Where Life Arose

Life could not have come into being on Earth until after about 4 billion years ago,when bombardment by asteroids had subsided substantially.Even after Earth’s surface had stabilized,certain conditions were required for the origin of life.Some researchers have concluded that precursor compounds,and ultimately life itself,arose in small bodies of water that were struck by lightning and turned into what is sometimes referred to as the “primordial soup”of organic compounds.The problem with that idea is that it would have required an atmosphere lacking oxygen,because even a small amount of oxygen would have oxidized,and thereby destroyed,chemical raw materials necessary for the production of essential organic compounds.

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Knowing that photosynthesis produces the preponderance of oxygen in Earth’s atmosphere today,scientists once assumed that the atmosphere lacked free oxygen before the origin of photosynthetic organisms.We now know,however,that ultraviolet light breaks down water vapor in Earth’s upper atmosphere,slowly liberating oxygen, which spreads in small quantities throughout the atmosphere.This process would have contributed a small amount of oxygen to the ancient atmosphere-enough to destroy chemical compounds essential to life.Life must have originated not in a small body of water that was exposed to atmospheric oxygen but in some environment that was isolated from Earth’s atmosphere.The most likely setting was a warm area beneath the seafloor in the vicinity of a mid-ocean ridge (submarine mountain range).

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The heat that rises from Earth’s mantle (the layer between the crust and the planet’s central core)along mid-ocean ridges warms seawater that has percolated into the crust through pores and cracks. Because heating causes water to expand,and so reduces its density this water rises back to the ocean.In some areas it flows from the seafloor through large vents as columns of very hot water.Bacteria of many kinds inhabit the warm water of ridge environments,occupying pores,cracks,and vents.They live in a variety of ways,but most of them derive energy from the chemicals that the hot water has dissolved while moving through the ridge system.Some of these bacteria live in water warmer than 100 degrees Celsius,which remains in a liquid state because of the great pressure applied by the ocean above.Others live in lukewarm water farther from ridge axes. In general,these high-temperature bacteria may be inhabiting the kind of setting where life originated.

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The principle that most warm-adapted bacteria put into practice to obtain energy is quite simple:they harness the energy of naturally occurring chemical reactions.Many of the chemical compounds that emerge from deep within mid-ocean ridges are not stable after the rising water in which they are dissolved cools and mixes with seawater;and so they will enter into chemical reactions.Many such reactions do not occur quickly,however,and the bacteria take advantage of this situation.The bacteria consume the chemical compounds and simply allow chemical reactions that would have occurred in seawater to take place inside their cells.These reactions release energy.The bacteria harness the energy for their metabolism and excrete the chemical products of the reactions.

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Some of the warm-adapted bacteria actively carry out chemical synthesis,but unlike photosynthetic organisms,they do not use light as an energy source.Other kinds of bacteria get energy by combining hydrogen or sulfur with oxygen in the way that higher organisms obtain energy by oxidizing sugars.

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There is evidence that the warm-adapted bacteria that live in the vicinity of mid-ocean ridges are actually the most primitive living bacteria.Biologists have compared the genetic material,DNA and RNA,of all kinds of bacteria to reconstruct their evolutionary relationships.Those features of DNA and RNA that are shared by many bacterial groups are regarded as primitive features inherited from very ancient ancestors.Traits shared by few bacterial groups are regarded as indicating more recent branchings of the evolutionary tree.Using this method,it turns out that the most primitive living bacteria (Archaebacteria)are adapted to warm habitats.This finding suggests that bacteria evolved in such habitats,perhaps in the vicinity of mid-ocean ridges.?

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