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--------------------- 2372 - CHEMISTRY - when did it become biology?
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- This all started on the early Earth some 4,000,000,000 yeas ago. How could all the rest of us have come from this one single cell? Because that is what happened. All life on Earth came from one single first living cell.
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- Therefore, all life on Earth is related. We are related to the frogs, the pond scum, the whales, the petunias. How do we know this?
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- A strong bit of evidence that we all descend from the same single cell is the existence of old genes, shared by virtually every form of life on Earth. These old genes were the genes the cell we evolved from.
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- Earth formed, along with the rest of our home solar system, about 4,600,000,000 years ago from coalescing gases and debris whirling about, the result of a supernova, or star explosion. Compare this to a long time later with Homo sapiens evolving just 200,000 years ago.
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- From the Earth’s formation to about 3.8 billion years ago was an era called the Hadean Eon. Hadean, meaning Hades, Hell. It was hot around here. At a certain point the Earth was all magma. During the Hadean Eon of Late Heavy Bombardment, Earth was regularly struck by comets, planetesimals, and even planets.
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- One of these hits was the Giant Impact that happened about 105 million years after the beginning. A Mars-sized planet collided with Earth. Earth got bigger, and debris spewed out and began orbiting and eventually coalesced into the Moon.
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- At first, the Sun was dimmer and the Moon was closer to Earth. Earth lost its entire atmosphere, originally hydrogen and helium. At some point, likely after another big smashup, Earth’s rocks liquefied and iron and nickel sank to the core, leaving a crust composed of lighter, silicon-based rocks like granite and quartz.
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- Heat rises. Deep within the Earth, semi-liquid iron heats up, rises, then cools and sinks. It heats up again, rises again, cools again, sinks again. These convection currents create a magnetic field around Earth that protects all living things from deadly Sun flares.
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- Colliding comets donated water vapor. Or, the large, soaking-wet meteorites known as carbonaceous chondrites. These were chips and chunks off asteroids orbiting halfway to Jupiter.
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- Collisions were common. Earth cooled, and water vapor turned to water. The rains came, and it rained for centuries. The oceans filled. Earth was covered with ocean, but with volcanic islands sticking out of the water.
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- Volcanoes vomited methane, sulfur, hydrogen, carbon dioxide, nitrogen, water vapor, and chlorine, making a new atmosphere. That was the Hadean Eon. Toward the end of that eon was the beginning of the Archean Eon when life first appeared. Very old rocks hold the evidence for life starting at this time.
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- In 1952, at the University of Chicago experiments made the discovery of a lifetime. They made a glass tube-and-bulb trap for four gases. Water vapor, ammonia, methane, and hydrogen were each prominent in the gaseous giants Jupiter and Saturn, and thought to be the ones that composed Earth’s early atmosphere.
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- Putting water into the bottom bulb represented the original freshwater ocean. Heat was applied. The water evaporated and rose through the tubes to the top bulb. The top bulb had electrical charges attached to it, simulating lightning, an energy source on early Earth.
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- The atmosphere cooled, condensed, rained into the bottom bulb, heated up again, evaporated again. This cycle run for a week. At the end of the week, they discovered black muck, sugars!
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- And amino acids, the components of proteins, those essential engines of all bodily functions formed. This type of amino acids was coded by old genes, the genes shared by virtually every form of life on Earth.
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- Subsequent experiments using the various gases spewed by volcanoes also got sugars and also got amino acids and also did not make life. As it turned out, these organic compounds, molecules containing carbon, were common, not only on Earth but in space.
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- They were common, but they did not commonly occur in the long molecular chains called polymers that life requires.
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- Life began in a world of violent storms, volcanoes, gigantic tides pulled by the much-closer moon, tsunamis beyond imagination, and comet and meteorite impacts.
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- Living microbes can survive in space for eons. So it’s still possible that we are all Martians. We also know that Archaea, the most primitive microbes in existence, can survive deep in hot lavas spewing from vents at the mid-oceanic ridges, or in hot springs such as those shooting up in Yellowstone.
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- Life requires polymers. Life metabolizes, meaning it obtains and uses energy. Life replicates itself, makes copies of itself, but with variation in the copies. If there were no variation in the copies, then the first living entity would have produced duplicate after exact duplicate.
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- Evolution could not have begun with duplicates. We could never have evolved. Evolution based on natural selection, in which organisms with features most suited to the given environment survive to reproduce, depends on variation.
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- Life requires a membrane, a wall that separates inside from outside. Every living thing on Earth has two separate chemical systems that interact; one cannot function without the other. These are genes and metabolism.
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- DNA holds the gene codes for amino acids that make proteins that set off metabolism. Metabolism is the chemical process by which all living things gather the energy and atoms required for the maintenance of life, including DNA.
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- How could life have begun with only one of these? Yet how could genetics and metabolism have sprung up together? Or did they develop separately and then meet up? And how did polymers form?
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- When researchers stir all the molecules that were present on prebiotic Earth into a soup and just let it sit, or even heat it up, these small molecules do not form longer chains.
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- Something must have forced these small molecules to polymerize. Quite possibly that something was clay. Clay takes the form of minuscule compartments that carry a slight charge. Over millions of years, random molecules were presumably jammed together with other random molecules within billions of tiny clay compartments. Some would have polymerized. It may be that a clay vessel was the chemical urn in which life began.
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- Metabolism is the battery that makes life run. And it looks very similar in all forms of life. Metabolism breaks apart some substances to release energy, and it synthesizes other substances required by living cells.
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- All life today, every cell, is run by ATP, adenosine triphosphate. But ATP’s series of steps to store energy and release it to the cell are far too complex to have sprung into use by first life.
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- This metabolism requires an inorganic molecule, a phosphate. Phosphate is one atom of phosphorus hitched to four atoms of oxygen. Electrons have negative charges and protons have positive charges and these charges want to bond.
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- In triphosphate the oxygen atoms taken together have four extra negative electrons that want to attach to some positive proton. In that urge of unbonded electrons to bond lies the principle of chemical energy.
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- Add water to ATP. In our own cells, ATP becomes ADP, adenosine diphosphate, with two instead of three phosphate molecules, plus water, plus energy. Then this ADP is returned to the mitochondria . Within the mitochondria, a molecule of phosphate is added back to make a new ATP molecule, and the process begins again.
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- Each step in this many-step process is catalyzed by proteins called enzymes. Life today runs on lots of proteins, including lots of enzymes. One single bacterium cell, an E. coli cell, contains some 4,000 proteins, most of them enzymes.
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- Then, no protein molecule existed to catalyze anything. But organic molecules such as adenosine did exist. And minerals such as iron and sulfur existed in ocean-bottom hydrothermal vents, and might have donated electrons in a simple metabolic process.
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- One principle in the search for first life is that whatever is thought to have happened to begin life must be compatible with life as we know it. And, one of the metabolic steps in the ADP-ATP cycle is quite ancient and does not require oxygen. This step is called glycolysis.
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- A phosphate molecule is added to the sugar glucose, and that breaks the glucose molecule and releases its energy for use by the cell. Glycolysis is ubiquitous in life. And the step immediately following glycolysis, the citric acid cycle or Krebs cycle, is also ancient and also ubiquitous. And, it can replicate itself!
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- It starts with acetic acid, which contains only two carbon atoms. Acetic acid reacts with CO2 to form pyruvic acid , with three carbon atoms, which in turn reacts with more CO2 to make the four carbon oxaloacetic acid.
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- Other reactions produce progressively larger molecules, up to citric acid, with its six carbon atoms. The cycle becomes self-replicating when citric acid spontaneously splits into two smaller molecules, acetic acid ,two carbon atoms, plus oxaloacetic acid ,four carbon atoms, which are also part of the molecular loop.
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- The molecules in these reactions synthesize sugars and at least one amino acid. Is this life? No.
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- The RNA counter hypothesis is that the first self-replicating molecular entity was some type of RNA. RNA ,ribonucleic acid, is similar to DNA, deoxyribonucleic acid, but composed of only one strand, in contrast to DNA’s double strand.
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- RNA is made up of linked nucleotides. A nucleotide is a molecule composed of a base, a sugar, and phosphate. The RNA hypothesis imagines an RNA polymer enclosed in some sort of membrane. This RNA would carry a genetic code, later taken over by DNA, and would replicate itself.
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- As in metabolism, enzymes set off each reaction that enables RNA to copy the code, assemble the correct amino acids, and synthesize a new protein. But first life had no proteins. So how could any of this have gotten started?
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- On early Earth, charged magnesium ions may have catalyzed RNA molecules into replicating themselves. But magnesium also rips up the fatty acid membrane and destroys growing RNA chains as fast as it catalyzes their duplication.
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- Adding a citric acid derivative to the mix bonded to the magnesium enough But, no one has gotten RNA to form all by itself in a proto-cell using only elements available on prebiotic Earth.
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- All the successes have been achieved by using lab-supplied templates that did not exist on prebiotic Earth.
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- Cells today have membranes made of fat molecules called lipids. Lipids repel water. This is good for preventing the surrounding water from dissolving the contents of the cell. The problem is that the insides of a cell are also mostly water.
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- In modern cells, a molecule comprising phosphorus and a lipid makes a phospholipid. The phosphorus part of the molecule adores water, and the lipid part abhors it. Dump these molecules into water and the lipid parts clump together to escape the water.
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- In this way they form a vesicle, a double-layered membrane that has an outer layer that adores water and an inner layer that repels water, letting it through its structures only very selectively.
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- Cell membranes have elaborate protein-controlled pores and pumps to allow for the entrance of food and exit of waste. The first cells, the cells before proteins, could not have had protein-controlled pores and pumps. How could they eat? How could they excrete?
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- How did the long chains of molecules assemble from the many smaller organic molecules floating about? How did metabolism begin? How did replication begin? When did the membrane materialize to contain the molecules that carried out metabolism and replication, and to separate outside from inside? Where did this happen?
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- Life may have started in hot hydrothermal vents that open in the floor of the ocean. Iron or sulfur or both could have catalyzed reactions. Or , life may have began in a hot springs environment that alternated between wet and dry.
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- When water is present, trillions of fatty-acid molecules form vesicles, enclosing trillions of random organic molecules. When the rocks dry up, the fatty acids coagulate into layers. These layers trap organic molecules between them, monomers that are thereby forced together, making polymers. Then the water spouts once again, releasing trillions of new fatty-acid vesicles enclosing new random polymers.
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- Trillions of new vesicles surround trillions of random organic molecules each time the hot spring spouts. If, over millions of years, one or more of these enclosed polymers began chemical reactions for splitting and metabolizing, there you would have life?
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- During the Great Oxygenation Event plate tectonics shifted land masses around, making and remaking continents. Multicellular life began some 700 million years ago. Land plants evolved from pond scum which is green algae.
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- The explosion of multicellular life forms began in the Cambrian period, about 542 million years ago. Several disastrous extinction events changed everything, each time.
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- The most catastrophic was the end-Permian extinction, the mother of all extinctions, which, 251 million years ago, wiped out 90 to 95 percent of life on Earth. After the end-Permian extinction, dinosaurs evolved. Birds evolved. Small mammals evolved.
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- A disaster occurred when the Chicxulub asteroid smacked into Earth 65.5 million years ago. Hundreds of species of dinosaurs were wiped out. Slowly mammals diversified further and took up more niches.
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- Primates evolved. One early proto-primate made it through the Chicxulub catastrophe. True primates evolved some 55 million years ago. These squirrel-sized mammals had forefeet and hind feet good at grasping things.
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- Then came the monkeys. Then came the great apes. In the 1970s, the bones of our cousin Lucy, a bipedal upright primate with a small brain, were found and dated to about 3.2 million years ago.
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- Walking releases the hands. Hands are useful. Hands gave us superpowers. Hands could extend their usefulness by using tools. Figuring out tools requires brainpower, and those with more of that survived better to reproduce more.
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- Homo habilis, an early member of our genus, means “handyman.” We evolved from Homo habilis through Homo erectus through a possible Homo gensis, ending up about 200,000 years ago as ourselves, Homo sapiens.
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- The first cells, when chemistry became biology, were simple systems, protocells, barely alive, and were different each from the other. Most had their components dispersed, but, a few of these happened to find a way to grow by polymerization, like RNA. These cells evolved for perhaps a billion years into an ancestor to all life existant on Earth today.
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- A very incomplete story, but, the best we can do right now. Here are some more Reviews available if you want to learn more:
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- 2361 - How did God start life on Earth?
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- 2288 - The diversity of life. From microbes to monkeys we still have a lot to learn.
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- 2165 - Evolution of life.
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- 2148 - The origins of life in the Universe.
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- 2122 - Are we alone in the Universe?
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- 1942 - What was the earliest life on Earth?
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- 1874 - Will we likely find life on exoplanets and exomoons? This review lists 17 more reviews on this subject.
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- 844 - Drake’s equation for putting odds on finding life on other planets.
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- 340 - Life
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- 342 - The whole Shebang.
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- May 19, 2019.
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--------------------- Friday, May 17, 2019 -------------------------
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