This document provides a training manual for content in the Human Evolution Exploration Station at the Denver Museum of Nature and Science. It includes an overview of specimens to be displayed, such as various hominin species and chimpanzees. The manual then covers topics to be explained in the station, including studying human evolution, identifying fossils, dating techniques, genetics and evolution, bipedalism in humans, and the species Homo sapiens. It provides details on key hominin specimens and how to classify and identify human remains based on anatomical features.

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2016
Human Evolution
Training Manual
DENVER MUSEUM OF NATURE AND SCIENCE
MEGAN MURPHY
EditedbySamanthaRichards andTylerLyson

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Table of Contents
I. Specimens in the Human Evolution Exploration Station
II. Content for the Human Evolution Exploration Station
2.1 Things to keep in mind
2.2 The basics
i. Studying human evolution
ii. Science
iii. Darwin and natural selection
2.3 Finding fossils
2.4 Human taxonomy and identifying the remains
i. Teeth
ii. Physical bipedal identifiers
iii. Flaws to bipedalism
iv. Sex of a skeleton
v. Age of death
2.5 Dating fossils
2.6 Genetics
i. Evolutionary forces
ii. Smithsonian National Museum of Natural History: What does it mean to
be human?
2.7 Becoming bipedal
2.8 Homo sapiens
III. Supplemental Information
3.1 Australopithecus afarensis – Lucy
3.2 Stone tools
3.3 Neanderthals
3.4 Language

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I. Specimens in the Human Exploration Station
Pan paniscus (bonobo chimpanzee)
Bonobos, along with the common chimpanzee
Pan troglodytes,are our closest living relatives.
They are quadrupeds via knuckle walking, and as
such, their foramen magnum is placed at the rear
of the skull. Chimps have a protruding face with
large canines and a honing mechanism on the
lower jaw. Their average brain size is 305-485
cubic cm. Additionally, bonobos are sexually
dimorphic with males weighing on average 86
pounds and standing between 2.4-2.7 feet tall,
and females average 68 pounds at 2.3-2.5 feet
tall.
Sahelanthropus tchadensis
This is oldest hominid on record and lived
around 6-7 million years ago. Fossils were
found in Chad in 2001. Sahelanthropus
demonstrates an evolving animal with a brain
size similar to chimps at 320-380 cubic cm.
Additionally they have a relatively flat face
compared to chimps; yet, a protruding face
compared to modern humans. Additionally, the
foramen magnum is becoming more centrally
located—but compared to modern humans is still
pretty far back. Based on this placement, they
were probably bipedal walkers but also regularly
moved around on all four limbs. (Fossil record:
nine cranial specimens that include a relatively
complete skull, four jawbones, and a few teeth.
No other body parts present.)
Homo erectus
Homo erectus is the oldest hominin species
found inside and outside Africa. Fossils have
been found in Europe and far east Asia. H.
erectus lived between 1.9 million years- 14,000
years ago. Although earlier species of hominids
were equally as efficient walkers, scientists
believe H. erectus was the first one smart enough
to actually travel long distances. Moving takes a
lot of forethought and the ability to carry or find
sustenance for survival. H. erectus’s brain size is
roughly 750-1,300 cubic cm. Additionally, the
foramen magnum is centrally located, and the
face and teeth are relatively modern looking.
(Fossil record: one nearly complete skeleton
missing just the hands and feet, as well as several

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incomplete craniums, teeth, jaws, and a few limb
bones.)
Modern Homo sapiens
Modern humans display moderate skeletal
variation and slight sexual dimorphism. They
have large, rounded skulls, with a flat face
tucked underneath. Their average brain size is
roughly 1,000-2,000 cubic cm. Additionally, the
foramen magnum is centrally placed and the
canine teeth are drastically reduced compared to
chimpanzees. Oldest Homo sapiens lived around
200,000 years ago in Africa.
Laetoli Footprints
In 1976 Mary Leaky discovered these prints in
Laetoli, Africa. The prints are in fossilized
volcanic ash and date to 3.6 million years old.
Along with many species of animal prints
including elephants and giraffes, are footprints of
2-3 Australopithecus afarensis’ (Lucy’s kin).
Based on size, there is an adult walking with a
child, and scientists believe there is a probable
third adult following. In this topographic image,
we see that the A. afarensis moved with a similar
heel-to-toe (push off) strategy like modern
humans and that the humanlike gait developed
around 3-4 million years ago.
II. Content for the Human Evolution Exploration Station
2.1 Things to keepin mind
 Biological evolution simply refers to the change in the genetic and morphological
makeup of populations over time.
 Evolution via natural selection is a testable theory that has yet to be disproven. Because
evolution is a testable theory, it is different than religion which is faith based.
 A hominin refers to the taxonomic group of modern humans and all extinct primates that
are more closely related to humans than to chimps, including other Homo species,
Australopithecus, Paranthropus, Orrorin, Sahelanthropus and Ardipithecus. A new
classification system, taking into account the close relationship between humans and
chimps, places orangutans, gorillas, and chimps together with humans in the family
Hominidea (hominids); chimps and humans in the subfamily Homininae (hominines);
and humans in the tribe Hominini (hominins).2
1 “Human Family Tree Now a Tangled, Messy Bush,” last modified August 31, 2007,
http://www.livescience.com/7376-human-family-tree-tangled-messy-bush.html.
2 “Hominid, hominin, hominoid, human,” accessed February 4, 2016, http://stylemanual.ngs.org/home/H/hominid.

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 One possible reason why hominins thrived is because they were diverse eaters. Truly, no
group was limited by their food choice and each used their omnivore teeth to the fullest.
2.2 The basics
i. Studying human evolution
Anthropology is the science that investigates human biological and cultural variation and
evolution. There are four subfields to anthropology: linguistics, archeology, cultural, and
biological. For the purposes of this exploration station, we will mainly be focusing on cultural
and biological anthropology, with minor attention to archeology, and little towards linguistics.
ii. Science
When considering human evolution, it is important to understand exactly what science is and the
underlying relationship between facts, hypotheses, and theories. Understanding science will
clarify the differences between what we are teaching in Prehistoric Journey and religion. The
scientific process starts by forming an answerable question, known as a hypothesis. Hypotheses
are tested by collecting data and must have the potential to be rejected. If a hypothesis cannot be
tested or falsified, then is it not a scientific hypothesis, and rather a supernatural study. Again, a
hypothesis must be testable in which the research will prove or disprove the original question or
statement. The data scientists collect can be in the form of observation or in a controlled activity
termed experiments. Once enough data is gathered, scientists are then able to form a theory
explaining the hypothesis. A scientific theory is not speculation or a random guess; rather, a
theory is a set of hypotheses that have been tested repeatedly and have not been rejected. In other
words, scientists do not prove theories, simply, they continuously fail to reject them. Because of
this, scientific theories are not static and still have the potential to be rejected as new discoveries
are made.
As of today, evolution is both a fact and a theory. As a fact, there are forms of life today that did
not exist millions of years ago and there are forms that existed in the past but do not live today.
This can be seen in the fossil record. As a theory, various hypotheses have been suggested in the
past to explain how and why evolution has occurred. Over time, Charles Darwin’s hypothesis of
natural selection has yet to be rejected.
iii. Darwin and natural selection
While working on the Galapagos Islands, Darwin observed that the islands’ finches varied
considerably from the ones he witnessed at home. He came to wonder why their appearances
differed even though they were the same species. If we look at modern humans, the same is true.
We are all Homo sapiens, yet some are tall, some short; some are light, some are dark, etc.
Darwin attributed the different appearances to the finches’ different environments. He found that
organisms seem well adapted to specific environments, like finches with downy feathers dwell in
colder climates, and finches with longer beaks tend to live near insects that live deep in tree
trunks.

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Additionally, after reading the works of economist Thomas Malthus (1766-1834), Darwin knew
more individuals are born to a species than can possibly survive. Because of this, Darwin
observed that the individual who is more likely to feed itself, like the finches with long beaks
near the tree trunk dwelling insects, are most likely to survive and reproduce. For an inherited
trait like this, in an optimal environment for the individual, Darwin saw that nature (a.k.a. the
environment) selected those individuals to survive and as such passed on those advantageous
genes to the next generation.
Perhaps the most important aspect to natural selection is that it operates on genetic variation that
is already present in a species. As with the finch example, even though all the finches are of the
same species, their genetic makeup is different. Let’s say all the finches originated in a warmer
climate where insects live in tree trunks. Then a group of 100 finches decided to fly north and
enter a cooler climate. Of the original 100 traveling birds, a random number of them were
already born with thicker downy feathers. While this adaptation probably did not hurt them in
the warmer climates, it definitely was not advantageous. Only when they traveled to colder
weather did this trait become beneficial. At this point, the finches with the thicker downy
feathers become more likely to survive and pass on this genetic, inherited trait. Because traits
are relative, scientists cannot state which traits are “good” or “bad”, only which traits seem better
or worse for the organism’s environment. Once the environment changes, different traits that are
already expressed in species via variation, become advantageous or disastrous. Specifically for
human evolution, scientists can only infer what was genetically “advantageous” by what is still
present today. There is not a complete hominin fossil record, thus we do not know all of the
inherited characteristics that have gone extinct in our past.
2.3 Finding fossils
The evidence for evolution remains in the fossil record. Many species no longer exist, like the
Gomphotherium, but its relatives do (i.e. modern elephants). A similar case can be found with
the changing of certain features. For instance, the early proto horse, Hyracotherium, possessed
five toes, while the modern horse only has one. Although there is a plethora of evolutionary
evidence in the fossil record, fossils are rare and hard to find. “It takes a lot of luck and special
circumstances for any creature to get preserved as a fossil. When an organism dies, its body
immediately begins to decompose. Bacteria and insects get right to work, breaking down the
plant or animal’s organic material. Scavengers come running, grabbing arms and legs, dashing
off with body parts to munch the meat off the bones. Fluctuating temperatures stretch and shrink
the body’s tissues. Rain and sun degrade skin and bones. Herds of animals trample the
remaining structures, and beetles chew up whatever happens to be left.”3 Luckily, some animals
die and are rapidly buried by water, ash, lava, sand, a collapsed cave, or even an avalanche. In a
majority of environments, soft tissue like skin, muscle, and DNA deteriorate (via bacteria or
other animal consumption, and/or the minerals dissolve) and leave little to nothing behind.
Scientists (like paleontologists, paleoanthropologist, archeologists, etc.) are then largely left with
the hard parts such as bones and teeth. Today, many scientists try to maximize their odds of
3 “Conditions of Fossil Preservation: Rapid Burial, Hard Parts & the Elements,” accessed February 4, 2016,
http://study.com/academy/lesson/conditions-of-fossil-preservation-rapid-burial-hard-parts-temperature.html.

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success by looking for places that have rocks of both the right age and right type (i.e.
sedimentary) to preserve fossils.
All of this is important to keep in mind while explaining human evolution. The hominin fossil
record is largely incomplete (note: the fossil record for any group including invertebrates, plants,
etc. is not 100% complete). This is not because the fossils are not there, we just have not found
them or the right aged rocks yet. Humans have been relatively small creatures, and it is highly
probable that many animals scavenged hominin bodies after death, making their remains even
harder to find.
2.4 Human taxonomy and identifying the remains
Modern human taxonomy under kingdom, phylum, class, order, family, genus, species, sub-
species: Animalia, Chordata, Mammalia, Primate, Hominoidea, Homo, sapiens, sapiens.
Based on genetic similarities, humans are classified with the great apes under primates.
Although we did not evolve from any living primate today, our closest living relatives are
chimpanzees and bonobos. In other words, we share a common ancestor with chimpanzees and
bonobos from which humans, chimps, and bonobos evolved! One modern species does not
“evolve” into or from another modern species. The below image shows the primate lineage with
common ancestors.
4
In regards to DMNS facilitation, it is crucial to explain that humans are apes, not monkeys.5 The
last common ancestor for Old World Monkeys (baboons and macaques) and apes lived 25
4 “Genetic Evidence,” last modified February 2, 2016, http://humanorigins.si.edu/evidence/genetics.
5 Humans are vertebrates because we have a backbone, we are tetrapods because we have four limbs, we are
amniotes because we have an amniotic egg/sac,we are mammals because we have hair/milk, etc.

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million years ago. One of the main differences between the two is that monkeys have tails, while
apes do not. The last common ancestor (a hominid) between chimpanzees/bonobos and humans
lived between 6 to 8 million years ago. The earliest known hominin is Sahelanthropus
tchadensis that lived about 7 million years ago.
Often times, human evolution is daunting because of the complicated speciation present in our
family bush (it’s not a ‘tree’ because numerous species were alive at the same time and evolution
is not linear). To be part of the same biological species, two individuals must be able to
interbreed in nature and produce fertile offspring. Even though this seems pretty basic, in the
fossil record, identifying an individual’s species and whether it fits in with an existing species or
is a new one altogether, can be very difficult. DNA comparison can sometimes help scientists
declare separate species—the amount of difference, for example, can lead researchers to species
separations—but DNA is rarely preserved. Yet, most often, fossil discoverers declare species
names based on physical appearance and dating rather than mating and offspring fertility, which
is impossible to test with fossils. This is why some species names change over time—science is
never static and new findings change our understanding throughout time. Below is our current
family bush (as of January 2016).
6
For scientists looking at early hominin life, they largely focus on two major areas: anatomy
related to walking upright and on two legs (i.e. bipedalism) and the shape and size of the jaw and
6 “Anthropology Online Courses,” last modified July 10, 2013, http://anthropology.msu.edu/anp264-
us13/2013/07/10/week-2/.

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teeth. Larger brains did not appear in hominins until about one million years ago (H. erectus’s
brain size is roughly 750-1,300 cubic cm, whereas Lucy’s is about 387-550 cubic cm).
i. Teeth
When looking at teeth, humans and chimpanzees are very similar. Both have the same dental
formula (the number of each type of tooth in one-half of the jaw). The formula is 2-1-2-3,
indicating each have 2 incisors, 1 canine, 2 premolars, and 3 molars. But there are some key
differences that help anthropologists separate early hominins from our ape relatives. First, human
jaws are shaped more like a parabola, whereas chimps are more rectangular. Another difference
is that chimpanzees have large canines, with space on their lower jaw for honing. Honing is the
process of the canine being sharpened on the lower premolars as the jaw is opened and closed.
Even in early hominins, their canines are reduced, and over time, the honing space becomes
smaller (look at the A. afarensis and see the change). A bit more subtle variation is that
chimpanzees have a 4 cusp molar, whereas humans have 5—typically referred to as a Y-5 molar
because it resembles the letter “Y”. Finally, with most of the jaw, scientists observe that human
teeth do not protrude as far forward as chimpanzees.
7
ii. Physical bipedal identifiers
There are quite a few anatomical differences that come with walking on two limbs (biped)
instead of four (quadruped). Within the upper body, one of the best characteristics is the foramen
magnum. The foramen magnum is the point where the spinal cord passes though the skull
providing balance while moving. In quadrupeds, the foramen magnum is located towards the
back or rear of the skull, whereas in humans, it is more centrally located. Over time, particularly
between a bonobo chimp to the earliest hominid, Sahelanthropus, and then again from
Sahelanthropus to A. afarensis, the foramen magnum continues to move more centrally,
indicating a more habitual bipedal locomotion.
In addition, there are numerous bipedal characteristics in the lower body. For one, the pelvis is
reconfigured. Like that in a chimp, the quadruped pelvis has a long and narrow top blade (ilium)
that connects to the base of the pelvic bone (ischium), which creates a flat plane. Conversely, the
biped pelvis has a short and broad ilium that appears to flare out at the top and connects to the
ischium at a curve.
7 “Methods in Biology,” accessed February 4, 2016, https://www.studyblue.com/notes/note/n/ant-285-methods-in-
biol-anth-test-2-/deck/6122372.

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Moreover, the biped foot changed. Chimps have an opposable big toe that sticks out and helps
them grasp while climbing. Human feet, on the other hand, require the big toe to align with the
others and literally “push off” the ground while walking.
Additionally, apes and humans have a different “carrying angle” between the femur and lower
leg bones. The human femur slants in, bringing the knees closer together and directly passing
the body weight to the ankle joints. In doing so, walking becomes much more energy (a.k.a.
calorically) efficient and better balanced. The image below shows the described differences.
9
iii. Flaws to bipedalism
Although bipedal locomotion comes with many benefits like freeing the hands for building or
carrying things, there are a few downsides that follow it as well. One example is a prolonged
birth process. For one, bipedal locomotion demanded a pelvis reconfiguration that narrowed the
birth canal. This combined with the enlarged brain and skulls of modern humans, makes having
a child remarkably more difficult. For a modern human head to pass the birth canal, it must
rotate three times to fit through the narrow, bipedal pelvis. The average birth labor time for a
8 “Analysis of Early Hominins,” last modified 2012, http://anthro.palomar.edu/hominid/australo_2.htm.
9 “What’s New in Human Evolution and the Monkeys in my Family Tree,” last modified January 2015,
http://janetkray.com/2015/01/.
Ilium
Ischium

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chimpanzee is 2 hours, and for humans it is about 14 hours. The below image shows the birth
process between a chimp, A. afarensis, and modern human.
10
Another example is back pain. In order to support the head and balance our weight, the spine
evolved to resemble more of an “S” curvature instead of an arch like quadrupeds. This change
places more pressure on the lower back and causes back pain.
iv. The sex of a skeleton
Sexual dimorphism helps scientists distinguish between male and female skeletons. Males tend
to be larger, with denser, heavier, and rougher bones than females. Additionally, some females
like the A. afarensis Lucy, have scaring on their pelvis indicating child birth.
v. Age of death
Determining the age of an individual ultimately depends on what pieces of the skeleton have
been found. The human body goes through a series of developmental changes that can be seen
through tooth eruption and bone fusion. The image below shows when the different changes
occur. Important to note, the age of development changes among species, including the human
lineage. Earlier hominin species and other apes have quicker development stages than modern
humans (the sooner you grow/develop, the sooner you can care for yourself).
10 “Anatomy and Behavioral Strategies of Human and Nonhuman Primate Parturition,” last modified December 19,
2014, https://newlifeinburiedbones.wordpress.com/2014/12/19/anatomy-and-behavioral-strategies-of-human-and-
nonhuman-primate-parturition/.
Modern humanLucyChimp

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2.5 Dating fossils
Originally, late eighteenth and early nineteenth century scientists utilized stratigraphic layering
to date rocks. Stratigraphic layering typically follows that the older rocks are on the bottom and
the newer rocks are on top—excluding tectonic activities such as mountain building events that
can twist and fold the rock layers. The rocks’ chronological order gives fossils relative dates.
But today, many researchers rely on radiometric dating that gives the rock an absolute date.
These techniques are based on known rates of decay for several radioactive (unstable) isotopes in
common elements like carbon, uranium, and potassium/argon. Isotopes are unstable and decay
into other isotopes; the time required for one half of an unstable isotope to decay is constant and
is referred to as a half life.12 Original isotopes, called the parent, gradually decay to form a new
isotope, called daughter. When a parent decays to a daughter, it has reached a half life. Most
importantly, isotope decay is unique to the element and the rate/type of decay is known. If
scientists can measure the portion of parent and daughter isotopes in rocks, they can calculate
when the rocks formed.13 The ratio of parent to daughter isotopes is determined using a mass
spectrometer. Below are a few examples provided by DMNS Dr. Bob Raynolds:
 Carbon-14 decays to nitrogen 14 5,730 years, used to date organic material like
bone. The technique is good to about 60,000 years after which the amount of remaining
11 Kenneth Feder, The Past in Perspective: An Introduction to Human Prehistory (Oxford University Press, 2010),
43.
12 For more information, refer to Notes on Dating of Rocks by Bob Raynolds on the DMNS volunteer portal.
13 “Relative dating,” last modified May 18, 2011, http://sciencelearn.org.nz/Contexts/Dating-the-Past/Science-Ideas-
and-Concepts/Relative-dating.

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carbon 14 is so miniscule as to be hard to measure. Carbon 14 is continuously created by
collision of cosmic rays with neutrons in the upper atmosphere and makes up about 1 part
per trillion of the carbon in the atmosphere.
 Uranium 234 decays to thorium 230 80 thousand years
 Uranium 235 decays to lead 206 700 million years, often used with zircon
crystals.
 Potassium 40 decays to argon 40 1.3 billion years, used with micas and
hornblendes.
 Uranium 238 decays to lead 206 4. billion years
 Rubidium 87 decays to strontium 87 50 billion years
 Samarium 147 decays to neodymium 143 106 billion years
Radiometric dating is commonly used in eastern Africa hominin sites because of the amount of
volcanic activity. Volcanic eruptions essentially ‘reset’ the process of isotope decay of
potassium to argon. The high level of heat in east Africa causes the argon gas to add from a
baseline of zero. The fossils themselves are not dated with these methods but the rock layers in
which they are found are. Simply, scientists date the layers above and below a fossil and
determine the level of isotope decay.14
2.6 Genetics
Biological variation and evolution stems from genetics—that is the change in genes, or genetic
frequency that is passed down to offspring (known as Mendelian genetics). Scientists study
genetics by analyzing DNA. Simply put, DNA provides information for building, operating, and
repairing organisms. In many organisms—including humans—most of the DNA is found in the
cell’s nucleus, known as nuclear DNA that makes up an organism’s genome, with a small
amount existing in the mitochondria, known as mitochondrial DNA or mtDNA. The nuclear
DNA sequences are bound together by proteins in long strands called chromosomes and the
specific position of a gene or DNA sequence on a chromosome is call a locus (or loci for plural).
The alternative form of a gene or DNA sequence on a locus is known as an allele. During
reproduction, one allele is passed down from each parent (according to Mendel’s Law of
Segregation) and the two alleles lay together on a single locus. These alleles define an
organism’s genotype, or their genetic makeup, by carrying either dominant and/or recessive
genes. A dominant gene masks the effects of the other allele at any given locus, whereas, a
recessive gene can be masked. This is important when looking at an organism’s phenotype,
which is their observable appearance. Understanding how genes are passed on to offspring helps
scientists predict the possible genotypic and phenotypic results in the offspring. These results
are random, therefore, scientists can only guess at the probability. The image (a Punnett square)
below demonstrates this probability.
14 Charles Lockwood, The Human Story: Where we come from and how we evolved (Sterling Publishing, 2008), 15.

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15
In this image, “T” is the dominant allele and “t” is recessive. The only time an individual will
demonstrate the recessive phenotype is with two recessive traits, “tt”. With regards to evolution,
natural selection operates on the dominant and recessive alleles. As an example, let’s say being
tall allowed you to scare off more predators and you were therefore 100% more likely to survive.
Conversely, being short resulted in certain death. Based on this, 75% of the individuals will
survive—that is everyone containing a “T” allele, while 25% will die—those individuals with
“tt”. When discussing the variations within any given species, these are the types of differences
we get. It explains why a child could have blue eyes while both of their parents have brown
eyes, why some offspring inherit a disease that neither parent show symptoms of, and how
individuals of the same species can look so different. Humans have 46 chromosomes and
depending on the genotypic mix, individuals will display different phenotypes. As natural
selection operates on the “advantageous” or “disastrous” traits (whatever that may be in a given
environment or scenario), it does not introduce new genetic codes, simply, it exploits the various
traits already in a population.
i. Evolutionary forces
Thus far, this manual has explained that biological evolution simply refers to the change in
genetic makeup over time and how natural selection contributes to this process. But as
previously stated, Darwin’s natural selection does not create new alleles or traits for a
population, it only acts on the variation that is already there. But is it enough to literally
transform organisms until they are completely unrecognizable from their original beings? The
answer is no. Changing a genetic makeup for an entire population takes more than a few lucky
alleles that are silently sleeping within the species, other evolutionary forces help the biological
variation. Besides natural selection, there are three other evolutionary forces: mutation, genetic
15 “What determines if a gene is dominant or recessive?” last modified October 22, 2013,
http://biology.stackexchange.com/questions/11009/what-determines-if-a-gene-is-dominant-or-recessive.
Mother
Father

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drift, and gene flow. While this manual will outline them separately, all four forces typically
operate at the same time.
Perhaps the best source for genetic variation, mutation actually changes an organism’s genetic
code. Mutations are random (and therefore do not appear out of necessity) and can be caused by
a number of environmental factors like exposure to radiation from the earth’s crust and cosmic
rays. They can take place in any cell of the body but are only of evolutionary importance when
the mutation occurs in the sex cell so that the variation can be passed on to offspring.
Importantly, mutations can be advantageous, disastrous, or neutral. Much like the alleles
exploited by natural selection, mutated alleles depend on the environment and their contributions
to making an organism have a better or worse chance of survival and reproduction. Mutations
are vital to evolution because they provide new variations that can potentially contribute to major
changes in allele frequencies.
Whereas mutations affect an organism’s genetic makeup, genetic drift is the random change in
allele frequency from one generation to the next. Mostly, the random changes are a result of
probability. Keep in mind that the effects of genetic drift depend on the size of the population—
the larger the population size, the less change will occur from one generation to the next.
Referring back to the Punnett square on page 13, let’s say both the mother and father have the
genotype “Aa”. “During the process of sex cell replication (meiosis), only one allele out of two
at a given locus is used. The probability of either allele being passed on is 50 percent, just like a
coin toss…The man can pass on either an “A” allele or an “a” allele. Likewise, the woman can
pass on either an “A” allele or an “a” allele…the probable distribution of genotypes among the
children is 25 percent “AA”, 50 percent “Aa”, and 25 percent “aa”. If the couple has four
children, you would expect one with “AA”, two with “Aa”, and one with “aa”.”16 However, due
to random chance with genetic drift, the couple may not get this genotype—allele frequencies
can change because of random chance. Genetic drift occurs in each generation.
Often termed migration (even though it is slightly different), gene flow is the movement of
alleles from one population to another. When gene flow occurs, the two populations mix
genetically and change the genotypic frequencies within each population. In other words, gene
flow introduces new variation into the existing populations. As an example, let’s say a few of
Darwin’s cold climate, thick downy feathered finches fly to a tropical island where all the
finches there have short feathers and long beaks. If these two populations mix, and by chance,
the dominant long beak allele and the dominant thick feather allele are passed down from each
parent, then the offspring with be a thick feathered finch with a long beak. Over time, if these
two populations keep mixing, they will most likely look more similar to each other (meaning
there will be more thick feathers and long beak finches) than to the originally separated
populations.
Again, although the four evolutionary forces have been discussed separately within this training
manual, in reality, they act together to produce biological change. “Mutation acts to introduce
16 John Relethford, The Human Species:An Introduction to Biological Anthropology (McGraw-Hill, 2013), 75.

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new genetic variants; natural selection, genetic drift, and gene flow act to change the frequency
of the mutant allele.”17
ii. Smithsonian National Museum of Natural History, What does it mean to be human?
DNA
Genetics have come up with a variety of ways of calculating the percentages, which give
different impressions about how similar chimpanzees and humans are. The 1.2% chimp-human
distinction, for example, involves a measurement of only substitutions in the base building blocks
of those genes that chimpanzees and humans share. A comparison of the entire genome,
however, indicates that the segments of DNA have also been deleted, duplicated over and over,
or inserted from one part of the genome into another. When these differences are counted, there
is an additional 4 to 5% distinction between the human and chimpanzee genome…No matter how
the calculation is done, the big point still holds: humans, chimpanzees, and bonobos are more
closely related to one another than either is to gorillas or any other primate.18
IMPORTANT NOTE: When comparing the entire modern human genome to others, we are
about 94-96% similar to chimpanzees, which is the closet relative we have. This percent is the
same for all hominin species regardless of when they lived. However, within specific sections of
the genome, like those present in mtDNA, scientists have been able to identify genetic markers
and shown that modern humans interbred with archaic human populations. Perhaps the most
notable is with Neanderthals. In an article produced by National Geographic News, Ker Than
reported that “in all probability, there was gene flow from Neanderthals to modern humans,” and
that at least 1 to 4 percent of a person’s genetic makeup is Neanderthal. 19 Strictly when
comparing modern human mtDNA to Neanderthal mtDNA, we are at least 1-4% similar; the
entire Neanderthals and humans genome is 4-5% different from chimps. While these numbers
seem the same, they are completely different—we are talking about specific genetic markers
versus an entire genome.
Modern Human Diversity - Skin Color
As early humans moved into hot, open environments in search of food and water, one big
challenge was keeping cool. The adaptation that was favored involved an increase in the number
of sweat glands on the skin while at the same time reducing the amount of body hair. With less
hair, perspiration could evaporate more easily and cool the body more efficiently. But this less-
hairy skin was a problem because it was exposed to a very strong sun, especially in lands near
the equator. Since strong sun exposure damages the body, the solution was to evolve skin that
was permanently dark so as to protect against the sun’s more damaging rays.
17 Relethford, 84.
18 “Genetic Evidence.”
19 “Neanderthals, Humans Interbred — First Solid DNA Evidence,” last modified May 8, 2010,
http://news.nationalgeographic.com/news/2010/05/100506-science-neanderthals-humans-mated-interbred-dna-
gene/.

17. 16
Melanin, the skin's brown pigment, is a natural sunscreen that protects tropical peoples
from the many harmful effects of ultraviolet (UV) rays. UV rays can, for example, strip away
folic acid, a nutrient essential to the development of healthy fetuses. Yet when a certain amount
of UV rays penetrates the skin, it helps the human body use vitamin D to absorb the calcium
necessary for strong bones. This delicate balancing act explains why the peoples that migrated
to colder geographic zones with less sunlight developed lighter skin color. As people moved to
areas farther from the equator with lower UV levels, natural selection favored lighter skin which
allowed UV rays to penetrate and produce essential vitamin D. The darker skin of peoples who
lived closer to the equator was important in preventing folate deficiency.20
2.7 Becoming bipedal
21
Today, there are four Great Apes–the chimpanzees, bonobos, gorillas, and orangutans; however,
during the Miocene epoch, scientists have discovered more than 20 genera of extinct apes.
Proconsuloids, the earliest of the apes, lived around 23 to 16 million years ago. Around 16
million years ago, African climates became drier and more seasonal, altering the forests. By the
late Miocene (roughly 10-8 million years ago), apes like the proconsuloids “developed mobile
arms that could freely rotate at the shoulder joint, allowing efficient suspension of the body
beneath tree branches and imparting all-around greater agility. These early hominoids also
typically had molar teeth with thick enamel that were set in robust jaws, allowing them to tackle
a broad range of seasonally available forest foods as they began spreading beyond the Afro-
Arabian region into Eurasia.”22 Then again, around 6 to 4.5 million years ago, there was another
major climate change. “Oceanic cooling affected rainfall and temperatures on continents
worldwide, giving rise in tropical regions to an exaggerated form of seasonality often known as
the “monsoon cycle”.” 23 In Africa, the environmental change caused the forests to be replaced
by savannas or grasslands. By 5 million years ago, most of the ape species that had once thrived
20 “Modern Human Diversity – Skin Color,” last modified February 2, 2016,
http://humanorigins.si.edu/evidence/genetics/skin-color/modern-human-diversity-skin-color.
21 “Miocene,” last modified June 18, 2014, http://www.eoearth.org/view/article/154642/.
22 Ian Tattersall, Masters of the Planet: The Search for Our Human Origins (St. Martin’s Griffin, 2013), 2-3.
23 Tattersall, 3.

18. 17
there became extinct. Many believe that this type of climate change caused the natural selection
of bipedalism.
2.8 Homo sapiens
Knowing that many early Homo species existed, and some even existed at the same time, it is
right to wonder; how did the jumble of premodern human species in Africa, Europe, and Asia
become the single species of modern Homo sapiens? Currently, there are three theories
explaining modern human evolution: 1) replacement model, 2) multiregional model, and 3) a
mixture of both. Based on the existing skeletal record, artifacts, and genetics, many scientists
prefer the replacement model.
1. Replacement model: Also known as the out-of-Africa theory, the replacement model
states that the evolution of anatomically modern human beings occurred in Africa
between 100,000 and 200,000 years ago. Accordingly, the first modern humans
expanded out from Africa and slowly spread into Asia and Europe. At first, the modern
humans would have encountered other archaic humans like Homo erectus, and for some
reason, modern humans possessed an advantage (maybe they were smarter or could
communicate more efficiently) and essentially replaced all premodern humans.
Scientists believe that modern humans were able to out compete their archaic comrades
for resources thus causing the archaic people to go extinct.
2. Multiregional model: For the multiregional model, or regional continuity theory,
proponents believe that the modern human anatomical evolution happened in separate
geological locations, not just Africa. Through migration, intermarriage, and gene flow,
scientists believe numerous archaic populations evolved together and simultaneously
throughout Europe, Asia, and Africa. No population replaced another, simply via gene
flow, they all came to resemble each other as modern humans.
3. Mixture of both-the middle ground: Combining both the replacement and multiregional
models, this approach claims that the first anatomically modern human evolved in one
place (Africa), but rather than replacing other archaic populations, they mated and
participated in gene flow.
The question about modern human origins continues to be a debate, but most evidence leans
towards the replacement model.
III. Supplemental Information
3.1 Australopithecus afarensis – Lucy
Currently, Australopithecus afarensis is one of the best known early human species consisting of
several hundred individuals, including males, females, and juveniles. Perhaps one of the best
known Australopithecines is Lucy. Lucy was discovered in 1974 by Donald Johanson in Hadar,
Ethiopia. The Beatles song Lucy in the Sky with Diamonds was playing at the archeology site,
and thus, Lucy has been her nickname ever since. At the time of her discovery, Lucy was the
oldest and most complete hominid with 40% skeletal structure found. The brownish/darker
bones on our Lucy skeleton represent the bones that were discovered. Lucy is roughly 3.2

19. 18
million years old and was fully grown when she died. Based on her pelvis, Lucy is female and
had a child. Due to her small size, she probably spent most of her time in the trees hiding from
the dangerous predators roaming the savanna. She most likely scavenged, but mainly ate plants
and fruits. There is no definitive answer as to how she died and her real skeleton is in Ethiopia.
A. afarensis are about 3.85-2.95 million years old and found throughout eastern Africa. They
have a variety of ape like and human characteristics. Their faces are very ape, with a flat nose
and protruding face, and their brain sizes average 387-550 cubic cm—a braincase closer to
chimpanzees than modern humans. Like chimps, A. afarensis’ are extremely sexually
dimorphic. Males average 5 feet tall weighing 93 pounds, whereas females stand 3.5 feet tall
weighing 64 pounds. Additionally, as proportioned to their body, they have long arms and
curved fingers indicating a high adaptiveness for climbing. As for human characteristics, A.
afarensis’ are bipedal. Their foramen magnum is relatively centrally located, with a short, broad
pelvis. Moreover, their canines have been reduced and resemble more modern human than ape.
For more information, please look at Shelly Knepley’s fact sheet on the DMNS Earth Science
Volunteer Portal.
3.2 Stone tools
As a rule of thumb, the older the tool, the less advanced it is. The oldest tools are known as
Oldowan technology and roughly date to 2.6 million years old. Around 1.5 million years ago,
Acheulean technology replaced the Oldowan model. One main difference between the two tools
is the amount of flakes chipped off the core. Acheulean makers knew preciously where to hit the
core for a precise flake to break off. The maker would continuously chip and shape the flake to
produce a sharper, more advanced tool. Finally, the last type of tool to emerge is Mousterian.
This technology surfaced roughly 100,000 years ago and exhibits even more complex and
precise flaking as compared to Acheulean work. Importantly, just because stone tools did not
appear until 2.6 million years ago, that does not mean that earlier hominins like Lucy were not
using them. Much like apes in the wild, early humans most likely used the various stones and
sticks at their disposal. Classifications for stone tools are items that show specific flaking marks,
which is a skill that takes planning and forethought.
24
24 “Archaic Human Culture,” last modified 2012, http://anthro.palomar.edu/homo2/mod_homo_3.htm.

20. 19
3.3 Neanderthals
“Homo neanderthalensis was the first fossil hominin discovered and described. Today, we have
thousands of fossil specimens excavated from hundreds of sites across Europe. This record is
remarkable, because it includes individuals of all ages, from premature fetuses to the very
elderly.”25 Neanderthals lived around 350,000- 28,000 years ago and spread across Europe,
Siberia, and even reached southwest Asia. Skeletally, these hominins look similar to Homo
sapiens, but tend to be broader throughout. They had large brains averaging 1,200- 1,750 cubic
cm and exhibited a pretty complex culture. They are believed to be the first hominins to
purposely bury their dead. Additionally, artistic shells, teeth, and stone pieces are found at many
Neanderthal sites, leading anthropologists to believe that they had a desire for symbolic
expression.
There is no definitive answer to explain why Neanderthals went extinct. Some studies, like that
conducted in 1987 by Erik Trinkaus and D.D. Thompson, show that nearly 43% of Neanderthals
died before reaching age 12, while only 10% lived past 40 years of age. They believe that this
species suffered dietary deficiencies that produced cracked bones and thin tooth enamel. Others
believe that Neanderthals became too adaptively advanced for their environment, and once the
climate changed, they were unable to adapt as easily as modern humans. This type of hypothesis
is similar to the climate change 5 million years ago that forced many forest apes into extinction,
while bipedal hominins continued to evolve.
3.4 Language
“Anthologists continue to debate when speech first developed among our ancestors. The larynx
is composed of soft tissue and does not fossilize, so our ancestors’ speech anatomy must be
reconstructed from other lines of evidence. The hyoid bone, from which the larynx hangs, does
sometimes survive and provide some clues, while the size and shape of the mouth and the size of
the holes in the skull through which the nerves controlling speech pass also reveal detail [sic]
regarding possible speech. But it is impossible to know the detailed vocal-tract anatomy of our
ancestors. Because even modern human language is not always spoken, and human vocal
anatomy is quite similar to that of apes, it seems likely that the key to mastering speech is not our
anatomy, but lies in the brain instead.”26
25 Dr. Alice Roberts, Evolution:The Human Story (DK, 2011), 152.
26 Roberts, 32.

21. 20
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