Did the common ancestor had a Y chromosome .Do we know how many chromosome did the common ancestor had ? At what point in time man started to have a Y chromosome ? Can animals with different number of chromosome mate and reprodice ?
Sigh. You are relying on misinterpretation of old data. This 2010 paper confirms the evolutionary model that apes and humans had a common ancestor but show that not all of the content of the Y chromosome of that common ancestor was strongly conserved in both lineages. http://www.nature.com/nature/journal/v463/n7280/full/nature08700.html But comparison with more distant cousins, the rhesus monkeys, shows that the good bits are strongly conserved. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4120474/
Yes, obviously the common ancestral population did. Yes -- genetic studies between chimps and humans have identified which chromosomes correspond to which and that the event that cause one lineage to have a different number of chromosome pairs than the other was a fusion event. The common ancestral population had 48 chromosomes and the the human lineage had an event where two chromosomes fused to form the ancestor of human chromosome pair #2. Roughly one hundred million years before there was such an animal as man. http://www.ncbi.nlm.nih.gov/pubmed/11306800/ Sometimes, yes. Between 1% and 5% of XXY males with 47 chromosomes are fertile and can produce offspring. Chromosomal number change is a barrier to reproduction, but not an absolute one. http://www.nichd.nih.gov/health/topics/klinefelter/conditioninfo/Pages/treatments.aspx
The topic title should either be changed to a question or a useful scientific definition of "uniqueness" should be immediately proposed and a fact-based argument for that proposition be advocated. Are you saying humans are unique for having some species specific variant of the Y chromosome or are you claiming that the Y chromosome of which humans just happen to have a homologous instance of is unique among chromosomes? If the former, how, for example, is the human Y chromosome more unique than human chromosome pair 2? Women are notably deficient in human Y chromosome; does that make them less unique than human males? Human males are more fragile because the genes on both the X and Y chromosome have no redundancy in individuals thus suffer a greater incident of deleterious mutations; is knowledge of this weakness why you initially titled the post in ALL CAPS? If so, how is that more unique than the similar sex-based weakness common to all great apes?
Going by the platypus - a majestic and underappreciated aberrant - there were probably several 'Y variants' which were reduced or degraded X chromosomes. The small Y is thus a little less old than the Mammalia, depending on whether or not one finds these beautiful, duckbilled abominations to be 'true mammals' or not. They are of course: the debut of the mammals isn't so much a point as a process. You know, like karate in The Karate Kid. Always. Quite a refined Y by the arrival of humanity. Yep, or several can. Interspecific polyploids in the amphibia, in fish, even a couple mammals. That said, I wouldn't recommend it.
Please, please: don't misunderstand me! It was never my intention to insult the existence of these playful, adorable mutated freaks. The honourable and industrious deviants known as the platypi have been maligned long and unfairly, and largely because of their unavoidable yet inexusable atrocities of design and function. To my mind, it is well past time to give these beautiful, abnormal throwbacks their just due. I hope that settles your concerns.
Those of us who have managed to spot one in the wild.. It is a privileged experience. Surreal to be honest. Because they are so rare. I actually cried when I saw it in the wild. I knew it was a once in a life time thing. Probably never to be repeated. The absolute silence of where we had walked to, the lighting. It was probably the closest thing I have ever come to what some would describe as a religious experience. Pure joy and pure emotion. Their 'design' suits their function. They are amazing animals. Hopefully the link helps.
No. Most mammals, and an assortment of other animals (including the often studied fruit fly Drosophila) have the XY sex determination system. Some plants (such as ginkoes) determine sex this way. There are other sex determination systems out there. For example, birds have a ZW system of chromosomal sex determination, where different chromosomes perform a similar function. There are a few animals (including some mammals) with an XO system, where females have two X chromosomes and males have just one, and Y chromosomes don't exist. There are lots of other variants too, such as animals where males and females have the same genetic complement and sex determination happens through environmental factors. (There are lizards where temperature performs this function.) I've even heard of animals where individuals are females during a portion of their lives, and males during other portions. Among ants and bees, unfertilized eggs become males and fertilized eggs females. https://en.wikipedia.org/wiki/Sex-determination_system The sex determination systems are an interesting problem for evolutionary biology. The different systems seem to be scattered seemingly randomly around the various taxa. The XY system is found in most mammals and it isn't hard to imagine that mammals share a common ancestral population if you go back far enough (probably before the dinosaurs) that we all inherited the XY system from. But if some insects and some plants also have the XY system, are we going to say that they are also descendants of the ancestral mammal population? Or vice-versa? And does the fact that some mammals don't use the XY system mean that they aren't really mammals at all and have a different ancestry? Apparently there's a lot of convergent evolution taking place here. So is there some kind of pattern to it? Does evolution select for different sex determination systems in different circumstances? Do different systems confer different advantages? There are some nice research topics for future biologists in all this. Probably long before human beings were human beings. We probably inherit the XY system from some Opossum-like ancestor that might predate the dinosaurs. That doesn't necessarily mean that all animals with Y chromosomes have the same genes on those Y chromosomes. It's well known that genes (short segments of DNA) can move around and even jump from chromosome to chromosome. https://en.wikipedia.org/wiki/Transposable_element
That it is. It seems that while the Platypus has the XY system of sex determination, it has no less than ten sex chromosomes. Males are XYXYXYXYXY and females are XXXXXXXXXX. And interestingly, some of the genes found on Platypus sex chromosomes aren't found in other mammals, but are found in the birds' ZW sex determination system. (Were the dinosaurs ZW's?) I'm guessing that latter aberration is likely the result of these egg-laying mammals' early divergence from the evolutionary line that led to the other mammals. https://en.wikipedia.org/wiki/Platypus
I wonder why timojin has spent two days on other threads but has not yet returned to this one. I assume that the answers above satisfied the curiosity about sex chromosomes in human ancestors.
Apparently males of species with short Y chromosomes are endangered; in humans the Y has reached about the limit, any smaller and it will become less reliable--male humans will then start to disappear, evolutionarily. At least, that's something like some thing I remember reading about some time ago . . .
That claim has no basis in empirical science and is directly rebutted in one of the articles I cited. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4120474/#S13title But even if, in some unlikely distant future, the Y chromosome did disappear from a large population of human descendants, it would mean that the population had moved from an XY sex chromosome system to a XO system. It would not necessarily mean the end of human males, but in such a distant future it is unlikely that the biological meaning of "human" would still apply.
It wouldn't happen, simply for the fact that countermutations would ensure a pool of male-survivable Ys to work from. A non-functioning Y couldn't go to fixation.
