Ancient mtDNA from a Megalithic tomb near Atapuerca

A new study has produced 22 mitochondrial sequences from a Megalithic tomb (dolmen) in Alto del Reinoso, some 7 Km NE of Atapuerca.

Kurt W. Alt, Stephanie Zech et al., A Community in Life and Death: The Late Neolithic Megalithic Tomb at Alto de Reinoso (Burgos, Spain). PLoS ONE 2016. Open access → LINK [doi:10.1371/journal.pone.0146176]

Abstract

The analysis of the human remains from the megalithic tomb at Alto de Reinoso represents the widest integrative study of a Neolithic collective burial in Spain. Combining archaeology, osteology, molecular genetics and stable isotope analysis (87Sr/86Sr, δ15N, δ13C) it provides a wealth of information on the minimum number of individuals, age, sex, body height, pathologies, mitochondrial DNA profiles, kinship relations, mobility, and diet. The grave was in use for approximately one hundred years around 3700 cal BC, thus dating from the Late Neolithic of the Iberian chronology. At the bottom of the collective tomb, six complete and six partial skeletons lay in anatomically correct positions. Above them, further bodies represented a subsequent and different use of the tomb, with almost all of the skeletons exhibiting signs of manipulation such as missing skeletal parts, especially skulls. The megalithic monument comprised at least 47 individuals, including males, females, and subadults, although children aged 0–6 years were underrepresented. The skeletal remains exhibited a moderate number of pathologies, such as degenerative joint diseases, healed fractures, cranial trauma, and a low intensity of caries. The mitochondrial DNA profiles revealed a pattern pointing to a closely related local community with matrilineal kinship patterns. In some cases adjacent individuals in the bottom layer showed familial relationships. According to their strontium isotope ratios, only a few individuals were likely to have spent their early childhood in a different geological environment, whilst the majority of individuals grew up locally. Carbon and nitrogen isotope analysis, which was undertaken to reconstruct the dietary habits, indicated that this was a homogeneous group with egalitarian access to food. Cereals and small ruminants were the principal sources of nutrition. These data fit in well with a lifestyle typical of sedentary farming populations in the Spanish Meseta during this period of the Neolithic.

While the nutritional part has some interest, it is ultimately not too conclusive (high protein diet, similar to that of Dordogne Neolithic, high incidence of caries, three individuals who may have been raised outside the "closely knit" community), so I will focus my attention on the mitochondrial lineages.

These are:

• U5b - 2
• U5b2b3 - 4
• U5b3 - 1*
• U4 - 1*
• V - 2
• K - 4
• K1a - 1
• K1a1 - 3
• X - 3
• T2b - 3
• T2a1b - 1*
• H3 - 1
• J - 1

The three individuals marked with an asterisk (*) might have been raised in other villages, two of them are adult men and the third one a teen-ager of unknown gender. While the possible immigration of men could suggest matrilocality, the reality is that most individuals buried whose gender could be discerned are men anyhow, and all the rest seem to share the same kind of diet (i.e. probably raised in the Alto del Reinoso community), so most unclear.

Notice also that, because of the limitations in haplogroup identification in ancient DNA, apparent "upstream" lineages such as U5b or K can perfectly be the same as "downstream" ones like U5b2b3 or K1a1 respectively. We just do not know for sure.

My preliminary diagnostic was that it looks a quite typical "mixed Neolithic" pool, much like the one of El Portalón in nearby Atapuerca, with dominance of "Neolithic" lineages (K, X, T, J - maybe also V and H3, not yet detected in Western hunter-gatherers) but also a high frequency of "Paleolithic" ones of the U typology (U5 and U4). It is almost absolutely "pre-modern", lacking the high frequencies (40-60%) of mtDNA H found today (and also found in the Neolithic of Paternabidea and Gurgy, in Navarre and Burgundy respectively) and therefore having great excess of both "Neolithic" and U frequencies.

It must be said that Northern Burgos is even today quite low in mtDNA H, with only 33% of this widespread lineage (half of it H3), being also unusually high (30%) in haplogroup U frequencies (ref.), but in any case the mtDNA pool is at the very least not standard for the wider geography and must have experienced some changes in the meantime therefore.

For that reason I considered comparing with the other nearby sites within this small Atapuerca-La Brújula mountain gates area that divides the Duero from the Ebro basins. The result (in percentages) is as follows:

Site	date (BCE)	n	U5	U4	H*	H1	H3	V	J	T2	X	K	U3
Reinoso	~3700	22	27	5	-	-	5	9	5	18	14	18	-
Portalón	~3000	7	29	-	-	-	29	-	14	-	14	14	-
Mirador	~2500	20	-	-	5	20	-	-	10	20	20	20	5

[Ref. links for El Portalón and El Mirador (both in Atapuerca)].


The three sites provide jointly a most interesting sequence for the district. I would say that Reinoso and Portalón seem quite similar, especially considering that the latter sample is very small, allowing for some random fluctuations (decrease in V and T2, increase in H3 and J). However in El Mirador, some 500 years later, we just cannot ignore that there are notable changes.

While the main "Neolithic" lineages (J, T, X, K) remain pretty much the same, all the rest is completely different: U5 (and its faithful sidekick U4) has vanished, has happens with H3 and V, instead we see a sudden outburst of H1 (and H*) and also the less impressive appearance of U3.

What does this mean? Let's go back to the modern mtDNA pool in Northern Burgos Province (n=24) as per Behar 2012:

• H1: 2 (8%)
• H3: 4 (17%)
• U: 8 (33%)
• K: 2 (8%)
• T: 2 (8%)
• J: 2 (8%)
• Singletons: H*, H4, V, L2

It seems to me that those ancient genetic pools are still very present: although there are greater frequencies of H and U than the average produced by merely admixing Reinoso and Mirador, the underlying typology that we can discern (within H most clearly) seems to conform to what those ancient populations already had. Instead the "Neolithic" lineages are less common.

Why?

My guess is that these ancient sites lay all in a key passage of a most strategic route, Spain's National Road 1, St. James' Way, Roman road Ab Asturica Burdigalam and surely much older trade and cultural routes went through that series of mountain passes. This may have attracted more immigrants from the Neolithic settler populations from the Mediterranean, who may have been surrounded by others of more "aboriginal" roots (mostly Paleolithic ascendancy) and rather low visibility for archaeologists.

It remains to be confirmed if Bell Beaker (associated with El Mirador site) may be directly blamed for the introduction of mtDNA H1 in the district. It is indeed possible but in any case the long term impact was limited.

It must also be understood that all this has very little to no relationship with what may have happened in the Basque Country, in spite of being not far away: the genetics involved, both ancient and modern, are quite different. It may be surprising how much the genetic pool can vary in just 200 Km but the rugged geography and diverse ecology seem to favor this kind of sharp distinctions. And, regardless of the causes and the surprise, it may cause, the data is there and is very clear.