Mastering Concepts
15.1
1. How were conditions on Earth before life began different from current conditions?
Before life began on Earth, the atmosphere contained a mixture of methane, ammonia,
water and hydrogen gases; volcanic eruptions, earthquakes, and meteor and asteroid
strikes were common; there was no atmospheric layer of ozone shielding Earth’s surface
from cosmic radiation; Earth’s surface temperature and atmospheric pressure were
dramatically higher than they are today.
2. What types of information can prebiotic simulations provide?
Prebiotic simulations can demonstrate what might have happened on early Earth. They
demonstrate that given the right environment, organic molecules can arise from simple
inorganic chemicals.
3. Why is RNA likely to have been pivotal in life’s beginnings?
RNA is an extremely versatile molecule that stores information encoding proteins, can
act as an enzyme that can catalyze chemical reactions, and can duplicate on its own.
4. How did early life change Earth?
Early life introduced O2 into the atmosphere and reduced levels of CO2. Eventually ozone
formed in the upper atmosphere, making Earth’s surface more habitable.
15.2
1. How might the endoplasmic reticulum, nuclear envelope, and other internal
membranes have arisen?
The endoplasmic reticulum, nuclear membrane, and other membranes may have arisen
from the infolding of the outer membrane of an early cell.
2. What is the evidence that mitochondria and chloroplasts descend from simpler cells
engulfed long ago?
Mitochondria and chloroplasts have their own DNA, RNA, and ribosomes; they replicate
by binary fission; their size, shape, and membrane structure are similar to those of
prokaryotes; the pigments in chloroplasts are similar to the pigments of cyanobacteria;
DNA sequences of mitochondria and certain bacteria are similar.
3. What are two ways that multicellular organisms may have originated?
Multicellular organisms may have originated by individual cells merging to form a single
organism, or by cells remaining stuck together after division.
15.3
1. When did the Ediacarans live, and what were they like?
Ediacarans lived during the Precambrian, about 600 to 543 MYA. No one is sure exactly
what they were like.
2. What types of organisms flourished in the Cambrian?
In the Cambrian, life was plentiful in the seas, including Ediacarans, red and green algae,
sponges, jellyfish, worms, trilobites, nautiloids, eurypterids, brachiopods, mollusks, and
the unique animals preserved in the Burgess Shale.
3. How did Paleozoic life diversify during the Ordovician, Silurian, Devonian,
Carboniferous, and Permian periods?
The Ordovician period was characterized by huge sea communities of algae and
invertebrates. The first jawless fishes appeared in the Ordovician; life also moved onto
land as primitive plants appeared. The Silurian saw the rise of vascular plants, terrestrial
invertebrates, and fungi. Fishes with jaws appeared, as well as freshwater fishes. The
Devonian was dominated by fishes. Lobe-finned fishes appeared, as did amphibians.
Plants diversified into ferns, horsetails, and seed plants. Amphibians flourished in the
Carboniferous, as did coal forests and many large land insects. The first reptiles appeared
in the Carboniferous as well, and bony fishes and sharks lived in the Carboniferous
oceans. The Permian saw the rise of gymnosperms and reptiles.
4. How did the Paleozoic era end?
The Paleozoic period ended with a mass extinction. The Permian extinctions may have
been caused by a drop in sea levels, which dried out coastline communities. The
concurrent rise in the level of atmospheric carbon dioxide raised global temperatures.
5. Which organisms came and went during the Mesozoic era?
During the Mesozoic era, dinosaurs flourished and later went extinct, as did therapsids,
pterosaurs, and other non-dinosaurian lineages of reptiles. The first flowering plants also
originated during the Mesozoic, as did the first mammals.
6. Which new organisms arose during the Cenozoic era?
During the Cenozoic era, mammals (especially placental mammals) underwent a huge
adaptive radiation. Many forests gave way to grasslands, providing habitat for herds of
grazing mammals. Humans also originated in the Cenozoic era.
15.4
1. Name and describe the three groups of contemporary primates. To which group do
humans belong?
The three groups of contemporary primates are prosimians (lemurs and others), monkeys,
and hominoids (apes). Humans are hominoids.
2. What can skeletal anatomy and DNA sequences in existing primates tell us about the
study of human evolution?
The skeletal anatomy of existing primates can help us understand trends in human
evolution, including the diets, body positions, sizes, and many other aspects of fossil
humans and their ancestors. DNA sequences in existing primates can reveal which
species are most closely related to one another and to humans.
3. What are the four groups of hominins in the human family tree, and which still exist
today?
The four groups are Ardipithecus, Australopithecus, Paranthropus, and Homo. Only
Homo sapiens exists today.
4. Which conditions may have contributed to the evolution of humans?
Conditions that led to the origin of humans are rooted in plate tectonics. After India and
the Himalayan region collided with Eurasia, the rise of the Himalayas altered Africa’s
climate. It became cooler and some tropical forests became grasslands. Ancestors of
humans began to live in grasslands instead of in trees; the new habitat selected for the
bipedalism that characterizes humans. The degree of exposure to ultraviolet radiation has
selected for different skin colors; migration and culture have also affected human
evolution.
15.5
1. What criteria did the researchers use when choosing the subjects and the DNA
sequences for their study?
They compared DNA segments from 10 individuals native to different regions of each of
three continents: Africa, Europe, and Asia. The 50 DNA sequences analyzed for each
person were randomly selected from the noncoding regions of the genome.
2. Explain how the data in figures 15.29 and 15.30 support the “out of Africa” model of
human evolution.
Figure 15.29 shows that Africans have more genetic diversity than do Asians or
Europeans. Figure 15.30 shows that Asians and Europeans are more genetically similar to
each other than either group is to Africans. Both observations are consistent with the idea
that a subset of Africans migrated to the Asian and European continents.
Write It Out
1. Explain how the origin of self-replicating molecules was critical to life’s origin.
Two of the properties of life are reproduction and evolution. Self-replicating molecules
would have been an important step in reproduction; the inevitable errors in selfreplication would have made natural selection possible.
2. List three ways that studying the history of life helps us understand life’s current
diversity, and predict how diversity might change in the future.
Fossils and biogeography reveal when and where the ancestors of existing species lived,
which helps us place each organism on an evolutionary tree. In addition, extinction and
adaptive radiations have been prominent features in life’s history; studying the history of
life helps us determine what conditions lead to extinction and adaptive radiation. Finally,
understanding how environmental change has affected species in the past can help us
understand how future changes are likely to affect future species and to understand the
implications of species losses and gains.
3. Review the structures of nucleic acids and proteins in chapter 2. What chemical
elements had to have been in primordial “soup” to generate these organic molecules?
Carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur must have been present to
have generated all of the amino acids and nucleotides currently found in life.
4. Describe the role of lipids in the origin of life.
Lipids would have been critical to the formation of the membranes that surrounded the
first protocells.
5. The amoeba Pelomyxa palustris is a single-celled eukaryote with no mitochondria, but
it contains symbiotic bacteria that can live in the presence of O2. How does this
observation support the endosymbiont theory?
The endosymbiont theory proposes that mitochondria (and chloroplasts) originated as
bacteria living inside of eukaryotic cells. The bacteria inside Pelomyxa show that it is
possible not only for such a symbiotic relationship to occur but also for the bacteria to
carry out a function that benefits the host cell.
6. The antibiotic streptomycin kills bacterial cells but not eukaryotic cells; diphtheria
toxin kills eukaryotic cells but not bacteria. Which of these two substances do you predict
would kill mitochondria and chloroplasts? Explain your answer.
Mitochondria and chloroplasts arose from bacterial ancestors. They should therefore be
susceptible to streptomycin but not diphtheria toxin.
7. List the major events of the Precambrian supereon and of the Paleozoic, Mesozoic, and
Cenozoic eras.
Precambrian: Life originated, reproduced and diversified; photosynthesis evolved; O2
accumulated in Earth’s atmosphere; eukaryotes arose.
Paleozoic: Sea supported huge communities of algae and invertebrates; vertebrates such
as jawless fishes appeared; life ventured onto land in the form of primitive plants and
arthropods; the first vascular plants evolved; the first freshwater fishes appeared; the
oceans contained corals, trilobites, and mollusks; fishes with skeletons of cartilage or
bone appeared; lobe-finned fishes appeared; ferns, horsetails, and seed plants appeared;
scorpions, millipedes, and other invertebrates lived on the land; amphibians emerged;
bony fishes and sharks were beginning to resemble modern forms; reptiles introduced the
amniote egg, in which an embryo could develop completely in dry land; a major
extinction event occurred.
Mesozoic Era: Thecodonts shared forests with cycads, ginkgos, and early conifers;
ancestral mammals appeared; dinosaurs dominated for over 120 million years; first
flowering plants appeared; Archaeopteryx and ancestral birds appeared; an extinction
event occurred in the Cretaceous period, ending the age of dinosaurs.
Cenozoic Era: Marsupials (pouched mammals) appeared; 15 of the 18 modern orders of
placental mammals arose; modern mountains and coastlines formed; grasslands began to
replace the forest in some areas; Homo sapiens appeared in Africa and eventually
colonized most of the planet.
8. Distinguish between the terms primate, hominid, hominin, and Homo.
Primates are a group of mammals consisting of prosimians (tarsiers, lemurs, and the like),
monkeys, and apes. Hominids are the great apes (orangutans, gorillas, chimpanzees, and
humans). Hominins are a subgroup of hominids consisting of extinct and modern
humans, including Ardipithecus, Australopithecus, Paranthropus, and Homo. Homo is a
genus of hominins that includes several extinct and modern humans.
9. Explain how opposable thumbs, large brains, and binocular vision are adaptive to
primates.
Opposable thumbs enable primates to grasp branches, foods, and other objects with
precision. Large brains enable primates to solve problems, to work together to acquire
food and defend themselves, and (in humans) to develop culture. Binocular vision
provides excellent depth perception, which is useful in feeding and defense.
10. What can scientists learn by comparing the fossilized skeletons of extinct primates
with the bones of modern species?
The fossilized skeletons can yield important information regarding locomotion, posture,
diet, and brain size.
11. How have DNA analyses changed how scientists study the evolutionary history of
humans?
Before DNA analysis, scientists studying human evolutionary history relied mainly on
comparisons of fossils and the skeletons of existing species to infer species relationships.
DNA allows for more direct observations of relationships among existing primates and
among diverse human subpopulations. In addition, DNA has been extracted from some
human fossils, which has enabled scientists to make direct comparisons between the
DNA of modern humans and Neandertals.
12. Use the Internet to learn about National Geographic’s Genographic Project. What are
the main objectives and components of the project, and how are researchers using the
information they gather to learn more about human evolution?
The Genographic Project is trying to map the migration patterns of humans from our last
common ancestral population to today. It analyzes DNA samples from hundreds of
thousands of volunteers around the world.
13. In what ways has culture been an important factor in human evolution?
Culture has facilitated the development of language, knowledge, art, and our way of life.
The ability to transmit knowledge from generation to generation has enabled us to learn
from the past and has contributed greatly to our success as a species.
14. At one time, several species of Homo existed at the same time. Propose at least two
hypotheses that might explain why only Homo sapiens remains.
Perhaps Homo sapiens had greater intelligence and therefore greater reproductive success
than the other species. Perhaps Homo sapiens interbred with the other species,
eliminating the distinctions among them. Perhaps Homo sapiens killed off the other
species.
15. Compare culture to evolution by means of natural selection. How are they similar?
How are they different? Can you think of examples of knowledge, beliefs, and behaviors
that were appropriate in one set of conditions but that humans selected against as
conditions changed?
Culture changes over time, as do species. People “release” many new ideas, songs,
dances, TV shows, movies, videos, and inventions into the marketplace; many fail, but
some are successful. In natural selection, organisms produce many more offspring than
can survive and reproduce; many fail to reproduce, but some are successful. However,
cultural innovations are intentional human creations that are designed to “catch on” with
the public, whereas the properties of each offspring are not created deliberately.
(Examples will vary.)
16. The video game “Spore” invites players to design creatures and guide them through
“five stages of evolution.” Search the Internet for information about “Spore,” then
describe how evolution in this game is similar to, and different from, the evolution of life
on Earth.
In the game, organisms start with basic cells. Players can spend DNA points to mutate
their organism and change its traits, similar to how in evolution mutations can lead to
new adaptive traits. Unlike evolution, each stage of the game, and game as a whole, has
a goal. Also, throughout the game, the choices and actions of the player determine what
the player starts with in the next stage. Such a transmission of traits is more Lamarckian
than it is evolutionary.
Pull It Together
1. Arrange the 14 major events at the bottom of the concept map in chronological order,
then indicate when the major extinction events occurred. What was the implication of
each extinction?
Origin of life; origin of three domains; origin of photosynthesis; origin of eukaryotes;
origin of multicellularity; origin of Ediacarans; origin of vertebrates; origin of land
plants; origin of land animals; age of reptiles; origin of flowering plants; diversification
of flowering plants; rise of mammals; origin of humans. Major extinction events occurred
twice during the Paleozoic, at the end of the Paleozoic, once during the Mesozoic, and at
the end of the Mesozoic. Each mass extinction resulted in the death of many species, but
also provided the opportunity for other species to diversify in major adaptive radiations.
2. How has the emergence of new species changed Earth’s history?
New species have contributed to the formation of the biosphere, changing the
environment and providing food and habitat for other species. Some species have had a
particularly large impact; for example, the aquatic photosynthetic microbes that first
released oxygen into the atmosphere set into motion the evolution and diversification of
many other species, and the first land plants opened new food sources and habitats for a
huge variety of animal species.
3. Create an additional concept map that depicts the evolution of humans.
One possible strategy is to use figure 15.23 as a guide. “Ancestral primate” gave rise to
“Prosimians,” “Monkeys,” and “Apes.” “Monkeys” and “Prosimians” lack long arms, but
they have long legs and a tail. “Apes” have long arms, but they lack long legs and a tail.
“Apes” include “Gibbons,” “Orangutans,” “Gorillas,” Bonobos,” “Chimpanzees,” and
“Humans.” “Gorillas,” “Bonobos,” and “Chimpanzees” move by “Knuckle-walking.”
“Humans” are “Bipedal.” (Other answers are possible.)