Science Practice Challenge Questions

In addition to biology, evidence drawn from many different disciplines, including chemistry, geology, and mathematics, supports models of the origin of life on Earth. In order to determine when the first forms of life likely formed, the rate of radioactive decay can be used to determine the age of the oldest rocks (see optional problems C and D, below) exposed on Earth’s surface. These are found to be approximately 3.5 billion years old. The age of rocks can be correlated to fossils of the earliest forms of life.

A. The graph compares times of divergence from the last common ancestor based on the fossil record with a "molecular time" constructed by comparing sequences of conserved proteins to determine a mutation rate (after Hedges and Kumar, Trends in Genetics, 2003). Explain how such a molecular clock could be refined to infer time for the evolution of prokaryotes.

B. Using a molecular clock constructed from 32 conserved proteins, Hedges and colleagues (Battistuzzi et al., BMC Evol. Biol. 2004) estimated the times during which key biological processes evolved. A diagram based on their work is shown. Connect the time of the origin of life inferred from this diagram with the age of the oldest fossil stromatolites and the age of the oldest exposed rock to show how evidence from different scientific disciplines provides support for the concept of evolution. Evaluate the legitimacy of claims drawn from these different disciplines (biology, geology, and mathematics) regarding the origin of life on Earth.

The oldest known rocks are exposed at three locations: Greenland, Australia, and Swaziland. The following application of mathematical methods provides the essential evidence of the minimum age of Earth. The mathematics is appropriate for students who have completed a second year of algebra. However, it is not illustrative of the type of item that could appear on the AP Biology Exam.

The exposed rocks contain a radioactive isotope of rubidium, ⁸⁷Rb, which decays into a stable isotope of strontium, ⁸⁷Sr. An ⁸⁷Rb atom with 37 protons and 50 neutrons decays when a proton is converted into a neutron to produce an atom, ⁸⁷Sr, with 36 protons and 51 neutrons. As time passed, the number of each isotope changed from its initial value. When a crystal containing ⁸⁷Rb atoms formed from the molten surface of the hot, early Earth during the Hadean eon, the number of these atoms at that initial time can be represented as N_87Rb,0. As time passed, the number of atoms of this isotope changed to N_87Rb.

C. Justify the relationship between the number of each isotope at any time and the number of each at the time that the molten rock solidified (denoted by the subscript 0):

The decay of unstable radioisotopes is exponential with a half-life of T_1/2, which for ⁸⁷Rb is 4.88 × 10¹⁰ years:

This can be used to replace the initial number of ⁸⁷Rb atoms, which cannot be measured, with the present-day value:

When the measurements of the numbers of ⁸⁷Rb and ⁸⁷Sr were made (Moorbath et al., Nature, 1972), measurements of a second stable isotope of strontium, ⁸⁶Sr, were also made. The ratio of the initial number of ⁸⁷Sr and ⁸⁶Sr atoms is the same as today, since the isotopes are both stable. The value of this ratio is 0.71.

This is a linear equation in the form y = ax + b, where a is the term in parenthesis containing the half-life of ⁸⁷Rb. If $$Y=N_{{87}_{Sr}}/N_{{86}_{Sr}}$$ is graphed versus $$N_{{87}_{Rb}}/N_{{86}_{Sr}},$$ the slope can be used to determine the time, t, that has passed since the rock formed from melting: $$a=e^{0.683t/T_{1/2}}-1$$, so $$t=\ln (a+1)•T_{1/2}/0.693$$

D. Data on the rubidium and strontium isotopes at Isua in Greenland are provided in the table. Analyze these data to obtain the age of formation of these rocks.

The solidification of the molten surface of Earth at the end of the Hadean eon (4 to 4.6 billion years ago) and the condensation of liquid oceans provided a medium from which life emerged. The most ancient fossils are colonial, photosynthetic cyanobacteria called stromatolites. As climate change melted the perennial snow covering Greenland, new geologic evidence of the time of that origin was obtained (Nutman et al., Nature 2016) with the discovery of the most ancient stromatolites. These fossils record communities of photosynthetic bacteria embedded in Isua sediments 3.7 billion years ago. Worldwide stromatolite fossils show a decline between 1 and 1.3 billion years ago.

In 1952, the Miller-Urey experiment showed that an electrical discharge in a gas-phase mixture of ammonia, hydrogen, methane, and water produced five amino acids. When the experiment was conducted, evidence indicated that this mixture was representative of the Hadean (early Earth) atmosphere. The experiment was repeated in the presence of jets of hot steam, simulating Hadean volcanic eruptions and producing an even larger variety of amino acids.

A. Consider the following criticisms of the “organic soup” model and justify the selection of data that other experiments might provide regarding the origin of life on Earth.

• Biopolymers on Earth have a left-hand symmetry at the carbon adjacent to the carboxylic acid carbon, and these experiments produced mixtures of both left- and right-hand symmetries.
• No peptide bonds between amino acids were observed.
• Early Earth’s atmospheric oxygen concentration is known to have been very low, implying the absence of an ozone layer to filter high-energy ultraviolet (uv) radiation.
• Ammonia decomposes when it absorbs high-energy uv radiation, but diatomic nitrogen does not.

Models of the abiotic synthesis of biomolecules suffer from a “chicken and egg” dilemma. Proteins are needed to synthesize DNA and RNA, and DNA and RNA are needed to synthesize proteins. Which molecules came first?

B. In light of the following observations, evaluate the hypothesis that nucleotides arose from a prebiotic mixture.

• Nuclei acids are not found in experiments like those of Miller and Urey.
• Purines and pyrimidines decompose at high temperature, and Earth was bombarded by meteors and comets during the Hadean eon.
• Bonds in the purine and pyrimidine rings of nucleic acids are broken by high-energy uv radiation.
• Carl Sagan and colleagues synthesized ATP from a mixture of adenosine, ribose, and phosphate when exposed to uv radiation.
• Ribose has never been synthesized in experiments like those conducted by Miller and Urey.
• Ribose has a left/right symmetry, and the right-handed form occurs in Earth organisms.

Continuing with the analogy, if neither the chicken nor the egg came first, then both must have arisen together. Some regard simultaneous innovations in both catalysis and information storage and retrieval as too improbable. In samples of meteorites, both amino acids and nucleic acids have been found. The amino acids are mixtures of left- and right-handed symmetries, although some have shown a significant bias toward the left-handed form (J. Elisa et al., ACS Central Science, 2016). The arrival from space of the seeds of biomolecules is called panspermia. Carl Sagan (1966) and Francis Crick (1973), one of the first to describe the structure of DNA, regarded panspermia as the only plausible origin of life on Earth. In fact, their belief was in directed panspermia, the intentional seeding by intelligent aliens.

C. Describe the questions that must be addressed for panspermia to be a scientific hypothesis about the origin of life on Earth and describe the reasons for the directed panspermia revision of this hypothesis.

To avoid the conflicting chicken-and-egg claims that “protein catalyst was first” and “DNA information storage was first,” two alternatives have emerged regarding the origin of life on Earth. Consider two simple ideas: 1) water blocks uv radiation, and cracks in the ocean floor (hot vents) provide a temperature difference that generates a source of entropy; and 2) ribosomes are composed of RNA.

D. Describe one of the following as a hypothesis concerning the origin of life on Earth:

• Reactions among molecules in the vicinity of hot vents became organized in space and time, eventually developing structures that foreshadow the proton gradient upon which metabolism is based. This alternative is the basis for what is referred to as the metabolism-first hypothesis.
• The catalytic properties of the ribosome reflect the self-catalytic polymerization of nucleotides with sequential structures conserved in modern DNA, the catalytic properties conserved in proteins, and the catalytic properties of the ribosome whose core structure is RNA. This alternative is the basis for what is referred to as the RNA-first hypothesis.

The radiant energy emitted by a star gradually increases after its birth. During the Hadean eon, while the molten Earth cooled and life emerged, the Sun provided approximately 25% less radiant energy than it does now. Ignoring effects due to differences in the composition of Earth's atmosphere between then and now, this means that the average surface temperature of the surface would be about 25 °C below the freezing temperature of water. Evidence of liquid water on Earth during the Hadean eon is provided by geologic structures known only to form in liquid water, such as lava pillows and the stromatolites that are the fossilized layers of photosynthetic cyanobacteria.

Pose a scientific question that guides inquiry into early Earth conditions that supported the innovation of photosynthesis.

Connect the techniques of radiometric measurement, anatomy, and molecular biology to the supporting evidence of the theory of evolution provided.

Describe reasons for the revision of scientific hypotheses of the origin of life on Earth.

Directed evolution is an inquiry strategy that is usually used to investigate gene expression or the function of proteins that are expressed. The investigator imposes a selection pressure and observes the evolution of a population. In one investigation, unicellular yeast were allowed to sediment in a column of a nutrient-containing solution. Individuals that traveled furthest towards the bottom of the column were removed and placed in a new column. After 60 generations of repeated selection, yeast became multicellular. In this experiment, selection was acting on the collection of cells and not on the individual. To test the claim that selection was acting on the multicellular system and not just individual cells, the investigators compared the effects on a population of yeast that had acquired multicellularity by strong selection (allowing only 5 minutes to settle) and weak selection (allowing 25 minutes to settle). A strong selection increased cluster size, and a weak selection decreased cluster size.

A. Evaluate the claim that the use of both a strong and weak selection demonstrates that evolution is an ongoing process that, under artificially imposed conditions, led to the emergence of multicellularity in a single-celled organism.

B. In this directed evolution study, the selection pressure imposed by the investigators led to a new phenotype. Consider a situation in which there is a vertical variation in the density of nutritional resources. Analyze the advantages and disadvantages of cooperative behavior, including changes in the likelihood of replication of the individual and population genomes.

Selection processes in changing and unchanging environments differ. Connect the effects of negative and positive selection pressures to changes in the environment.

In biology, the word “race” is rarely used. It could be imagined to be synonymous with a subspecies. Species is well defined, at least when horizontal gene transfer is not taken into account, by reproductive isolation. Speciation may arise through geographic isolation.

A. Aside from geographic isolation leading to reproductive isolation, predict two other mechanisms of speciation in a population and how these mechanisms can lead to a scientific definition of a subspecies.

The use of the term “race” with regard to human populations might be a reference to cultural or socioeconomic isolation, and has often been mistaken to have biological significance. Rosenberg et al. (Science, 2002) sampled the genes of 1,056 people from 52 populations. They compared genetic variations within each population to variations among populations. They found that differences between individuals in two different populations were, on average, roughly 20 times smaller than differences between two individuals in the same population.

B. Groups of humans have often been geographically isolated for long periods of time until isolation is broken by invasion, enslavement, migration, or another similar event. Invaders have traditionally been male. Predict the effect of invasion on the differential inheritance of genes in X- and autosomal chromosomes.