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Are we alone in the Universe? Searching for Life on Other Planets The image to the right is our first glimpse of the surface of another planet. The image was sent to Earth by NASA’s Viking I in 1976, the first of two robotic probes which landed on Mars that year to search for life in the Martian surface soil. When no definitive signs of life were detected, many scientists concluded that although Mars may have once harbored life, today it is likely to be a cold, dry, “dead” planet. However, in the three decades since the Viking mission, new discoveries of life right here on earth have caused us to dramatically alter our notions of the physical conditions which can sustain life. What were these discoveries and how have they changed our understanding of the very nature of life itself? They’re Alive! Or Are They? Minuscule and mysterious, they had remained hidden since time immemorial, buried deep within Earth, 3 miles beneath the ocean floor off the coast of Western Australia. Then, several years ago, a team of scientists spotted the tiny oddities while using ultra-high- powered microscopes to study ancient rocks retrieved from an oil- drilling site. Researchers dubbed the miniature things “nanobes” because they are so small that they measure only billionths of a meter (nanometers), with a million or so able to fit comfortably in the dot over this “i.” The discovering scientists proclaimed these tiny creatures to be the world’s smallest living organisms. When brought into the laboratory nanobes will grow so quickly that within weeks they can go from being visible only with the world’s most powerful microscopes to forming fast- expanding threadlike mats that are easily visible to the naked eye. More recently, scientists have reported finding these minuscule forms in eggshells, blood, and decaying leaves. Some have hailed this work as a fundamental and important discovery, even suggesting that nanobes are likely to be widespread—possibly the most abundant form of life in and on Earth. Although nanobes are indisputably tiny, scientists cannot seem to agree on whether they are the world’s smallest organisms. That is because there still is no agreement on whether nanobes are actually alive. Isn’t it a simple matter to tell living from nonliving, after all, any schoolchild can distinguish between an inanimate stone and the living being who skips it across a pond? The Mysterious Life of Caves Deep in the rainforest of southern Mexico, a small milky white stream emerging from the depths of Cueva de Villa Luz (Lighted House Cave) beckons travelers to enter one of its many sky lights connecting the cave to the surface. Just inside the cave, a gas smelling of rotten eggs bubbles up from crystal clear pools of water, a sure sign that the cave poses invisible and potentially deadly risks. Today we know this gas to be hydrogen sulfide. It emerges from deep underground and combines with water vapor in the cave air to produce sulfuric acid and without respirators this corrosive acid would quickly eat away the lungs of anyone who dared to venture further. Wearing adequate protective gear, cave researchers have ventured deep into Cueva de Villa Luz where they have encountered strands of mucous dangling from the cave ceiling. They refer to them as “snotites” because of their resemblance to the stalactites commonly found in caves, but which instead drip like a runny nose. To their surprise, dripping from them is more sulfuric acid, and despite the presence of this caustic acid, toxic fumes and total darkness life is thriving in this “extreme” environment. Examined under a microscope the mucous strands, along with other residues found on cave walls, reveal diverse colonies of bacteria which have been found to feed on hydrogen sulfide and minerals in the rock walls such as manganese. These bacteria produce the sulfuric acid as a by-product of their metabolism. Geologists, working with biologists who study cave formation, theorize that this acid eats away at the cave walls, thereby enlarging the cave over vast expanses of time. In the years since Viking first landed on Mars, biologists have found life, sometimes termed “extremophiles”, thriving in the near boiling temperatures of hot springs, in the snow-packed ice of glaciers, and at depths of 5000 meters below the ocean surface around active undersea volcanic (hydrothermal) vents. But are these environments really “extreme”? From our perspective they certainly are, but what about from the perspective of the life that inhabits these places? What do these discoveries (extremophiles and nanobes) say about what we must now consider in the search for life on other worlds? Connecting Concepts - Organize your knowledge of the characteristics of life by completing the following incomplete table. First try to complete the table on your own from you textbook reading, and then if you get stuck, feel free to discuss it with classmates. As an example, the first row is completed for you. Unifying Definition or explanation of concept Example characteristic Life is hierarchically organized into levels that Atoms form organic molecules which are organized into intracellular organelles which build on previous simpler levels. Moreover, at collectively form cells that are then organized into tissues which form organs and Emergent each of these levels, properties or organ systems comprising an organism. Organisms then interact with other organisms hierarchical characteristics emerge that are not displayed in habitats and ecosystems within the earth’s biosphere. Life’s emergent properties properties at simpler levels. are exemplified by cells which exhibit properties (metabolism, movement, reproduction) that are not displayed by the organelles that comprise a cell. Complexity Nucleic Acids, Carbohydrates, ____________, ____________ Prokaryote- Cell Structure Eukaryote – Metabolism Homeostasis Life’s diversity comes from __________? Revisiting Nanobes Now that we have a list of shared characteristics of living things, how do nanobes stack up? Measuring from 20 to 150 nanometers across, are they the smallest living things? Use the table below to organize your knowledge about what is currently known about the “life-status” of nanobes. Some knowledge is already included. Based on your research what decision would you make? Are nanobes alive, not alive, or do they deserve to be classified somewhere in between? Nanobes appear to be highly structured and have very much the “look” of a complex microscopic organism— appearing as spheres, chains of beads, or bean- or sausagelike shapes, as many microscopic organisms do—as opposed to looking like an array of lifeless crystals or disorganized nonliving material. Scientists say that the nanobes also appear to have a cell-like structure, with a cell-like outer surface, or membrane, encircling the nanobe. Cell Structure Application of chemical stains for DNA to nanobes reveals the presence of DNA. Molecular structure After seeing the enlargement of nanobe colonies in the laboratory, some scientists have concluded that nanobes can grow. Other scientists observed that nanobes increase in size similar to the way drying salt water can create a growing crust of salt crystals under certain conditions. Reproduction Metabolism Evolution Sense and response to environmenta l cues? Closing Thoughts Why should we even worry about whether or not nanobes should be classified as alive? Well, it is important to understand that the name biologists attach to something has meaning. In this case “alive” means possessing all of the diverse and unifying characteristics of life which resides on planet Earth. Among biologists, arguments about whether or not something is alive are about more than just attaching the name “alive” to it; they are really about the very nature of life itself.
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