From the mightiest blue whale to the most miniscule paramecium, life as we know it takes dramatically different forms. Nonetheless, all organisms are built from the same six essential elemental ingredients: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur (CHNOPS). Why those elements? To find out, Life's Little Mysteries consulted Matthew Pasek, a biogeochemist at the University of South Florida. "First off, carbon enters easily into bonds with other carbon atoms. This means it forms vast chains that act as a nice skeleton for other atoms to bond to," Pasek said. In other words, carbon atoms are the perfect building blocks for large organic molecules. "This lends itself to complexity." But what explains the other five chemical ingredients of life? "One thing that makes nitrogen, hydrogen and oxygen good is that they're abundant," Pasek said. "They also exhibit acid-base effects, which allows them to bond with carbon to make amino acids, fats, lipids and the nucleobases from which DNA and RNA are built." "Sulfur provides electron shuffle," Pasek continued. "Basically, with their surplus of electrons, sulfides and sulfates help catalyze reactions. Some organisms use selenium in place of sulfur in their enzymes, but not many." Last but not least, phosphorus, usually found in the molecule phosphate, is vital to metabolism, because polyphosphate molecules such as ATP (adenosine triphosphate) are able to store a huge amount of energy in their chemical bonds. Breaking the bond releases its energy; do this enough times in, say, a group of muscle cells, and you can move your arm. Late last year, NASA scientists discovered the only known exception to the phosphorus requirement in an arsenic-rich California lake. They found a strain of microbes able to substitute arsenic atoms for phosphorus in their molecules when supplies of phosphorus are low. Arsenic is chemically similar to phosphorus, making it poisonous to most life forms because it disrupts metabolic pathways. In summary, "With a few exceptions, what you need for life is CHNOPS, plus a dash of salt and a few metals," Pasek said. "Of course, those ingredients do have to be in the correct bonding structure, but this seems to occur naturally. Amino acids occur spontaneously, as do sugars and lipids and nucleobases." That's true, at least, on Earth. For the necessary molecular structures to form, a planet must be just the right distance from its sun it can't be too hot or too cold for liquid water to exist. Having an abundant supply of water also helps, because it makes it easier for the ingredients to move around and bump into each other to form interesting compounds. Gravity must be just right, too. Finally, a dash of lightning can provide the much-needed energy to catalyze a reaction that will ultimately lead to the production of the complex moleculesamino acids, proteins, fats, carbohydrates, RNA and DNAthat lend themselves to producing life. At least as we know it. Got a question? Send us an email and we'll look for an expert who can crack it. Follow Natalie Wolchover on Twitter @nattyover
How did life begin? We will never know with certainty what the Earth was like four billion years ago, or the kinds of reactions that led to the emergence of life at that time, but there is another way to pose the question. If we ask “how can life begin?” instead of “how did life begin,” that simple change of verbs offers hope. It does seem possible we can demonstrate a series of obvious steps toward the origin of life, perhaps leading to a synthetic version of life in the laboratory. We will then be able to provide a satisfactory answer to the second question: How can life begin on the Earth and other habitable planets? The first step toward life involves a fundamental question we can answer: Where did the elements of life come from? Take a look at the simplified periodic table below. See the six elements in green? Those are called the biogenic elements.  If you add up all the atoms composing a living cell, those six represent close to 99% of the elemental composition of proteins, nucleic acids, and cell membranes. Life does need these six elements, but it only works if the elements have combined into molecules. Let’s consider what happens if we put two or more of the elements together in a compound. Carbon and hydrogen, for instance, become hydrocarbons, and the hydrocarbon chains in cell membranes are an essential component of life. If we let three elements combine, such as carbon, hydrogen, and oxygen, we get carbohydrates like sugar and cellulose. Five elements—carbon, hydrogen, oxygen, nitrogen and sulfur—form the amino acids of proteins, and if we exchange phosphorus for sulfur five elements also compose nucleic acids like DNA. Even though we know the ultimate source of biogenic elements, we also need to know how they become compounds, and then how the compounds became sufficiently complex for life to emerge on the sterile Earth four billion years ago. Now we can return to the source of the biogenic elements. With one exception, the biogenic elements in all life on Earth, including elemental silicon and iron (which compose the Earth itself) were synthesized in stars. The exception is hydrogen, and the only reason it is present on Earth as one of the biogenic elements is that the hydrogen in water–H2O–had the good fortune not to be caught up in the sun when our solar system formed. In fact, in terms of numbers of atoms, hydrogen makes up about 70 percent of all the atoms in life on the Earth. How could the elements of life possibly come from stars? In 1946, Fred Hoyle, a young British astronomer, had an idea. Hoyle was full of ideas, and boldly published most of them, but only one has survived experimental and theoretical testing. To understand his idea, we need to recall a little high school chemistry. All matter is composed of atoms, and all atoms have a tiny nucleus composed of particles called protons and neutrons, which are surrounded by orbital clouds of much lighter electrons. But in stars, the temperature is so high that the electrons fall off, so stars like our sun are composed of a gas of naked atomic nuclei, mostly hydrogen and helium. Hydrogen is the lightest element, with a single proton in its nucleus, and helium is the second lightest element, with two protons and two neutrons in its nucleus. When the temperature is high enough, around 10 million degrees, hydrogens combine to form helium and release an enormous amount of energy. This is the energy that make stars shine. Hoyle’s brilliant insight was that a second fusion reaction begins when a star approaches the end of its life and its temperature approaches 100 million degrees. At that point two helium nuclei fuse to form beryllium, the lightest metallic element, which then can fuse with another helium nucleus to produce carbon. Earlier theoretical models had already shown that if carbon is available in a star, nitrogen and oxygen can form in a process called the carbon-nitrogen-oxygen cycle, which is the primary source of fusion energy in large, hot stars on their way to becoming novas and supernovas. Those models did not include a source of carbon, and this is where Hoyle filled in a gap. To sum up, the atoms of carbon, nitrogen, oxygen, sulfur, and phosphorus that comprise all life on the Earth were forged in stars at temperatures hotter than any hydrogen bomb. As living organisms, we are not in any way separate from the rest of the universe. Instead we borrow a tiny fraction of its atoms for a few years and incorporate them into the transient molecular structures of cells that are the living unit of all life on Earth. Featured image credit: “Butterfly Nebula in narrow band of Sulfur, Hydrogen and Oxygen” by Stephan Hamel. CC BY-SA 4.0 via Wikimedia.
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The six most common elements of life on Earth (including more than 97% of the mass of a human body) are carbon, hydrogen, nitrogen, oxygen, sulphur and phosphorus. The colors in the spectra show dips, the size of which reveal the amount of these elements in the atmosphere of a star. The human body on the left uses the same color coding to evoke the important role these elements play in different parts of our bodies, from oxygen in our lungs to phosphorous in our bones (although in reality all elements are found all across the body). In the background is an artist’s impression of the Galaxy, with cyan dots to show the APOGEE measurements of the oxygen abundance in different stars; brighter dots indicate higher oxygen abundance. Click on the image for a link to download a larger version. Image Credit: Dana Berry/SkyWorks Digital Inc.; SDSS collaboration To say “we are stardust” may be a cliche, but it’s an undeniable fact that most of the essential elements of life are made in stars. “For the first time, we can now study the distribution of elements across our Galaxy,” says Sten Hasselquist of New Mexico State University. “The elements we measure include the atoms that make up 97% of the mass of the human body.” The new results come from a catalog of more than 150,000 stars; for each star, it includes the amount of each of almost two dozen chemical elements. The new catalog includes all of the so-called “CHNOPS elements” – carbon, hydrogen, nitrogen, oxygen, phosphorous, and sulfur – known to be the building blocks of all life on Earth. This is the first time that measurements of all of the CHNOPS elements have been made for such a large number of stars. How do we know how much of each element a star contains? Of course, astronomers cannot visit stars to spoon up a sample of what they’re made of, so they instead use a technique called spectroscopy to make these measurements. This technique splits light – in this case, light from distant stars – into detailed rainbows (called spectra). We can work out how much of each element a star contains by measuring the depths of the dark and bright patches in the spectra caused by different elements. Astronomers in the Sloan Digital Sky Survey have made these observations using the APOGEE (Apache Point Observatory Galactic Evolution Experiment) spectrograph on the 2.5m Sloan Foundation Telescope at Apache Point Observatory in New Mexico. This instrument collects light in the near-infrared part of the electromagnetic spectrum and disperses it, like a prism, to reveal signatures of different elements in the atmospheres of stars. A fraction of the almost 200,000 stars surveyed by APOGEE overlap with the sample of stars targeted by the NASA Kepler mission, which was designed to find potentially Earth-like planets. The work presented today focuses on ninety Kepler stars that show evidence of hosting rocky planets, and which have also been surveyed by APOGEE.
“For the first time, we can now study the distribution of elements across our Galaxy. The elements we measure include the atoms that make up 97% of the mass of the human body.” While the Sloan Digital Sky Survey may be best known for its beautiful public images of the sky, since 2008 it has been entirely a spectroscopic survey. The current stellar chemistry measurements use a spectrograph that senses infrared light – the APOGEE (Apache Point Observatory Galactic Evolution Experiment) spectrograph, mounted on the 2.5-meter Sloan Foundation Telescope at Apache Point Observatory in New Mexico. Jon Holtzman of New Mexico State University explains that “by working in the infrared part of the spectrum, APOGEE can see stars across much more of the Milky Way than if it were trying to observe in visible light. Infrared light passes through the interstellar dust, and APOGEE helps us observe a broad range of wavelengths in detail, so we can measure the patterns created by dozens of different elements.” The new catalog is already helping astronomers gain a new understanding of the history and structure of our Galaxy, but the catalog also demonstrates a clear human connection to the skies. As the famous astronomer Carl Sagan said, “we are made of starstuff.” Many of the atoms which make up your body were created sometime in the distant past inside of stars, and those atoms have made long journeys from those ancient stars to you. While humans are 65% oxygen by mass, oxygen makes up less than 1% of the mass of all of elements in space. Stars are mostly hydrogen, but small amounts of heavier elements such as oxygen can be detected in the spectra of stars. With these new results, APOGEE has found more of these heavier elements in the inner Galaxy. Stars in the inner galaxy are also older, so this means more of the elements of life were synthesized earlier in the inner parts of the Galaxy than in the outer parts. While it’s fun speculate what impact the inner Galaxy’s composition might have on where life pops up, we are much better at understanding the formation of stars in our Galaxy. Because the processes producing each element occur in specific types of stars and proceed at different rates, they leave specific signatures in the chemical abundance patterns measured by SDSS/APOGEE. This means that SDSS/APOGEE’s new elemental abundance catalog provides data to compare with the predictions made by models of galaxy formation. Jon Bird of Vanderbilt University, who works on modelling the Milky Way, explains that “these data will be useful to make progress on understanding Galactic evolution, as more and more detailed simulations of the formation of our galaxy are being made, requiring more complex data for comparison.”
“we are now able to map the abundance of all of the major elements found in the human body across hundreds of thousands of stars in our Milky Way.” “It’s a great human interest story that we are now able to map the abundance of all of the major elements found in the human body across hundreds of thousands of stars in our Milky Way,” said Jennifer Johnson of The Ohio State University. “This allows us to place constraints on when and where in our galaxy life had the required elements to evolve, a sort ‘temporal Galactic habitable zone’”. The catalog of chemical abundances from which these maps were generated has been publicly released as part of the Thirteenth Data release of the SDSS, and is available freely online to anyone at www.sdss.org. Images
The six most common elements of life on Earth (including more than 97% of the mass of a human body) are carbon, hydrogen, nitrogen, oxygen, sulphur and phosphorus. The colors in the spectra show dips, the size of which reveal the amount of these elements in the atmosphere of a star. The human body on the left uses the same color coding to evoke the important role these elements play in different parts of our bodies, from oxygen in our lungs to phosphorous in our bones (although in reality all elements are found all across the body). In the background is an artist’s impression of the Galaxy, with cyan dots to show the APOGEE measurements of the oxygen abundance in different stars; brighter dots indicate higher oxygen abundance. Click on the image for a link to download a larger version. Image Credit: Dana Berry/SkyWorks Digital Inc.; SDSS collaboration
About the Sloan Digital Sky SurveyFunding for the Sloan Digital Sky Survey IV has been provided by the Alfred P. Sloan Foundation, the U.S. Department of Energy Office of Science, and the Participating Institutions. SDSS acknowledges support and resources from the Center for High-Performance Computing at the University of Utah. The SDSS web site is www.sdss.org. SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS Collaboration including the Brazilian Participation Group, the Carnegie Institution for Science, Carnegie Mellon University, the Chilean Participation Group, the French Participation Group, Harvard-Smithsonian Center for Astrophysics, Instituto de Astrofísica de Canarias, The Johns Hopkins University, Kavli Institute for the Physics and Mathematics of the Universe (IPMU) / University of Tokyo, Lawrence Berkeley National Laboratory, Leibniz Institut für Astrophysik Potsdam (AIP), Max-Planck-Institut für Astronomie (MPIA Heidelberg), Max-Planck-Institut für Astrophysik (MPA Garching), Max-Planck-Institut für Extraterrestrische Physik (MPE), National Astronomical Observatories of China, New Mexico State University, New York University, University of Notre Dame, Observatório Nacional / MCTI, The Ohio State University, Pennsylvania State University, Shanghai Astronomical Observatory, United Kingdom Participation Group, Universidad Nacional Autónoma de México, University of Arizona, University of Colorado Boulder, University of Oxford, University of Portsmouth, University of Utah, University of Virginia, University of Washington, University of Wisconsin, Vanderbilt University, and Yale University. |
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