Discusión:Fósil

message: Copio algs citas y refs: hola esperamos esta información les sea de mucha ayuda.
*Para el 97 existían dudas acerca de la autenticidad de las secuencias de  ADN fósil. Por ejemplo:  Jeremy J. Austin, Andrew B. Smith and Richard H. Thomas Palaeontology in a molecular world: the search for authentic ancient DNA. Trends in Ecology & Evolution. Volume 12, Issue 8, August 1997, Pages 303-306. Abstract:
The survival of ancient DNA in specimens up to several thousands of years old is established. However, there have been several claims concerning the recovery of geologically ancient DNA from fossil material many millions of years old. The authenticity of these fossil DNA sequences is questionable on theoretical and empirical grounds, and the existence of authentic geologically ancient DNA remains to be proven. 
*Para el 2000 ya habia revisiones sobre el tema y algunas definiciones, métodos y precauciones sobre el uso y abuso. Ejemplo, Fossil DNA. por Tomas Lindahl. Current Biology Volume 10, Issue 17, 1 September 2000, Page R616  
"Also known as… ancient DNA. The New Shorter Oxford Dictionary offers the precise explanation of ‘ancient’ as ‘belonging or pertaining to the period before the fall of the Western Roman Empire in AD 476.’ DNA can be recovered from fossils much older than 1500 years, but the inevitable time-dependent decay of the DNA structure due to oxidation and hydrolysis makes retrieval increasingly difficult with age.
How is it recovered? The advent of the polymerase chain reaction (PCR) allowed for amplification and sequencing of the minute amounts of DNA fragments sometimes found in fossils. DNA is better preserved in ancient bones than in soft tissues and multicopy mitochondrial DNA is more easily recovered than nuclear DNA; some fascinating sources have been used.
So what is possible? The oldest authentic samples are short sequences of 100–250 nucleotides from Neandertal and mammoth fossils, which had remained in a cold environment for 40,000–100,000 years. Low temperature slows the DNA decay process.
But hasn’t dinosaur DNA been reported? Attempts in many laboratories to isolate DNA from dinosaur bones and other antediluvian sources have yielded nothing but modern contaminants. As late as April 2000, one of the major scientific journals contained as a News item a breathless account on the similarity between retrieved dinosaur DNA and bird DNA, starting “They said it couldn’t be done. But …” Scientists at University of Alabama sequenced a 130-nucleotide long mitochondrial DNA sequence from dinosaur vertebrae, and found that it was 100% homologous to mitochondrial DNA from turkeys. However, the scientists themselves “remain quite sceptical of our own work” and noted that they had been consuming turkey sandwiches in the laboratory.
Are there other such reports? Unfortunately, yes. Insects entombed in amber are a perennial favourite. A 1996 paper in the same journal from another group described how a piece of amber containing an insect was cracked open over an agar plate. Subsequently, a yeast colony grew out on the plate, which was interpreted as a Jurassic yeast, and therefore published.
So how do you avoid contamination problems? Fossils are regularly contaminated with present-day DNA. For this reason, it seems difficult to be certain of DNA purported to come from, for example, ancient bacteria in fossil glacier ice, primitive human tribes, or other likely antecedents of extant modern species. Extinct animals with a good taxonomic record offer a safer source. Svante Pääbo’s group in Leipzig has set the standards for everybody else by their meticulous and highly credible work.
What extinct species have been analyzed? The most archaic are mammoths, which turn out, not surprisingly, to have had DNA more similar to that of elephants than other big mammals. Being more reminiscent of Asian than African elephants also fits with the known geographic distribution of mammoths. Another well studied example is the giant ground sloth, which was eradicated in connection with the human invasion of Latin America 11,000 years ago.
Interesting, but are there any revelatory scientific achievements in this field? Yes. Palaeontologists had been unable to agree from the fossil record whether Neandertals were a separate species or direct precursors of humans. Isolation and sequencing of DNA from Neandertal bones over the last couple of years has shown that those anthropologists were right who subscribed to the former hypothesis. Note that the palaeontologists continue their skirmishes just as before, essentially ignoring the new molecular evidence.
Can the complete genome of a Neandertal be sequenced? Possibly, although it would be very difficult to recover all single-copy nuclear DNA sequences, and fossils are scarce. Also, the DNA fragments are so short that it would be a major computing task to assemble all those bits into a correct sequence.
Can ancient DNA be repaired? Some base damage could most likely be corrected, offering modest quality improvement. But the main problem is extensive fragmentation of very old DNA by double-strand breaks, apparently occurring as a consequence of prolonged drying.
Don’t say… ‘We have found a frozen mammoth and intend to clone it’
Do say… ‘From DNA sequencing we now have irrefutable evidence that Neandertals represented a distinct species, different from humans.’
Where can I find out more?
Greenwood AD, Capelli C, Possnert G, Pääbo S: Nuclear DNA sequences from late pleistocene megafauna. Mol Biol Evol 1999, 16:1466-1473.
Höss M: Neanderthal population genetics. Nature 2000, 404:453-454.
Kimberly KO, Robertson HM: Ancient DNA from amber fossil bees? Mol Biol Evol 1997, 14:1075-1077.
Willerslev E, Hansen AJ, Christensen B, Steffensen JP, Arctander P: Diversity of Holocene life forms in fossil glacier ice. Proc Natl Acad Sci USA 1999, 96:8017-8021." 
*Para el 2005 el tema de los artefactos en los papers que tratan con ADN fósil aparenta seguir siendo un problema: Ver "Geologically ancient DNA: fact or artefact? Trends in Microbiology, Volume 13, Issue 5, May 2005, Pages 212-220 por Martin B. Hebsgaard, Matthew J. Phillips and Eske Willerslev" (abstract: "Studies continue to report ancient DNA sequences and viable microbial cells that are many millions of years old. In this paper we evaluate some of the most extravagant claims of geologically ancient DNA. We conclude that although exciting, the reports suffer from inadequate experimental setup and insufficient authentication of results. Consequently, it remains doubtful whether amplifiable DNA sequences and viable bacteria can survive over geological timescales. To enhance the credibility of future studies and assist in discarding false-positive results, we propose a rigorous set of authentication criteria for work with geologically ancient DNA.")
*Papers qeu han informado el uso de ADN fósil cmo evidencia paleontológica: Studies of Avian Ancient DNA: From Jurassic Park to Modern Island Extinctions. Avian Molecular Evolution and Systematics, 1997, Pages 345-373. Alan Cooper. Summary:
"During the first decade of ancient DNA analysis the concept of analyzing prehistoric genetic information has emerged from obscurity to become a relatively well-known and mainstream pursuit (Higuchi et al., 1984; Crichton, 1991; Pääbo, 1993). Ancient DNA techniques are particularly suited to the study of avian evolution since birds make up a disproportionate number of the world's recently extinct and currently threatened taxa. Extensive museum collections of avian skins and skeletons, often significant contributors to this situation, are now a considerable resource for systematics research. Many extinct avian taxa were the results of evolution within ecosystems that have since disappeared, and represent unique unrepeatable experiments. In these situations ancient DNA techniques may allow “lost” genetic information to be used to reconstruct these evolutionary paths and assist in the conservation of remaining biota."
*Ejemplos en plantas: acá va uno "DNA sequences from ancient fossils have great potential for studies of phylogeny, biogeography, and molecular evolution (Soltis et al., 1995  ); DNA from fossils also facilitates the rigorous testing and calibration of mutation rates among related taxa (Golenberg et al., 1990  ; Cano et al., 1992  , 1993  ; DeSalle et al., 1992  ; Soltis et al., 1992  ; Poinar et al., 1993  ). The Clarkia fossil beds in northern Idaho, USA (17–20 million years [my] BP; Miocene), bear some of the best preserved compression fossils from the Miocene. These fossils are believed to be chemically well preserved because of the low oxygen content and cold temperatures of the water in which they were deposited. Leaves apparently fell into an ancient lake and remained unoxidized and water-saturated until the present day (Smiley, 1985  ; Giannasi and Nicklas, 1977  ). In an ultrastructural study of these fossils, 90.1% of 2300 randomly sampled cells contained chloroplasts (Niklas et al., 1985  ). 
Golenberg et al. (1990)  obtained a partial DNA sequence of the chloroplast gene rbcL (the large subunit of ribulose 1,5- bisphosphate carboxylase/oxygenase) from a leaf specimen of Magnolia latahensis (Berry) Brown (Magnoliaceae) from the Clarkia fossil beds. Compared with the rbcL sequence of modern M. macrophylla (1428 base pairs [bp]), there were 17 base substitutions in the 759-bp region they sequenced, 12 of which were transitions and all but four of which were third-position substitutions. In their phylogenetic trees, M. latahensis was the sister to the remaining family members analyzed, M. macrophylla and Liriodendron tulipifera, when all three codon sites were included in the analysis (UPGMA using Kimura's 3 substitution type estimates). When third codon sites were eliminated, M. latahensis was sister to M. macrophylla, and L. tulipifera was sister to the M. macrophylla-M. latahensis clade. Successful extraction of DNA and sequencing of rbcL were repeated from a specimen of the gymnosperm Taxodium (Soltis et al., 1992  ) from the same site. Miocene fossil Taxodium differed from modern Taxodium distichum at 11 of the 1320 nucleotides sequenced; all substitutions were third-position changes. Phylogenetic analyses of Taxodiaceae and Cupressaceae placed the fossil sister to extant T. distichum (Brunsfeld et al., 1994  ). Despite these apparent successes, the validity of these first DNA sequences from the Clarkia fossil beds was challenged on theoretical grounds. Based on in vivo studies of DNA degradation (Lindahl and Andersson, 1972  ; Lindahl and Nyberg, 1972  ), Pääbo and Wilson (1991)  suggested that DNA should be completely degraded after 4 my. We report two additional examples of the successful amplification and sequencing of fossil genes obtained from leaves of the Clarkia beds: (1) a partial ndhF (the sixth subunit of NADH dehydrogenase) sequence (1528 bp) from fossil M. latahensis and (2) a partial rbcL sequence (699 bp) from fossil Persea pseudocarolinensis Lesquereux (Lauraceae). Recent molecular phylogenetic studies of Magnoliaceae (Kim, 2001  ; Kim et al., 2001  ) identified major clades of extant taxa and estimated substitution rates of various genes, including rbcL and ndhF. Complete rbcL sequences have also been reported for Persea americana and other genera of Lauraceae (Goldenberg et al., 1990  ; Soltis et al., 2000  ). We (1) reanalyzed the previously reported rbcL sequence of the fossil M. latahensis (Goldenberg et al., 1990  ) with data from phylogenetic studies of modern Magnoliales and Laurales (Qiu et al., 1999  ; Soltis et al., 2000  ; Kim, 2001  ), (2) analyzed the ndhF sequence of M. latahensis with previously published ndhF sequences for Magnoliaceae (Kim et al., 2001  ), and (3) analyzed the rbcL sequence of the fossil P. pseudocarolinensis with rbcL sequences for Magnoliales and Laurales." 
*Cano R. J. H. N. Poinar N. J. Pieniazek A. Acra G. O. Poinar Jr 1993 Amplification and sequencing of DNA from a 120–135 million-year-old weevil. Nature 363: 536-538
*DeSalle R. J. Gatesy W. Wheeler D. Grimaldi 1992 DNA sequences from a fossil termite in Oligo-Miocene amber and their phylogenetic implications. Science 257: 1993-1936
*Golenberg E. M. D. E. Giannasi M. T. Clegg C. J. Smiley M. Durbin D. Henderson G. Zurawski 1990 Chloroplast DNA sequence from a Miocene Magnolia species. Nature 344: 656-658
*Soltis P. S. D. E. Soltis C. J. Smiley 1992 An rbcL sequence from a Miocene Taxodium (bald cypress). Proceedings of the National Academy of Sciences, USA 89: 449-451
*en bacterias: "217 000-year-old DNA sequences of green sulfur bacteria in Mediterranean sapropels and their implications for the reconstruction of the paleoenvironment" acá — El comentario anterior sin firmar es obra de 2806:105E:19:19D2:451E:87BE:50B4:52C8 ( • contribs • bloq). 00:12 11 jun 2019 (UTC)
thread_title: ADN en fósiles

message: 13 referencias y esto es un articulo destacado?? como verificar la informacion aqui??.. esta es una de las normas que son inviolables.. la de PONERLE REFERENCIAS A TODO!!! WP:VER.. SI van a hacer las cosas bien HAGANLAS NO SEAN HOLGAZANES!!--76.19.149.244 (discusión) 05:04 28 abr 2008 (UTC)
thread_title: Y LAS REFERENCIAS QUE???????????=

message: En vista del vandalismo que tiene este artículo últimamente y que parte de la información ha sido desvirtuada (intentaré reponerla en cuanto tenga tiempo), creo que sería buena idea poner esta página como "semiprotegida". Fiquei (discusión) 08:45 21 abr 2008 (UTC)
thread_title:  Semiprotección 

message: El Diccionario panhispánico de dudas dice: "A menudo la disyuntiva que plantea esta conjunción no es excluyente, sino que expresa conjuntamente adición y alternativa: En este cajón puedes guardar carpetas o cuadernos (es decir, una u otra cosa, o ambas a la vez). En la mayoría de los casos resulta, pues, innecesario hacer explícitos ambos valores mediante la combinación y/o". Así que lo aplico a la intro. --Edescas (discusión) 09:12 29 abr 2008 (UTC)
replies: text: Muchas gracias por la precisión y, sobre todo, por tu documentado argumento (fui yo el que puso "y/o"). —PePeEfe (discusión) 19:25 29 abr 2008 (UTC)
thread_title:  o