Does the Bronze Age Herald a Major Transformation in Human Dispersal Patterns?

Humans–Homo Sapiens or Anatomically Modern Humans–have been around for about 200,000 years. We have only recently–for the past few thousand years or so–begun making a serious effort at recording human history and figuring out what happened before our own times.

Most of what we know about major migrations and changes among human populations come from three major sources: written records, archaeology, and genetics.

Written records are (usually) the easiest to work with. We know when the Spaniards discovered Cuba because we have written records of the event, for example. Unfortunately, written records go back only a few thousand years–covering a teeny portion of human history–and can be highly unreliable. After all, we thought the entire world was only 6 thousand years old for a while because a book that seemed to say so.

Archaeology lets us peer much further back than written records, but with much less detail. We don’t know a lot, for example, about the folks who made Aurignacian tools–what they called themselves, what sort of rituals they had, what they hoped or dreamed of. Without those details, it’s hard to care much about one culture or another. After a while, pots blend into pots, stone tools into stone tools.

Can you tell which one is Aurignacian, and which is Gravettian?

(Oh, I threw in a Mousterian tool, as well. Those were made by Neanderthals, not H. sapiens.)

I can’t, either.

It is difficult to tell whether a change in artifacts between one layer and the next reflects a change in people or a change in technology. The proliferation of steel artifacts in the archaeological record in Mexico circa 1500 reflects an influx of new people, but the proliferation of television sets in the future-archaeological record of my area merely reflects a technological development. Finding a lot of mass graves in an area is, of course, a tip-off that invasion and replacement happened, but invasions aren’t always accompanied by easily identified mass-internments.

This is where genetics comes in. If we can find some skeletons and sequence their DNA, and then find some later or earlier skeletons in the same area and sequence their DNA, then we can get a pretty good idea of whether or not the later people are descended from the earlier people. This probably doesn’t always work (if the people in question are under some kind of selective pressure–which we all are–then their descendants might look genetically different from their ancestors simply due to evolution rather than replacement,) but it is a pretty darn good tool.

As I discussed back in “Oops, Looks Like it was People, not Pots,” archaeologists have fiercely debated over the decades whether the replacement of Narva Pots with Corded Ware Pots circa 3750 ago represented a population replacement or just a change in pot-making preferences:

    

Corded Ware on the left, Narva on the right.

Luckily for us, genetics has now figured out that the Corded Ware people are actually the Yamnaya, aka the Proto-Indo-Europeans, and that they expanded out of the Eurasian Steppe about 4,000 years ago, replacing much of the native population as they went.

So it’s starting to look like there were quite a few conquering events of this sort.

From, A recent bottleneck of Y chromosome diversity coincides with a global change in culture

from A Handful of Bronze Age Men Could have fathered two-thirds of Europeans

In general, if you see a lot of mtDNA and only a little Y-DNA, that means there were a lot of women around and only a few men. And that generally means those men just killed all of the other men and raped their wives and children.

Which appears to have happened on a massive scale throughout much of the world around 10,000-4,000 years ago.

Just off the top of my head, recent large-scale migrations and at least partial replacements include the arrival of Indians in Australia around 4,230 years ago; replacement of the Thule people by the Inuit (aka Dorset aka Eskimo) around 1,000 ago; successive waves of steppe peoples like the Turks and Mongols invading their neighbors; the Great Bantu Migration that began about 3,500 years ago; the spread of Polynesians through areas formerly controlled by Melanesians starting around 3,000 BC; displacement of the Ainu by the Japanese over the past couple thousand years; etc.

Replacement of the Thule by the Dorset, from The genetic prehistory of the New World Arctic

Paths of the great Bantu Migration

Map is in French. Negative numbers are years BC; positive numbers are years CE.

And of course, we know of many more recent migrations, like the one kicked off by Columbus.

So it looks like people have moved around a lot over the past 10,000 years.

Terms like “bronze age” are a little problematic because people adopted different technologies at different times. So the “bronze age” began around 5,300 years ago in the Middle East, 4,000 years ago in Ireland, and skipped the Inuit entirely (they basically went straight from stone and bone tools to guns.)

Agriculture emerged in the Middle East circa 11,500 years ago; followed by the wheel, 8,500 years ago; carts, 6,500 years ago; and domesticated horses about 6,000 years ago. These technologies made the world ripe for warfare–riders on horseback or in chariots were great at conquering, and agricultural settlements, with their large population centers and piles of food, were great for conquering.

Our conventional views of prehistory are tainted, I suspect, by a mis-perception of time. This is probably basically a quirk of perception–since we remember yesterday better than the day before yesterday, and that day better than last week, and last week better than last year, we tend to think of more recent time periods as longer than they really are, and older time periods as relatively shorter. Children are most prone to this; ask a child to make a numberline showing events like “Last week, my last birthday, the year I was born, and the year mommy was born,” and you’ll tend to get a very distorted number line. Grown ups are much better at this task (we can count the time-distance between these events,) but we’re not perfect.

We show this same tendency when thinking about human history. Our written documents barely go back past 3,000 years, and as far as most people are concerned, this is the beginning of “history”. Nevermind that humans have been around for 200,000 years–that’s 197,000 years of human history that we tend to condense down to: humans evolved, left Africa, and invented agriculture–then came us. We tend to mentally assign approximately equal chunks of time to each phase, which leads to things like people thinking that the Basques–who speak a language isolate–are an ancient, archaic people who hail directly from the first humans, or Neanderthals, or somesuch. Neanderthals disappeared around 40,000 years, and the Indo-European language expansion probably cut the Basques off from their fellow-language speakers about 3,000 years ago. Of course, the Basques could have been cut off since the Neanderthal age, but that’s a jump of 37,000 years (or more) on very little evidence. Likewise, we tend to assume that people just spread out from their original African homeland, got to where they were going, sat down, and never moved again. With the exception of Columbus and his European co-ethnics, everyone is sort of assumed to have gotten where they are now about 100,000-40,000 years ago. (Or the equivalent time period for people who think humanity is much younger or older than it is.)

But the emerging picture is one of conquering–lots of conquering, at least in the time periods we’ve been able to get details on. But go back more than 10,000 years or so, and the records start petering out. We’ve got no writing, far fewer artifacts, and even the DNA breaks down. The technology we’ve developed for extracting and sequencing ancient DNA is amazing, but I suspect we’ll have a devil of a time trying to find any well-preserved 40,000 year old DNA in the rainforest.

So what did the human story look like between 200,000 and 10,000 years ago? Have humans been conquering and re-conquering each other from the beginning? Is it ethnic group after ethnic group, all the way down? Or did lower population density in the pre-agricultural era make it easier to spread out and avoid one’s neighbors than to bother fighting with them? Certainly armies would have spread much less slowly before the domestication of the horse and invention of the chariot. (Not to mention that they require quite a bit of food, which is a tough sort of thing to get in large, easily-transportable form if you’re a hunter-gatherer.)

Certainly prehistoric peoples slaughtered (or slaughter) each other with great frequency–we can tell that:

It doesn’t take a lot of technology to go put a spear into your neighbor’s chest. Even bands of chimps go smash other bands of chimps to bits with rocks.

We also have genetic evidence emerging from further back, ie, An Older Layer of Eurasian Admixture in Africa. As Dienekes summarises:

The authors propose that a genetic component found in Horn of Africa populations back-migrated to Africa from Eurasia ~23 thousand years ago. … For a time, there was a taboo against imagining back-migration into Africa; in a sense this was reasonable on parsimony grounds: Africans have most autosomal genetic diversity and the basal clades of mtDNA and Y-chromosomes; a model with Out-of-Africa is simpler than one with both Out-of and Into-Africa. However, we now know that pretty much all Africans have Eurasian ancestry, ranging from at least traces in theYoruba and Pygmies (to account for the Neandertal admixture) to intermediate values in East Africans, to quite  a lot in North Africans.

Eurasian admixture in Africa seems to be general, variable, and to have occurred at different time scales. It’s still the best hypothesis that modern humans originated in Africa initially and migrated into Eurasia. However, it is no longer clear that Africa was always the pump and never the destination of human migrations.

Whether this was “conquering” or just wandering remains to be discovered.

As for me, my money’s on horses and agriculture making warfare and dispersal faster and more efficient, not fundamentally changing our human proclivities toward our neighbors.