When G2a Neolithic farmers started advancing from the Near East into Europe, they encountered indigenous hunter-gathering tribes belonging to various haplogroups (C1a2, F, I*, I1, I2a, I2b, I2c, and possibly even H). Interestingly, most of these lineages didn't survive in significant number today, except I1 and I2a. I1 had a late start and only prospered in the North European Plain and Scandinavia from the Chalcolithic onwards.
The great winner during the Neolithic period was haplogroup I2a, which consistently shows up alongside G2a in most Neolithic sites tested to date (Starčevo, Körös, Lengyel, LBK, Cardium Pottery, Megalithic), and seem to increase in frequency over time and as one moves towards Northwest Europe. Based on the few samples available it appears to have been particularly common in the Megalithic culture. All four Megalithic Y-DNA samples (from France and Spain) belonged to I2a1 or I2a2.
Nowadays I2a is more common than G2a everywhere in Europe except in insular Greece and Cyprus, peninsular Italy and Sicily (but not Sardinia), Tyrol, south-eastern France (Auvergne, Provence), and about half of Iberia (Portugal, Asturias, Cantabria, Castile, Murcia). Everywhere else I2a dominates, even in regions with high Neolithic ancestry and Sardinia and Corsica (which have respectively 3x and 2x more I2a1a1 than G2a).
Therefore, although I2a was just one of many Mesolithic hunter-gatherers' lineages in Europe when agriculturists arrived, it is the only one that readily embraced the new lifestyle and managed to supersede the original farmers in number. I2a's destiny was not only linked to its ability to chum with G2a, but we could say that G2a farmers catalysed I2a's success. I2a people integrated G2a tribes, learned the new Neolithic techniques from them and became so good at them that over time the student overtook the master.
What is amazing too is the symbiosis that operated between I2a and G2a throughout the Neolithic period. It's hard to find one without the other. Their destiny was intertwined as soon as the two met in the Balkans. That would explain also why Italy and Greek islands have so much more G2a than I2a. If G2a spread directly from Anatolia by boat to the Greek islands and then over to Italy, I2a wouldn't have been part of the migration. On the other hand, I2a probably re-expanded many times from the Danube region to continental Greece since the Neolithic so that the whole Balkans became somewhat homogenised. As for Sardinia, it appears to have been colonised from the Iberia peninsula via the Balearic Island, following the west to east currents in this part of the Mediterranean. If the winds were the same in the Neolithic as they are today it would have been impossible to sail directly from Italy to Sardinia, except perhaps from Liguria via Corsica (but Neolithic farmers didn't have maps or compasses and Liguria wasn't the naval hub it later became under the auspices of the Genoan Republic).
This symbiotic pairing between G2a and I2a is not unique in prehistory. I can think of several other cases in which a small group of men belonging to haplogroup brought a more advanced technology or lifestyle to another region, which were enthusiastically adopted by local men, who eventually spread it well beyond their ancestral borders.
This is what happened with R1b and R1a in the Early Bronze Age. The R1b cattle pastoralists from the Yamna culture expanded north into the forest-steppe of R1a tribes, who quickly adopted their lifestyle, technologies (bronze working, wagons) and apparently also their Indo-European language, and spread them to the Baltic region, Siberia, Central Asia, South Asia and the Middle East. An R1b minority always accompanied these great R1a migrations, but the further they went and the higher the proportion of R1a increased against R1b (from 7:3 in Corded Ware to perhaps 30:1 in India).
What is fascinating is that this adoption and lifestyle and technologies doesn't always happen, and actually is the exception rather than the rule. Only R1a tribes adopted the ways of R1b Yamna people. The cultures of Neolithic Europe collapsed when R1b tribes moved into their regions, and eventually R1b came to replace over half of the indigenous lineages in central and western Europe, and in some regions up to 80% of all lineages.
One of the few native Mesolithic/Neolithic paternal lineages that seems to have benefited from the Indo-European migrations is I1. Now the dominant lineage in Scandinavia, it was apparently absent from Scandinavia during both the Mesolithic and Neolithic periods, and fairly rare in Neolithic Central Europe. It first appears in Scandinavia during the Corded Ware period. In all likelihood I1 picked up around Poland or North Germany by the invading Corded Ware people, and I1 was among the few indigenous lineages who managed to adapt to the new lifestyle and make the most of it. It probably took only one man, as all modern I1 people descend from a common ancestor who lived approximately 4700 years ago, at the beginning of the Corded Ware period. I1 had existed for thousands of years before that, but all other side lineages are now extinct.
Another fascinating case is the pairing of Proto-Semitic haplogroup E-M34 and indigenous Southwest Asian haplogroup J1-P58. E-M34 is thought to have arrived from Northeast Africa to the Levant around 3750 BCE, during the Copper Age. The local Levantine lineages would have comprised haplogroups G2, J1, J2 and T. The proportions are unknown at present, but what seems clear is that J1 later became the dominant paternal lineage of Semitic people, even though Afro-Asiatic languages originated with haplogroup E1b1b, and Proto-Semitic with E-M34.
In South Levantine countries like Lebanon and Jordan, E-M34 and J1-P58 are found in equal proportions (around 20% each in Lebanon and 30% each in Jordan). But everywhere else in the Middle East J1-P58 is 3 to 6 times more common than E-M34. This could obviously be a result of a strong founder effect during the Muslim expansion from Saudi Arabia, which could have spread mostly J1 lineages. But even Saudi Arabia has 4.5 times more J1 than E-M34 to start with, so J1 must already have superseded E-M34 by the Bronze Age, when Semitic speakers moved from Levant to Saudi Arabia.
The great winner during the Neolithic period was haplogroup I2a, which consistently shows up alongside G2a in most Neolithic sites tested to date (Starčevo, Körös, Lengyel, LBK, Cardium Pottery, Megalithic), and seem to increase in frequency over time and as one moves towards Northwest Europe. Based on the few samples available it appears to have been particularly common in the Megalithic culture. All four Megalithic Y-DNA samples (from France and Spain) belonged to I2a1 or I2a2.
Nowadays I2a is more common than G2a everywhere in Europe except in insular Greece and Cyprus, peninsular Italy and Sicily (but not Sardinia), Tyrol, south-eastern France (Auvergne, Provence), and about half of Iberia (Portugal, Asturias, Cantabria, Castile, Murcia). Everywhere else I2a dominates, even in regions with high Neolithic ancestry and Sardinia and Corsica (which have respectively 3x and 2x more I2a1a1 than G2a).
Therefore, although I2a was just one of many Mesolithic hunter-gatherers' lineages in Europe when agriculturists arrived, it is the only one that readily embraced the new lifestyle and managed to supersede the original farmers in number. I2a's destiny was not only linked to its ability to chum with G2a, but we could say that G2a farmers catalysed I2a's success. I2a people integrated G2a tribes, learned the new Neolithic techniques from them and became so good at them that over time the student overtook the master.
What is amazing too is the symbiosis that operated between I2a and G2a throughout the Neolithic period. It's hard to find one without the other. Their destiny was intertwined as soon as the two met in the Balkans. That would explain also why Italy and Greek islands have so much more G2a than I2a. If G2a spread directly from Anatolia by boat to the Greek islands and then over to Italy, I2a wouldn't have been part of the migration. On the other hand, I2a probably re-expanded many times from the Danube region to continental Greece since the Neolithic so that the whole Balkans became somewhat homogenised. As for Sardinia, it appears to have been colonised from the Iberia peninsula via the Balearic Island, following the west to east currents in this part of the Mediterranean. If the winds were the same in the Neolithic as they are today it would have been impossible to sail directly from Italy to Sardinia, except perhaps from Liguria via Corsica (but Neolithic farmers didn't have maps or compasses and Liguria wasn't the naval hub it later became under the auspices of the Genoan Republic).
This symbiotic pairing between G2a and I2a is not unique in prehistory. I can think of several other cases in which a small group of men belonging to haplogroup brought a more advanced technology or lifestyle to another region, which were enthusiastically adopted by local men, who eventually spread it well beyond their ancestral borders.
This is what happened with R1b and R1a in the Early Bronze Age. The R1b cattle pastoralists from the Yamna culture expanded north into the forest-steppe of R1a tribes, who quickly adopted their lifestyle, technologies (bronze working, wagons) and apparently also their Indo-European language, and spread them to the Baltic region, Siberia, Central Asia, South Asia and the Middle East. An R1b minority always accompanied these great R1a migrations, but the further they went and the higher the proportion of R1a increased against R1b (from 7:3 in Corded Ware to perhaps 30:1 in India).
What is fascinating is that this adoption and lifestyle and technologies doesn't always happen, and actually is the exception rather than the rule. Only R1a tribes adopted the ways of R1b Yamna people. The cultures of Neolithic Europe collapsed when R1b tribes moved into their regions, and eventually R1b came to replace over half of the indigenous lineages in central and western Europe, and in some regions up to 80% of all lineages.
One of the few native Mesolithic/Neolithic paternal lineages that seems to have benefited from the Indo-European migrations is I1. Now the dominant lineage in Scandinavia, it was apparently absent from Scandinavia during both the Mesolithic and Neolithic periods, and fairly rare in Neolithic Central Europe. It first appears in Scandinavia during the Corded Ware period. In all likelihood I1 picked up around Poland or North Germany by the invading Corded Ware people, and I1 was among the few indigenous lineages who managed to adapt to the new lifestyle and make the most of it. It probably took only one man, as all modern I1 people descend from a common ancestor who lived approximately 4700 years ago, at the beginning of the Corded Ware period. I1 had existed for thousands of years before that, but all other side lineages are now extinct.
Another fascinating case is the pairing of Proto-Semitic haplogroup E-M34 and indigenous Southwest Asian haplogroup J1-P58. E-M34 is thought to have arrived from Northeast Africa to the Levant around 3750 BCE, during the Copper Age. The local Levantine lineages would have comprised haplogroups G2, J1, J2 and T. The proportions are unknown at present, but what seems clear is that J1 later became the dominant paternal lineage of Semitic people, even though Afro-Asiatic languages originated with haplogroup E1b1b, and Proto-Semitic with E-M34.
In South Levantine countries like Lebanon and Jordan, E-M34 and J1-P58 are found in equal proportions (around 20% each in Lebanon and 30% each in Jordan). But everywhere else in the Middle East J1-P58 is 3 to 6 times more common than E-M34. This could obviously be a result of a strong founder effect during the Muslim expansion from Saudi Arabia, which could have spread mostly J1 lineages. But even Saudi Arabia has 4.5 times more J1 than E-M34 to start with, so J1 must already have superseded E-M34 by the Bronze Age, when Semitic speakers moved from Levant to Saudi Arabia.
