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CHAPTER OUTLINE
32.1 Chordates
To be classified as a chordate, an animal must have, at some time during its life history, a dorsal
supporting rod called a notochord, a dorsal tubular nerve cord, pharyngeal pouches, and a
postanal tail.
Evolutionary Trends among the Chordates
The tunicates and lancelets are nonvertebrate chrodates, meaning that they do not have
vertebrae. The vertebrates include the fishes, amphibians, reptiles, birds, and mammals.
Several characteristics, including jaws, lungs, jointed appendages, invading land, and
reproduction suitable for land, evolutionarily distinguish each group of animals from the
preceding one.
Nonvertebrate Chordates
In nonvertebrate chordates the notochord never becomes a vertebral column. Lancelets
and tunicates are nonvertebrate marine chordates.
32.2 Vertebrates: Fish and Amphibians
In vertebrates the notochord is replaced by a vertebral column, which is part of the flexible but
strong, jointed endoskeleton. A high degree of cephalization is accompanied by complex sense
organs. Vertebrates have efficient respiration and excretion, and a closed circulatory system.
Fishes: First Jaws, Then Lungs
The first vertebrates were jawless fishes. Today there are three living groups of fishes:
jawless fishes, cartilaginous fishes, and bony fishes. The latter two groups have jaws.
Jawless Fishes
Jawless fishes have smooth, scaleless skin and no jaws or paired fins. These
include the hagfishes and lampreys.
Cartilaginous Fishes
Cartilaginous fishes are the sharks, the rays, and the skates. They have skeletons
of cartilage instead of bone.
Bony Fishes
Bony fishes are by far the most numerous and diverse of all the vertebrates.
Most of the bony fishes we ear are ray-finned fishes. The lobe-finned fishes are
the ancestors of amphibians; they had fleshy appendages that could be adapted to
land locomotion and most had a lung that was used for respiration.
Amphibians: Jointed Appendages
Amphibians include frogs, toads, newts, and salamanders. Amphibians have jointed
appendages and usually four limbs, they also have eyelids, ears, and a voice-producing
larynx. Adult amphibians usually have small lungs. Most live in the water as larvae and
on land as adults.
32.3 Vertebrates: Reptiles and Mammals
Reptiles: Amniotic Egg
The reptiles include the dinosaurs, which are now extinct, turtles, alligators, snakes,
lizards and birds. The body is covered with hard, keratinized scales that protect it from
desiccation and predators, one of their adaptations for life on land. Reptiles have welldeveloped lungs. Fertilization is internal and the female lays leathery, flexible, shelled
eggs. This amniotic egg made development on land possible by providing the developing
embryo with oxygen, food and water and protects it from drying out and injury.
Feathered Reptiles
Based on fossils and molecular studies, birds are now considered part of the Reptilia.
Their legs have scales and their feathers are modified reptilian scales, but they lay a hardshelled amniotic egg. With the exception of birds, reptiles are ectothermic.
Diversity of Birds
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The majority of birds are able to fly. Some are flightless. Traditionally they have
been classified based on beak and foot type and, to some extent, on their habitat
and behavior.
Anatomy and Physiology of Birds
Nearly every anatomical feature of a bird can be related to its ability to fly. Birds
have a four-chambered heart and are endothermic.
Mammals: Hair and Mammary Glands
Mammals evolved from the reptiles. The chief characteristics are body hair and milkproducing mammary glands.
Monotremes
Monotremes are mammals that have a cloaca, a common chamber for feces,
excretory wastes, and sex cells. They lay hard-shelled amniotic eggs. They are
represented by only five species: four spiny anteaters and the duckbill platypus.
Marsupials
The young of marsupials begin their development inside the female’s body, but
they are born in a very immature condition. Newborns crawl up into a pouch on
their mother’s abdomen, attach to the nipples of mammary glands, and continue
to develop. They are found mainly in Australia and include koalas and
kangaroos.
Placental Mammals
The vast majority of living mammals are placental mammals. The
extraembryonic membranes have been modified for internal development within
the uterus of the female. With the exception of marine mammals, mammals are
adapted to life on land. The mammalian brain is well-developed and enlarged
due to the expansion of the cerebral hemispheres. Mammals also have
differentiated teeth.
Primates
Primata are members of the order Primates. They are adapted to an arboreal life (living
in trees). Primate limbs are mobile, and the hands and feet both have five digits each.
The evolutionary trend among primates is toward a larger and more complex brain. One
offspring per birth interval is the norm. Primates has two suborders: the strepsirhini
(lemurs, aye ayes, bush babies, and lorises) and the haplorhini (monkeys, ape, and
humans).
32.4 Evolution of the Hominins
All primates share a common ancestor and over time they diverged, producing various lineages.
Humans are most closely related to African apes, but the two lineages split about 6 to 8 MYA
according to molecular data. Currently, humans and chimpanzees are probably the most closely
related, sharing more than 90% of our DNA. Hominins include humans and species very closely
related to humans and evolved about 5 MYA. Hominins, chimpanzees, and gorillas are now
grouped together as hominines. The hominids include the hominines and the oranguatan. The
hominoids include the gibbon and the hominids.
Evolution of Humanlike Hominins
The biggest derived characteristic that separates modern humans is bipedalism, an
anatomy suitable for standing erect and walking on two feet.
Australopithecines
Australopithecus anamensis, a fossil discovered in Middle Awash, Ethiopia, may
be the missing evolutionary link between Ardipithecus and the
australopithecines, the possible direct hominin ancestors for humans.
Evolution of Early Homo
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Early Homo species appear in the fossil record approximately 2 MYA, they all have a
brain size that is 600 cc or greater, their jaw and teeth resemble those of humans, and tool
use is in evidence.
Homo habilis and Homo rudolfensis
H. habilis is credited as being the first members of the genus to use tools. H.
habilis and H. rudolfensis were socially organized, probably scavengers, had
smaller cheek teeth, and were omnivorous. Their face protruded less and their
brains were larger than australopiths.
Homo ergaster and Homo erectus
Compared to H. habilis, H. ergaster had a larger brain, rounder jaw, prominent
brow ridges, and a projecting nose. Most likely H. erectus evolved from H.
ergaster after H. ergaster arrived in Asia. They had a knowledge of fire and may
have been the first to cook meat.
Homo floresiensis
It appears that H. floresiensis coexisted with modern H. sapiens and used tools
and fire.
32.5 Evolution of Modern Humans
The most widely accepted hypothesis for the evolution of modern humans from archaic humans
is referred to as the replacement model, or out-of-Africa hypothesis, which proposes that modern
humans evolved only in Africa and then migrated to Asia and Europe. This is supported by the
fossil record and DNA data. The opposing multiregional continuity hypothesis proposes that
modern humans arose from archaic humans in essentially the same manner in Africa, Asia, and
Europe.
Neandertals
The Neandertal brain was slightly larger than that of Homo sapiens. They had massive
brow ridges, wide, flat noses, a forward-sloping forehead and a receding lower jaw.
Evidence suggests that Neandertals were culturally advanced.
Cro-Magnon
Cro-Magnons are the oldest fossils to be designated Homo sapiens. They had lighter
bones, high foreheads, domed skulls, small teeth, and a distinct chin. They hunted
cooperatively and may have been the first to have language and their culture included art.
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