Learning Objectives
Three domains of life on EarthDNA sequence comparisons and structural and biochemical comparisons consistently categorize all living organisms into 3 primary domains: Bacteria, Archaea, and Eukarya (also called Eukaryotes; these terms can be used interchangeably). Both Bacteria and Archaea are prokaryotes, single-celled microorganisms with no nuclei, and Eukarya includes us and all other animals, plants, fungi, and single-celled protists – all organisms whose cells have nuclei to enclose their DNA apart from the rest of the cell. The fossil record indicates that the first living organisms were prokaryotes (Bacteria and Archaea), and eukaryotes arose a billion years later. Show
Study Tip: It is suggested that you create a chart to compare and contrast the three domains of life as you read. The information below was adapted from OpenStax Biology 22.2Archaea and Bacteria share a number of features, but are also distinct domains of life:
The features of a typical prokaryotic cell are shown. Image credit: OpenStax Biology 22.2 Phylogenetic relationships between Archaea, Bacteria, and EukaryaWhile the term prokaryote (“before-nucleus”) is widely used to describe both Archaea and Bacteria, you can see from the phylogenetic Tree of Life below that this term does not describe a monophyletic group: A phylogenetic tree of living things, based on RNA data and proposed by Carl Woese, showing the separation of bacteria, archaea, and eukaryotes. By This vector version: Eric Gaba (Sting – fr:Sting) – NASA Astrobiology Institute, found in an article, Public Domain, https://commons.wikimedia.org/w/index.php?curid=1201601In fact, Archaea and Eukarya form a monophyletic group, not Archaea and Bacteria. These relationships indicate that archaea are more closely related to eukaryotes than to bacteria, even though superficially archaea appear to be much more similar to bacteria than eukaryotes. Metabolic diversity of prokaryotesThe information below was adapted from OpenStax Biology 22.3 Prokaryotes have been and are able to live in every environment by using whatever energy and carbon sources are available. Prokaryotes fill many niches on Earth, including being involved in nutrient cycles such as nitrogen and carbon cycles, decomposing dead organisms, and thriving inside living organisms, including humans. The very broad range of environments that prokaryotes occupy is possible because they have diverse metabolic processes. Phototrophs (or phototrophic organisms) obtain their energy from sunlight. Chemotrophs (or chemosynthetic organisms) obtain their energy from chemical compounds. Prokaryotes not only can use different sources of energy but also different sources of carbon compounds. Recall that organisms that are able to fix inorganic carbon (for example, carbon dioxide) into organic carbon (for example, glucose) are called autotrophs. In contrast, heterotrophs must obtain carbon from organic compounds. The terms that describe how prokaryotes obtain energy and carbon can be combined. Thus, photoautotrophs use energy from sunlight, and carbon from carbon dioxide and water, whereas chemoheterotrophs obtain energy and carbon from an organic chemical source. Chemoautotrophs obtain their energy from inorganic compounds, and they build their complex molecules from carbon dioxide. Finally, photoheterotrophs use light as an energy source, but require an organic carbon source (they cannot fix carbon dioxide into organic carbon). In contrast to the great metabolic diversity of prokaryotes, eukaryotes are only photoautotrophs (plants and some protists) or chemoheterotrophs (animals, fungi, and some protists). The table below summarizes carbon and energy sources in prokaryotes. The videos below provide more detailed overviews of Archaea and Bacteria, including general features and metabolic diversity: Key events and evidence in the evolution of the three domains of life on EarthEarly life on Earth: The Earth is approximately 4.6 billion years old based on radiometric dating. While it is formally possible that life arose during the Hadean eon, conditions may not have been stable enough on the planet to sustain life because large numbers of asteroids were thought to have collided with the planet during the end of the Hadean and beginning of the Archean eons. Evidence from microfossils (literally “microscopic fossils”) suggests that the life was present on Earth at least 3.8 billion years ago. The earliest chemical evidence of life, in the form of chemical signatures produced only by living organisms, dates to approximately 3.6 billion years ago. What were these early life forms like? For the first billion years of Earth’s existence, the atmosphere was anoxic, meaning that there was no molecular oxygen (O2). Thus the first living things were single-celled, prokaryotic anaerobes (living without oxygen) and likely chemotrophic. The Oxygen Revolution: The evolution of water-splitting and oxygen-generating photosynthesis by cyanobacteria led to the first free molecular oxygen about 2.6 billion years ago. The free oxygen produced by cyanobacteria immediately reacted with soluble iron in the oceans, causing iron oxide (rust) to precipitate out of the oceans. Oxygen didn’t accumulate all at once, and evidence indicates that the oceans weren’t fully oxygenated until 850 million years ago (Mya). Today we see evidence of the slow accumulation of oxygen in the atmosphere through banded iron formations present in sedimentary rocks from that period. Banded iron formation, Karijini National Park, Western Australia. By Graeme Churchard from Bristol, UK – Dales GorgeUploaded by PDTillman, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=30889569 The increase in oxygen, called “The Oxygen Revolution,” enabled the evolution of larger bodies and organs and tissues, such as brains, with high metabolic rates. The increase in oxygen is a dramatic example of how life can alter the planet. Evolution of oxygenic photosynthesis changed the planet’s atmosphere over billions of years, and in turn caused radical shifts in the biosphere: from an anoxic environment populated by anaerobic, single-celled prokaryotes, to eukaryotes living in a micro-aerophilic (low-oxygen) environment, to multicellular-organisms in an oxygen-rich environment. The video below provides an overview of the Oxygen Revolution (aka, the Oxygen Catastrophe), including its detrimental effects on the organisms that lived at the time: Origins of eukaryotes: How did eukaryotes arise? The leading hypothesis, called the endosymbiotic theory, is that eukaryotes arose as a result of a fusion of Archaean cells with bacteria, where an ancient Archaean engulfed (but did not eat) an ancient, aerobic bacterial cell. The engulfed (endosymbiosed) bacterial cell remained within the archaean cell in what may have been a mutualistic relationship: the engulfed bacterium allowed the host archean cell to use oxygen to release energy stored in nutrients, and the host cell protected the bacterial cell from predators. Microfossil evidence suggests that eukaryotes arose sometime between 1.6 and 2.2 billion years ago. The descendants of this ancient engulfed cell are present in all eukaryotic cells today as mitochondria. We’ll discuss the endosymbiotic theory for the origin of eukaryotes more in the next reading. Complex life forms: Much of the life on Earth was singled celled until shortly before the Cambrian “explosion,” when we see emergence of all modern animal phyla. The Cambrian radiation (meaning rapid evolutionary diversification) occurred approx. 540 Mya. The “explosion” term refers to an increase in biodiversity of multicellular organisms at the start of the Cambrian, 540 million years ago. Multicellular life appeared only several tens of millions of years before the start of the Cambrian, as bizarre-looking fossils (Ediacaran biota/Doushantuo fossils) and exhibiting body plans unlike anything seen present-day animals. These species largely disappeared and were replaced by Cambrian fauna, whose variety includes all of the body plans found in present-day animal phyla. The appearance of Cambrian fauna span millions of years; they did not all appear simultaneously as the term “explosion” inaccurately implies. Placing key events on the geologic time scaleHow do each of these events map onto geologic time? Most of them are not “instantaneous” events, and so they span multiple time periods as follows:
Links to human health and environmental processesThe information below was adapted from OpenStax Biology 22.4 Some prokaryotic species can harm human health as pathogens: Devastating pathogen-borne diseases and plagues, both viral and bacterial in nature, have affected humans since the beginning of human history, but at the time, their cause was not understood. Over time, people came to realize that staying apart from afflicted persons (and their belongings) tended to reduce one’s chances of getting sick. For a pathogen to cause disease, it must be able to reproduce in the host’s body and damage the host in some way, and to spread, it must pass to a new host. In the 21st century, infectious diseases remain among the leading causes of death worldwide, despite advances made in medical research and treatments in recent decades. The information below was adapted from OpenStax Biology 22.5 Not all prokaryotes are pathogenic; pathogens represent only a very small percentage of the diversity of the microbial world. In fact, our life would not be possible without prokaryotes. Some prokaryotic species are directly beneficial to human health:
Carbon cycle; Image modified from “Nitrogen cycle” by Johann Dréo (CC BY-SA 3.0). The modified image is licensed under a CC BY-SA 3.0 license._ Other prokaryotes indirectly, but dramatically, impact human health through their roles in environmental processes:
a) Cleaning up oil after the Valdez spill in Alaska, workers hosed oil from beaches and then used a floating boom to corral the oil, which was finally skimmed from the water surface. Some species of bacteria are able to solubilize and degrade the oil. (b) One of the most catastrophic consequences of oil spills is the damage to fauna. (credit a: modification of work by NOAA; credit b: modification of work by GOLUBENKOV, NGO: Saving Taman; from https://cnx.org/resources/b3178fe3228bf3c1f1ce0feae58ed67d7d1dad07/Figure_22_05_03ab.jpg) Which of the following is characteristic of archaea?The common characteristics of Archaebacteria known to date are these: (1) the presence of characteristic tRNAs and ribosomal RNAs; (2) the absence of peptidoglycan cell walls, with in many cases, replacement by a largely proteinaceous coat; (3) the occurrence of ether linked lipids built from phytanyl chains and (4) in ...
Which of the following groups of archaea plays an important role in sewage treatment?Methanogenic archaea play a crucial role during anaerobic wastewater treatment. The most abundant acetoclastic methanogens in the anaerobic reactors for industrial wastewater treatment are Methanosarcina sp.
Which of the following is a phylum in domain archaea?Most taxonomists agree that within the Archaea, there are currently five major phyla: Crenarchaeota, Euryarchaeota, Korarchaeota, Nanoarchaeota, and Thaumarchaeota.
What is archaea quizlet?Archaea are organisms that have many unique molecular traits. Archaea are prokaryotes, but their cell walls are chemically different from those of bacteria. Some olecules in archaea are similar to molecules in eukaryotes. Some molecules in archaea are not found in other living things.
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