Below is a map of confirmed instances of I-M26 found in prehistoric remains. Lots of others could have potentially been added -- ancestral clades, closely related sister clades, and ones where the coverages is insufficient to determine whether it is indeed M26 (or L672 L160 etc.) But we decided to err on the side of being conservative.
Going chronologically, we have M26 in what is now Sweden, at the Motala site, at 7730 BP (Before Present). These were Hunter/Gatherers.
Next, we have it at the La Spina site, in modern Spain, at 5765 BP. This was a Megalithic site, during the Neolithic. Farming was already in Spain at that time.
Next we find it at Treilles, modern France, at 5015 BP. This was a Megalithic/Neolithic site of farmers, near the coast.
Next we find it in the Remedello culture, of Northern Italy, at 4758 BP. This was a Chalcolithic (Copper Age) site. Per Robert S.P. Beekes, "bears
all the marks of an Indo-European invasion: a new style of ceramics, a new burial rite,
changes in the social structure, the introduction of a warrior aristocracy, the intro-
duction of metallurgy, the horse and the chariot. But it is still not possible to assign
language groups to particular culture."
Finally, for now, researchers have found it in the Megalithic culture of central France, near the Dolmen of Villaneuve-Sur-Yonne, 4753 BP, and again, of course, farming was present.
In modern times, we find M26 at 37% in Sardinia, certainly an outlier due to substantial founder effect.
But it is also found at up to 10% in Samnite country in Molise, Italy (and in significant numbers in Cosenza province of northern Calabria). It is found at 5-9% in Spain, including Basque country. At up to 7% on Sicily. And 3% in many areas of England and Ireland, especially places like the Channel Islands. It is still found at 1% in Southern Sweden.
So, given all that we know, what is a sensible theory for M26's distribution and spread?
We've heard them all, and each has merit: that is represents Megalithic Mariners, who went around old Europe converting local populations and building huge monuments like Stonehenge and the Nuraghe. That it represents the spread of Cardial Ware culture, along the western Mediterranean seaboard. That it represents the spread of farming, either as hunter/gatherers who adopted farming quickly, or as a rare clade of Haplogroup I that was predominantly farmers since the dawn of agriculture. Finally, some posit that it represents a caste (or not) of people embedded in other haplogroups in motion, most often listed as G2a or R1b.
All have merit, none are perfect. Let's go through the logical conclusions and form a model.
The presence of M26 amongst the hunter/gatherers of Motala, and its widespread ancient distribution by the dawn of the Neolithic tell us that it is a most ancient indigenous European clade.
We believe that it is safe to say that the first modern humans in Europe, Aurignacians, aka Cro-Magnons, bore haplogroup C. (Although during the Paleolithic, that far back, it is really anyone's guess whether Hg C came in during one the next phases).
M26, along with its brother clades within Haplogroup I2, most likely formed a part of the second wave of European hunter/gatherers, and was presumably present among the Epi-Gravettians and the Magdalenians.
How then does one explain its wide distribution and adaptive nature throughout the continent during the Neolithic?
Perhaps the answer is simple. These were people who have been in Europe for a long time, and are adaptable, and are survivors.
Europe was settled in waves, and not just the three big ones (Hunter/Gatherer, Farmers, Steppe Horsemen), which is an oversimplification.
Imagine Europe as a pipe. It has three entry points: the steppes of Russia from people heading due west, the Balkans/river corridors from people heading northwest, and the Mediterranean from people heading north.
Imagine a party. The "Emtwentysix" family was among the first to arrive. When they got to the house, they represented maybe 15% of the guests. But as more guests arrived, some through the front door, some through the back door, and some through the side door, the Emtwentysix family became a smaller percentage of the total guests, now just 0.5-3%.
When the family arrived, they were playing foosball together. But at some point, some members of the family joined a group of other guests who were dancing, and some joined groups of other guests who were playing video games.
You get the analogy.
None of the above theories may be 100% accurate, and yet all theories may be accurate in their own way.
For example, it is doubtful that I-M26 first showed up in Europe with the Cardium Pottery culture, but it could have been among the groups (as it was clearly in Italy for a long time) that encountered the Cardial peoples, and then became a component of said peoples, heading west along the western Mediterranean seaboard.
Similarly, in Northern Italy, M26 peoples could have been among the first who were Indo-Europeanized, in the Remedello culture, and then part of the secondary expansion into Italy, which formed the Oscan-speaking Sabellic tribes (Samnites, Brutti, and Sicels).
In other places, clearly M26 was Megalithic acculturated, and its odd distribution in places where Megaliths appear is intriguing for sure.
And lastly, some of its distribution could reflect later movements by obsidian traders or something similar.
Prehistory is a series of periods of demographic expansion followed by demographic crisis. Good hunting, good weather, good crops, absence of disease, and other factors make people have more babies. Then luck turns, and some lines die out, while other lines come to the party. Since the Bronze Age in Europe, it has mostly been a demographic march to more population as time goes by.
We would like to see a study comparing the M26 in Sweden, Ireland, Spain, the Italian mainland, and Sardinia, to see who is ancestral to whom, or how and when the different groups separated.
But the bottom line appears to be that M26 has been in Europe a long time, and like all lineages that were there a long time, its distribution will have changed a bit and its absolute numbers will have gone down, but these Most Adaptable Hunter Gatherers (MAHGs) continue to intrigue.