree includes ancestors who walked upright but had brains much less
developed than ours.
°
After
dismissing some of the folklore on human evolution, we must admit that many
questions about our own ancestry remain.
°
Hominid
diversity increased dramatically between 4 and 2 million years ago.
·
The
various pre-Homo hominids are
classified in the genus Australopithecus
(“southern ape”) and are known as australopiths.
°
The
first australopith, A. africanus, was
discovered in 1924 by Raymond Dart in a quarry in
§
From
this and other skeletons, it became clear that A. africanus probably walked fully erect and had humanlike hands
and teeth.
§
However,
the brain was only about one-third the size of a modern human’s brain.
·
In
1974, a new fossil, about 40% complete, was discovered in the Afar region of
°
This
fossil, nicknamed “Lucy,” was described as a new species, A. afarensis.
·
Based
on this fossil and other discoveries, this species had a brain the size of a
chimpanzee, a prognathous jaw, longer arms (for some level of arboreal
locomotion), and sexual dimorphism more apelike than human.
°
However,
the pelvis and skull bones and fossil tracks showed that A. afarensis walked bipedally.
°
Two
lineages appeared after A. afarensis:
the “robust” australopithecines with sturdy skulls and powerful jaws and teeth
for grinding and chewing hard, tough foods; and the “gracile”
australopithecines with lighter feeding equipment adapted for softer foods.
·
Combining
evidence from the earliest hominids with the fossil record of australopiths
makes it possible to consider hypotheses about trends in hominid evolution.
·
Why
did hominids become bipedal?
°
Our
anthropoid ancestors of 30–35 million years ago were tree dwelling.
§
Twenty
million years ago, the forests contracted as the climate became drier.
§
The
result was an increased savanna with few trees.
§
For
decades, paleontologists thought that bipedalism was an adaptation to life on
the savanna.
°
All
early hominids show indications of bipedalism, but they lived in forests and
open woodlands, not savanna.
°
An
alternate hypothesis is that bipedalism allowed hominids to reach low-hanging
fruits.
°
About
1.9 million years ago, hominids living in arid environments walked long
distances on two legs.
·
The
manufacture and use of complex tools is a derived human character.
°
When
and why did tool use arise in the human lineage?
°
Other
hominoids are capable of sophisticated tool use.
§
Orangutans
can fashion probes from sticks for retrieving insects from their nests.
§
Chimps
use rocks to smash open food and put leaves on their feet to walk over thorns.
°
The
oldest generally accepted evidence of tool use is 2.5-million-year-old cut
marks on animal bones found in
§
The
australopith fossils near the site had relatively small brains.
§
Perhaps
tool use originated before large hominid brains evolved.
·
The
earliest fossils that anthropologists place in our genus, Homo, are classified as Homo
habilis.
°
These
fossils range in age from 2.4 to 1.6 million years old.
°
This
species had less prognathic jaws and larger brains (about 600–750 cm3)
than australopiths.
°
In
some cases, anthropologists have found sharp stone tools with these fossils,
indicating that some hominids had started to use their brains and hands to
fashion tools.
·
Fossils
from 1.9 to 1.6 million years ago are recognized as a distinct species, Homo ergaster.
°
H. ergaster had a larger brain than Homo habilis, as well as long slender
legs well adapted for long-distance walking.
°
This
species lived in more-arid environments and was associated with
more-sophisticated tool use.
°
Its
reduced teeth suggest that it might have been able to cook or mash its food
before eating it.
·
Specimens
of early Homo show reduced sexual
dimorphism, a trend that continued with our species.
°
Sexual
dimorphism is reduced in pair-bonding species.
°
Male
and female Homo ergaster may have
engaged in more pair-bonding than earlier hominids, perhaps in order to provide
long-term biparental care of babies.
·
Some
paleontologists still think that Homo
ergaster were merely early specimens of Homo
erectus.
·
Homo erectus was the first hominid
species to migrate out of Africa, colonizing Asia and
°
They
lived from about 1.8 million to 500,000 years ago.
§
Fossils
from Asia are known by such names as “
§
In
Europe, Neanderthals arose from an earlier species, Homo heidelbergensis, which arose in Africa about 600,000 years ago
and spread to
·
The
term Neanderthal is now used for humans who lived throughout
°
Fossilized
skulls indicate that Neanderthals had brains as large as ours, though somewhat
different in shape.
°
They
made hunting tools from stone and wood.
°
Neanderthals
were generally more heavily built than modern humans.
·
Neanderthals
apparently went extinct about 30,000 years ago, contributing little to the gene
pool of modern humans.
·
Evidence
of the extinction of Neanderthal can be found in their DNA.
°
Scientists
have extracted DNA from four fossil Neanderthals living at different times and
places in
§
All
Neanderthals formed a clade, while modern Europeans were more closely related
to modern Africans and Asians.
·
In
2003, researchers in
°
These
early humans were slender and lacked brow ridges.
·
Evidence
suggests that all living humans are more closely related to each other than to
Neanderthals.
·
Europeans
and Asians share a relatively recent common ancestor and many African lineages
branched off from more ancient positions on the human family tree.
°
This
is supported by analysis of mDNA and Y chromosomes of various populations.
·
These
findings strongly suggest that all living humans arose from
·
Our
ancestors emerged in one or more waves, spreading into Asia, then Europe, and
·
The
rapid expansion of our species may have been spurred by the evolution of human
cognition.
°
Neanderthals
produced sophisticated tools, but had little creativity or capacity for
symbolic thought.
·
In
2002, researchers found 77,000-year-old from
·
By
36,000, humans were producing spectacular cave paintings.
·
Symbolic
thought may have emerged along with full-blown human language, raising the
reproductive fitness of humans by allowing them to construct new tools and
teach others how to build them.
·
Population
pressure may have driven humans to migrate into Asia and then
·
In
2002, geneticists found that FOXP2, a
gene essential for human language, experienced intense natural selection after
the ancestors of humans and chimps diverged.
°
Comparisons
of flanking regions of the gene suggest that most changes took place within the
past 200,000 years.
°
The
evolutionary change in FOXP2 may be
the first genetic clue about how our own species came to be.