Answers to End-of-Chapter Questions – Brooker et al ARIS site
Chapter 22
Test Yourself Questions
1. The prebiotic soup was
a. the assemblage of unicellular prokaryotes and eukaryotes that existed in the oceans of
primitive Earth.
b. the accumulation of organic molecules in the oceans of primitive Earth.
c. the mixture of organic molecules that was found in the cytoplasm of the earliest cells on
Earth.
d. a pool of nucleic acids that contained the genetic information for the earliest organisms.
e. none of the above.
Answer: b. The prebiotic soup was the accumulation of organic molecules in the oceans of primitive
Earth.
2. Which of the following is not a characteristic of protobionts that was necessary for the evolution of
living cells?
a. a membrane-like boundary separating the external environment from an internal
environment
b. polymers capable of functioning in information storage
c. polymers capable of enzymatic activity
d. self-replication
e. compartmentalization of metabolic activity
Answer: e. Compartmentalization of metabolic activity was not a characteristic of protobionts that was
required for the evolution of cells. Compartmentalization is a characteristic of eukaryotic cells.
3. RNA is believed to be the first functional macromolecule in protobionts because it
a. is easier to synthesize compared to other macromolecules.
b. has the ability to store information, self-replicate, and perform enzymatic activity.
c. is the simplest of the macromolecules commonly found in living cells.
d. all of the above
e. a and c only
Answer: b. RNA is believed to be the first functional macromolecule in protobionts because RNA has the
ability to store information, self-replicate, and perform enzymatic activity. DNA has the ability to
store information, but does not perform enzymatic activity. Proteins, however, perform enzymatic activity
but do not store information or have the ability to self-replicate.
4. The movement of landmasses that have changed their positions, shapes, and association with other
land masses is called
a. glaciation.
b. Pangea.
c. continental drift.
d. biogeography.
e. geological scale.
Answer: c. Continental drift is the movement of landmasses that have changed their positions, shapes
and association with other land masses.
5. Paleontologists estimate the dates of fossils by
a. the layer of rock in which the fossils are found.
b. analysis of radioactive isotopes found in the rock.
c. the complexity of the body plan of the organism.
d. all of the above
e. a and b only
Answer: e. Paleontologist date fossils relative to the layer of rock in which the fossil is found and/or the
use of radioisotope dating.
6. The fossil record does not give us a complete picture of the history of life because
a. not all past organisms have become fossilized.
b. only organisms with hard skeletons can become fossilized.
c. fossils of very small organisms have not been found.
d. fossils of early organisms are located too deep in the crust of the Earth to be found.
e. all of the above.
Answer: a. The fossil record is not a complete picture of the history of life because not all past
organisms have become fossilized.
7. The endosymbiosis hypothesis explaining the evolution of eukaryotic cells is supported by
a. DNA-sequencing analysis comparing bacterial genomes, mitochondrial genomes, and
eukaryotic nuclear genomes.
b. naturally occurring examples of endosymbiotic relationships between bacterial cells and
eukaryotic cells.
c. the presence of DNA in mitochondria and chloroplasts.
d. all of the above
e. a and b only
Answer: d. The endosymbiosis hypothesis is supported by DNA comparison between prokaryotic and
eukaryotic cells, the presence of mitochondrial and chloroplast DNA, and natural examples of
endosymbiosis among prokaryotic and eukaryotic cells.
8. Which of the following explanations of the evolution of multicellularity in eukaryotes is seen in the
development of complex, multicellular organisms today?
a. endosymbiosis
b. aggregation of cells to form a colony
c. division of cells with the resulting cells sticking together
d. multiple cell types aggregating to form a complex organism
e. none of the above
Answer: c. Complex multicellular organisms today form by cell division and adhesion of the resulting
cells.
9. The earliest fossils of vascular plants occurred during the __________ period.
a. Ordovician
b. Silurian
c. Devonian
d. Triassic
e. Jurassic
Answer: b. The earliest fossils of vascular plants are found during the Silurian period.
10. The appearance of the first hominids date to the __________ period.
a. Triassic
b. Jurassic
c. Cretaceous
d. Tertiary
e. Quaternary
Answer: d. Hominids came into existence toward the end of the Tertiary period.
Conceptual Questions
1. What are the four stages in which the origin of life occurred?
Answer:
 Nucleotides and amino acids were produced prior to the existence of cells.
 Nucleotides and amino acids became polymerized to form DNA, RNA, and proteins.
 Polymers became enclosed in membranes.
 Polymers enclosed in membranes evolved cellular properties.
2. Define radioisotope dating and half-life.
radioisotope dating: A common way to estimate the age of a fossil by analyzing the elemental isotopes
within the accompanying rock.
half-life : The length of time required for exactly one-half of the original isotope to decay.
3. Briefly discuss the Cambrian explosion.
Answer: The Cambrian period lasted from 543 million years ago to 490 million years ago. The Cambrian
explosion was an abrupt increase on a geological scale in the diversity of animal species. During the
Cambrian period, all of the existing major types of marine invertebrates arose as well as many other
major animal groups.
Experimental Questions
1. What is chemical selection? What was the hypothesis tested by Bartel and Szostak?
Answer: Chemical selection occurs when a particular chemical in a mixture has advantageous
properties that allow it to increase in number compared to the other chemicals in the mixture. Bartel and
Szostak hypothesized that variation in the catalytic abilities of RNA molecules would allow for chemical
selection in the laboratory. Bartel and Szostak proposed to select for RNA molecules with higher
catalytic abilities.
2. In conducting the selection experiment among pools of long RNA molecules with various catalytic
abilities, what was the purpose of using the short RNA molecules?
Answer: The short RNA molecules were not considered part of the pool of the RNA molecules for
selection. Instead, these short RNA molecules allowed the researchers to physically separate the
mixture of longer RNA molecules based on catalytic properties. Long RNA molecules with catalytic
abilities would covalently bond with the short RNA molecules. The short RNA molecules had a specific
region that caused them to be attracted to column beads in the experimental apparatus. The long RNA
molecules that did not have catalytic abilities passed through the column, and therefore could be
separated from the ones that had catalytic activity and became bound to the column beads.
3. What were the results of the experiment conducted by Bartel and Szostak? What impact did this
study have on our understanding of the evolution of life on Earth?
Answer: The researchers found that with each round of selection, the enzymatic activity of the selected
pool of RNA molecules increased. These results provided evidence that chemical selection could
improve the functional characteristics of a group of molecules. Much of the explanation of the evolution
of life on Earth is theoretical, meaning it is based on scientific principles but has not been experimentally
verified. Researchers are attempting to develop laboratory experiments that test the explanations of the
evolution of life. The experiment conducted by Bartel and Szostak provided experimental data to
support the hypothesis of chemical selection as a possible mechanism for the early evolutionary
process that lead to living cells.
Collaborative Questions
1. Discuss three possible hypotheses of how organic molecules were first formed.
Answer:
The reducing atmosphere hypothesis - This hypothesis is based largely on geological evidence. By
examining what chemicals were in the primitive atmosphere at the time when life arose, scientists have
determined that the basic chemicals needed to form organic molecules were present. By combining
these primitive inorganic molecules, more complex molecules such as amino acids and nucleotides
could be formed.
The extraterrestrial hypothesis - This hypothesis postulates that the first organic molecules may have
come from fallen bodies from space such as meteorites. Because certain meteorites called
carbonaceous chondrites contain organic carbon, these bodies may have added the first organic
molecules to our planet.
Deep sea vents - This hypothesis proposes that the key organic molecules which started life may have
originated in deep sea vents. Because these vents have the necessary chemicals and high
temperatures, life on our planet may have began here.
2. Discuss three periods in geological time.
Answer:
Ordovician - The Ordovician period ranged from 490 to 443 million years ago (mya). This period of time
had a very diverse group of marine invertebrate life such as trilobites, brachiopods, mollusks, and
corals. In addition, the first primitive vertebrates, the jawless fish, made their debut during the
Ordovician. Fossil evidence also indicates that primitive plants and arthropods colonized the land during
this period.
Silurian - The Silurian period ranged from 443 to 417 mya. During this period no new invertebrate
groups appeared but significant changes occurred in the body plans of existing invertebrates. Many new
vertebrates in the form of fishes arose during this period and many plants and animals colonized the
land. The fossil record indicates that the first vascular plants evolved during the Silurian.
Devonian - The Devonian period ranged from 417 to 354 mya. The Devonian saw major increases in
the number of terrestrial species and the first seed plants arose. By the end of the Devonian, the first
trees and forests were formed. Invertebrates continued their expansion across the land and insects
emerged during this period. In addition, tetrapods, animals with four limbs came into existence during
the Devonian. These were in the form of amphibians which inhabited the land but still needed water for
reproduction.
Note: many other periods are possible to discuss.