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[h] Origin of Life at Alkaline Hydrothermal Vents

[i]

[q] What’s the key idea of the “metabolism-first” camp?

[a] The “metabolism-first” camp promotes the idea that organized chains of chemical reactions were the initial systems that led to the emergence of living systems.

[q] Briefly describe “Black Smoker” vent ecosystems

[a] “Black smoker vents” are powered by rising magma. The magma superheats ocean water in the ocean floor, which rises full of dissolved minerals. Mineral deposition leads chimneys to form. The chimneys emit a rich, black smokey fluid (which is why these systems are called “Black Smokers.”
The food webs in these communities are based on chemosynthetic bacteria, which can use hydrogen sulfide as an electron and proton donor to create carbohydrates. The community supports worms, clams, crabs, shrimp, and other organisms.

[q]How are alkaline hydrothermal vents different from “Black smoker vents?”

[a]Alkaline hydrothermal vents are much cooler than Black smoker vents. They emit fluids that are alkaline, and rich in hydrogen. Their spires last much longer than “Black smoker” chimneys. Their internal structure is riddled with tiny pores.

[q]List four reasons why ancient alkaline hydrothermal vents are a promising site for a “metabolism first” origin of life.

[a]Ancient alkaline hydrothermal vents

1. Naturally produce hydrogen gas, a rich energy source.
2. Exist in an environment that would have been a stable refuge for life during the hellish Hadean era when life first emerged.
3. Contain spires that are riddled with tiny pores that could have acted as reaction chambers, concentrating monomers into polymers.
4. Would have produced metal catalysts that would promote the formation of monomers and polymers.

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[h]Origin of Life: Cumulative Flashcards

[i]

[q json=”true” yy=”4″ dataset_id=”AP_Bio_Flashcards_2022|14184585d8110″ question_number=”341″ unit=”7.Evolution_and_Natural_Selection” topic=”7.13.Origin_of_Life”] When did life first evolve on Earth? What’s the evidence?

[a] There’s widespread evidence for life on Earth as long ago as 3.4 billion years ago (bya), and life might have emerged even earlier than that. The consensus date for the emergence of life is about 3.8 bya.

The evidence for this claim consists of 1) fossilized bacterial mats (stromatolites), 2) microfossils of bacterial cells and 3) chemical traces of what’s called “biogenic carbon” (carbon of biological origin), all of which date back to 3.4 bya (or earlier).

[q json=”true” yy=”4″ unit=”7.Evolution_and_Natural_Selection” dataset_id=”AP_Bio_Flashcards_2022|140a02bd03910″ question_number=”342″ topic=”7.13.Origin_of_Life”] Describe the fossilized stromatolites and fossilized cells that serve as evidence for the earliest traces of life on Earth.

[a]

Stromatolites are layered bacterial mats, consisting of layers of cells interleaved with layers of sediments. These are formed in some areas on Earth today (such as Shark Bay in Australia). The oldest fossilized stromatolites have been dated (through radiometric dating) as 3.4 billion years old.

Inside these fossilized stromatolites are microfossils. These are fossils that are cellular in size, and can only be visualized with an electron microscope. When ancient fossil stromatolites are sectioned and viewed under an electron microscope, structures that match cells in size and form can be seen.

[q json=”true” yy=”4″ unit=”7.Evolution_and_Natural_Selection” dataset_id=”AP_Bio_Flashcards_2022|13fc7a37e6510″ question_number=”343″ topic=”7.13.Origin_of_Life”]Isotopic signatures can serve as evidence for the earliest traces of life on Earth. Explain.

Illustrative example. Evidence for the origin of life

[a] When living things take in carbon, they preferentially take in carbon-12 over a heavier (and much rarer) isotope, carbon-13. In carbon that’s biological in origin, the ratio of carbon 12 to carbon 13 is much higher than in carbon samples of non-biological origin. Samples of carbon from 3.4 billion years ago (and even up to 3.8 billion years ago) show that this carbon was captured by living things. Note that using isotopic signatures alone, claims have been made for an origin of life that occurred over 4 billion years ago, but these are not widely accepted.

[q json=”true” yy=”4″ dataset_id=”AP_Bio_Flashcards_2022|13ef16f387510″ question_number=”344″ unit=”7.Evolution_and_Natural_Selection” topic=”7.13.Origin_of_Life”] It’s been said that life emerged on Earth about as early as it possibly could have. Explain.

[a]Our solar system, including our Earth, formed about 4.6 bya from a stellar nebula. As the planets formed, there was a period of massive collisions when the early Earth was under continual bombardment by comets, asteroids, and even planet-sized objects. These bombardments would have made life impossible.  Yet signs of life are unambiguous at 3.4 bya, and life might have evolved as early as 3.8 bya, which is close to when the period of heavy bombardment was coming to a close. Hence, life emerged on our world just about as early as it could have.

[q json=”true” yy=”4″ unit=”7.Evolution_and_Natural_Selection” topic=”7.13.Origin_of_Life” dataset_id=”AP_Bio_Flashcards_2022|1781950e37dcd5″ question_number=”345″] Describe some the conditions on the early Earth during the period when life first emerged.

[a]Life on Earth emerged when conditions were quite different from conditions today. First of all, Earth had very little land: just a few islands above a world ocean. The young moon was much closer, causing massive tides in these oceans. The Earth spun more quickly on its axis: a day might have been as short as eight hours. The atmosphere was different, consisting of carbon dioxide and nitrogen, and no molecular oxygen (O₂).

[q json=”true” yy=”4″ dataset_id=”AP_Bio_Flashcards_2022|13e1d8efe6910″ question_number=”346″ unit=”7.Evolution_and_Natural_Selection” topic=”7.13.Origin_of_Life”] In any origin of life scenario, what’s the first step? Explain.

[a] In any origin of life scenario, the first step is the formation of monomers: the amino acids, nucleotides, monosaccharides, and fatty acids that serve as the building blocks of life. Only if these molecules are present can more complex molecules (like proteins and nucleic acids) come into being.

Monomer formation is difficult to explain because these monomers don’t spontaneously form. Rather, these energy-rich molecules are generated biologically by plants, cyanobacteria, and algae during photosynthesis. So the challenge in explaining the origin of life is to explain how to generate monomers abiotically (in the absence of life).

[q json=”true” yy=”4″ unit=”7.Evolution_and_Natural_Selection” dataset_id=”AP_Bio_Flashcards_2022|13d888c056910″ question_number=”347″ topic=”7.13.Origin_of_Life”] The Miller-Urey experiment was an attempt to validate Oparin and Haldane’s idea, proposed in the 1920s, of monomers emerging in a primordial soup. What was the primordial soup?

[a] The primordial-soup model posits that in the Earth’s early oxygen-free atmosphere, available energy could cause molecules like methane and ammonia to combine to form monomers (monosaccharides, amino acids, etc.), which would accumulate in the early oceans, forming a primordial soup. This, in turn, would set the stage for more complex molecules to form, leading to the emergence of life.

[q json=”true” yy=”4″ unit=”7.Evolution_and_Natural_Selection”  topic=”7.13.Origin_of_Life” dataset_id=”AP_Bio_Flashcards_2022|d112b223e40ea” question_number=”348″] Describe the Miller-Urey experiment and its results.

[a]The Miller-Urey experiment validated the idea of the abiotic formation of monomers. The experiment combined methane, ammonia, and hydrogen (thought at that time to be a plausible mix for the Early earth’s atmosphere) in a reaction chamber (5). The chamber was connected to a flask containing heated water (2) which represented the early Earth’s oceans. Sparks (5) simulated lightning. After several days, the circulating mixture in the apparatus was sampled, and analysis showed the presence of several amino acids (the monomers of proteins).

[q json=”true” yy=”4″ unit=”7.Evolution_and_Natural_Selection” dataset_id=”AP_Bio_Flashcards_2022|13c404178d510″ question_number=”349″ topic=”7.13.Origin_of_Life”] Why is the Miller-Urey experiment widely regarded as a keystone in origin of life research, and why has it been widely criticized?

[a] The overall achievement of the Miller-Urey experiment was validation of the idea that monomers can be formed abiotically. In subsequent years, this has been confirmed in a variety of experimental settings, with many different starting compounds and energy sources. This idea has also been validated by the discovery of monomers on meteorites (meaning that certain monomers can form out in space). However, the experiment has been criticized for using an atmosphere with highly reduced gases (methane, ammonia, and hydrogen), which are now thought to have been unlikely on the early Earth, and which made it much easier to produce compounds such as amino acids.

[q json=”true” yy=”4″ unit=”7.Evolution_and_Natural_Selection”  topic=”7.13.Origin_of_Life” dataset_id=”AP_Bio_Flashcards_2022|d0f5978f420ea” question_number=”350″] The Miller-Urey experiment was an attempt to validate the  “primordial soup” model of monomer formation. But while life requires water, the presence of water is also a problem in any origin of life scenario. Explain.

[a]The problem is that the polymerization of monomers to create polymers such as RNA and protein would be difficult in a watery environment. That’s because monomers combine to form polymers through dehydration synthesis, a reaction that only occurs in a watery solution with the help of enzymes. Without enzymes (which are themselves polymers), it’s hard to imagine how polymer formation could have happened. In other words, life’s first nursery was unlikely to have been any type of soupy, open body of water. Rather, a system like a hydrothermal vent is a much more likely venue for the origin of life.

[q json=”true” yy=”4″ dataset_id=”AP_Bio_Flashcards_2022|13bafe6979d10″ question_number=”351″ unit=”7.Evolution_and_Natural_Selection” topic=”7.13.Origin_of_Life”] How was the early earth’s atmosphere different from the atmosphere today?

[a] The Earth’s earliest atmosphere (like that of Mars and Venus) lacked molecular oxygen (O₂). That’s because O₂ only exists on Earth because of photosynthesizing organisms that oxidize water and release O₂ as a waste product. Thus, free O_2, which makes up about 21% of the atmosphere today, is a planetary-scale biogenic signature: a feature that arises only in the presence of life. If in the future, a space probe finds itself orbiting a planet with an O₂-rich atmosphere, we’ll be fairly certain that this is a planet where life (at least microbial life) is flourishing (even before our probe lands and takes samples).

[q json=”true” yy=”4″ unit=”7.Evolution_and_Natural_Selection” dataset_id=”AP_Bio_Flashcards_2022|13abb6db71910″ question_number=”352″ topic=”7.13.Origin_of_Life”] The early Earth’s atmosphere was quite different from the atmosphere today. Why is that important for the origin of life?

[a] Unlike today’s atmosphere, which consists of about 21% oxygen, the early Earth’s atmosphere has no free oxygen (O₂). The reason why this is important to the origin of life is that for life to arise, its molecular components, biological monomers, need to arise. These monomers are reduced molecules, and in the presence of oxygen, their spontaneous tendency is to become oxidized. A world with abundant oxygen is not a world where new life could arise, and the presence of a reducing (or at least non-oxidizing) atmosphere and ocean was probably a precondition for the emergence of the monomers (amino acids, nucleotides, fatty acids, and monosaccharides) that were combined to create the polymers in the first living cells.

[q json=”true” yy=”4″ dataset_id=”AP_Bio_Flashcards_2022|139bff8b2e910″ question_number=”353″ unit=”7.Evolution_and_Natural_Selection” topic=”7.13.Origin_of_Life”] What is the RNA world hypothesis?

[a] The RNA world hypothesis is the idea that the first genetic molecule was RNA, not DNA. That’s because DNA is too protein-dependent for its replication and transcription to have arisen on its own. Without proteins (like DNA polymerases), you can’t replicate DNA. But polymerases are proteins and are spelled out by DNA. How could we have one arise without the other? RNA is thought to be a better candidate as the first genetic molecule because RNA can be both information (as in messenger RNA) and act as a catalyst (as in ribosomal RNA). According to the theory, if a self-replicating RNA could arise, then that would set the stage for the emergence of evolution and life.

[q json=”true” yy=”4″ unit=”7.Evolution_and_Natural_Selection” dataset_id=”AP_Bio_Flashcards_2022|13918555ac910″ question_number=”354″ topic=”7.13.Origin_of_Life”] What are two attributes of RNA that make it a better candidate than DNA for playing the role of the first genetic molecule?

[a] RNA is thought to be a better candidate for the first genetic molecule than DNA because RNA can serve as both genetic information (as it does in RNA viruses and mRNA) and as a catalyst for chemical reactions (as it does in spliceosomes and as the catalytic part of ribosomes). DNA, by contrast, is purely informational.

[q json=”true” yy=”4″ unit=”7.Evolution_and_Natural_Selection” dataset_id=”AP_Bio_Flashcards_2022|13823dc7a4510″ question_number=”355″ topic=”7.13.Origin_of_Life”] Using the RNA-world hypothesis, sketch out the steps to the emergence of DNA-based and cell-based life.

[a] The RNA-world hypothesis posits that as life was arising, a population of self-replicating RNAs emerged (1 and 2). During self-replication, some errors would arise, leading to diverse RNAs that could be subject to natural selection. Those RNA replicators that could replicate more quickly and efficiently would come to dominate their populations.  At some point, self-replicating RNA polymers became paired with amino acids and polypeptides, leading to the emergence of RNA-protein systems (3). Next, these self-replicating RNA-protein systems would have had DNA replace RNA as the genetic material (4). Finally, encapsulation by lipid membranes would lead to the first cells (5).

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