Everywhere we look, life exists in both the most hospitable of environments and in the most extreme. Yet we have only ever found life on our planet. How did the stuff of stars come together to create life as we know it? What do we really mean by ‘life’? And will unlocking this mystery help us find life elsewhere?
Where Do We Come From?
By Jacob Silverman, HowStuffWorks.com
In the Beginning
About 4.6 billion years ago, our solar system resembled a giant cloud of swirling cosmic dust, hydrogen and other gases. As with the thousands of other such clouds in our galaxy, some of these molecules began condensing, gathering and creating their own gravity. Eventually these small clumps formed what became our sun — a star surrounded by a quickly moving, flat disc made up of the cloud’s leftovers. These leftovers also developed into our solar system’s planets, asteroid belt and other interstellar bodies.
Earth’s relative proximity to the sun meant that gases were largely burned away in those early days, leaving a rocky, metal-rich planet made from planetesimals, or smaller cosmic bodies. These same planetesimals also may have brought water and gases later. Often made of ice, they helped to plant the seeds for what would become a fertile, water-rich planet with a healthy atmosphere, capable of protecting life from the sun’s harmful rays.
Although scientists generally agree upon the story of Earth’s formation, no widely accepted scientific consensus exists over the origins of life on Earth, although most hypotheses contain common elements. It’s thought that up to 4 billion years ago, nucleic acids (DNA and RNA are both nucleic acids) began combining. Following that, these primitive bundles somehow developed into enzymes and later single-celled organisms. A lot of the critical intermediate steps have flummoxed researchers.
But it was these early single-celled bacteria that probably formed the basis for all subsequent life. What is life? The biologist Andrew Knoll has defined it as something having the ability to grow, reproduce and engage in Darwinian evolution. This last feature is made possible when some source of variation is introduced, such as a genetic mutation, and it survives the process of natural selection. Over several billion years, these early organisms adapted and evolved innumerable times, producing millions of highly diverse and complex species.
Still, a number of unanswered questions about the origin of life on Earth and where we come from remain. For one, how did these early nucleic acid combinations develop in a methane-dominated atmosphere yet develop into organisms that require an oxygen-dominated atmosphere?
Questions like these drive experiments looking at how life began on Earth and how it might develop elsewhere. One well-known experiment took place in 1952, when scientists Stanley Miller and Harold Urey placed water, hydrogen, ammonia and methane in a beaker in order to approximate the most common elements in the early Earth atmosphere. They then applied an electrical charge — imitating lightning — that led to the formation of amino acids. In this way, Miller and Urey showed that life, or at least the building blocks of life, could form out of the basic chemistry and conditions of the planet, and that life is a process deriving in large part from chemical interactions.
This notion of life as chemistry has been profoundly influential, and it also extends more generally into how life and the Earth are interconnected — for example, how plants use photosynthesis to produce oxygen and regulate carbon dioxide levels, thereby creating a healthy atmosphere for other planetary residents.
Yet life on other planets may not look like what we have here. Most organisms on Earth essentially are made up of hydrogen, carbon and oxygen, but life elsewhere may be non-carbon-based — perhaps silicon-based, as silicon is comparable chemically to carbon.
Or, extraterrestrial life may be even more bizarre than we imagine. In 2007, a study found interstellar dust, made up of plasma crystals, that organized itself into helical shapes, the same physical structure of DNA. Falling under the theoretical category of “weird life,” these inorganic crystals even seemed to evolve, with stronger crystals replacing weaker ones that broke down. However, it’s important to note that we’re looking at these crystals through our definition of life — reproduction, development, evolution. In fact, true extraterrestrial life may be (relatively) stranger than anything we have yet imagined.
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