EXTENDED LECTURE OUTLINE
17.1 Fertilization
Fertilization results in a zygote.
Steps of Fertilization
Several sperm penetrate the corona radiata and zona pellucida, but only one sperm enters the egg. Sperm gain entry through
mechanism of contacting and then fusing with the egg plasma membrane before the sperm nucleus enters the cell.
17.2 Pre-Embryonic and Embryonic Development
Processes of Development
Embryonic development of humans and all other animals includes the following processes:
Cleavage
Cleavage begins right after fertilization as the zygote divides and divides again. The size of the cell does not
increase during this stage.
Growth
Growth accompanies cell division during embryonic development.
Morphogenesis
Morphogenesis is the reshaping of the embryo as cells migrate to different areas.
Differentiation
Differentiation occurs as cells take on specific structures and then functions.
Extraembryonic Membranes
Extraembryonic membranes extend out beyond the embryo. The amnion envelops the fetus in protective fluid. The yolk sac
is the first site of red blood cell formation, and part of this membrane eventually becomes a portion of the umbilical cord.
The allantois contributes to the circulatory system, and its vessels become the umbilical blood vessels. The chorion, the
outermost membrane, contributes to the placenta.
Stages of Development
Pre-Embryonic Development
This stage encompasses the events of the first week. The zygote divides repeatedly as it passes down the oviduct. A
morula is a compact ball of cells that becomes a blastocyst. In the blastocyst, there is an inner cell mass surrounded
by a layer of cells called the trophoblast. The inner cell mass will become the embryo and the trophoblast will
become the chorion.
Embryonic Development
Second Week
At the end of the first week, the embryo begins the process of implantation. The trophoblast begins to
secrete HCG, the hormone that is the basis of the pregnancy test. The inner cell mass detaches from the
trophoblast and becomes the embryonic disk. During gastrulation, the three primary germ layers,
endoderm, mesoderm, and ectoderm develop. As differentiation continues throughout development, the
three germ layers give rise to all other tissues and organs of the body.
Third Week
During the third week, the nervous system and the heart develop.
Fourth and Fifth Weeks
At the end of the fifth week, organs are developed and the placenta is forming. Limb buds are present, and
eyes, ears, and a nose appear.
Sixth through Eighth Weeks
During the sixth through eighth weeks the embryo changes to form the shape of an easily recognized
human being.
17.3 Fetal Development
By the tenth week the placenta is full formed and begins to produce progesterone and estrogen. The placenta has a fetal side
contributed by the chorion and a maternal side consisting of uterine tissues. The blood of the mother and the fetus never mix.
Path of Fetal Blood
Umbilical arteries carry oxygen-poor blood to the placenta while the umbilical vein carries blood rich in nutrients
and oxygen away from the placenta to the fetus. The umbilical vein enters the liver and then joins with vessels that
return blood to the right atrium. This mixed blood enters the heart and is shunted to the left atrium through the oval
opening. The left ventricle pumps this blood into the aorta.
Events of Fetal Development
Fetal development extends from the third to the ninth month.
Third and Fourth Months
During the third and fourth months, the body increases in size, and epidermal refinements (eyelashes, nipples)
become apparent. Bone is replacing cartilage. During this time, it becomes possible to distinguish males from
females.
Fifth through Seventh Months
The mother can feel fetal movement. The fetus’s thin skin is covered with lanugos, and the eyelids open fully.
Eighth through Ninth Months
During the last two months, the fetus grows greatly in size. It rotates its head down toward the cervix.
Development of Male and Female Genitals
The sex of an individual is determined at the moment of fertilization. Males have a pair of chromosomes designated as X and
Y, and females have two X chromosomes.
Normal Development of the Genitals
Internal Genitals
Gonads begin developing during the seventh week of gestation. Genes on the Y chromosome code for
testes development. In the absence of the Y chromosome, fetuses are female and develop a vagina, uterus,
and ovaries. Males and females have somewhat analogous development during various fetal stages.
External Genitals
The external tissues are also indifferent at first—they can develop into either male or female genitals. By
week 14, it is possible to determine whether there is a scrotum or labia.
Abnormal Development of Genitals
It is not correct to say that all XY individuals develop into males. The SRY gene located on the Y chromosome is
able to turn on other genes that cause testes to form and secrete the appropriate hormones.
Ambiguous Sex Determination
The absence of any one or more of these male hormones results in ambiguous sex determination. True
gonadal hermaphroditism in which a person has both ovarian and testicular tissue is rare.
17.4 Pregnancy and Birth
Major changes take place in the mother’s body during pregnancy due to placental hormones.
The Energy Level Fluctuates
When first pregnant, the mother may experience nausea and vomiting loss of appetite, and fatigue.
The Uterus Relaxes
Blood volume increases along with the number of red blood cells. Smooth muscle relaxation occurs in the uterus
and the gastrointestinal tract.
The Pulmonary Values Increase
There is an increase in vital capacity and tidal volume during pregnancy. The uterus comes to occupy most of the
abdominal cavity.
Still Other Effects
Other changes including stress incontinence, edema, and varicose veins occur during pregnancy.
Birth
Oxytocin and prostaglandins cause the uterus to contract and expel the fetus. The expulsion of a mucous plug from the
cervical plug marks the beginning of the first stage of parturition, or giving birth.
Stage 1
Stage 1 labor involves contractions that move the baby’s head downward, enhancing effacement and dilation of the cervix.
The amnion breaks, releasing amniotic fluid. This stage ends when the cervix is completely dilated.
Stage 2
Stage 2 labor has frequent contractions of longer duration. The mother experiences a desire to push. An episiotomy is often
performed to prevent tearing. The baby is pushed out during this stage.
Stage 3
Stage 3 is the delivery of the afterbirth or placenta.
17.5 Development After Birth
Development is a lifelong process into adulthood. After that period, aging occurs. Gerontology is the study of aging.
Hypotheses of Aging
Genetic in Origin
One theory of aging suggests that aging has a genetic basis. Cells can divide only so many times. As we grow older,
it may be that more cells age and die. Also, some cell lines may die before that maximum number of cell divisions
has been reached. In addition, offspring of long-lived people also tend to be long-lived. Some people may have
genes that code for efficient enzymes that remove free radicals, causing the individuals to live longer.
Whole-Body Process
A second theory of aging suggests that a hormonal decline can affect many different organ systems. The immune
system no longer performs as well, which is perhaps why cancer is more prevalent in the elderly. Aging may be due
to the failure of a particular tissue type found throughout the body.
Extrinsic Factors
A third theory on aging suggests that years of poor health habits contribute most to aging. Osteoporosis is a good
example.
Effect of Age on Body Systems
Skin
Skin loses elasticity and becomes thinner with age, resulting in sagging and wrinkling. Fewer sweat glands are
present, so temperature regulation is less efficient.
Processing and Transporting
Deterioration of the cardiovascular system is the leading cause of death among the elderly. The heart shrinks with
age, and fatty deposits clog arteries. Low-cholesterol, low-fat diets slow degenerative changes. Lungs lose some
elasticity, so ventilation is reduced. A reduced blood supply to the kidneys results in the kidneys becoming smaller
and less efficient. The digestive tract may lose muscle tone but still absorbs nutrients efficiently.
Integration and Coordination
Normal aging results in the loss of few nerve cells. Short- term memory may decline, but overall cognitive skills
remain. After age 50, there is a slow decline in the ability to hear higher frequencies, and the lens of the eye does not
accommodate as well. Loss of skeletal muscle mass is common but can be controlled through exercise. Bone density
declines, which can be slowed by adequate calcium intake and exercise.
The Reproductive System
Females undergo menopause and are no longer reproductive. In males, sperm production continues until death.
Conclusion
Good health habits started when young slow the aging process and contribute to a long, healthy life span.
EXTENDED LECTURE OUTLINE
18.1 Chromosomes and the Cell Cycle
Humans have 46 chromosomes that occur in 23 pairs. Twenty-two of these pairs are called autosomes. One pair of chromosomes is
called the sex chromosomes. A karyotype is a picture of the chromosomes present in a cell. Staining causes the chromosomes to have
cross-bands of varying widths and intensity and these, along with size and shape, help identify the individual chromosomes.
A Karyotype
A normal karyotype is diploid, meaning that it has the full complement of 46 chromosomes. Chromosomes in dividing cells
are composed of two identical parts, called sister chromatids. The chromatids are held together at a region called the
centromere.
The Cell Cycle
The cell cycle is an orderly process that results in the division of one cell into two identical cells. It has two parts: interphase
and cell division.
Interphase
Most of the cell cycle is spent in interphase which is divided into three stages: The G 1 stage occurs before DNA
synthesis, the S stage includes DNA synthesis, and the G2 stage occurs after DNA synthesis.
Cell Division
Cell division, consisting of mitosis (the division of the nucleus) and cytokinesis (the division of the cytoplasm),
increases the number of somatic cells. Apoptosis or programmed cell death decreases that number.
18.2 Mitosis
Mitosis is duplication division. The nuclei of the two new daughter cells have the same number and kinds of chromosomes as the
parent cell.
Overview of Mitosis
When mitosis is going to occur, chromatin in the nucleus becomes highly condensed, and the chromosomes become visible.
Each chromosome is composed of two sister chromatids. During mitosis, the centromeres divide and the sister chromatids
separate. Each daughter cell gets a complete set of chromosomes and is 2n.
The Spindle
The centrosome is the microtubule organizing center of the cell. After centrosomes duplicate, they separate and
form the poles of the mitotic spindle which is responsible for separating the chromatids during mitosis. The
centrioles are short cylinders of microtubules that are present in centrosomes.
Phases of Mitosis
Mitosis is divided into four phases: prophase, metaphase, anaphase, and telophase.
Prophase
Spindle fibers appear, the chromosomes condense, the nuclear envelope fragments, and the nucleolus disappears.
Spindle fibers attach to the centromeres of the chromosomes.
Metaphase
Metaphase involves a lining up of chromosomes along the cell equator.
Anaphase
At the start of anaphase, sister chromatids split, and then are pulled toward respective poles of the cells. The
chromatids are now chromosomes.
Function of the Spindle
The spindle moves chromosomes during this process. One type of spindle fibers extends to the equator of
the spindle where they overlap. These fibers increase in length and push the chromosomes apart. The
other type of fibers is attached to the centromeres. These fibers short and pull the chromosomes apart.
Telophase
When chromosomes arrive at each pole, telophase begins. The spindle disappears, the nucleoli reappear, and nuclear
envelopes form.
Cytokinesis
Cytokinesis is the division of the cytoplasm and organelles. A slight indentation, called a cleavage furrow, passes around the
circumference of the cell. Actin filaments form a contractile ring, and as the ring gets smaller, the cleavage furrow pinches
the cell in half.
The Importance of the Cell Cycle and Mitosis
The cell cycle and mitosis are responsible for our growth as well as the repair of injury. Ordinarily, a cell cycle control
system works perfectly to produce more cells only to the extent necessary. A tumor develops when the cell cycle control
system is not functioning properly.
Cell Cycle Control System
The cell cycle control system extends from the plasma membrane to particular genes in the nucleus. Growth factors
stimulate a cell-signaling pathway that results in the activation of genes whose products either stimulate or inhibit
the cell cycle.
18.3 Meiosis
Meiosis is reduction division. Because meiosis occurs twice, there are four daughter cells, each with half as many chromosomes as
the parent cell.
Overview of Meiosis
At the start of meiosis, the parent cell is 2n or diploid, and the chromosomes occur in pairs. The members of a pair are called
homologous chromosomes.
Meiosis I
During meiosis I, homologues pair and synapse, during which time nonsister chromatids exchange genetic material
in crossing-over. Next, the homologous chromosomes of each pair separate so that one chromosome from each pair
will be in the daughter cell. This reduces the number of chromosomes to half. The cell waits momentarily during
interkinesis between meiosis I and meiosis II. There is no duplication of DNA.
Meiosis II and Fertilization
During meiosis II, the haploid number of chromosomes per cell is still in duplicated condition. This division
separates the sister chromatids. Fertilization restores the diploid number of chromosomes in the zygote.
Stages of Meiosis
Both meiosis I and meiosis II have the same four stages of nuclear division as did mitosis: prophase, metaphase, anaphase,
and telophase.
Prophase I
In prophase I, the spindle appears, nuclear envelopes disappear, homologous chromosomes pair and synapse to form
tetrads, and crossing-over occurs. This means that chromatids held together by a centromere are no longer identical.
Metaphase I
In metaphase I, the homologous pairs line up along the equator. The maternal and paternal homologous pairs align
independently at the equator, meaning that each gamete will have a different combination of maternal and paternal
chromosomes.
Significance of Meiosis
Meiosis is part of gametogenesis, the production of sperm and egg. Meiosis keeps the chromosome number constant
from generation to generation. An easier way to keep the chromosome number constant is to reproduce asexually as
uniceullar organisms such as bacteria, protozoans, and yeasts do.
18.4 Comparison of Meiosis and Mitosis
Mitosis and meiosis are both nuclear division, but there are several differences between them. DNA replication takes place only once
prior to both, however meiosis requires two divisions while mitosis only requires one. Mitosis produces two daughter cells while
meiosis produces four. The four daughter cells of meiosis are haploid as compared to the diploid cells after mitosis. The daughter
cells after mitosis are genetically identical while the daughter cells after meiosis are not.
Occurrence
Meiosis occurs only at certain times during the life cycle of sexually reproducing organisms. Mitosis occurs in all tissues
during growth and repair.
Process
Comparison of Meiosis I with Mitosis
Homologous chromosomes pair and undergo crossing-over during prophase I of meiosis, but not mitosis. Paired
homologous chromosomes align at the equator during metaphase I of meiosis and homologous chromosomes with
their centromeres intact separate and move to opposite poles during anaphase I.
Comparison of Meiosis II with Mitosis
The events of meiosis II are just like those of mitosis except that in meiosis II the nuclei contain the haploid number
of chromosomes.
Summary
Tables 18.1 and 18.2 compare meiosis I and meiosis II with mitosis.
Spermatogenesis and Oogenesis
Spermatogenesis is the production of sperm in males and oogenesis is the production of eggs in females.
Spermatogenesis
Spermatogenesis begins with a primary spermatocyte and ends with four haploid sperm.
Oogenesis
Oogenesis begins with a primary oocyte and ends with one secondary oocyte (also called an egg). At each division,
a polar body containing the separated chromosomes but little or no cytoplasm is produced. The polar bodies
disintegrate.
18.5 Chromosome Inheritance
Normally each individual receives 22 pairs of autosomes and two sex chromosomes.
Changes in Chromosome Number
Sometimes individuals are born with too many or too few autosomes or sex chromosomes. This is most likely due to
nondisjunction during meiosis when either homologous chromosomes (meiosis I) or sister chromatids (meiosis II) fail to
separate. In a trisomy, one chromosome is present in three copies where in a monosomy, one chromosome is present in only
one copy. The chances of survival are greater when these abnormalities involve the sex chromosomes. In normal XX
females, one of the X chromosomes becomes a Barr body, an inactivated X chromosome that is highly condensed.
Down Syndrome, an Autosomal Trisomy
The most common autosomal trisomy seen among humans is Down syndrome, also called trisomy 21. Down
syndrome is easily recognized by these common characteristics: short stature; an eyelid fold; a flat face; stubby
fingers; a large, fissured tongue; a round head, a palm crease; and unfortunately, mental retardation which can vary
in intensity. The genes that cause the symptoms of Down syndrome are located on the bottom third of the
chromosome.
Changes in Sex Chromosome Number
An abnormal sex chromosome number is the result of inheriting too many or too few X or Y chromosomes.
Turner Syndrome
An individual with Turner syndrome has only one X chromosome. They can lead fairly normal lives if
they receive hormone supplements.
Klinefelter Syndrome
A male with Klinefelter syndrome has two or more X chromosomes in addition to a Y chromosome. No
matter how many X chromosomes there are, an individual with a Y chromosome is usually a male.
Poly-X Females
A poly-X female has more than two X chromosomes and extra Barr bodies. Females with more than three
X chromosomes occur rarely.
Jacobs Syndrome
Males with two Y chromosomes in addition to an X have Jacobs syndrome.
Changes in Chromosome Structure
Various agents in the environment, such as radiation, certain organic chemicals, or even viruses, can cause chromosomes to
break. If the broken ends do not rejoin in the same pattern as before, various chromosomal mutations, such as deletions,
duplications, inversions and translations, result.
Human Syndromes
Deletion Syndromes
Williams syndrome occurs when chromosome 7 loses a tiny end piece.
Translocation Syndromes
A person who has both of the chromosomes involved in a translocation has the normal amount of genetic
material and is usually healthy. The person who inherits only one of the translocated chromosomes will no
doubt have only one copy of certain alleles and three copies of certain other alleles.
EXTENDED LECTURE OUTLINE
19.1 Cancer Cells
Cancer is actually over a hundred different diseases. However, these characteristics are common to cancer cells.
Characteristics of Cancer Cells
Cancer is a cellular disease.
Cancer Cells Lack Differentiation
Cancer cells are nonspecialized and do not contribute to the functioning of a body part.
Cancer Cells Have Abnormal Nuclei
The nuclei of cancer cells are enlarged and may contain an abnormal number of chromosomes.
Cancer Cells Have Unlimited Replicative Potential
Cancer cells are immortal and keep on dividing for an unlimited number of times.
Cancer Cells Form Tumors
Cancer cells pile on top of one another and grow in multiple layers, forming a tumor.
Cancer Cells have No Need for Growth Factors
Cancer cells keep on dividing, even when stimulatory growth factors are absent, and they do not respond to
inhibitory growth factors.
Cancer Cells Gradually Become Abnormal
The process of carcinogens is a multistage process that can be divided into three phases: initiation, promotion, and
progression.
Cancer Cells Undergo Angiogenesis and Metastasis
Tumors require a well-developed capillary network to bring nutrients and oxygen. Angiogenesis is the formation of
new blood vessels. When cancer cells begin new tumors far from the primary tumor, metastasis has occurred.
Cancer is a Genetic Disease
Proto-oncogenes encode proteins that promote the cell cycle and prevent apoptosis. Tumor-suppressor genes encode proteins
that inhibit the cell cycle and promote apoptosis.
Proto-Oncogenes Become Oncogenes
When proto-oncogenes mutate, they become cancer causing genes called oncogenes. These would be considered
“gain of function” mutations. Some proto-oncogenes encode growth factors or growth factor receptors.
Tumor-Suppressor Genes Become Inactive
When tumor-suppressor genes mutate, their products no longer inhibit the cell cycle nor promote apoptosis. These
mutations can be called “loss of function” mutations.
Types of Cancer
Oncology is the study of cancer. Tumors are classified according to their place of origin: carcinomas are cancers of epithelial
cells, sarcomas are cancers that arise in muscles and connective tissue, leukemias are cancers of the blood, and lymphomas
are tumors of lymphatic tissue.
Common Cancers
Cancer can occur in all parts of the body, but some organs, such as the lungs, the colon/rectum, the blood, breast,
and skin are more susceptible than others.
19.2 Causes and Prevention of Cancer
Cancer is caused by a combination of heredity and environmental factors.
Heredity
Certain cancers, such as breast, lung, and colon cancers, run in families. Some childhood cancers are inherited as a dominant
gene.
Environmental Carcinogens
A mutagen is an agent that enhances the chance of a DNA mutation. A carcinogen is an environmental agent that can trigger
cancer. Carcinogens are frequently mutagenic.
Radiation
Ultraviolet radiation in sunlight and tanning lamps triggers the development of skin cancers. Melanoma is the
spreading form of skin cancer. Radon gas can lead to lung cancer. X rays and nuclear radiation can lead to cancer.
Organic Chemicals
Tobacco Smoke
Tobacco smoke contains numerous organic chemicals that can lead to cancers of the lung, mouth, larynx,
bladder, kidney, and pancreas.
Pollutants
Exposure to pollutants such as metals, dust, chemicals, or pesticides can increase the risk of cancer.
Viruses
At least four types of DNA viruses, hepatitis B and C viruses, Epstein-Barr virus, and human papillomavirus, are
directly believed to cause human cancers.
Dietary Choices
Nutrition is emerging as a way to help prevent cancer. The consumption of fruits and vegetables, whole grains instead of
refined grains and a limited consumption of red meats are recommended.
19.3 Diagnosis of Cancer
The earlier a cancer is detected, the more likely it can be effectively treated.
Seven Warning Signs
The seven warning signs of cancer spell the word CAUTION (change in bowel or bladder habits, a sore that does not heal,
unusual bleeding or discharge, thickening or a lump in breast or elsewhere, indigestion or difficult in swallowing, obvious
change in wart or mole, nagging cough or hoarseness.)
Routine Screening Tests
Pap smears for cervical cancer are an example of a routine screening test. For breast cancer, routine self-exam, exam by a
doctor, and mammography are recommended. Colon cancer screening involves a digital rectal exam, sigmoidoscopy, a fecal
occult blood test, and colonoscopy. Diagnosis of cancer in other parts of the body may involve other types of imaging.
Tumor Marker Tests
Blood tests for tumor antigens/antibodies produced against tumors are called tumor marker tests. They can be used to detect
first-time cancer and cancer relapses.
Genetic Tests
When individuals test positive for the presence of marker genes, such as the BRCA1 breast cancer oncogene, they should be
vigilant for signs of cancer. Microsatellite abnormalities and the presence of telomerase indicate that cancer is present.
19.4 Treatment of Cancer
Standard Therapies
The following are the standard methods of cancer therapy.
Surgery
Surgery is sufficient for cancer in situ. Surgery followed by radiation is recommended when cancer cells may have
been left behind.
Radiation Therapy
Ionizing radiation causes chromosomal breakage and cell cycle disruption. Therefore, dividing cells, such as cancer
cells, are more susceptible to its effects than other cells.
Chemotherapy
Chemotherapy is used for metastatic cancers that may have spread throughout the body. Chemotherapeutic drugs
kill cells during cell division. Certain types of cancers are now successfully treated by combination chemotherapy
alone. Bone marrow transplants are used when a patient is to receive high doses of radiation or chemotherapy.
Newer Therapies
Several therapies are currently in clinical trials.
Immunotherapy
The use of monoclonal antibodies designed to combine with receptors on cancer cells is under investigation. A
cancer vaccine that stimulates the body’s immune system to attack cancer cells has promise but has yet to be highly
effective.
p53 Gene Therapy
Adenoviruses are used to carry a normal copy of the p53 gene into cancerous tissues.
Other Therapies
Other therapies such as the use of antiangiogenic drugs are being investigated.
EXTENDED LECTURE OUTLINE
22.1 Origin of Life
It is suspected that chemical evolution produced the first cells on earth.
The Primitive Earth
The sun and planets formed from aggregates of dust and debris 4.6 billion years ago. The primitive atmosphere on earth
was produced by outgassing from the earth’s interior and contained very little free oxygen. As the earth cooled over
millions of years, water vapor condensed and produced the earth’s oceans.
Small Organic Molecules
As the earth cooled, clouds of water vapor condensed and rained down on the earth, bringing with them atmospheric
gases. Energy from lightning and volcanic heat triggered the gases to react, producing simple organic compounds.
Macromolecules
Small molecules reacted and formed larger ones, and RNA likely formed. RNA can act both as a substrate and an
enzyme, which supports this RNA-first hypothesis. The protein-first hypothesis suggests that dry heat, such as on a
rocky shore, caused proteins to form from amino acids.
The Protocell
A protocell with a lipid-protein membrane must have evolved first. Small organic molecules would have served as food
for this heterotroph.
The True Cell
A true cell carries on protein synthesis to produce enzymes that allow DNA to replicate. If the first cell had RNA genes,
it could have directed protein synthesis. A reverse transcriptase would have produced DNA in multiple copies. If the cell
began with proteins, it could function as enzymes, guiding the synthesis of nucleotides, and eventually, nucleic acids.
22.2 Biological Evolution
The first true cells were most likely prokaryotes. Eukaryotic cells, with nuclei, evolved later. All living things can trace their
biological evolution back to the first cells. Descent from the original cell(s) explains why all living things have a common chemistry
and a cellular structure. Differences among living things can be attributed to adaptation to the environment.
Common Descent
Charles Darwin was one of the first scientists to accumulate data that supported the idea of common descent.
Fossil Evidence Supports Evolution
Fossils are the best evidence for evolution because they are the actual remains of species that lived on Earth at
least 10,000 years ago and up to billions of years ago. When an organism dies, the hard parts are preserved as
fossils. Paleontologists have spent considerable time in the field looking for fossils. The fossil record is far
more complete now than it was when Darwin formulated his theory of evolution. For example, transition
fossils have been found.
Other Evidence Supports Evolution
Many different types of evidence support the hypothesis that organisms are related through common descent.
Biogeographical Evidence
Biogeography is the study of the distribution of plants and animals throughout the world. Darwin noted
that South America had no rabbits although the environment could have supported them. He concluded
that rabbits evolved elsewhere.
Anatomical Evidence
Many diverse organisms show anatomical similarities, such as vertebrate forelimbs. Similar structures
that were inherited from a common ancestor are called homologous structures. The unity of plan seen
in all vertebrates is evident in their common stages of embryological development. Analogous
structures have the same function but do not share a common ancestry. Vestigial structures are
anatomical features that are fully developed in one group of organisms but that are reduced and may
have no function in similar groups.
Biochemical Evidence
Nearly all organisms on earth use the same biochemical molecules (DNA, ATP), all use the same
triplet code for amino acids, and many share similar gene sequences. The degree of relatedness
between organisms is reflected in the similarity of their DNA base sequences.
Intelligent Design
Evolution is a scientific theory that has been supported by repeated scientific experiments and observations. Many
scientists, and even religions, argue that intelligent design is faith based and not science.
Natural Selection
Charles Darwin’s theory of evolution through natural selection was based on the idea that a species becomes adapted to
its environment over time. The environment selects the individuals that are best adapted. This idea contrasts with the
teleological notion of Lamarck that organisms acquired characteristics throughout their life spans. Darwin’s ideas on
natural selection are based on variations within the population, competition for limited resources, and adaptation.
Natural selection accounts for the great diversity of life on earth because of its diverse environments. Natural selection
has been occurring for a very long time.
22.3 Classification of Humans
Biologists classify organisms based on evolutionary relationships. Organisms are named using their genus and species, a binomial
system of classification.
DNA Data and Human Evolution
Researchers are depending more and more on DNA data to trace the history of life. DNA data is particularly useful
when anatomical differences are unavailable.
Humans are Primates
Primates are adapted to an arboreal life—that is, for living in trees. The prosimians include the lemurs, tarsiers, and
lorises. The anthropoids include the monkeys, apes, and humans.
Mobile Forelimbs and Hindlimbs
Primate limbs are mobile, and the hands and feet both have five digits each.
Binocular Vision
In chimps, like other primates, the snout is shortened considerably, allowing the eyes to move to the front of
the head.
Large, Complex Brain
The evolutionary trend among primates is toward a larger and more complex brain. The portion of the brain
devoted to smell gets smaller, and the portions devoted to size increase in size and complexity during primate
evolution.
Reproduced Reproductive Rate
One birth at a time is the norm in primates.
Comparing Human Skeleton to the Chimpanzee Skeleton
Humans, as opposed to chimpanzees, are adapted for an upright stance.
22.4 Evolution of Hominids
Our evolutionary tree shows that all primates share one common ancestor and that the other types of primates diverged from the
human line of descent over time. The split between the ape and human lineage may have occurred about 7 MYA.
The First Hominids
Hominid is a term that refers to our branch of the evolutionary tree. Any fossil placed in the hominid line of descent is
closer to us than to one of the African apes. Molecular data involves genetic changes that accumulate at a fairly constant
rate. Such changes can be used as a type of molecular clock.
Hominid Features
One of the hominid features is bipedal posture. Two other hominid features of importance are the shape of the face and
brain size.
Suggested Fossils
Several fossils have been found that date to between 5 and 7 MYA including Sahelanthropus tchadensis, Orrorin
tugenensis, Ardipithecus kadabba, and Ardipithecus ramidus.
Evolution of Australopithecines
The hominid line led directly to modern humans and began with the australopithecines in Africa. Both gracile and robust
australopithecines may have existed simultaneously.
Southern Africa
Australopithecus africanus, a gracile type, and A. robustus, a more robust form are both believed to have walked upright
but had apelike limbs.
Eastern Africa
Australopithecus afarensis, or “Lucy,” was small but walked upright and had a heavy jaw and smaller brain than modern
humans. All human characteristics did not evolve at the same time but instead exhibited mosaic evolution.
22.5 Evolution of Humans
Fossils are from the genus Homo if the brain size is 600 cc or more, if the jaws resemble those of humans, and if tool use is evident.
Early Homo
Homo habilis
Homo habilis (“handy man”) made stone tools. Stone flakes were used to clean hides and remove meat from
bones. Speech was likely in this group, which also probably possessed attributes of culture and cooperation.
Homo erectus
Homo erectus had an even larger brain and traveled extensively throughout Europe, Africa, and Asia. It
probably first appeared in Africa. It was the first hominid to use fire and to make axes and cleavers. It was a
good hunter. Evidence indicates the use of “home bases” and social interaction. Language and culture were
likely.
Evolution of Modern Humans
The multiregional continuity hypothesis suggests that modern humans arose simultaneously in several different places.
The out-of-Africa hypothesis suggests that Homo sapiens arose from H. erectus in Africa and then migrated to other
areas of the world about 100,000 years ago.
Neandertals
The Neandertals (H. neanderthalensis) from 200,000 years ago had massive brow ridges and protruding facial features,
and were, perhaps, an archaic H. sapiens. They were heavily muscled and had larger brains than modern humans. They
were culturally advanced and buried their dead with flowers, indicating a possible religion.
Cro-Magnons
Cro-Magnons (H. sapiens), from 100,000 years ago, had a modern appearance, were accomplished hunters, and likely
caused the extinction of many large mammals. They painted and sculpted, and lived in small groups.
Human Variation
All humans on earth today belong to one species, Homo sapiens, even though differences occur. Such phenotypic
differences like skin color are most likely due to climatic differences. Differences in stature could reflect climatic
temperature differences.
Genetic Evidence for a Common Ancestry
Molecular data do not support the notion of separate “races” of people. The majority of genetic various occurs
within ethnic groups, not among them.