tag:blogger.com,1999:blog-38085367.post2672856513322823637..comments2024-03-19T09:06:21.507-04:00Comments on Irtiqa: Bubonic plague, inbreeding Neanderthals and shipwrecked marble for a Roman-era templeSalman Hameedhttp://www.blogger.com/profile/04327330113822656571noreply@blogger.comBlogger5125tag:blogger.com,1999:blog-38085367.post-38451907220263293102013-06-10T16:28:23.597-04:002013-06-10T16:28:23.597-04:00Salman and Charles Ross, many thanks for taking th...Salman and Charles Ross, many thanks for taking the time to answer my question and for the Zimmer article link. I appreciate that we don’t have exact answers yet (and there’s a chance that we may never will) but we have learned a lot more about Neanderthals (and our own species) in the last decade alone and hopefully the works of Svante Paabo and other scientists will unravel a lot more clues in the near future.Asad Mhttps://www.blogger.com/profile/00590746399686326935noreply@blogger.comtag:blogger.com,1999:blog-38085367.post-81296986550112383412013-06-05T22:19:20.774-04:002013-06-05T22:19:20.774-04:00Charles Ross' response continues:
-----------...Charles Ross' response continues: <br />-----------------<br />Part 2: <br /><br />What can these patterns tell us about potential hybridization between H. sapiens and H. neanderthalensis specifically? From general patterns in nature, it certainly would seem possible, and it's hard to imagine why a hypothesis of isolation or hybridization should be preferred. However, there also are potential reasons why H. sapiens and H. neanderthalensis would not interbreed. Culture is a strong evolutionary influence and in Homo it clearly can affect things like mating preferences and non-random associations. Another potential barrier is the karyological (chromosomal) compatibility between the two species. H. sapiens have 46 chromosomes (23 'pairs'); great apes (and early Homo?) have 48 chromosomes. I don't think it is established how many H. neanderthalensis had, but if they had 48, hybridization could be difficult, though not impossible. For example, a horse has 64 chromosomes and a donkey has 62, though they can produce a mule with 63 chromosomes (though the sterility of the mule probably results from this). However, if there was a high level of synteny along various chromosomes that were different among species, it could be very possible to successfully hybridize and produce viable offspring. We know there is a high level of genomic synteny between humans and chimps, for instance (The second chromosome in humans is actually two in chimps), and though there are no human-chimp hybrids, it seems reasonable to think that even if H. neanderthalensis had 48 chromosomes, hybridizing and backcrossing with H. sapiens wouldn't be prevented by pairing chromosomes during metaphase. So, I would say that there is no 'genetic' reason to suspect H. sapiens and H neanderthalensis could or could not hybridize, though there might well be barriers at this level. Also, it would be hard to assert that a H. sapien - H. neanderthalensis hybrid would or would not be sterile. If there was some partial hybrid sterility, it likely would be expressed in male hybrids. However, hybridization itself could have been prevented by strong cultural barriers, but these may never be known.<br /><br />For the second question about how we can tell if H. sapiens and H. neanderthalensis did in fact hybridize, the best evidence is currently emerging from comparing variation and characteristics of human and neanderthal genomes. We have lots of human genomes sequenced at this point, and Paabo and colleagues are sequencing the few neanderthal genomes available. They and others are looking for neanderthal 'genetic signatures' in our own genome. Finding genetic evidence of neanderthal genes in our genome is extremely hard to do, as we share at least 99.5% of our genome with neanderthals already due to recent divergence. That is, how do you know if a particular genetic variant is due to common ancestry or hybridization? It is possible, but it's like looking for a specific kind of needle in haystack of needles, which is why many researchers find the evidence equivocal. Consequently, thoughts about hybridization seem to flip back and forth with new evidence, but currently I feel there is some stronger evidence being presented, especially in conjuction with data from Denisovan genomes, suggesting that we did in fact hybridize with H. neanderthalensis, at least rarely. Here is a url of an article in Discover magazine that highlights some of the recent research in hominin hybridization:<br /><br />http://discovermagazine.com/2013/march/14-interbreeding-neanderthals#.UaygNWQ_8b1<br /><br /><br />Hope this gives some insight into the question. I apologize for the uncertainty in my answer, but I think that is the state of the evidence right now. <br /><br />Charles Ross. Salman Hameedhttps://www.blogger.com/profile/04327330113822656571noreply@blogger.comtag:blogger.com,1999:blog-38085367.post-70893362461334011982013-06-05T22:18:03.873-04:002013-06-05T22:18:03.873-04:00Asad,
Here is a response from evolutionary biolog...Asad,<br /><br />Here is a response from evolutionary biologist. Charles Ross from Hampshire College:<br /><br />----------------------<br />Part 1: <br />Great question! The short answer, as you might expect, is "We don't know." In general, the fertility of hybrid offspring usually is variable both among individuals within a two-species hybrid cross, and among different combinations of hybridization between two species. That is, not all hybrid offspring are equivalent in their viability or fertility when two species cross, and hybridizations among different pairs of species may produce a variety of outcomes.<br /><br />I think there really are two things to consider. First, Is it possible for Homo sapiens sapiens and Homo neanderthalensis to hybridize and what would be the state of the hybrid offspring? Second, how could we tell if this happened?<br /><br />For the first question, we can consider the general phenomena of hybridization as well as the specific case of H. sapiens and H. neanderthalensis. <br />In general, hybridization is more prevalent among species than many people might guess. For information on this, see Arnold 1996, " Natural Hybridization and Evolution" and his later book in 2006, "Evolution Through Genetic Exchange", which compile some estimates on the extent of hybridization in nature. Overall, I believe he estimated about 19% of all species may have hybrid origins, with some groups approaching 50%. I don't have the books in front of me, so I can't say those numbers are accurate, but it probably is reasonable to believe that hybridization is not a rare occurrence. Additionally, the phylogenetic distance among hybridizing taxa can sometimes be moderately distant. That is, they don't have to be sister taxa. Rieseberg et al showed this with sunflowers, though I can't recall the specific journal article (2003?). While the results of hybridization are widely variable among cases, there are numerous cases where the hybrid offspring seem perfectly viable and fertile, with some hybrids even having higher fitness than their parental types in certain environments. Of course, there also are many cases where the hybrids exhibit some form of disfunction or unfitness. This usually takes the form of hybrid inviability or hybrid sterility, as in the case of sterile mules, which are the offspring of a male donkey and female horse (there actually are some cases of female mules siring offspring). These 'post-zygotic' barriers are almost always expressed more in the heterogametic sex (in this case, the male, who has XY sex chromosomes), something called "Haldane's Rule". There are many hypotheses about why Haldane's Rule exists, such as the expression of deleterious recessive alleles in a hemizygous state on the X, but overall it suggests some incompatibility among genomes of the hybridizing species. So, in general, hybridization doesn't appear to be a rare occurrence, and the outcome of hybridization is not always unfit offspring, though if the offspring do have low fitness, patterns of unfitness often take characteristic forms. <br /><br />(cont.)<br />-----<br />Part 2 coming up.Salman Hameedhttps://www.blogger.com/profile/04327330113822656571noreply@blogger.comtag:blogger.com,1999:blog-38085367.post-26440959429804770032013-05-31T22:15:23.031-04:002013-05-31T22:15:23.031-04:00Asad,
good question. I will also get a more detail...Asad,<br />good question. I will also get a more detailed answer from a biologist friend of mine and will get back. As far as I know, in most cases the hybrid offsprings are sterile - but not in all. But then there is also a question of how "species" are defined. But the details of these debates are beyond my understanding, but I do think the Neanderthal chromosomes are equal to humans. You should also check out <a href="http://discovermagazine.com/2013/march/14-interbreeding-neanderthals#.UalYpOuhNw0" rel="nofollow">"Inbreeding with Neanderthals></a> by Carl Zimmer.Salman Hameedhttps://www.blogger.com/profile/04327330113822656571noreply@blogger.comtag:blogger.com,1999:blog-38085367.post-85311988519755062342013-05-31T11:06:18.521-04:002013-05-31T11:06:18.521-04:00Salman, just a silly question out of curiousity: w...Salman, just a silly question out of curiousity: would the offspring of a Sapien and a Neanderthal be able to reproduce coz usually the hydrid offspring of closely related mammals are sterile (e.g. horse & donkey or lion & tiger)? Also how many chromosome pairs did Neanderthal have? (I’m guessing 23 like us otherwise they wouldn’t have been able to produce anything at all).Asad Mhttps://www.blogger.com/profile/00590746399686326935noreply@blogger.com