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New light on how life originated on Earth
We all wonder how life originated on our planet from the primordial soup. Scientists proposed several theories.
Life on Earth began more than 4 billion years ago(1), evolving from the most basic of microbes into a dazzling and stunning array of complexity over time. First let us see what these theories are...
1. Electric sparks can generate amino acids and sugars from an atmosphere loaded with water, methane, ammonia and hydrogen. Over millions of years, larger and more complex molecules could form. Although research has revealed the early atmosphere of Earth was actually hydrogen-poor, scientists have suggested that volcanic clouds in early atmosphere might have held methane, ammonia and hydrogen and been filled with lightning as well.
2. Clay might have provided a platform to meet the first molecules of life according to an idea elaborated by organic chemist Alexander Graham Cairns-Smith at the University of Glasgow in Scotland. These surfaces might not only have concentrated these organic compounds together, but also helped organize them into patterns much like our genes do now. He suggests that mineral crystals in clay could have arranged organic molecules into organized patterns. After a while, organic molecules took over this job and organized themselves.
3. The deep-sea vent theory suggests that life may have begun at submarine hydrothermal vents, spewing key hydrogen-rich molecules. Their rocky nooks could then have concentrated these molecules together and provided mineral catalysts for critical reactions. Even now, these vents, rich in chemical and thermal energy, sustain vibrant ecosystems.
4. Ice might have covered the oceans 3 billion years ago, as the sun was about a third less luminous than it is now. This layer of ice, possibly hundreds of feet thick, might have protected fragile organic compounds in the water below from ultraviolet light and destruction from cosmic impacts. The cold might have also helped these molecules to survive longer, allowing key reactions to happen.
5. DNA needs proteins in order to form, and proteins require DNA to form, so how could these have formed without each other? The answer may be RNA, which can store information like DNA, serve as an enzyme like proteins, and help create both DNA and proteins. Later DNA and proteins succeeded this "RNA world," because they are more efficient. RNA still exists and performs several functions in organisms, including acting as an on-off switch for some genes. The question still remains how RNA got here in the first place. And while some scientists think the molecule could have spontaneously arisen on Earth, others say that was very unlikely to have happened. Other nucleic acids other than RNA have been suggested as well, such as the more esoteric PNA or TNA.
6. Instead of developing from complex molecules such as RNA, life might have begun with smaller molecules interacting with each other in cycles of reactions. These might have been contained in simple capsules akin to cell membranes, and over time more complex molecules that performed these reactions better than the smaller ones could have evolved, scenarios dubbed "metabolism-first" models, as opposed to the "gene-first" model of the "RNA world" hypothesis.
7. Perhaps life did not begin on Earth at all, but was brought here from elsewhere in space, a notion known as 'panspermia'. For instance, rocks regularly get blasted off Mars by cosmic impacts, and a number of Martian meteorites have been found on Earth that some researchers have controversially suggested brought microbes over here, potentially making us all Martians originally. Other scientists have even suggested that life might have hitchhiked on comets from other star systems. However, even if this concept were true, the question of how life began on Earth would then only change to how life began elsewhere in space.
My art work : ORIGIN OF LIFE - SCIENTIFIC POINT OF VIEW
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Scientists all the while are trying to simulate the conditions of earlier Earth and its oceans to figure out how complex life forming chemicals came into existence here. They are also trying to test various theories by conducting experiments to figure out the exact conditions in which life originated on Earth or elsewhere.
Another study done recently showed an interesting aspect in this context.
"Nucleobase and amino acid formation through impacts of meteorites on the early ocean"
A new study has shown that meteorite impacts on ancient oceans may have created nucleobases and amino acids. Researchers from Tohoku University, National Institute for Materials Science and Hiroshima University discovered this after conducting impact experiments simulating a meteorite hitting an ancient ocean.
With precise analysis of the products recovered after impacts, the team found the formation of nucleobases and amino acids from inorganic compounds. The research is reported this week in the journal Earth and Planetary Science Letters.
The emergence of life's building blocks on the prebiotic Earth was the first crucial step for the origins of life. Extraterrestrial delivery of intact amino acids and nucleobases is the prevailing hypothesis for their availability on prebiotic Earth because of the difficulties associated with the production of these organics from terrestrial carbon and nitrogen sources under plausible prebiotic conditions. However, the variety and amounts of these intact organics delivered by meteorites would have been limited. Previous shock–recovery experiments have demonstrated that meteorite impact reactions could have generated organics on the prebiotic Earth. Now the researchers report on the simultaneous formation of nucleobases (cytosine and uracil) found in DNA and/or RNA, various proteinogenic amino acids (glycine, alanine, serine, aspartic acid, glutamic acid, valine, leucine, isoleucine, and proline), non-proteinogenic amino acids, and aliphatic amines in experiments simulating reactions induced by extraterrestrial objects impacting on the early oceans.
In 2009, this team reported the formation of the simplest amino acid, glycine, by simulating meteorite impacts. This time, they replaced the carbon source with bicarbonate and conducted hypervelocity impact experiments at 1 km/s using a single stage propellant gun.
They found the formation of a far larger variety of life's building blocks, including two kinds of nucleobases and nine kinds of proteinogenic amino acids. The results suggest a new route for how genetic molecules may have first formed on Earth.
References:
1. http://www.sciencedaily.com/releases/2015/10/151019154153.htm?utm_s...
New Research Sources: Journal: Earth and Planetary Science Letters (2015) and
http://www.sciencedirect.com/science/article/pii/S0012821X15004926
The seven theories on the origin of life source: http://www.livescience.com/13363-7-theories-origin-life.html
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Replies to This Discussion
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Permalink Reply by Dr. Krishna Kumari Challa on
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Originally an answer to "What is the current theory of abiogenesis, and is there any evidenc..."
First of all, "abiogenesis" is almost exclusively a term used derisively by people who don't believe it's possible -- so I'm already a little worried that this question isn't being asked in good faith. Scientists almost universally prefer the term "origin of life", as it isn't loaded with the implicit assumption that something is either life or it isn't with no room in between.
Secondly, there isn't just one theory. The origin of life took place so long ago and left little to no physical evidence (like fossils), so there are necessarily multiple speculative theories to address the lack of "proof." It's entirely possible that we'll never know what the very first "living" entity was; part of that is driven by the fact that it's quite difficult to reach universal consensus on what is life and what is not.
That being said, while we don't know what the first living entity was, we can make some educated guesses about some of the features it must have had. It absolutely must have had the capacity to store, replicate, and transmit information in the form of molecular interactions of some sort. Without this capacity, there would be no room for either persistence or inheritance, which are both essential characteristics of life. This suggests that you need one or multiple types of molecules that are capable of being both "read" and chemically "written."
It's widely believed that some of the earliest forms of life (though not necessarily the first) accomplished all of this by virtue of being made of RNA. Modern RNA is a polymer of four different building blocks that have sufficient chemical diversity to be both a template (for storing and reading information) as well as a source of enzymatic activity (for making something out of the information being read.)
There's no doubt that RNA is an exceptionally ancient ancestor of all contemporary life, as enzymes made predominantly or exclusively out of RNA, as well as small molecules that use RNA pieces to do chemistry exist in all extant species. Perhaps the greatest example of this is the enzyme complex central to virtually all modern chemistry in the cell -- the ribosome. This uses a catalytic RNA to read an RNA template to assemble proteins, and some form of it exists in every single species ever found.
Because it is effectively universal, the easiest and most elegant argument you can make about its origins is that it was "invented" once in a very ancient organism and has been passed down from generation to generation (with slight modifications) ever since. Arguments like these are what allow us to survey the species that are currently living and speculate about what common ancestors must have looked like.
RNA-only life forms are extraordinarily ancient, but there's no evidence that they were first. We may never know what came first, in part because our information about the conditions in which the first living entity arose is incomplete. We also don't know if there was a precursor to RNA because that precursor has left no traces of itself in contemporary life.
Does this weaken the theory that life arose spontaneously and independently from a rich, complex bath of organic compounds? Not really, because the alternatives that the "abiogenesis" crowd like to propose are even harder to substantiate. As lack of support for a competing theory is not evidence in the slightest for a theory, anyone who wants to invoke a designer needs to do a lot better than pointing out that science "can't prove abiogenesis."-- -
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The hypothesis advanced by Nick Lane and Bill Martin in their beautiful hypothesis article in the journal Cell: The Origin of Membrane Bioenergetics (it may require a subscription to access).
My summary: the other common property to all life, besides that of having a genetic code, is that energy is stored in the form of ion gradients over membranes. Without this harnessing, energy would be dissipated and could not sustain life. Nick Lane and Bill Martin propose a completely natural origin for this membrane bioenergetics from completely inert materials, basically rocks, carbon dioxide and water. The origin would have been in hydrothermal wells, where natural proton gradients across the thin ferrous sulfide (FeS) walls that exist in deep-sea alkaline (high PH) hydrothermal vents could drive the assimilation of carbon, giving rise to proto-cells. These proto-cells would have created a proton gradient and concentrated simple organic molecules formed by carbon assimilation. Once a sodium-proton pump evolved (and these are relatively simple proteins, the proto-cells could close their membranes completely, and still derive energy from the sodium pump. At this point, they could have become left the vent and still capture energy.
Lane was inspired by the bacteria and Archaea that live in these deep sea hydrothermal wells, they use iron and sulphur-containing proteins to convert hydrogen and carbon dioxide into organic molecules.
I think its a very elegant hypothesis because it could explain how life arose from very simple components, in the extreme conditions of ancient Earth, through a completely natural process, one that is still used by microorganisms living today in those deep-sea vents.
Nature News and Views had a good article when the original paper came out: How life emerged from deep-sea rocks.
http://www.cell.com/cell/abstract/S0092-8674%2812%2901438-9?_return...
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How life emerged from deep-sea rocksThe origin of ion-pumping proteins could explain how life began in, and escaped from, undersea thermal vents.
http://www.nature.com/news/how-life-emerged-from-deep-sea-rocks-1.1...--
First Life on Land?
440 million-year-old fungus fossils may be remnants of earliest land-dwelling organism
March 2 (Reuters) - At first glance, they do not look like much: tiny fragments of a primordial fungus shorter than a single hair's width. But these fungal remnants possess the unique distinction of being the oldest-known fossils of any land-dwelling organism on Earth.
A study published on Wednesday described microfossils of a subterranean fungus called Tortotubus that was an early landlubber at a time when life was largely confined to the seas, including samples from Libya and Chad that were 440 to 445 million years old.
The fossils represented the root-like filaments that fungi use to extract nutrients from soil. Tortotubus possessed a cord-like structure similar to some modern fungi. It was unclear whether it produced mushrooms.
Tortotubus helped set the stage for complex land plants and later animals by triggering the process of rot and soil formation.
"By building up deeper, richer, more stable soils, Tortotubus would have paved the way for larger, more complex green plants to quite literally take root, in turn providing a food source for animals and allowing the escalation of terrestrial ecosystems," said paleontologist Martin Smith of Britain's Durham University, who conducted the research while at the University of Cambridge.
These fossils, also discovered in other places including Sweden, Scotland and New York state, reflect the humble beginnings of life on the land.
While the primeval oceans were teeming with life including jawless fish, arthropods, squid relatives, jellyfish and more, the land was barren and void.
To survive on land, organisms had to be able to tolerate desiccation, ultraviolet light exposure and limited nutrients.
Tortotubus may not have been the very first land pioneer, but no fossils have been found of earlier terrestrial organisms.
"By the time Tortotubus went extinct, the first trees and forests had come into existence," Smith said. "This humble subterranean fungus steadfastly performed its rotting and recycling service for some 70 million years, as life on land transformed from simple crusty green films to a rich ecosystem that wouldn't look out of place in a tropical greenhouse today."
Smith studied fossil filaments so small that thousands would fit on the head of a pin. The filaments would have gone through the ground in search of food in the form of dead organic matter. The original, non-fragmented organism could have been a fungal network measuring yards (meters) across, Smith said.
The research was published in the Botanical Journal of the Linnean Society.
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Life on Earth likely started 4.1 billion years ago, much earlier than scientists thought
Evidence that early Earth was not dry and desolateDate:
October 19, 2015
Source:
University of California - Los Angeles
Summary:
Geochemists have found probable evidence for life on Earth at least 4.1 billion years ago -- 300 million years earlier than previously documented, pushing the origin of life close to when the planet formed, 4.54 billion years ago.
UCLA geochemists have found evidence that life likely existed on Earth at least 4.1 billion years ago -- 300 million years earlier than previous research suggested. The discovery indicates that life may have begun shortly after the planet formed 4.54 billion years ago.The research is published today in the online early edition of the journal Proceedings of the National Academy of Sciences.
"Twenty years ago, this would have been heretical; finding evidence of life 3.8 billion years ago was shocking," said Mark Harrison, co-author of the research and a professor of geochemistry at UCLA.
"Life on Earth may have started almost instantaneously," added Harrison, a member of the National Academy of Sciences. "With the right ingredients, life seems to form very quickly."
The new research suggests that life existed prior to the massive bombardment of the inner solar system that formed the moon's large craters 3.9 billion years ago.
"If all life on Earth died during this bombardment, which some scientists have argued, then life must have restarted quickly," said Patrick Boehnke, a co-author of the research and a graduate student in Harrison's laboratory.
Scientists had long believed the Earth was dry and desolate during that time period. Harrison's research -- including a 2008 study in Nature he co-authored with Craig Manning, a professor of geology and geochemistry at UCLA, and former UCLA graduate student Michelle Hopkins -- is proving otherwise.
"The early Earth certainly wasn't a hellish, dry, boiling planet; we see absolutely no evidence for that," Harrison said. "The planet was probably much more like it is today than previously thought."
The researchers, led by Elizabeth Bell -- a postdoctoral scholar in Harrison's laboratory -- studied more than 10,000 zircons originally formed from molten rocks, or magmas, from Western Australia. Zircons are heavy, durable minerals related to the synthetic cubic zirconium used for imitation diamonds. They capture and preserve their immediate environment, meaning they can serve as time capsules.
The scientists identified 656 zircons containing dark specks that could be revealing and closely analyzed 79 of them with Raman spectroscopy, a technique that shows the molecular and chemical structure of ancient microorganisms in three dimensions.
Bell and Boehnke, who have pioneered chemical and mineralogical tests to determine the condition of ancient zircons, were searching for carbon, the key component for life.
One of the 79 zircons contained graphite -- pure carbon -- in two locations.
"The first time that the graphite ever got exposed in the last 4.1 billion years is when Beth Ann and Patrick made the measurements this year," Harrison said.
How confident are they that their zircon represents 4.1 billion-year-old graphite?
"Very confident," Harrison said. "There is no better case of a primary inclusion in a mineral ever documented, and nobody has offered a plausible alternative explanation for graphite of non-biological origin into a zircon."
The graphite is older than the zircon containing it, the researchers said. They know the zircon is 4.1 billion years old, based on its ratio of uranium to lead; they don't know how much older the graphite is.
The research suggests life in the universe could be abundant, Harrison said. On Earth, simple life appears to have formed quickly, but it likely took many millions of years for very simple life to evolve the ability to photosynthesize.
The carbon contained in the zircon has a characteristic signature -- a specific ratio of carbon-12 to carbon-13 -- that indicates the presence of photosynthetic life.
"We need to think differently about the early Earth," Bell said.
Wendy Mao, an associate professor of geological sciences and photon science at Stanford University, is the other co-author of the research.
The research was funded by the National Science Foundation and a Simons Collaboration on the Origin of Life Postdoctoral Fellowship granted to Bell.
Story Source:
The above post is reprinted from materials provided by University of California - Los Angeles. The original item was written by Stuart Wolpert. Note: Materials may be edited for content and length.
Journal Reference:
Elizabeth A. Bell, Patrick Boehnke, T. Mark Harrison, and Wendy L. Mao. Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon. PNAS, October 19, 2015 DOI: 10.1073/pnas.1517557112
http://www.sciencedaily.com/releases/2015/10/151019154153.htm?utm_s...
http://www.sciencedaily.com/releases/2015/10/151019154153.htm?utm_s... -
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