Untitled Flashcards Set
LO 1: Identify the four eons of geologic time by the major events of life (or absence thereof) that define them, and list the eons in chronological order.
The geologic timescale is divided into four eons, each defined by significant events in the history of life on Earth [2]. These eons, in chronological order, are the Hadean, Archean, Proterozoic, and Phanerozoic [3].
Hadean Eon (4.6-4.0 BYA): This is the earliest eon, occurring before there is compelling evidence for life [3]. The Earth was still forming, and it is believed that frequent asteroid collisions made conditions unstable for life to exist [4].
Archean Eon (4.0-2.5 BYA): This eon is characterized by the evolution of early life, including bacteria, archaea, and the first cyanobacteria capable of oxygenic photosynthesis [3]. The earliest undisputed evidence for life, microfossils, dates to this time, suggesting life arose between 3.5 and 3.8 BYA. Early life forms were single-celled, prokaryotic anaerobes that likely used chemical sources of energy [4]. Fossilized stromatolites, which are layered sedimentary structures produced by microbes, also date back to this eon [5].
Proterozoic Eon (2.5 BYA-542 MYA): A key event in this eon was the "Oxygen Revolution," the accumulation of oxygen in the atmosphere due to oxygenic photosynthesis [3, 6]. This eon also saw the first single-celled and multicellular eukaryotes arise [3, 7]. The oceans were not fully oxygenated until 850 MYA [6].
Phanerozoic Eon (542 MYA to present): This eon begins with the Cambrian explosion, a period of rapid diversification of animal life [3, 8]. It is characterized by the proliferation of plant and animal life [3].
LO 2: Identify the fossil, chemical, and genetic evidence for key events for evolution of the three domains of life (Bacteria, Archaea, and Eukarya).
The three domains of life are Bacteria, Archaea, and Eukarya [9]. All three share a single common ancestor [9].
Fossil Evidence:Microfossils: The earliest undisputed evidence for life is microfossils, dating back to 3.5 to 3.8 BYA during the Archean eon, which indicates the presence of early prokaryotic life [4].
Stromatolites: These fossilized layered structures, produced by microbial communities, are another early form of fossil evidence dating from 3.48 to 3.7 BYA [5]. These suggest the presence of early photosynthetic bacteria, though not necessarily oxygenic photosynthesis [5].
Eukaryotic Microfossils: Eukaryotic microfossils are found from the Proterozoic eon dating between 1.6 and 2.2 BYA [7]. Multicellular fossils are found from near the end of the Proterozoic, around 600 MYA [7].
Chemical Evidence (Biosignatures):Isotopes and Molecules: Biosignatures, also known as chemical fossils, include specific carbon isotopes and components of fatty acids, nucleic acids, and proteins, suggesting that life may have been present as early as 4.1 BYA in the Archean eon [4].
Banded Iron Formations: The presence of banded iron formations in sedimentary rocks provides evidence for the slow accumulation of oxygen in the atmosphere during the Oxygen Revolution [6]. The iron reacted with oxygen and precipitated out of the ocean [6].
Genetic Evidence:DNA Sequence Comparisons: Comparisons of DNA sequences and structural and biochemical comparisons consistently categorize all living organisms into the three domains [9]. The phylogenetic relationships derived from this information show that Archaea and Eukarya form a monophyletic group, indicating they are more closely related to each other than to Bacteria [10, 11].
Horizontal Gene Transfer: The prevalence of horizontal gene transfer among prokaryotes makes it difficult to resolve phylogenetic relationships using strictly genetic approaches because a particular gene in two species might be from horizontal gene transfer, not shared ancestry [12]. Phylogenetic analyses use genes that are less likely to be exchanged via HGT such as ribosomal gene sequences [12].
LO 3: Describe the processes that caused the oxygenation of the atmosphere and the evidence for it.
The oxygenation of the atmosphere, known as the Oxygen Revolution, was caused by the evolution of oxygenic photosynthesis in early cyanobacteria [6]. In this process, cyanobacteria split water molecules, producing free molecular oxygen (O2) as a byproduct [6].
This free oxygen initially reacted with soluble iron in the oceans, leading to the precipitation of iron oxide (rust), which resulted in banded iron formations in sedimentary rocks [6]. This is key evidence for oxygen accumulation in the atmosphere [6, 7].
The accumulation of oxygen was not immediate; the oceans were not fully oxygenated until 850 MYA near the end of the Proterozoic eon [6].
The increase in oxygen is an example of how life can alter the planet [8].
LO 4: Describe the hypotheses explaining the causes and results of the Cambrian explosion.
The Cambrian explosion, also called the Cambrian radiation, is a period of rapid diversification that marks the beginning of the Phanerozoic eon, about 542 MYA [8]. The term "explosion" is inaccurate, as it occurred over millions of years, which is still very rapid in the context of geologic time [8].
Hypotheses for the Cambrian Explosion:Oxygen Accumulation: The leading hypothesis is that the accumulation of oxygen in the atmosphere (due to the Oxygen Revolution) allowed for the evolution of larger bodies and more complex tissues and organs, such as brains, which require higher metabolic rates [8]. The increase in oxygen allowed for more complex lifeforms [8].
The availability of oxygen also resulted in mass extinction of many obligate anaerobic organisms that could not survive in the presence of oxygen [8].
LO 5: Place and identify the three domains of life on a phylogenetic tree.
The three domains of life are Bacteria, Archaea, and Eukarya [9].
Phylogenetic analysis, based on RNA data, shows that Archaea and Eukarya form a monophyletic group, meaning they share a more recent common ancestor with each other than with Bacteria [10, 11]. Although the term "prokaryote" is used for both Archaea and Bacteria, it does not describe a monophyletic group [10].
This indicates that Archaea are more closely related to Eukarya than to Bacteria, even though Archaea and Bacteria share more similar superficial characteristics [11].
LO 6: Use cellular traits to differentiate between Bacteria, Archaea, and Eukarya.
Cell Structure:Bacteria and Archaea are both unicellular prokaryotes, meaning their cells lack nuclei and other internal membrane-bound structures [9, 11].
Eukarya include both unicellular and multicellular organisms. Their cells have a nucleus that separates their DNA from the rest of the cell, as well as membrane-bound organelles [9, 11].
Chromosomes:Bacteria and Archaea typically have a single, circular chromosome [11].
Eukarya usually have multiple, linear chromosomes that are wrapped around proteins called histones [11].
Reproduction:Bacteria and Archaea reproduce asexually through binary fission, where a cell divides in two after replicating its chromosome [11].
Eukarya reproduce asexually through mitosis, which involves the precise duplication and separation of multiple chromosomes. Many eukaryotes can also reproduce sexually through meiosis [11].
Bacteria and Archaea cannot reproduce sexually [11].
Cell Walls:Bacteria have cell walls made of peptidoglycan [13].
Archaea have cell walls made of polysaccharides (sugars) [13].
Eukaryotic cell walls, when present, are made of cellulose in plants and chitin in fungi [13].
Horizontal Gene Transfer:Bacteria and Archaea can share genetic material between individuals through horizontal gene transfer, which is a primary way that antibiotic resistance spreads [11].
Eukaryotes primarily use vertical gene transfer in which offspring inherit genes from their parents [11].
LO 7: Define photo-, chemo-, auto-, and hetero-trophy, and identify which domains of life can accomplish each metabolic strategy.
Energy Source:Phototrophs obtain energy from sunlight [14].
Chemotrophs obtain energy from chemical compounds [14].
Carbon Source:Autotrophs can fix inorganic carbon (e.g., carbon dioxide) into organic compounds [14].
Heterotrophs must obtain carbon from an organic source [14].
Combinations:Photoautotrophs use sunlight for energy and carbon dioxide for carbon (e.g., plants and cyanobacteria) [14].
Chemoheterotrophs obtain both energy and carbon from organic compounds (e.g., animals and fungi) [14].
Chemoautotrophs obtain energy from inorganic compounds and carbon from carbon dioxide [14].
Photoheterotrophs use light as an energy source but require organic carbon [14].
Metabolic Diversity Across Domains:Prokaryotes (Bacteria and Archaea) show all four metabolic types: photoautotrophs, chemoheterotrophs, chemoautotrophs, and photoheterotrophs [14, 15].
Eukaryotes are only photoautotrophs (plants and some protists) or chemoheterotrophs (animals, fungi, and some protists) [16].
LO 8: Describe the importance of prokaryotes (Bacteria and Archaea) with respect to human health and environmental processes.
Prokaryotes and Human Health:Pathogens: Some prokaryotes are pathogens that cause disease in humans [17]. Antibiotic resistance is an increasing concern in the 21st century [17].
Beneficial Roles: Many prokaryotes are beneficial [18]. They protect us from pathogens, help us digest food, produce vitamins, and influence our moods and metabolism [18]. The absence of key microbes can cause health problems including allergies and some autoimmune disorders [18]. Fecal transplants have been successfully used to reestablish normal gut bacteria in patients suffering from Clostridium difficile overgrowth [18].
Prokaryotes and Environmental Processes:Nutrient Cycling: Prokaryotes play a critical role in biogeochemical cycling of nitrogen, carbon, phosphorus and other nutrients [19]. Biological nitrogen fixation (BNF), which converts atmospheric nitrogen to ammonia, is exclusively carried out by prokaryotes [19].
Bioremediation: Prokaryotes are used in bioremediation to remove pollutants such as pesticides and toxic metals [19]. They are also important in cleaning up oil spills, where they can degrade hydrocarbons [19, 20].