Vocabulary Test 46

Passage 1

      The origins of life on Earth are shrouded in mystery. Scientists agree that life arose almost 4 billion years ago from non-living chemicals, a process called abiogenesis. However, many competing hypotheses exist to explain how this might have happened. Because Earth is the only planet in the universe known to harbor life, studying the unique chemical environment of early Earth can allow us to develop a deeper understanding of the causes of abiogenesis.

     During the earliest phase of Earths existence, the Hadean eon, conditions on the newly formed planet were very different from those found today. The young Earth was intensely hot, with highly active volcanoes and frequent meteorite impacts. Unlike today's atmosphere, which is predominantly made of nitrogen and oxygen, the Hadean atmosphere is thought to have consisted mainly of carbon dioxide, hydrogen, water vapor, and volcanic gases. Thanks to the intense pressure of this thick atmosphere, liquid water oceans probably existed despite the boiling temperatures on Earths surface.

      Although these conditions would be totally inhospitable to modern life, this unique environment could have produced many of the building blocks of life. Scientists have discovered this by replicating the conditions of the Hadean eon in laboratories. The earliest and most famous of these experiments, conducted by Stanley Miller in the 1950s, involved passing electricity through the particular mixture of gases in the early Earths atmosphere. Miller found that electricity, such as that delivered by lightning strikes, could have triggered chemical reactions in the Hadean atmosphere producing amino acids, the building blocks of proteins, as well as the nitrogenous bases and sugars that make up nucleic acids such as DNA and RNA. More recent experiments using ultraviolet light, a major component of sunlight, have found that it too could have caused organic compounds to form on Earth during the Hadean eon. 

        This has led to speculation on the part of many scientists that these molecules, once synthesized in the early Earths oceans, could have become organized into self-replicating structures that developed into life as we know it. Nucleic acids, for instance, can both carry genetic information and catalyze chemical reactions; simple nucleic acids thus could have replicated themselves and even created proteins from amino acids, like modern life forms do. Indeed, many scientists now believe that today's life descends from an "RNA world" that formed in this way.

Passage 2

        It turns out that the conditions for life to arise may actually be quite common throughout the universe. At the very least, the building blocks of life as we know it as amino acids, simple sugars, and other organic compounds seem to show up wherever we point our telescopes.

        For instance, organic molecules form quite readily in the clouds of dust and gas that hang between and around stars. A number of studies have found that certain organic molecules, called PAHs, may be present in nebulae and star systems all over the universe. These molecules, made up of rings of carbon and hydrogen, have structures that might allow them to help RNA strands self-assemble in the oceans of planets; NASA scientists estimate that these molecules contain as much as 20% of the universe's carbon and may have formed shortly after the universe began.

        Scientists have also found organic molecules closer to home, within our own galaxy and solar system. In the massive nursery of new star systems at the heart of the Milky Way, a simple form of sugar has been detected. The 65 formation of this sugar is a key step in the creation of the more complex sugars in nucleic acids. This suggests that the raw materials for nucleic acids, and perhaps other key components of life, might be commonly incorporated into forming star systems. This certainly seems to have happened around our Sun. A number of Solar System bodies, such as the Murchison meteorite, have crashed to Earth bearing nitrogenous bases and amino acids that were formed in space, and comets currently orbiting our Sun have been found to carry amino acids as well. If the early Earth was seeded with organic molecules, either during its formation or by meteorite and comet impacts, it is plausible that this could have paved the way for abiogenesis to take place soon thereafter.

       Taken together, this evidence suggests that the building blocks of life appear throughout the Milky Way galaxy and elsewhere in the universe. Earths status as the cradle of life may not be so special after all.