Assuming evolution is the reason we are here, there must have been a “final” genetic mutation which can be found in every human alive today. What I don’t understand is that the children of the parent who had this mutation would have inherited his or her DNA code (I believe we get the half of our DNA code from our father, the other half from our mother).
This would result in a common genetic code for all humans who who were the first to receive the last common genetic mutation.
So my question is… what’s with all the differing DNA codes we see in humans today?
(I was going to throw this in to GQ, but figured it would probably end up in GD).
Speciation doesn’t work like that. There is no mutation that breaks the camel’s back.
Here’s how I conceptualize speciation. You start with one species. A subset of that species somehow becomes reproductively isolated from the main group (let’s say a few pioneers traveled across a moutain ridge and never returned). Initially, the genetic composition of the sub-group is identical to the larger group. Between individuals in either the sub or main group, a fair amount of genetic diversity exists, but on average, the populations are identical…
Over time, even without selective pressures (which is probably an impossibility) or with similar selective pressures (more likely) the two groups will come to differ. Even if selection is neutral, replication of DNA is inexact and different mutations will accumulate within our now separate populations. Severe selective pressures will hasten this process.
The two populations will gradually drift away from one another genetically speaking and at some (somewhat arbitrary) point, we call them separate species. Between populations, individuals will still share many characteristics (a2b2 structured hemoglobulin will not likely be scrapped). How they differ will be far more complex and interesting than just one lil’ ol’ gene.
Firstly, there will be profound differences in DNA sequences that don’t make a difference. So-called “junk DNA” comprises a large portion of the eukaryote genome and tends to be highly mutable since such mutations carry no cost to the organism.
How the populations will differ at expressed loci is hard to predict. Generally a trend of, “What works well is conserved,” is observed throughout evolution. However, given the complexities and redundancies of biology, considerable room for embellishment exists.
To be sure, however, there would be no single gene underlying the differences between the original population and the new species.
Evolution thrives on genetic mutations. It is lots of these ‘mutations’ that create new species. Therefore for the human species, there must have been a last common mutation, no?
Not correct. Or at least not always or most of the time. Sometimes speciation can result from a single mutation ( i.e. spontaneous polyploidy in selfing plants ). But more commonly speciation proceeds through a variety of mechanisms involving change ( gradual or quick ) of a multitude of gene ( allele ) frequencies.
Now mutations provide the building blocks by producing genetic variation. But it isn’t necessarily an accumulation of mutations that creates a new species. By sheer “luck” you may have three times as many mutations as me ( every cell in our bodies produces an average of 120 new mutations in the course of developing ) - Doesn’t mean you are a different species. Mutations, in of themselves, don’t necessarily define anything.
No, not necessarily. A possibility exists that hominids at some level are the result ( in a sense ) of a single mutation - Perhaps a translocation event that winnowed our chromosome count to 23 pairs vs. the 24 seen in other primates. But modern humans didn’t necessarily arise at the moment ( though some ancestor of ours might have ).
As to our genetic diversity - It actually isn’t very high compared to closely related taxa like other primates. I’m not sure exactly what differences you are talking about. All non-clonal species have a certain level of internal genetic diversity.
—It is lots of these ‘mutations’ that create new species.—
Not really. It’s wrong to think of evolution as simply waiting around for mutations to accumulate. Evolution, by which I mean selection (whether adaptive or random drift) is usually a limiting factor on the speed of mutation, not the other way around. The variation necessary for speciation can even already exist in a given population, but just isn’t fully expressed until selection shaves stuff away. On a very simple scale, think, for instance, about recessive genes becoming selected for and eventually becoming de facto inevitable, with every animal in a breeding population being a carrier.
Taking all of that in to consideration, must there have been a last common human genetic mutation?
Eg, all humans have opposable thumbs. Our ancestors didn’t have them some 10 million years ago, but at some point between then and now this genetic trait became common in ALL humans alive today. It seems to have been one of the last common genetic mutations.
So, yes or no, was there a last common human genetic mutation, even if we don’t know what it is?
Think of it this way (thanks to the doper who first suggested this analogy, whose name is lost in the mists of time).
When did Old English become Modern English? If you were time traveled back to medieval England (sorry; don’t know exactly when), you would not be able to communicate with or understand anyone you met. It was an entirely different language.
But gradually, over time, it evolved into the language we speak today. As an analogy to biological evolution, it’s now a different species. But everywhere along the line of evolution, children were able to speak to their parents, right? There’s no single point in time that you can name that is the dividing line between the two different “species” of English.
Biological speciation works exactly the same way, only over a much longer period of time.
It’s also helpful to remember that there’s no such thing as species.
“Species” and “speciation” are an entirely artificial concept invented by man to describe an entirely arbitrary process. Since it is, after all, impossible to discover a specific point along an evolutionary path where speciation occurred, it’s somewhat open to interpretation. So we come up with even more arbitrary shades of gray like “subspecies” and “races.” And it’s not at all uncommon for “official” species names to be revised over and over again
So in other words, since Nature (note the capital N) has no idea what a “species” is, she doesn’t make it very easy for us to define it, either.
It is possible that one mutation caused a finality of speciation to Homo sapiens from our progenitor species. It is unlikely, though: we are not 100% sure if evolution travels in big leaps versus small steady progress. A plausible big leap would be large scale genome rearrangements which destroy any interfertility possibilities. I see this as relatively unlikely. Even if it were the case, not knowing from what we came from directly (the closest living things from us forked off dozens of evolutionary steps ago) means that we have nothing to compare ourselves to, and thus we can’t say what the last big change was.
It is possible to go through and catalog the genes more divergent between humans and chimpanzees than between chimpanzees and other mammals. This gives an idea of the genes that have changed the most in the shortest amount of time. A short article published in Nature two weeks ago identified one such gene: FOXP2. Apparently, there has been as much change in the gene between humans and chimpanzees and orangutans than between chimpanzees and mice (I think about 10 Myr versus 70 Myr of evolution, so a 7x rate approximately). Interestingly, people with loss-of-function mutations in FOXP2 have a problem with speech. Whether that problem is in the neck, the mouth, the jaw, or the brain has not been well characterized. But it did allow a tenuous hypothesis (and therefore guarantee the Nature letter): FOXP2 is one of the genes that has been under the most selective pressure as humans started to communicate. Since our communication skills are one of the major differences between us and the rest of the great apes, these mutations in FOXP2 are one of the major differences between us and the great apes.
Here is a cite: Molecular evolution of FOXP2, a gene involved in speech and language
WOLFGANG ENARD, MOLLY PRZEWORSKI, SIMON E. FISHER, CECILIA S. L. LAI, VICTOR WIEBE, TAKASHI KITANO, ANTHONY P. MONACO & SVANTE PÄÄBO Nature418 869-872
—is unlikely, though: we are not 100% sure if evolution travels in big leaps versus small steady progress.—
Actually, we have a fairly good idea that it works via small steady process regardless of whether or not it works in fits and starts, because if it worked in “big leaps,” it wouldn’t plausibly work at all (it would be the creationist paraody of evolution). Gould and Leotowin have caused a great deal of confusion with their presentation of punctuated equillibrium, but even they never intended to advocate saltation.
If we were able to trace back through our ancestry, all humans alive today would have a common ancestor, right?
By a common ancestor, I mean an individual, single animal (not a species, but one member of a species) that would no doubt have looked very much like a modern man.
Now, this animal would have had DNA code that it would have passed on to its offspring.
This DNA code would continue to be passed along through all generations, meaning that humans of today should all share this part of code in their DNA.
But looking at human DNA code today we see that it is very different between humans. My question is… why?
—all humans alive today would have a common ancestor, right?—
Are you talking about mitochondrial “Eve”? Every human on the planet is descended from her (and by definition we know that she must have had two daughters that each had a line of progeny that survives to this day, and so on). But this designation (which must refer to an actual historical individual), can only be awarded retroactively: there was nothing necessarily special or even new about this particular “common (female) ancestor” at the time that she lived.
There is, of course, also a Y chromosome “Adam” (who, we might add, is very likely to have been a most prodigious lover and fatherer of babies by many different women). It’s important to see that it is highly unlikely, and totally unecessary, that these two common ancestors (and there well could be others for other traits) lived at the same time. And that’s sort of key to your question. Despite being everyone’s common ancestor, there were changes in the sorts of traits in the population key to what we know as “humanity” both before and after the woman who (only later) turned out to be M.Eve. Likewise with YAdam.
One obvious difference is skin color. Another is something like sickle cell: these differences may seem small, and they are certainly superficial and well below any range of species classification: but they are undeniably examples of genetic diversity that has to have arisen well AFTER we crowned a certian breeding population as being the first example of homo sapiens (which is quite differnt from MEve or YAdam: homo sapiens developed long long before either of them is likely to have lived (YAdam could even had lived as few as tens of thousand years ago, and MEve three hundred thousand years ago)
So maybe what you mean is “is there a “last” (i.e., most recent) common ancestor?” In which case, yes: though that doesn’t necessarily have anything to do with that ancestor having any new “mutation” per se.
—But looking at human DNA code today we see that it is very different between humans. My question is… why?—
Well, first of all, it’s NOT very different. In fact it’s surprisingly undifferentiated compared to most life on earth: which suggests that we (the breeding population alive today) are a fairly recent species (“recent” still being measured in geological terms).
But second of all, everyone having a common ancestor does not mean that this ancestor is the only contributor to our genetic line. It’s not like ALL diversity had to come from MEve: only her mitochondria are “common” to everyone.
But a further elucidation might be, Beastal, that even though there must have been a ‘Mitochondrial Eve,’ those kinds of genetic drift and accumulated mutations that eventually accrue to the threshold of speciation did not stop with her, and of course continues from generation to generation today. That would of course account for any variations in individual human DNA (which, as has been noted above, is not perhaps as great as you might think), but has not ‘yet’ [whether it ever will is a whole nother long-ass, contentious debate) led to further speciation.
And also, to riff on Apos’s final paragraph, just because we all spring from this M.Eve doesn’t mean that we came just from her. Remember the language analogy? Her children would still have been able to breed with the offspring of other women. So even as her line, our line, began, genes from other lines–children of her contemporaries–began mixing right back in the following generation. She was unique, but not isolated.
Not really on topic here, but I just heard something interesting, but unconfirmed, about mtDNA. A Danish poster on an e-mail list I belong to said that there was a very brief blurb in her newspaper about the discovery of male mitochondria being passed on to a descendent. The information allegedly came from a John Vissing, Neurological Clinique, Center of Muscle Research, Danish State Hospital. According to Vissing’s statement, they have found only this single example and don’t know if it is just some extremely rare occurence or if this happens more frequently, and that it was too early to speculate on what this might mean in re: inheritance, evolution, etc.
The poster didn’t have a link to an online article, and said she had checked the hospital’s website but found no mention there. Has anyone else heard of this? Anyone care to speculate on the mode of transmission? Whether or not this is a human error and will be discredited later, an extremely rare event, or something that happens with some frequency and simply hasn’t been discovered yet? If true, what effects might this have on tracing inheritance through mtDNA identification?
Brand new research, hard to say where it will lead.
European Congress on Human Genetics 2002: http://www.medacad.org/eshg/indexeshg.htm
P0782: “Paternal inheritance of mtDNA in a patient with mitochondrial myopathy” M. Schwartz (1), J. Vissing (2);
(1)Department of Clinical genetics, Rigshospitalet, Copenhagen, DENMARK, (2) Department of Neurology, Rigshospitalet, Copenhagen, DENMARK.
Abstract:
Research was published as: Marianne Schwartz and John Vissing “Paternal Inheritance of Mitochondrial DNA” NE JM 22/8/2002 347: 576-580.
With an accompanying editorial: Williams R. S “Another Surprise from the Mitochondrial Genome” NEJM 22 /82002; 347:609-612.
In re mtDNA Eve: others have noted, but it is necessary to point out that mtDNA even and Y-Adam do not mean that only these two ‘individuals’ lived or somehow gave rise to humanity a la Adam and Eve of Judeo-Xtian-Islamic mythology. Rather they represent two lines of descent which gradually (and seperately) dominate. Artefact of descent lines, rather like the 7 Sisters in re Europe, which again is widely misinterpreted.
Outstanding, Collounsbury! Thank you very much - I wasn’t expecting such a quick answer. If I understand what I’m reading - I’m just a cat breeder, so some of this is over my head - the paternal mtDNA is only found in muscle cells, which is why it hasn’t been noticed previously (most experimental samples being taken from blood/bone marrow). I would think that the only real effect of this discovery (on this topic) would be to enable us to track inheritance more precisely when the appropriate tissue is available for testing.
Nah, what you are seeing is a selection event. The mother’s mitochondria are sick, and the muscle needs lots of mitochondria. So the small fraction of paternal mitochondria present in every cell are selected for, and they expand to take up the role. There are perhaps upward of 100,00 mitochondria in the oocyte and only a few (which are perhaps usually eliminated) in the sperm. If the mother’s mitochondria are healthy, then there is no change in this ratio. If the mother’s mitochondria don’t work, you see an expansion of paternal mitochondria in tissues requiring a lot of respiration (the muscles and brain especially). This is called mitochondrial heteroplasmy. If you do a PubMed search under mitochondrial heteroplasmy, you will pull out a few hundred hits.
Also, let me steer this back to the OP. First, great apes share many of our mtDNA and Y chromosome polymorphisms, and we often utilize these in the studies of human descent. There has been no real significant change probably in these tiny mostly insignificant pieces of DNA (they total under 100 genes combined) to make us human, that separates us from the great apes. The change was almost certainly autosomal. Thus, the mitochondrial Eve or the Y chromosome Adam are totally arbitrary points of divergence, when humans acquired one or another polymorphism that separates our haplotype from that of the great ape. These exact changes probably had no significant role in making us more human than more hominid – those changes were almost certainly autosomal and cannot be followed by mtDNA or Y chromosome studies.
Also. Some reading on this has shown that one of the major chromosomal changes that happened between us and the great apes is a Xq21.3 to Yp translocation (a bit of the terminal X was transposed to the Y chromosome). I had never heard of this, and I will do some reading to see what I can make of it.
BTW, of interest as I recall (having looked at this quite rapidly - or was it a discussion of the underlying article?) was the initial hypothesis that the FOXP2 change occured recently c. 100 kya +/- and rapidly diffused through the population.
Suggestive of the speed, if it holds up, of spread of advantageous mutations.
Just FYI, our lab is mapping SNPs on X and Y and are specifically looking for the regions found on both for some project the higherups are doing (i just do the busywork on that one…).
that mtDNA article was news to me. Are there mitochondria in the sperm head? i thought they were exclusively in the tail which got dropped off when the head entered the egg cell. (i’m no expert on reproductive biology) There doesn’t appear to be any in the photos from this site. (or am i blind?)
Oh, I see - it only happens in special circumstances, so, even if “more common than generally believed”, it’s still a pretty rare event? But, still - oh, heck, I’m off to PubMed and other points of interest to bone up on mitochondrial inheritance and such.