Understanding early human migrations

I wish to explain why I do not worry that my beliefs concerning the initial colonisation of the world--i.e. that it occurred in a sudden pulse 40-50,000 years ago--are at variance with current mainstream thinking on this subject. In a nutshell, it is because mainstream thinking is undoubtedly immature and will be revised extensively as time goes on.

In the 1970s, archaeologists thought they had a good idea of how the world was settled, and, in particular, their understanding seemed to supersede the older idea that humans had spread out from a single source during the upper palaeolithic.

[Today] many authorities believe that...leptolithic [upper palaeolithic] man (of the modern species, Homo sapiens sapiens) was the direct descendant of the Neandertalers. Such a view contrasts strongly with the earlier opinion that the leptolithic cultures were brought to Europe by an immigration from the Near East of H. sapiens, who wiped out the earlier and 'inferior' Neandertal population.

David and Ruth Whitehouse, Archaeological atlas of the world, 1975, p. 39.

This view has itself now been superseded, and the latest genetics research is taking us back to the original idea, that humans originated in one place and, from there, colonised the rest of the planet.

Clear genetic trees for both modern Y chromosomes and mtDNA point back to a recent common ancestor of all modern humans within the last 200,000 years and a migration out of Africa less than 100,000 years ago. This new line rather quickly replaced all pre-existing human genetic lines, including the Neanderthals...[T]here is no convincing evidence for [interbreeding] in our male and female gene lines.

Stephen Oppenheimer, Out of Eden: The peopling of the world, 2004, p. 347.

It would be premature to assume that we now have perfect knowledge and that the latest ideas will not be superseded in their turn. Genetics is a new tool, and in the rush to apply it there has been little attention paid to its limitations, which will gradually come to light. The development of understanding is a process of successive refinement. It will not be complete until history itself comes to an end.

A major reason why ideas about early human migrations (or any other aspect of the past) keep changing is that they focus on material evidence and often on only one kind of such evidence (pots, genes). Material remains can never provide a full record of the living, breathing past, and will always leave ambiguities. To round them out, it is necessary to take account of everything we know, from contemporary and historical experience, about how societies function. Although some archaeologists do study modern farmers and hunter-gatherers in order to understand those of the past, what I am really referring to is the kind of total historical theory based on abstract theoretical principles to which this website is devoted. Such a theory, treating all human experience as one, will allow us to build a more reliable understanding than comes from concentrating on pots, genes or whatever kind of evidence might come along in future.

Humans have always wondered how they got to be where they are now. Many peoples have foundation myths in which their ancestors arrived from somewhere else. The Aztecs recalled their origins on the North American plains. Many Andean societies believed they had emerged from underground. Some Maori tribes celebrate the ancestor Kupe who first came to New Zealand. The English, and to a lesser extent some continental European nations, link themselves to barbarian invaders who plundered the collapsing Roman Empire.

These traditional stories are often fantastical in their details, and they leave such questions as whether an invading group replaced the original population or merely superimposed the thin layer of a conquering elite.

In the early twentieth century, archaeology appeared to be shining the light of science on the problem. The distinctive styles of ancient artefacts (e.g. shape/decoration of pots and tools) seemed to demarcate distinctive cultures associated with different human groups. When artefacts of a particular style were found to spread from one area to another, this was assumed to reflect a displacement or expansion of the people of the corresponding culture.

By the later twentieth century, theorists began to doubt the formerly confident conclusions they had arrived at in this way. It was recognised that the diffusion of cultural objects did not necessarily require the movement of people. Artefacts might travel along trading networks, or one group's styles might simply be adopted by its neighbours. Archaeologists went almost to the opposite extreme, denying the possibility of population movement at all.

Today the evidence and its implications remain uncertain. The main conclusion is the sheer complexity of the interactions and travels of prehistoric peoples.

• The Beaker Folk were once deemed to be recognisable by their pottery (see picture, right) and related artefacts, and the spread of Beaker objects in the third millennium BC was thought to represent a wave of technologically advanced people colonising much of north-western Europe. Later, it was thought to represent just the transmission of the technology and an associated belief system. The modern thinking is that both aspects played some part, though any migration was tentative and targeted, not the sweeping motion once envisaged.
  
  
• The Anglo-Saxons were long thought to have invaded Britain after the collapse of the Roman Empire, bringing a new language, religion and way of life, and driving the original Britons west into Wales and Cornwall. This is the story we have from Bede, who lived within a couple of hundred years of the events. It seemed to be confirmed by the appearance of new kinds of artefacts in the archaeological record, around the relevant time. However, the archaeological evidence has come to look problematic, with close intermingling of the supposed 'British' and 'Anglo-Saxon' material, and some of the changes taking place apparently before the time of the alleged invasion. In the last decade or so, it has been increasingly argued (by both professional and amateur theorists) that there never were any Anglo-Saxon invasions, and the distinction between eastern 'English' and western 'British' has existed in Britain from Mesolithic times. This is despite explicit references to an invasion in contemporary sources (e.g. the Gallic Chronicle of 452, under the year 408). Most historians and archaeologists would today be cautious about making firm statements in any direction.

While the archaeology has looked increasingly debatable, it is now genetics that seems to have all the answers, and again it seems to be based on clear-cut scientific reasoning.

The genetics approach makes use of haplotypes, i.e. specific markers in mitochondrial and Y-chromosome DNA, which are inherited in only the female and male lines respectively, and which embody a precise record of a person's ancestry. For a clear description, see this charts and diagrams website. The technique was also discussed in an earlier post, where I mentioned my suspicion that the flaws in its assumptions will be revealed in due course.

The Genographic Project, a National Geographic website, presents an Atlas of the Human Journey, based on the new genetic findings, and allows you to get your own DNA tested and your origins worked out.

Before the use of haplotypes came along, human genes had already been studied for a long time by another method. This is population genetics, which considers the genes on ordinary chromosomes that can be passed down by either the male or female line. Unlike the haplotype technique, population genetics offers few easy answers and creates a complicated picture.

Population geneticists are sceptical of the new arguments of the haplotype researchers. At a 2005 symposium, geneticists working on haplotypes were invited to analyse an artificial dataset whose 'ground truth' was known. While it was agreed the precise results would not be revealed, it has been admitted they were less than spectacularly successful (see Simulations, genetics and human prehistory, ed. S. Matsumura et al., p. 192).

Another indication that haplotype data may be more ambiguous than its practitioners care to acknowledge is the fact that commercial companies offering ancestral DNA testing have been found to give inconsistent results. The biochemical reactions extracting the DNA profile are presumably precise and repeatable, but the significance of the results is more a matter for opinion and interpretation.

None of this is to say the new genetic methods, or indeed traditional archaeology, are useless for understanding early human migrations. The point is simply that we should take the latest pronouncements on this subject with a pinch of salt. We are still in the early stages of unravelling this aspect of our past.

To all corners of the earth

Long before Europeans developed an ocean-going capability, humans had reached the furthest corners of the earth. This included not only every continent but even the remote Easter Island, thousands of miles from habitation in every direction. Some other Pacific islands, such as Christmas Island, showed signs of human settlements that had died out before Europeans first arrived.

This tells us something about people. Humanity, like nature, abhors a vacuum. Humans do not naturally stick together and congregate in large numbers. There is rather a continual outward pressure as people try to get away from each other.

The stone age lifestyle, based on hunting and gathering, requires a large amount of land area per person. If human numbers were going to grow, people had the choice of staying where they were and developing more advanced food production technologies (to support more people from the same land area) or move steadily out into virgin territory. In reality, people did not make a conscious choice. Moving to new territory would have seemed like the only option, and it was only when the world had filled up that humans had to confront and solve the problem of feeding more people from the same area of land.

The hunting-gathering lifestyle naturally draws people on. Rather than moving a little each day, hunter-gatherers tend to stay in one place for a while and then relocate in one big jump. It has been said they eat their way out of camp. The longer they stay, the further they have to travel to find new food sources. In a matter of weeks, the daily round trip exceeds twenty or thirty miles, and it becomes necessary to make a major move.

This outward pressure can take people a long way in a short time. It often seems to be assumed that for early humans to migrate, say, a thousand miles would have taken many generations. Yet even in the stone age, people could in principle have travelled from one end of the earth to the other in their own lifetimes. Consider that the circumference of the earth is roughly 25,000 miles. For a stone age person with an active life of 25 years, it would be necessary to travel 1000 miles per year, or under 3 miles per day, to cover this distance. People who follow stone-age lifestyles today can easily do that kind of mileage.

Of course, humans would not have gone directly around the equator, with several oceans in the way. Nor would they have walked directly across the continents, where the way can be barred by mountains and deserts.

Their easiest route would have been along the coastline. Since people reached Australia very early, they may have had simple boats from the very beginning, as they came out of Africa. This would have sped them on their way.

At the beginning of the upper paleolithic, 40,000 years ago, sea level was lower than it is today. This left a continuous land route from Africa to the Americas, with no ice barriers. Here is a map of the world at that time:

The above image was created using a program at the site of Sebastien Merkel. This allows you to enter a given sea level and view the resulting map. Information about how sea level has changed over time is available at this site.

With sea level as above, travel to Australia still required crossing open water. However, this could be done by island-hopping, with each island visible from the previous one. Only the final crossing, of about 100 miles, required a leap into the unknown, but Australia was a large target and people could have spotted it during short sea trips to and from their home base. (Humans could also have arrived at Australia via New Guinea, which was then connected to Australia by land. However, it is believed they did not take that route and New Guinea was settled after Australia.)

I do not accept that humans only gradually diffused through the world. I believe that, following initial speciation in Africa, humans exploded across the planet, so that the world was occupied almost simultaneously at around 40,000 years ago.

We can estimate how long it would have taken people to spread to fill every continent.

Modern hunter-gatherers live at densities of around 1 person per 15 square miles (compared with 67,000 persons per square mile in Manhattan). However, they tend to occupy marginal environments, the best land having been taken over long ago by agriculturalists. Average population densities in the old stone age might have been higher, perhaps as high as 1 person per square mile. Since the habitable area of the world is about 15 million square miles, the total human population of the world, with paleolithic technology, would have been somewhere between 1 million and 15 million.

Let us define the following:

A =	total area of the world habitable by hunting and gathering (say, 15 million square miles)
ρ =	population density of hunter-gatherers (say, 1 person per square mile)
T =	time for an unconstrained human population to double in numbers (say, 10 years, which was observed on Pitcairn Island during the first thirty years after it was settled by the Bounty mutineers)
t =	time

The number of hunter-gatherers that would saturate the world is A ρ, while, starting from a population of 1 (a pregnant woman), the number of humans after time, t, would be 2^t/T.

The time taken to populate the world is therefore given by

A ρ = 2^t/T

or

t = T log₂(A ρ)

Putting in the numbers suggested above, this comes to about 240 years, for humans to cover the planet.

Of course, this figure should not be taken literally. It is just to give us a feel for the issue. A larger starting population (1 pregnant woman is a little unrealistic) would reduce the time, as would a lower population density, while a longer doubling time would increase it. The point is that humanity's spread through the world does not need to have taken a hugely long time, and in fact it is unlikely that it took a hugely long time.

Note that, once the world was fully populated, population growth would have had to stop until technological improvements allowed more people to be supported from the same land surface. Modern hunter-gatherers are adept at keeping their numbers in tune with their environment, or in fact in tune with what the environment can support in its leanest years. There would therefore have been a short population explosion, followed by near-stagnation (until the development of farming).

Three factors would have slowed down human expansion relative to this simple picture:

1. Obstructions, such as deserts, mountains and waterways. Crossing the Amazon at its mouth, for example, is like crossing the English channel.
2. Varying ecological conditions. People who knew how to exploit the flora and fauna of one environment (grassland, forest, seashore) would have had to learn new skills to survive in a different one.
3. Presence of other hominids. Neanderthals and other hominids already occupied the lands outside Africa. Modern humans would have been competing with them for resources, and might for some time have been kept out of their territories by fear or force.

Nevertheless, people could still have reached every continent in a short time, even if actually filling the continents was slower than the simple argument suggests. Having boats and following the coastline, humans would have remained within a familiar environment while being able to bypass obstacles. As for conflict with other hominids, it might even have been a factor drawing the more capable humans onwards.

The problem of the Australians

The (aboriginal) Australians worry me.

These folks settled their continent 40 thousand years ago -- or, as many scholars now believe, 65 thousand years ago.

Yet in all that time, they failed to develop beyond the old stone age. They developed no agriculture, metal, or permanent settlements. They scarcely even had clothing.

If Australia had not been discovered from outside, there is every reason to suppose its inhabitants might have remained at the stone age level for ever more -- or until the sun burned itself out.

It is frightening to think that a branch of the human species could have continued in this way forever, never realising its potential, and never knowing anything of the science and technology we have developed.

If the Australians had been typical of humanity as a whole, the human story would have looked something like this...

We can attempt to explain the Australians' lack of development in terms of dark age theory.

Scale (a measure of interactivity) is a critical determinant of a society's institutional and technological complexity. Low-scale societies are inevitably simple, as small groups of isolated people cannot support elaborate economies and political systems. Stuck at the end of the world's main landmass, and separated from it by a lengthy sea-crossing, Australia experienced low scale. It was exposed to few influences. The effective size of the population within reach, i.e. the scale, was too low to sustain development beyond paleolithic levels.

We can refine this argument. For an isolated region, scale depends on the population contained within that region. But the population that exists within the region depends on the technological level (because sophisticated food-production techniques support more people on the same amount of land). And the technological level depends on the scale. What appears to have happened in Australia is that humans achieved an equilibrium, or eigenmode, where technology supported a population that generated enough scale to support that level of technology...and no more. This eigenmode, i.e. self-consistent solution to the problems of social existence, was quite stable and there was no reason why the Australians should ever have broken out of it.

This argument suggests several points of concern:

1. New Guinea is also small and isolated, yet its inhabitants achieved the neolithic level, unlike the Australians. New Guinea was actually joined to Australia, as the continent of Sahul, until about ten thousand years ago, so why did the Australians not achieve the sophistication of New Guinea? The Polynesian islands are even smaller and even more isolated, yet many had chiefdoms, surpassing New Guinea in sophistication, let alone Australia. We might be able to explain this in terms of humans bringing the relevant technologies and institutions to these islands, so that they were already in a more complex eigenmode. But why were people not able to carry this eigenmode to Australia?
2. If Australia was too small to support developmental growth like that of the Afro-Eurasian and American world islands, it suggests that, had the world's landmass been more broken up than it is, then humans would not have been able to develop anywhere. The fact that humans have developed must then be seen, in part, as a geographical accident, not an inevitable result of human talents.
3. Australia is a big place. If we are saying Australia was too small, then just how big does a continent have to be before humans are able to develop beyond the paleolithic? It is true that Australia has large areas of desert, but the Nile Valley is surrounded by desert. And climatic conditions in Australia have changed a lot over the millennia. Why could civilisation not have developed along the valleys of Australia's Orange or Murray-Darling rivers? If we are saying environmental conditions here were never quite right, then just how flukey was the development of civilisation elsewhere?
4. If Australia reached equilibrium at a technological level commensurate with its size, then could this be the fate of the world as a whole? Will we stagnate at a (much higher) technological level where the planet is able to support just enough population to sustain that level of technology? Could the human race flat-line until the sun goes supernova or something else wipes us out?

The point of these questions is not to deny that Australia's lack of development can be related to its situation, but to show some of the complicating issues that a full theory must take into account and be able to explain.

We should also note two other points:

1. There is no reason to think, as some might, that the aboriginal Australians lacked the mental capacity of humans elsewhere. All humans today are members of a single species, descended from common ancestors living at most 100-200,000 years ago. On an individual level, aboriginal Australians operate perfectly competently in technologically advanced society; high achieving aborigines include academics, politicians and writers. Australian traditional culture is also sophisticated in its own way; languages and kinship systems are more complex than those of 'advanced' societies; art and mythology are well developed; the boomerang is a clever device; aborigines found honey by gluing feathers to bees, slowing them down so they could be followed. [As far as dark age theory is concerned, the sameness of humans everywhere and at all times is axiomatic. Only when it has proved impossible to build an adequate theory of history on that assumption will the axiom need to be abandoned - we are nowhere near that yet.]
2. Australians seem to have taken up agriculture at various points, then abandoned it again. Therefore, the situation is not as simple as achievement of an everlasting equilibrium. Stagnation was not total, and perhaps changing climatic conditions sometimes elevated scale sufficiently to promote development in some areas.

Finally, there is the argument of Jared Diamond, in Guns, germs and steel, which is that the move to neolithic (farming) lifestyles depended on the availability of crop plants and domesticable animals. While Eurasia had barley/wheat/rice on the one hand and sheep/cattle/horses/pigs on the other, suitable equivalents in the rest of the world were lacking.

Diamond's argument comes back to the issue of continental size. There is a well-known relationship between the size of an island/landmass and its biodiversity. Hence, the largest continent, with the greatest biodiversity, inevitably had the most suitable species for agriculture. The second largest continent was a runner up, while the smallest continent had too litte variety to provide species with the right characteristics for human exploitation.

This argument is not endorsed by dark age theory, which starts from the assumption that history is a sociological phenomenon, not dictated by random background features such as climatic conditions or availability of domesticable species. In dark age theory, necessity is the mother of invention, so that people would be expected to have found ways of supporting complex society if conditions were right for it. Diamond says attempts to use the zebra as a beast of burden have failed, which he suggests helps explain Africa's lack of development. However, over thousands of years the zebra might have been domesticated as the horse was, had people really needed such an animal. Dark age theory looks for explanations in terms of the inherent logic of human affairs, not in terms of chance, external factors. (This viewpoint may be wrong, but we start from it as an assumption, to be abandoned only when it has demonstrably failed.)

Genetic trees and the peopling of the world

I would like to go back to discussing how humans spread through the world.

Numerous books have appeared on this subject in recent years, stimulated by new techniques of genetic research. I find these interesting but I question their claimed definitiveness.

Imagine photocopying documents, where you photocopy the photocopies, perhaps making dozens of copies from each copy. In the course of this, blemishes will occasionally appear. When you photocopy the blemished copy, the same blemishes will be reproduced in all subsequent copies. If you look at two copies and see that they share the same blemish, you know they must "descend" from the same original, blemished copy. If you also see that the two copies have additional, different blemishes, and you know how often blemishes arise--say, once every thousand copies--you can work out how many copying generations it is since the copy from which they are descended. E.g. if they differ by three blemishes, it must be three thousand generations since the "ancestor" photocopy passed through the photocopier.

Photocopier genetics

A similar principle has been used to determine the family relationships between human populations.

It does not work with ordinary DNA, which is mixed and matched at every generation (half from each parent) by sexual reproduction--as if the photocopies were cut in half and taped together in new combinations, so that blemishes are spread around and clear distinctions between lines are not maintained.

Instead, it relies on parts of the genome that do not undergo recombination. These are mitochondrial DNA, which people inherit only from their mother, and the Y-chromosome, which is passed down intact (or almost so) from father to son. When these genes are passed down, copying mistakes occasionally occur, so that people today show many variations deriving from ancient errors. As with photocopier blemishes, such variations make it possible to estimate how long ago any two people shared a common ancestor (female ancestor for mitochondrial DNA and male ancestor for the Y-chromosome).

Stephen Oppenheimer, in his book Out of Eden: the peopling of the world, describes how geneticists have used this principle to trace the ancestral connections between human populations and to work out how long ago they split off from one another.

The message he likes to convey is that archaeologists and historians were fumbling around in the dark until these new genetic techniques came to shine the clear light of day on their disciplines.

For instance, the geneticists have apparently instantly and conclusively resolved an issue that archaeologists might have been arguing about until kingdom come. While archaeologists once saw changes in the distribution of pottery as indicating the wholesale movement of the people who produced the pottery, more recently they have argued that people remained put and only styles and technology spread from one region to another. The genetic evidence seems to show that, indeed, most modern populations have been where they are for a long time and the formerly hypothesised migrations never took place.

Nevertheless, I suspect that, in the fullness of time, the geneticists' arguments will come to seem as problematic as those of the archaeologists' and historians', which themselves once seemed so obvious and compelling. It will be revealed that there are other ways of interpreting the results and doubt will creep into this young discipline where today certainty is supreme.

After all, according to Oppenheimer, the entire non-African population of the world is descended from a single group that left Africa via the Arabian peninsula about 80,000 years ago. Yet the notion that, over the entire lifetime of the human species, no other people had the idea of leaving Africa is hard to credit. Right there lies a reason for having doubts about the completeness of the genetic picture.

Even if the geneticists prove to be completely correct in their picture of ancient population movements, there remains a vital role for historians and archaeologists in advancing understanding of the past. The things that make, say, Britain what it is today are Magna Carta, the Elizabethan Age, the Civil War, the Industrial Revolution, and so on--i.e. sociological things that are not passed down in our DNA. History and society, the concerns of this website, are the subject of cultural, not genetic, transmission, and the connection between genes and culture is probably looser than people like Oppenheimer tend to assume.

Outward pressure

Easter Island is a triangular speck in the Pacific Ocean, over a thousand miles from the nearest inhabited land. When it was 'discovered' by the Dutchman Jacob Roggeveen, on Easter Sunday 1722, it already had people living on it.

Humans had only been on Easter Island since about AD 1000. Nevertheless, that they had reached this remote spot points to humanity's strong centrifugal tendency. Humans were already in Australia a few thousand years after our emergence as a species, and that journey would have required a sea crossing at some point.

This shows that people do not naturally live together at high densities. They are driven, to some extent, to get away from each other. This outward pressure has several aspects:

1. For a given technology and way of life, a particular land area will support a limited number of people. As population increases, humans find it easier to move to empty land than to develop ways of supporting more people from the same territory.
2. Living together causes friction and conflict. Some people may bully others or may impose themselves on others in an effort to keep order. At least part of the population finds it preferable to move away, rather than stay and have its freedom curtailed.
3. While many humans are happy to drift along with what they know, others have a high degree of 'natural curiosity' that makes them want to explore beyond the known, both geographically and in other ways.

It is now fairly clear, based on genetic data, that humans arose in Africa recently, and spread out from there. Modern humans are therefore all quite closely related.

Nevertheless, I disagree in detail with current reconstructions of the peopling of the world (link requires Google Earth; a Google Maps version is here). According to the view presented in the preceding link, while people reached Australia about 40,000 BC, they did not reach North Africa until 20,000 BC, even though they had evolved on the African continent. Furthermore, while people had got half-way down the African coast by 40,000 BC, they supposedly did not reach the southern tip of Africa until 25,000 BC, even though they were in China and Spain by 40,000 BC and in Britain only a couple of thousand years later. Such a pattern makes little sense and rather shows the limitations of the gene-mapping methodology on which this reconstruction is based.

I suggest that, after emerging in central east Africa, the human species spread to and throughout every continent within a few thousand years, say around 40,000 BC. Only the peopling of oceanic archipelagoes that are not connected to the mainland by chains of intervisible islands did not occur until much later, say from about 5000 BC.

(Although mainstream opinion holds that humans did not move into the Americas until after about 15,000 BC, some archaeologists do place the entry into the Americas around 40,000 BC, on the basis of sites that remain controversial.)

I am still researching this issue...