Question 1

Several scientific laboratories across the globe are involved in research concerning the origin of life on Earth. Which graph below, if the results were produced abiotically, would have the greatest promise for revealing important information about the origin or Earth's first genetic system?

Accepted Answer

B

Question 2

What probably accounts for the switch to DNA-based genetic systems during the evolution of life on Earth?

Accepted Answer

DNA is chemically more stable than RNA, and it replicates with fewer errors (mutations) than RNA. These advantages likely led to the switch to DNA-based genetic systems during the evolution of life on Earth.

Question 3

How many half-lives should have elapsed if 6.25% of the parent isotope remains in a fossil at the time of analysis?

Accepted Answer

After four half-lives, 6.25% of the original parent isotope remains, as each half-life reduces the remaining amount by half (100% -> 50% -> 25% -> 12.5% -> 6.25%).

Question 4

Which of the following is the correct sequence of these events in the origin of life?
I. formation of protobionts
II. synthesis of organic monomers
III. synthesis of organic polymers
IV. formation of DNA-based genetic systems

Accepted Answer

The correct sequence is the synthesis of organic monomers, followed by the synthesis of organic polymers, then the formation of protobionts, and finally the formation of DNA-based genetic systems.

Question 5

Although absolute distinctions between the "most evolved" protobiont and the first living cell are unclear, biologists generally agree that one major difference is that the typical protobiont could not

Accepted Answer

The typical protobiont could not perform controlled, precise reproduction. This is a major difference between the protobiont and the first living cell, which had the ability to reproduce in a controlled and precise manner.

Answer: A, E
 The protobiont could not possess a selectively permeable membrane boundary, which would have allowed it to selectively absorb compounds from the external environment. Therefore, it also could not absorb compounds from the external environment in a controlled manner.

Question 6

The synthesis of new DNA requires the prior existence of oligonucleotides to serve as primers. On Earth, these primers are small RNA molecules. This latter observation is evidence in support of the hypothesized existence of

Accepted Answer

The fact that primers are small RNA molecules suggests that earlier genetic systems existed before the use of DNA as the primary genetic material. RNA is known to be capable of both storing genetic information and catalyzing chemical reactions, making it a plausible precursor to DNA-based genetic systems.

Question 7

Which of the following has not yet been synthesized in laboratory experiments studying the origin of life?

Accepted Answer

Protobionts that use DNA to program protein synthesis have not yet been synthesized in laboratory experiments studying the origin of life. While some researchers have attempted to create proto-cells with artificial membranes and rudimentary genetic systems, they have not yet been able to produce self-replicating entities that use DNA as their genetic material. Liposomes (A and B), oligopeptides and other oligomers (C), and amino acids (E) have all been synthesized in laboratory experiments studying the origin of life.

Question 8

Which measurement would help determine absolute dates by radiometric means?

Accepted Answer

Radiometric dating relies on measuring the accumulation of daughter isotopes and the loss of parent isotopes to determine absolute dates.

Question 9

If natural selection in a particular environment favored genetic systems that permitted the production of daughter "cells" that were genetically dissimilar from the mother "cells," then one should expect selection for I. polynucleotide polymerase with low mismatch error rates.
II. polynucleotide polymerases without proofreading capability.
III. batteries of efficient polynucleotide repair enzymes.
IV. polynucleotide polymerases with proofreading capability.
V. polynucleotide polymerases with high mismatch error rates.

Accepted Answer

Selection for genetic dissimilarity would favor polynucleotide polymerases without proofreading capability and with high mismatch error rates, as these would increase genetic variation.

Question 10

If relatively small carbonaceous chondrites from space were a significant source of Earth's original amino acids, then which two of these would have been most important in permitting their organic materials to survive impact with Earth?
I. Carbonaceous chondrites must contain no D-amino acids.
II. Earth's early atmosphere must have had little free oxygen.
III. The chondrites must have arrived on Earth before 4.2 billion years ago.
IV. Earth's early atmosphere must have been dense enough to dramatically slow the chondrites before they impacted.
V. The chondrites must have impacted land, rather than a large body of water.

Accepted Answer

A low-oxygen atmosphere (II) would prevent oxidation of organic materials, and a dense atmosphere (IV) would slow the chondrites, reducing impact damage.

Question 11

What is True of the amino acids that might have been delivered to Earth within carbonaceous chondrites?

Accepted Answer

Studies have shown that the proportion of amino acids found in carbonaceous chondrites is similar to those produced in the Miller-Urey experiment, which simulated the conditions on early Earth and resulted in the formation of amino acids.

Answer: C
 While many different types of amino acids are found in living organisms today, the amino acids found in carbonaceous chondrites are typically limited to a smaller subset.

Therefore, the correct answer is B, C.

Question 12

Which gas was least abundant in Earth's early atmosphere, prior to 2 billion years ago?

Accepted Answer

Oxygen (O₂) was least abundant in Earth's early atmosphere because it was primarily produced by photosynthetic organisms, which did not become widespread until later.

Question 13

The first genes on Earth were probably

Accepted Answer

The leading theory suggests that the first genes on Earth were autocatalytic RNA molecules. This hypothesis proposes that RNA was the first genetic material and acted both as an enzyme and a carrier of genetic information. These self-replicating RNA molecules evolved into more complex molecules that eventually led to the origin of life on Earth.

Question 14

RNA molecules can both carry genetic information and be catalytic. This supports the proposal that

Accepted Answer

The fact that RNA can both carry genetic information and be catalytic supports the proposal that RNA was the first hereditary information, as it suggests that RNA could have served as both a template for copying genetic information and as an enzyme for catalyzing the replication of that information.

Question 15

In their laboratory simulations of the early Earth, Miller and Urey observed the abiotic synthesis of

Accepted Answer

Miller and Urey's laboratory simulations of the early Earth resulted in the abiotic synthesis of amino acids, which are the building blocks of proteins. The other options listed (complex organic polymers, DNA, liposomes, and genetic systems) were not observed in their experiments.

Question 16

Several scientific laboratories across the globe are involved in research concerning the origin of life on Earth. Which of these questions is currently the most problematic and would have the greatest impact on our understanding if we were able to answer it?

Accepted Answer

Understanding how RNA sequences came to carry the code for amino acid sequences, also known as the transition from RNA to protein-based life, is currently the most problematic question in origin of life research. This would have the greatest impact on our understanding because it would explain how the genetic code that all living organisms use to make proteins arose, and how the first proteins were synthesized.

Question 17

Which is a defining characteristic that all protobionts had in common?

Accepted Answer

The defining characteristic that all protobionts had in common was the presence of a surrounding membrane or membrane-like structure. This allowed for separation from the external environment and the formation of a self-contained, semi-permeable system able to regulate internal conditions. The other options listed may have been present in some protobionts, but were not universal traits.

Question 18

Which of the factors below weaken the hypothesis of abiotic synthesis of organic monomers in early Earth's atmosphere?
1) the relatively short time between intense meteor bombardment and appearance of the first life forms
2) the lack of experimental evidence that organic monomers can form by abiotic synthesis
3) uncertainty about which gases comprised early Earth's atmosphere

Accepted Answer

The short time between meteor bombardment and the appearance of life (1) and uncertainty about the gases in early Earth's atmosphere (3) both challenge the hypothesis of abiotic synthesis of organic monomers.

Question 19

In what way were conditions on the early Earth of more than 3 billion years ago different from those on today's Earth?

Accepted Answer

Early Earth was intensely bombarded by large space debris, which created many impact craters and increased the temperature of the planet. Today, the frequency of large space debris impacting Earth is much lower.

Answer: D
 Less ultraviolet radiation penetrated Earth's early atmosphere because the ozone layer had not yet formed. This allowed for more radiation to reach the surface, which may have contributed to the origin of life through the formation of complex organic molecules.

Note: Choice A is incorrect as water vapor was present on both early Earth and today's Earth. Choice C is incorrect as early Earth had a reducing atmosphere, meaning there was little or no oxygen present. Choice E is incorrect as the ozone layer did not form until much later in Earth's history.

Question 20

If the half-life of carbon-14 is about 5,730 years, then a fossil that has one-sixteenth the normal proportion of carbon-14 to carbon-12 should be about how many years old?

Accepted Answer

Each half-life reduces the carbon-14 by half. One-sixteenth is 2^4, so 4 half-lives have passed. 4 x 5,730 years = 22,920 years, which is closest to 22,400 years.

Quiz 25: The History of Life on Earth