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|See Abiogenesis in Wikipedia, the free encyclopedia.|
The term abiogenesis simply means getting life from non-life and is used both for spontaneous generation and for chemical evolution. While chemical evolution refers to the process by which the first living entities are thought to have appeared on a lifeless Earth, spontaneous generation refers to the, now discredited, theory that small animals and bacteria are spontaneously generated at the present time.
Main article: Evolution is baseless without a theory of abiogenesis
It is worth noting that creationists frequently confuse abiogenesis with evolution. By pointing to problems or lack of knowledge about the former, they think they refute the latter. So, it is important to realise that how life arose is irrelevant to the validity of evolution. It matters not in the slightest to evolution whether the first living things came about through supernatural or natural means; the evidence for the common ancestry of populations (including species) that makes it possible to infer a single phylogenetic tree of all life is still the same in either case. Once there is life -- with heritable variation -- descent is allowed to accumulate modifications -- to evolve -- and populations will unavoidably eventually split, originating new species.
The notion of spontaneous generation has had a long history, going back at least as far as Aristotle, and has often been regarded as self-evidently true. Thus, around 1620, the physician and early chemist Jan Baptista van Helmont published this recipe for making mice:
- "for if you press a piece of underwear soiled with sweat together with some wheat in an open mouth jar, after about 21 days the odor changes and the ferment coming out of the underwear and penetrating through the husks of the wheat, changes the wheat into mice. But what is more remarkable is that mice of both sexes emerge (from the wheat) and these mice successfully reproduce with mice born naturally from parents? But what is even more remarkable is that the mice which came out were not small mice? but fully grown."
However, some of van Helmont's contemporaries started becoming skeptical. In 1672, the physician Sir Thomas Browne wrote a debunking book in which he stated:
- "Concerning the generation of Froggs, we shall briefly deliver that account which observation hath taught us. By Frogges I understand not such as arising from putrefaction, are bred ....but they let fall their spawn in the water... In this spawn of a lentous and transparent body, are to be discerned many specks, or little conglobulations, which in a small time become of deep black.... Now of this black or duskie substance is the Frogge at least formed; as we have beheld, including the spawn with water in a glass, and exposing it unto the Sun. For that black and round substance, in a few days began to dilate and grow longer, after a while the head, the eyes, the tail to be discernable, and at last to become that which the Ancients called Gyrinus, we a Porwigle or Tadpole. This in some weeks after becomes a perfect Frogg, the legs growing out before, and the tail wearing away, to supply the other behind." (Pseudodoxia Epidemica, 1672 6th edition)
He also became skeptical of whether mice are spontaneously generated (Pseudodoxia Epidemica, Bk 3, Ch 28), provoking this response by a certain Alexander Ross:
- "So we may doubt whether in cheese and timbers worms are generated, or if beetles and wasps in cow-dung, or if butterflies, locusts, shell-fish, snail, eels, and such life be procreated of putrefied matter, which is to receive the form of that creature to which it is by formative power disposed. To question this is to question reason, sense, and experience. If he doubts this, let him go to Egypt, and there he will find the fields swarming with mice begot of the mud of the Nylus, to the great calamity of the inhabitants." (Arcana Microcosmi, Bk 2, Ch 10, pp 151-156, 1652)
But in 1668, physician and biologist Francesco Redi decided to consider the question of rotting meat, which seemingly produces maggots, which ultimately become flies. But he noticed that rotting meat also attracts flies, so could those maggots be produced by those flies? He performed some classic experiments, exposing some rotting meat to air, while keeping flies away from some of it with gauze. Only the fly-accessible meat acquired maggots.
But spontaneous generation died a very slow death; it was reinforced by the discovery of numerous microscopic "animalcules", which often seemed to be spontaneously generated. But Redi-style experiments showed that even microorganisms are not spontaneously generated; the deathblow was delivered by Louis Pasteur's 1859 experiments with meat broth.
Some creationists wave around Pasteur's experiments as if they are some sort of absolute demostration, but that is not the case. He only showed that abiogenesis does not take place in certain commonplace circumstances.
However, traditional spontaneous generation has been revived by Dr. Periannan Senapathy, who expounds this view in his book Independent Birth of Organisms. Its cover features an illustration of his theory: a crab, a frog, a turtle, a butterfly, and an earthworm crawling out of a DNA-infused pond. But this is considered pseudo-science.
Biological evolution is a well supported and successful theory, regardless how life came about. Chemical evolution on the other hand addresses exactly the question of the origin of life. As there are no fossils preserved from this process, it is more speculative in nature.
The modern era of abiogenesis research can be considered to start from Charles Darwin's speculations about life emerging "in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc., present." He also speculated that no environments for abiogenesis would exist on the present-day Earth, because such environments would be quickly consumed by various organisms.
But there was no further progress since the 1920's, when Aleksandr Ivanovich Oparin and John Burdon Sanderson Haldane independently worked out scenarios of "chemical evolution". They concluded from various chemical grounds that the Earth had originally had hydrogen, ammonia, carbon dioxide, methane, and other simple compounds in its atmosphere -- but no oxygen molecules. And larger and larger molecules formed by various chemical processes until some of them succeeded in making copies of themselves, forming the first living things.
Much of the Oparin-Haldane scenario was little better than hand-waving, but the first concrete support was provided by the famous Urey-Miller experiments, conducted in 1953 by Stanley Miller and Harold Clayton Urey in 1953. These involved a simulated early Earth atmosphere and ocean, complete with simulated lightning: an electric spark. After about a week, the simulated ocean accumulated some brown tar -- and a variety of dissolved organic molecules, including several biological amino acids.
Urey-Miller experiments have been repeated with a variety of energy sources and other conditions, and as long as the source mixture is chemically reducing, organic molecules readily form. Contrary to creationists' claims, Urey-Miller experiments are not rigged to produce the "right" results; no rigging seems to be necessary.
Until 2005, the Earth's early atmosphere was thought to be neutral, consisting mostly of nitrogen and carbon dioxide, instead of hydrogen, ammonia, and methane (reducing), as had been suggested from cosmochemical grounds. Urey-Miller experiments performed with a neutral mixture are much less successful than those with a reducing mixture; however, even if the main atmosphere had been neutral, the early Earth could easily have had reducing microenvironments, like hot springs and hydrothermal vents. But outgasing experiments performed in late 2005 by Bruce Fegley, Ph.D. of Washington University now suggest that the earth's early atmosphere was in fact a reducing one. (See "Origins of life," September 07,2005 at PhysOrg.com.)
There is also the conundrum that bodies of water are poor places for the formation of biomolecules like proteins and nucleic acids, since the "primordial soup" is inevitably very dilute, making it difficult for molecules to "find" each other. This conundrum has led to the "primordial pizza" hypothesis of the origin of life on mineral surfaces like clay surfaces, which organic molecules can easily stick to, and which have catalytic properties that can easily assist in the formation of complex molecules. GÃ¼nter WÃ¤chtershÃ¤user has proposed that the Krebs Cycle (a.k.a. citric acid cycle, tricarboxylic acid cycle) had originated on such mineral surfaces, powered by iron-sulfur chemistry.
And while some biological molecules, like the smaller amino acids and nucleic-acid bases, are readily produced in Urey-Miller experiments, others, like sugars, are not. This would mean that nucleic acids are difficult to produce, since they contain the sugar ribose and its derivatives; this has been a major difficulty with the otherwise-very-attractive "RNA world" hypothesis. But in late 2003, Steven Benner of NASAâ€™s Astrobiology Institute found that ribose and other sugars are easily formed without being burned if they are captured by borax minerals. (See "UF Study Suggests Life on Earth Sprang from Borax Minerals," Jan. 8, 2004 at UF News.)
Thus, how to get from there to a complete self-reproducing system is still an unsolved problem, but this question is being actively researched.
Biogenesis, or life coming from life, is thought to be the absolutely most frequent mechanism through which living beings come to exist. Scientists such as Louis Pasteur and John Tyndall had performed experiments showing that in an adequately sterilized medium, life will not arise spontaneously from non-life. When it appears to occur, it can be shown that is in fact the result of the survival of preexisting living microorganisms or their heat-resistant spores, due to inadequate sterilization.
This does not prove, however, that life can never originate from non-life, in any condition, implying that life is eternal, or alternatively, that is necessarily first created by supernatural means. It just proves that in an adequately sterilized medium, life will not spontaneously arise.
Even though at the time of Pasteur and Tyndall there were those who believed that as life first originated from non-life billions of years ago, it could as well arise from non-life in the present days, this is no longer held by scientists, and not only because of TyndallÂ´s and PasteurÂ´s demonstrations. As the knowledge of the microbial life increased, became more clear that even microorganisms were formed by complex structural organization. Prior to that, many had thought that microbial life were simple enough to be possible to arise in a simple process.
But along with the increase of the knowledge about microbial life, new hypotheses about the ultimate origin of life were formulated, taking all the new data in consideration. The idea of a simple, "sudden", spontaneous origin of life gradually lost place for theories with multiple stages of pre-biotic systems with increasing complexity, a process that is sometimes called as chemical evolution, and it is thought to occur under conditions and timespans totally different from those studied by Pasteur and Tyndall.