But we can think in a more informed way about whether life might exist on other planets and under what conditions by considering how life may have arisen right here on our own planet.
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In this article, we'll examine scientific ideas about the origin of life on Earth. The when of life's origins 3. But the how is much less understood. In comparison to the central dogma or the theory of evolution, hypotheses about life's origins are much more No one is sure which hypothesis is correct — or if the correct hypothesis is still out there, waiting to be discovered. When did life appear on Earth? Geologists estimate that the Earth formed around 4. For many millions of years, early Earth was pummeled by asteroids and other celestial objects.
Temperatures also would have been very high with water taking the form of a gas, not a liquid. The first life might have emerged during a break in the asteroid bombardment, between 4. However, a second bombardment happened about 3. The earliest fossil evidence of life.
The earliest evidence of life on Earth comes from fossils discovered in Western Australia that date back to about 3. These fossils are of structures known as stromatolites , which are, in many cases, formed by the growth of layer upon layer of single-celled microbes, such as cyanobacteria.
Stromatolites are also made by present-day microbes, not just prehistoric ones. The earliest fossils of microbes themselves, rather than just their by-products, preserve the remains of what scientists think are sulfur-metabolizing bacteria. The fossils also come from Australia and date to about 3. Bacteria are relatively complex, suggesting that life probably began a good deal earlier than 3.
How might life have arisen? Haldane both separately proposed what's now called the Oparin-Haldane hypothesis: Oparin and Haldane thought that the early Earth had a reducing atmosphere, meaning an oxygen-poor atmosphere in which molecules tend to donate electrons. Under these conditions, they suggested that:. Simple inorganic molecules could have reacted with energy from lightning or the sun to form building blocks like amino acids and nucleotides, which could have accumulated in the oceans, making a "primordial soup.
The building blocks could have combined in further reactions, forming larger, more complex molecules polymers like proteins and nucleic acids, perhaps in pools at the water's edge. The polymers could have assembled into units or structures that were capable of sustaining and replicating themselves. The details of this model are probably not quite correct. For instance, geologists now think the early atmosphere was not reducing, and it's unclear whether pools at the edge of the ocean are a likely site for life's first appearance.
But the basic idea — a stepwise, spontaneous formation of simple, then more complex, then self-sustaining biological molecules or assemblies — is still at the core of most origins-of-life hypotheses today. From inorganic compounds to building blocks. They found that organic molecules could be spontaneously produced under reducing conditions thought to resemble those of early Earth.
Cartoon depiction of the apparatus used by Miller and Urey to simulate conditions on early Earth. After letting the experiment run for a week, Miller and Urey found that various types of amino acids, sugars, lipids and other organic molecules had formed. Large, complex molecules like DNA and protein were missing, but the Miller-Urey experiment showed that at least some of the building blocks for these molecules could form spontaneously from simple compounds. Were Miller and Urey's results meaningful? So, it's doubtful that Miller and Urey did an accurate simulation of conditions on early Earth.
A nucleotide is a relatively complex, three-part molecule with a sugar ring, a nitrogenous base, and one or more phosphate groups. As such, it's less likely to form through simple chemical reactions than a typical amino acid. From these experiments, it seems reasonable to imagine that at least some of life's building blocks could have formed abiotically on early Earth. However, exactly how and under what conditions remains an open question. From building blocks to polymers.
How could monomers building blocks like amino acids or nucleotides have assembled into polymers, or actual biological macromolecules, on early Earth? In cells today, polymers are put together by enzymes. But, since the enzymes themselves are polymers, this is kind of a chicken-and-egg problem! Monomers may have been able to spontaneously form polymers under the conditions found on early Earth. Fox suggested that, on early Earth, ocean water carrying amino acids could have splashed onto a hot surface like a lava flow, boiling away the water and leaving behind a protein.
The clay acts as a catalyst to form an RNA polymer. More broadly, clay and other mineral surfaces may have played a key role in the formation of polymers, acting as supports or catalysts. The image above shows a sample of a type of clay known as montmorillonite. What was the nature of the earliest life? If we imagine that polymers were able to form on early Earth, this still leaves us with the question of how the polymers would have become self-replicating or self-perpetuating, meeting the most basic criteria for life. This is an area in which there are many ideas, but little certainty about the correct answer.
One possibility is that the first life forms were self-replicating nucleic acids, such as RNA or DNA, and that other elements like metabolic networks were a later add-on to this basic system. This is known as the RNA world hypothesis. Perhaps the most important is that RNA can, in addition to carrying information, act as a catalyst. A catalytic RNA could, potentially, catalyze a chemical reaction to copy itself. Such a self-replicating RNA could pass genetic material from generation to generation, fulfilling the most basic criteria for life and, potentially, undergoing evolution.
In fact, researchers have been able to synthetically engineer small ribozymes that are capable of self-replication. Some highly conserved molecules found in the cells of many organisms are structurally related to RNA. In addition, functional RNA molecules play key roles in protein synthesis: These networks might have formed, for instance, near undersea hydrothermal vents that provided a continual supply of chemical precursors, and might have been self-sustaining and persistent meeting the basic criteria for life.
Eventually, the metabolic networks might have been able to build large molecules such as proteins and nucleic acids. What might early cells have looked like? A basic property of a cell is the ability to maintain an internal environment different from the surrounding environment.
In principle, this type of compartment could surround a self-replicating ribozyme or the components of a metabolic pathway, making a very basic cell. Though intriguing, this type of idea is not yet supported by experimental evidence — i. Organic molecules from outer space. But could they instead have come from space?
The idea that organic molecules might have traveled to Earth on meteorites may sound like science fiction, but it's supported by reasonable evidence. We also know that some organic compounds are found in space and in other star systems. Most importantly, various meteorites have turned out to contain organic compounds derived from space, not from Earth. One meteorite, ALH, came from Mars and contained organic molecules with multiple ring structures.
Another meteorite, the Murchison meteorite, carried nitrogenous bases like those found in DNA and RNA , as well as a wide variety of amino acids. One meteorite that fell in in Canada contained tiny organic structures dubbed "organic globules. There's no way for us to exclude the possibility that simple life forms were delivered to Earth on meteorites or other celestial objects early in Earth's history.
However, so far, there hasn't been clear evidence for this hypothesis.
Also, even if this turned out to be the case, we would effectively be "kicking the can down the road" on our question of how life arose. Even if life came to Earth from elsewhere in the universe, we would still be left wondering how it arose to begin with wherever it did arise. How life originated on our planet is both a fascinating and incredibly complex question. Scientific American Science Desk Reference.
The Editors of Scientific American. The Search for Our Cosmic Ancestry. Mysteries of the Universe and Planet Earth. Cosmology Evolution Spirit and Aliens. Science in Key Breakthroughs. Little Green Apples Publishing. Life and the Universe. Who Gives a Gigabyte? A Flash of Light. How to Live Forever. A Journey from Dust to Consciousness. Passing the State Science Proficiency Tests.
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Schill; Mats Harms-Ringdahl; et al. However, there are reasons to hypothesize that faster-than-light interstellar space travel might be feasible. Intelligent Life in the Universe. If you're seeing this message, it means we're having trouble loading external resources on our website. Under these conditions, they suggested that:. Planetary and Space Science.
Chi ama i libri sceglie Kobo e inMondadori. Home eBooks Nonfiction Panspermia: The Extraterrestrial Hypothesis Back to Nonfiction. Buy the eBook Price: Available in Russia Shop from Russia to buy this item. Or, get it for Kobo Super Points! Explore the solar system, and the role it plays in biotic molecule formation. Review the latest and the history of Mars research and what it means to life on Earth, the Galaxy, and the Universe. Read an actual hypothesis in scientific format, yet in the language that any one in the pop culture would appreciate as journalism consumers.