Out of Africa? Again?!? Y-not!

Evolutionary Tree
Get outta here! 

First thing’s first: What exactly is a Y-Haplogroup, and why on earth does it matter?

The Human Y-chromosome DNA haplogroup  is a haplogroup (<—which is a group of similar haplotypes [<—-which are a combination of alleles (or DNA sequences) at adjacent locations, or loci, on the chromosome and are transmitted together] )that is defined by differences in the non-recombining parts of the DNA from the Y chromosome (also dubbed Y-DNA).

That’s a fun definition, isn’t it? To me, this definition is not particularly helpful. For a start, what are non-recombining parts!? Here’s an easy way to think of it. Since every male (and only males!) have just one Y chromosome, he can only pass down that SINGLE chromosome, therefore, its not going to combine with anything else and change. So the Y-chomosome is passed down from father to son, nearly unchanged. Since this is true, the Y-chromosome can give us some info about our paternal ancestors, much like mitochondrial DNA can give us the maternal side. (Instead of Mitochondrial Eve, we now have Y-Chromosome Adam!)

(So ladies, if you’re curious about what your family’s Y-chromosome can tell you, sorry. You’ll have to beg your brothers, fathers, uncles, or grandfathers to help you out. )

Wait, what was a haplogroup again!?!?
A haplogroup, put sort of simply, is a group of similar combinations of alleles that at adjacent parts of the chromosome that get transferred together.

So! A Y-Haplogroup, then, is the alleles that get transferred together on the Y-chromosome only. This is important because it can give us a very specific part of our ancestral DNA, and can be traced.

NEXT UP: Nesting Clade Analysis.
Who or what is a nesting clade!? I’m sure we have all at least heard the world clade in our anthro classes, but just in case, a clade is a group consisting of an ancestor and all its descendants, a single “branch” on the “tree of life”. So what is a nesting clade, then? A clade located within a clade is said to be nested within that clade. To picture this better, let’s have a look at these nesting tables.

Nesting Tables
Pretend these are clades. Look how they fit into one another!

Here is a handy map of the Y-Haplogroups in their clades.

Y-Haplogroup Chart
Go clades, go!

Alright! So now that we’ve got the basics recapped, let’s go to the article: “Out of Africa Again and Again” by A.R. Templeton

In using DNA to trace major events in our genome, there is more power to detect older events and processes that influenced many populations/haplotypes rather than the more recent. Recent (relatively) events many not have had a chance to be marked by mutations in the genetic system, so they are harder (or impossible) to detect. Along these same lines, some major expansions that have occurred may not be detectable, either, if there were no mutations in the genes occurring at the same time to mark it. Haplotype trees contain information up to but not including the final coalescent event to a common ancestral molecule.

This means that events can be traced through haplogroups, depending on which groups were effected, and which groups were not, because they had not evolved yet. So the youngest clade (smallest nesting table) that marks an event or process is expected to be mostly concurrent with the time of the event itself.

Templeton argues that there were at least two major migrations of people out of Africa after the ORIGINAL spread of Homo erectus.  Analysis from GEODIS (a software program that runs analysis on nested clades) indicated that the most recent out-of-Africa expansion wasn’t a replacement event. It bases this off of the fact that if it had been, three significant genetic signatures of older gene flow would have been wiped away; yet they persist! Evidence also suggests that the middle out of Africa expansion was also not a replacement.

These arguments are also supported by archaeological evidence concerning cultural expansions or of Africa. The Acheulean culture expansions overlap the time frame of the intermediate out of Africa expansion. This is fascinating because it suggests expansion of people and ideas that would interact with those already expanded (maybe not the best word choice…with those who had already left Africa!) Fossil evidence also agrees! After the initial expansion, there wasn’t much change in modern human’s brain size up to 700,000 years ago. By about 4-500,000 years ago our brains got substantially bigger. This increase overlaps with the time period for the older expansion.

Since there were three expansions out of Africa, and none fully replaced the other, it resulted in extensive interbreeding which strengthened the genetic ties among the human populations in the world, which we can see through the Y-haplogroups.

Question for you: Do you think there are any drawbacks to the Y-DNA approach? Is it more useful than mitochondrial DNA, or are they about the same?

Map of YDNA expansion
Look at that Y-chromosome go!

9 Replies to “Out of Africa? Again?!? Y-not!”

  1. What a loaded question! Since I’m a lady, I can’t help but favor the mDNA analyses, but I’m sure they both are useful (and neither are really comprehensive). I know more about mDNA (which is not to say that I know very much, really), so I would be more inclined to site it as evidence… but I think there is something to be said in the Y-chromosomes usefulness for establishing the relationships of the out-of-Africa-waves with each other/other species running around contemporaneously. I CAN’T DECIDE.

  2. Correct me if I’m wrong but– mtDNA is passed from mother to child regardless of the sex of the child? But Y-DNA is only passed from father to son (because only guys have the Y chromosome). If this is true then I think mtDNA would be more useful. Even though the mtDNA that the boy recieves wouldn’t be passed on, his mtDNA would theoretically allow us to link him to his mother. Versus the Y-DNA which physically cannot be passed on to females at all.

    Kinda sexist…KIDDING

    • I think I understood this the same way and I agree. If only males carry y-chromosonal DNA (And they do, right?), then you are potentially excluding a large portion of the population (les femmes). Whereas, the males carry mtDNA but do not pass it along; however, lineage can still be determined and traced and organized into these haplogroups.

    • I think I understood this the same way and I agree. If only males carry y-chromosonal DNA (And they do, right?), then you are potentially excluding a large portion of the population (les femmes). Whereas, the males carry mtDNA but do not pass it along; however, lineage can still be determined and traced and organized into these haplogroups.

  3. I think that mtDNA and Y-DNA are useful together in that they measure different lines of inheritance, BUT they both look at single, tiny little portions of the whole picture. Even taken together, mtDNA and YDNA are very limited with what they can tell us (granted they tell us a heck of a lot more than we could figure out without them).

  4. Isn’t the Y chromosome kind of weak and puny? I know that there are some predictions that the y chromosome will eventually be gone forever because the genes needed will be crossed over onto an X chromosome. Weird. I think they tell us different things but with such a lack of information about our ancient histories I think any information is good information.

    I am a little interested in the extensive inbreeding -> it strengthened our ties in the past but now inbreeding does more harm than good. Isn’t there a natural inclination to avoid inbreeding?

  5. I think here we are talking about interbreeding (getting stronger with more genetic diversity) not inbreeding (shrinking the gene pool) LOL I read that wrong the first time too.

    The Y chromosome is dying out?! Really?! Tell me more!

    I also think it is important to look at both mtDNA and Y-DNA to look at overlapping patterns. Just looking at the Y leaves out a huge chunk of your population! Similarly, looking only at mtDNA, it may be harder to see some specific patterns.

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