Monday, December 28, 2015

The Cassidy Earthquake: Neolithic and Bronze Age migration to Ireland and establishment of the insular Atlantic genome

Lara Cassidy et al. just put out a paper that injects a bit of welcome science into the world of R1b fantasy theories.  Those theories, of marauding bands of R1b warriors, are popular on online messenger boards.  (One prominent board even maintains that most of Western Europe -- millions and millions of men -- are R1b because they are descended from royalty).

Here are the findings from this recent paper:

1.  The very derived downstream clades of R1b like R1b1a2a1a2c were well-established in Ireland by 3750 before the present.  There is no evidence the ancient specimens in the paper were the first generation in Ireland, so it is likely they were present by 2000 BCE.

2.  The population of the Central European migrants to Ireland, who were herders, and had some Steppe-derived ancestry, were MUCH higher, compared to hunter gatherers.  In other words, R1b is so common in Ireland because of massive migration of such people.

3.  This is emphatically NOT consistent with pioneer colonization and elite dominance.

4.  The current distributions in many parts of Western Europe are due to a LACK of invasions since (no Anglo-Saxon or Roman penetration.) In other words, this was a second but more prounounced founder effect of sorts.

5.  This is consistent with comparisons to more centrally located, easy to reach locales, like Italy, where the genomes show greater variability in both autosomes and Y DNA, due to introgressions that occurred after the late Neolithic and early Bronze Age migrations.  (Cavalli-Sforza's admonishment to understand the difference between an expansion and an "impansion" come to mind.)

6. In Western Europe, Bell Beaker culture is the most likely candidate for the spread of R1b and related autosomal genes.

7.  R1b and this Western European expansion is strongly scientifically correlated to lactose persistence, which likely provided the demographic advantage to propagate in larger numbers in places like Hibernia.

8.  As an addendum, the megaliths of Western Europe are indeed likely linked to early cardial cultures, who bore of mix of HG and farming genes, which correlate to I-M26 in Ireland and Sardinia.



Wednesday, December 23, 2015

The Spread of Haplogroups in Europe, Especially R1b

This post is intended to be a general foray into what I call "The Two -Ics" that explain modern haplogroup distributions: demographics and mathematics.  IMO, both are poorly understood.

It's been said, "to be an R1b Fantasist, you have to believe that I2-M26 came to predominate Sardinia by chance (e.g., Founder Effect and Drift) -- but that R1b came to predominate other locales (e.g., Ireland or Spain) by merit (e.g., military superiority or sexual selection)." 

It's also been said, "to be an R1b Fantasist, you have to now believe that R1b marks the spread of the first pastoralists, equestrians, and herders, and that you're now 100% correct that is right -- when just 2-3 years ago, you were 100% that Hg G2 was the mark of the first pastoralists and herders."

With respect to the first saying, I believe that most of the R1b apologists understand the former concepts (of chance as they apply to archaeogenetics), so this post is designed to build upon that knowledge, and add some demographics and mathematics too.

With respect to the second saying, I believe what is most key in a discipline like archaeogenetics is to recognize that theories and findings change from year to year, but the underpinnings of solid scientific method do not.

Let's get into it:

First, it is crucial to outline the possible outcomes.  Every generation, every clade and subclade of every Haplogroup has three "options" (or three outcomes).  Those are:

1.  Mutate (i.e., become something else)
2.  Propagate -- and, in more or less the same form, by having a male child who survives
3.  Die out, by having only daughters, or by having male children who fail to themselves breed

The "stakes" were more pronounced during prehistory than today, because the population sizes were so profoundly lower.  If you don't grasp this and accept it as fact, you can't grasp what I will detail later.

Population of Europe Over Selected Times  
(YBP = Years Before Present)

~50,000 YBP: No more than 10,000 (Neandertals)

~38,000 YBP - 19,000 YBP: No more than 37,000, likely population just 5,000

~12,000 YBP: About 28,000

~2000 YBP: About 35,000,000

-0 YBP: About 743,000,000
You can read more here.

In essence, you must remember that the population of Europe at the beginning of the time we are discussing (the post-glacial-maximum recolonization through the Bronze Age) was about 28,000 and peaked at maybe 100,000.  This is hard for the modern mind to comprehend, I know.  There were less people from Spain to Ukraine then, than there are in one city block in London now.

There are two takeaways:
1.  This made the population more susceptible to chance events, like a plague outbreak, or a famine in an area.

2.  This made the population more susceptible to massive dilution, when population started on its massive upward trajectory, after people started drinking milk, wine, and beer, when they started making cheese, when they started farming cereals and living in one spot, and when they started herding animals and having meat at will.

Going back to our three outcomes for Y Haplogroups, every generation: the first "takeaway" above should inform several likely mechanisms of how R1b spread over time.  If they entered a territory and had different disease resistance, it could have meant that large numbers of a tiny starting population would die off. 

Similarly, because the initial population was so small, when larger populations migrated for whatever reason, indeed possibly even as refugees from other regions, the other haplogroups would seem to have shrunk in size, whereas it really is different population sizes.

All this is just build up.  Our main focus, however, is the simple application of mathematics to Outcome 3 above.

This is what you need to know before we start:

1.  Hunter/gatherer women space babies on average 4.5 years apart, whereas farmers and moderns space them 1.5 years apart.

2.  The average paleolithic woman would have about 3.8 children.

3.  Infant mortality among hunter/gatherers is 30 times higher than among "civilized", and reached approximately 25% at many points during history.

4.  If the average hunter/gatherer family consisted of 3 children to live to adulthood, the odds of each family having just female kids survive was 12.5% each generation.  (.5 x .5 x .5)

Now just these numbers by themselves (HGs having fewer kids than farmers or pastoralists) explain a LOT. 

But the main point is thus: "older" non-mutated Y-chromosome haplogroups are found in lesser numbers simply because they are...older...

Every generation that a Hg exists and doesn't change, there is a 12.5% chance that those bearing it, in any one family, will not pass it along.  To be very clear: if a Hg does not mutate into something else -- or does not die entirely -- its numbers and distribution will decrease over time.  This applies to all except the most recent arrival, which is currently breeding like rabbits.  For example:

Many people believe that C1a was the first Y Hg in Europe.  There were probably just 5000-15,000 of them at any time.  By definition, the Hg C1a are folks that did not go on to mutate into any of the downstream clades.  Over time, the odds will catch up.

Many people believe that I2 was the next Y Hg in Europe.  There were probably just 10,000 - 50,000 of them at any time.  By definition, these are members of the IJ branch, and not members of F or K who mutated.  Over time, the odds will catch up.

These very simple concepts explain much of the modern distribution of haplogroups in Europe.  Is it more complex?  Sure.  Were there other factors?  Absolutely.  But over time, you cannot escape mathematics and demography being the biggest factors.

Tuesday, December 15, 2015

A Review of All Theories, on Why R1b Is So Common in Western Europeans

The great Roman historian Tacitus begins the Germania by discussing how the Germans are separated from certain peoples by mountains, and separated from other peoples by rivers -- and where there were no rivers or mountains, the peoples were separated by Fear.

A similar, intangible concept applies today, to understanding why R1b is so common in Western Europe.  Some of it can be gleaned by archaeology, some of it can be gleaned by DNA -- and where archaeology and DNA cannot provide an answer, we must resort to what makes us human: Logic.

Below is a review of all of possible models explaining why R1b and its subclades are common throughout Western Europe.  After reading it, you decide which is the most logical.

1.  The Bronze-Age Badasses.

The theory goes:R1b males were an awesome military force, who swept through Europe and killed the overwhelming majority of other males in their path.  They started in modern Ukraine as bad-ass horsemen.  But by the time they got to the coasts, they turned into bad-ass sailors and navigators.  These horsemen built boats, and Ireland and England were next to be mowed down by their genocidal awesomeness.  Despite traveling the length of Europe, they were still pure R1b by the time they reached Ireland.  Sufficiently that some counties in Ireland are 80-98% R1b today.  This R1b Empire was the largest that Europe ever knew.  Not even Caesar's stretched from Ireland to Ukraine!  Even though there was plenty of open space in Europe (the population being less than 1/1000th of what it is today), they decided to conquer an empire this vast expanse and risk the lives of themselves and their children, just because they were such badasses.  They were such efficient killers they left no trace in archaeological records in Western Europe of destruction or razing.  Despite well-established standards for evolution of language, the empire spoke vastly different languages (i.e. Latin and Ukrainian), despite this all happening just 1000 years or so before the beginnings of Rome and Greece.

Believe it or not, this theory is favored by some people today, who just happen to be R1b males.

2.  The Irresistible Indo-Europeans  

This theory goes: R1b males had uniformly gorgeous looks, tremendous wealth, and all-star qualities that made all hunter-gatherer women swoon with delight.  Whether they had bright red hair, or looked like James Buchanan, cavewomen of all groups throughout Europe dropped their guys and decided to procreate with these R1b studs.  None of the local guys resisted.  They too were enamored by the R1b good looks, and some kind of genetic superiority that made them and their genes irresistible.  

Believe it or not, this theory is favored by some people today.  No, really.  They actually posted it in comments below.  And they just happen to be R1b males.

3.  Colonizing Conquistadores

This theory is a variation of theory 1, minus the genocide.  The theory is: just model the R1b spread after that of the Spanish conquest of the New World.  Nevermind that the Spanish had guns, germs, and steel.  Nevermind that they had cannon, smallpox, and boats that could traverse oceans.  Nevermind that in most places in Latin America, the native haplogroups like Q and C still dominate.  Just ignore these things and model R1b after the Spanish.

4.  Lactase Persistence

At last we enter the realm of the plausible. 

This theory goes as follows: very basal subclades of R1b were present throughout Europe in tiny pockets for a very long time.  

This is why a slightly more downstream clade of R1b*, ancestral to modern lineages, was found, already in Els Trocs Spain, 7000 years ago.  

I mean think about it.  He couldn't have flown there.  And in 5100 BC, he couldn't have even ridden a horse.  

We know that R1 originated in Eurasia, and that it was present on both ends of Europe by 5100 BC.

If you adopt regular migration theories for on-foot migrations, these very basal R1b people in Spain were likely present in small pockets throughout Europe by 6000 BC.  Perhaps in the modern Czech Republic, perhaps in France, perhaps in modern Germany.  We only have ~400 aDNA samples from this epoch, and a smaller percentage of them have been tested for Y DNA.

Perhaps they lived in a moist climate less likely to preserve remains.  Perhaps they cremated their dead.  Perhaps archaeologists ignore their tiny region.  But one thing is certain:  Basal R1b was present in Europe, end to end, by about 6000 BC.

At some point, during a period of profound starvation, Western Europeans evolved a tremendous caloric advantage: the ability to digest milk.  No more killing the cow to eat and therefore live: you can live off of turning grass into protein.

Let's assume the first humans to evolve this, living in some nameless, forgotten pocket of Germany or England or France or Spain were majority R1b, then their population would EXPLODE.  In a time of mass starvation and famine, those with a caloric advantage would propagate exponentially.  

(Perhaps these people come to worship the cow to some degree, creating taboos to killing it, as we see in modern lands, creating idols of bulls, as we saw in many ancient cultures, and creating elaborate drinking vessels in the shape of Bell Beakers -- but I digress...)

The population of Europe at this time was maybe a million people across the whole continent.  If you figure that R1b people had a greater fertility rate (more kids per female, less time to wean because of cowmil availability, less time between kids, healthier kids, more kids reaching adulthood to propagate), then the simple math of exponential demography will show that within as few as 200 years, your uniparental markers will dominate the landscape.

It should be noted that the various genes for lactase persistence mirror closely the distribution of R1b-S21 even today.

5.  Refugees and Different Cultural Attitudes

If you know a little about history or current events, this one is not hard to imagine.  The historical example is the Goths; the modern example is what is happening in Lebanon with Syrian refugees.

People used to think the Goths were bad-ass, uncivilized, warlike, mighty (insert "supreme" adjective here) Germanic overlords who conquered much of the Roman world. But anyone who knows the history understands that the truth is a little kinder to them (kinder, depending on if you believe being peaceful and not purposefully killing people is a good thing).

The Goths were not some mighty tribe hell-bent on destruction, who willfully took over the Roman Empire. Just the opposite: they started out from modern South Sweden because of FAMINE. They were so weak, they were forced to WANDER for centuries. Finally, they invaded the Roman Empire, because the Huns EVICTED them from their steppe lands in modern Ukraine.

In other words, one of the baddest-ass people in most people's minds were refugees, forced to emigrate not because they wanted to conquer, but because they themselves had been evicted from their homelands by famine (first) and then another people (the Huns).

If that is too hard on you, let's imagine something happening today. The population of Lebanon is about 2 million people. Aside from the districts controlled by terrible people, many of the coastal folks are pretty wealthy, modern, and diverse. They don't have extraordinarily high birthrates.

All hell has broken loose near them, in a country you may have heard a lot of recently. It's called Syria. In the last two years, Lebanon...has been swamped with 2 million Syrian refugees.

In other words, the population of the country has doubled, in a generation, from an influx of refugees.

Now imagine the Lebanese bear Haplogroup L, we will call it. Imagine like many wealthier people today, they're not having 20 kids each. More like 1 or 2.

Imagine the Syrian refugees bear Haplogroup S, we will call it. Imagine like many poorer people today, they DO have many kids...

The "old" samples within this area we call Lebanon will all be Haplogroup L. A future archaeologist would find that to be the case.

The "new" samples, after a few generations, will be like 75-25%, with Haplogroup S clearly "winning out." The cause is a mix of migration -- plus different cultural attitudes toward having kids.

Did the Syrian refugees "conquer" the Lebanese?  (No.)

Is it safe to say that the Syrian refugees genes were "selected for?" (No).

That the Syrian men were "more attractive" to women? (No).

That they bore some kind of genetic advantage, that made them fitter? (Again, no.) 

6.  Different Starting Population Sizes, Different In Time

This one is the hardest to fathom almost, because it is almost circular.  It states simply that R1b is the most numerous in Western Europe because they started out more populous, and were the most recent immigrants.  

Western Europe is a cul-de-sac for overland migrations.  Almost all haplogroups originated in Africa or the Near East, but came into Western Europe via the eastern entry points into Europe.  Iberia is the end of the cul-de-sac.

Imagine a 100-acre parcel. At first, it is a hunting preserve of sorts. It is inhabited by 5 families who own 20 acres each. They love the deer and geese they harvest from said land.

Next some farmers move in. 50 acres are used for farming. They support 10 farming families, who each have 5 acres.

The land is supporting 5 hunters and 10 farmers. (Have the farmers been "selected for?" No.  They are more numerous and more recent migrants).

Finally, some others land in the area. There are 100 refugee families or maybe just people who tolerate living close to one another, so they squeeze into one acre of the land. They have metals, which they trade for food, so they are able to live in a much smaller parcel.

Have they been selected for?  Again, no.

I just described something that has happened in recorded history several times, and surely in prehistory too.

Older, less numerous populations will appear to be "drowned out," unless you are careful. It's just simple math.  Those who have been in a locale the longest will be diluted over time.  

7. Disease

Many plagues in Western Europe entered through the east.  Since R1b-bearing males were the largest migration from the east, it must be considered that different immune systems played a role in their spread.

In sum: R1b could be simply the most common haplogroup in Western Europe because it came there later, in greater numbers, and perhaps as part of a people who had different cultural attitudes toward having children.

In subsequent posts, I will link or recap demographic studies that show the clear power of exponential growth with even tiny differences in birthrates.


Another Way of Thinking About Ancient Populations (Autosomes versus just Y and mtDNA)

I don't think Neandertals died out at all.
No more so than any population that existed from 600,000 to 25,000 years ago.

If you tested ANY species of Homo that old, of course they wouldn't match us exactly. The genus has evolved.

Neandertal population size was tiny. Imagine that there were 40,000 of them in Europe. (That number is actually large.  Many aDNA experts believe that there were never more than 10,000 Neanderthals alive at any one time!)

Now imagine that 1 million modern humans come into Europe, during various phases.
You mix them together, and you get the 4% of Neandertal autosomes in our populations.

You also get drastically smaller odds that their sex-linked DNA survives over time.

Never, ever forget population size.

This "study" has been replicated in modern times.

Imagine 100 men are marooned on an island. 4 have the surname "Rarityrareness."

After generations, it is likely that the sons of those 4 will have a generation or two that produces only daughters. In fact, it's almost certain.

So the odds are that there will be no Y chromosomes of the Rarityrareness males.

But did they survive? Yes. Their descendants through their daughters are very much alive in the population.  And like Neandertals, large percentages of their genome would survive autosomally, perhaps as high as 80%!

Never ever forget initial and comparable population size. It explains just about every ratio of the newer versus older Y Chromosomes in Europe. 

It explains the lack of Neandertal sex-linked DNA, and it explains the smaller number of the Old Europe haplogroups from small hunter-gatherer populations.

Sunday, December 13, 2015

A Proposal for a New Lexicon for Ancient DNA "Components" Like WHG, EHG, EEF, ANE, and CHG

Some of us a few years back started to decry the ever-ongoing ISOGG renaming process, which coupled with the discovery of new subclades, meant that one year, someone might be deemed R1b1b1a2bab2ba11babd12ba2b1c, and the next year R1b1b2bab2f1faf1fafaf1f1f1a. 

People started saying that it would probably be better to say the first couple letters and the major terminal SNP. For example, R1b-U106 or I2-M26. This was logical and goodUnlike the terminology, the SNPs never change. And they're shorter to write.
Here I humbly propose a new terminology for ancient autosomal samples. I think picking terms like, "WHG" was a mistake, and now that I read about EHG and CHG, I really think so. For the uninitiated, these acronyms stand for "Western Hunter Gatherer," "Caucasus Hunter Gatherer," etc.

People compare their modern genomes, or the genomes of modern populations or ethnic groups, to these ancient samples. And then they use the shorthand, like, "Scottish average 19% CHG." This is highly misleading.

Let me give the reasons why I think it is deficient, and tell me if you disagree.

1. As we get more samples over time, it will be hard to keep renaming the different samples, if they form a different component. We just saw this with the recent CHG finds. Imagine if we find a detectable signal of ancient genes from Iberia. What will we call that component? "Really Western Hunter Gatherer?"

2. The shorthand is deeply misleading (i.e., "Scottish are 19% CHG.") This to me is the most important point. Most people reading this are experts. But I see on so many other boards people who seem to think that some scientist somewhere took a survey of a bunch of ancient samples, "averaged" it, and that we are comparing populations to populations.

We're not. We are not comparing Scots to Western Hunter Gatherers. We are comparing Scots (or any other modern individual or group) to ONE SAMPLE. For WHG, it's Loschbour. For EEF, it's Stuttgart. For ANE, it's Mal'ta. Etc.

3. We don't know that that one sample will turn out to be representative of "Western Hunter Gatherers" any more than we know that taking Danny Devito or the harlequin model Fabio is a representative of a modern Italian. Indeed, as the number of samples we get grows, we know the situation is infinitely more complex.

We all remember, for example, when the first farmers sampled had very unique mtDNA. For a while, people tried to read too much into it. "OMG, what if all farmers bore this odd mtDNA?" was the refrain. But it turned out to be a one-off. This can and will happen again and again as we get more samples over time.

4. The acronyms will get repetitive real fast. We are talking about aDNA, remember? Before farming, the whole world were hunter gatherers. So, many (most) aDNA samples will eventually have -HG after them, if we follow the current convention.

I imagine a world where we have found 26 slightly different hunter gatherer samples, and thus we have one different -HG for every letter in the alphabet! That'd be just silly.


For these reasons, but primarily numbers 2 and 3, I think the current practice is misleading and doomed to failure. Europe is a very complicated place. We will find ancient samples with very unique genomes, which are detectable in modern populations. They will all be slightly different from one another, because one sample is, well, one sample... It is highly misleading to say that "John Smith..." or "Estonians are more Western Hunter Gatherer than..." because we have not sampled all, most, or even many Western Hunter Gatherers. (I don't mean to pick on WHG. This applies equally, indeed MORE, with EEF and ANE!)

So, what is the solution?

I think if we purport to be scientific, we need to speak with scientific precision.

If an individual or a modern population bears resemblance to an ancient genome, we should state that it has a percentage similarity to that one sample. And not try to make it more than it is, like the very official and extensive term like, "Eastern Hunter Gatherers."

As for the sample, we should also include the year discovered, the situs of the discovery, and the years Before Present (BP). 

Remember, many of these sites are caves where there have been and will be more discoveries. In other words, I expect there will be many more Loschbours, more Stuttgarts, etc., and it will get quite confusing unless we speak with specificity about when something was discovered and when in time it came from.
Let's avoid a situation like we had with terms like R1b1b1b1a2a1b2bc3d, which lose meaning. Let's refer to things with scientific precision.


Instead of, "Scots are 19% Ancient North Eurasian."

SAY: "On average 19% of the genes of the modern Scottish population match 2013Mal'ta-24,000BP."

Instead of, "Southern European populations have a lot more CHG blood than I expected."

SAY: "Southern European populations bear many genes matching 2015Kotias-10,000BP."

Instead of, "Sardinians are 45% WHG."

SAY: "Approximately 45% of the genes in the modern Sardinian population resemble 2013Loschbour-6000BP."

This convention is much more accurate.

Saturday, December 12, 2015

How Little We Know About Ancient DNA - Part II

Earlier this year, I posted a series of maps graphically depicting the (at that time) exhaustive list of Ancient DNA finds, mapped out for both time and space.

The post, while now a bit dated due to additional finds, is still worth examining.  When reading it, it should becomes very apparent to you, the concept in the title of this post: How Little We Know About Ancient DNA.

As you can see from the maps, it appears that people bearing certain Y Chromosome haplogroups "flew" across time and space.  And that certain parts of Europe had nobody in them until someone flew across the distances.  

Of course, this is impossible.  It simply reflects the fact that we continue to have immense gaps in skeletal finds and in our knowledge.

Most importantly, it shows that today's conventional wisdom, popular at echo chambers like the Anthrogenica boards, will certainly change tomorrow -- revealing the adherents to such theories to be akin to arrogant fraudsters, peddling certainty where none is scientifically warranted.

Recently, a poster at Anthrogenica, with the handle Tomenable, posted an excellent visualization of the same gaps in knowledge that I referenced.

You can view a list of aDNA finds here, in chart form, courtesy of Tomenable.

And even better, here is a chart, in chronological order, showing the same.

What does the chart show?   By applying *scientific* methods of taking things to their logical conclusions, and reducing our knowledge to a set of provable statements, you can easily see how little we know.

For starters, based on these chrono maps, it looks like Haplogroup C1 made it to Europe after I2.  Yet almost everyone, from the scientific community to the "citizen" scientists (their term) on Anthrogenica, agree that C1 is a rare, likely "Cro-Magnon" marker, that represents the very first humans out of Africa and into Europe.  On the other hand, it has been accepted for over a decade that Haplogroup I, notably I2, represents the second (or third) wave of the population of Europe, associated with the Gravettian dispersions.  

But again, this is *not* "what the aDNA shows."

Ask the wiseguys on Anthrogenica, or search their posts.  They express with certainty that C1 came first, followed by I2.  And it probably is true.  But it is NOT born out by the aDNA evidence.  (Yet).

However, the same group of people turn to the aDNA evidence (blindly) to express 100% confidence in other theories, for example, everything from the notion that R1b xV88 couldn't be found west of modern Poland until the Indo-European expansions.  (I find this notion laughable.)  

They also rely on the aDNA evidence to express 100% confidence in wild notions of sex selection that have more in common with dimestore novels than anything scientific.  The proponents of said theories also happen to be mostly males bearing R1b.  Yes folks, in a world where racial identity is taboo, any sense of ingroup-outgroup dynamics for Western Europeans has simply been transferred to tiny markers on one chromosome.

In other words, many of these folks blindly turn to our meager aDNA evidence to justify their pet theory du jour, but choose to ignore what the aDNA evidence shows, when convenient.  

They cite gaps in data (i.e., a lack of samples) as evidence for proving a negative, as if that was possible -- when they want to.

But they ignore the lack of samples when convenient, if it doesn't fit in their narrative for that time or place.  

Doubt them, they revert to the argument "well, the aDNA shows..." but they are more than willing to fill in gaps in aDNA when convenient.

It's already been a rough year for the arrogantly certain in Ancient DNA.  Notably, past theories on the dispersion of Haplogroup J2 have fallen by the wayside.  Theories posted on Anthrogenica just a couple months ago, and accepted by the echo chamber as gospel fact, have been called into serious doubt by recent academic papers.  

I've also posted repeatedly on how difference in culture and hyperlocal topography can affect what aDNA survives into modern times.  The easiest example is one tribe burying their dead, while another tribe cremates it.  Anyone who knows anything about written history understands that the reason why we don't have m(any) ethnic Roman skeletons is because they cremated their dead.  To those who don't grasp this concept, it would be as if the Romans, a powerful, numerous, colonizing, widespread, important society -- didn't exist.  

I can just see Anthrogenica in the year 2515: "but there are no Roman samples in aDNA," they would maintain adamantly.  Yes, you would reply.  But the Romans existed.

The point is simple: approach any theories explaining what happened before written history with caution.  There are major gaps in the record, and it is far, far too early to approach things with the smug certainty one sees on these boards.  

Look at the samples across time and space (geography).  Don't hide behind relative, subjective terms like "Mesolithic" and "Neolithic."  Instead, look at how Europe was populated, the way it was populated --- in gradations, over (real) time.  You, too, will notice "How Little We Know (Still) About aDNA."