By Eric Vandenbroeck and co-workers

Where earlier we argued that if Homo sapiens somehow could have found a way to coexist with T. rex, it is unclear which would have become the more intelligent and dominant species. In some regions, growth-enhancing geography and diversity led to the rapid adaptation of cultural traits and institutional features to their surroundings, and the acceleration of technological progress. Centuries later, this process triggered an outburst of demand for human capital, a sudden drop in birth rates, and thus an earlier transition to the modern era of growth.

What happend because humans 'did 'survive whereby for most of this period, successful adaptation generated progressively better hunters and gatherers, which enabled a rise in the food supply and a significant increase in the size of the human population. Eventually, living space and natural resources available per person declined, and sometimes as early as 60,000 to 90,000 years ago, Homo sapiens embarked on a large-scale exodus out of the African continent in search of additional fertile living grounds. Due to the serial nature of this migratory process, it was inherently associated with a reduction in the diversity of populations that settled at greater migratory distances from Africa; the further away from Africa humans moved, the lower the degree of cultural, linguistic, behavioral, and physical diversity in their societies.

This phenomenon reflects the serial founder effect. Imagine an island that is home to five main breeds of parrot – blue, yellow, black, green, and red – equally well adapted for survival on this island. When the island is hit by a typhoon, a few parrots have swept away to a deserted, far-flung isle. This small subgroup is unlikely to contain parrots from all five of the original breeds. These parrots might be mostly red, yellow, and blue, for instance, and their chicks – which will soon fill the new island – will inherit their colors. The colony that will develop on the new island will therefore be less diverse than the original population. If a very small flock of parrots then migrates from the second island to a third, that group is likely to be even less diverse than those in each of the previous colonies. Thus, as long as the parrots migrate from each parental island more rapidly than the pace of potential mutations on the island, the further away the parrots migrate (sequentially) from the original island, the less diverse their population will be. 

Human migration out of Africa followed a comparable pattern. An initial group left Africa and settled in fertile regions nearby, carrying only a subset of the diversity that existed in their parental African population. Once the initial migratory group had grown to the extent that its new environment could no longer support any additional expansion, a less diverse subgroup departed in search of other virgin territory and settled in habitats further away. During this human dispersal out of Africa and the peopling of the continents, this process repeated itself: as populations grew, new subgroups containing only some of the diversity in their parental colony left again in a quest for greener pastures. Although some groups switched course, as will become apparent, the thrust of these migratory patterns was such that groups who left Africa and reached Western Asia were less diverse than the original human population in Africa, and their descendants who continued migrating east to Central Asia and ultimately to Oceania and the Americas, or north-west to Europe were progressively even less diverse than those who remained behind. This expansion of anatomically modern humans from the cradle of humankind in Africa has imparted a deep and indelible mark on the worldwide variation in the degree of diversity – cultural, linguistic, behavioral, and physical – across populations This decline in the overall level of population diversity with migratory distance from Africa is partly reflected in the reduction in genetic diversity among indigenous ethnic groups at a greater migratory distance from Africa. Based on a comparable measure of this type of diversity for 267 distinct populations, most of which can be associated with specific indigenous ethnic groups and their geographical homeland, it is apparent that the most diverse indigenous ethnic groups are those closest to East Africa, whereas the least diverse are the indigenous communities of Central and South America, whose overland migratory distance from Africa is the longest.

This negative correlation between diversity and migratory distance from East Africa is a pattern that is observed not only across continents. It is present within continents as well.

Broader evidence for the diminishing levels of diversity among indigenous groups at a greater migratory distance from Africa comes from the fields of physical and cognitive anthropology. Studies of particular features of body shape – for example, the bone structure pertaining to particular dental attributes, pelvic traits and the shape of the birth canal – as well as cultural distinctions, such as the differences between the fundamental units of speech (‘phonemes’) in different languages, also confirm the existence of a serial founder effect originating in East Africa; again, the greater the migratory distance from East Africa, the lower the diversity in these physical and cultural characteristics. Of course, a proper exploration of the impact of the overall level of population diversity in all its multifaceted forms on the economic prosperity of nations would require a far more comprehensive measure than geneticists and anthropologists provide. In addition, it would need to be independent of the population’s degree of economic development so as to be used to assess the causal effect of diversity on the wealth of nations. What might such a measure look like?

Migratory Distance from East Africa and Diversity among Geographically Indigenous Ethnic Groups:

Conventional measures of population diversity tend to capture only the proportional representation of the ethnic or linguistic groups in a population. These measures suffer therefore from two major deficiencies. One is that some ethnic and linguistic groups are more closely related than others. A society that consists of an equal proportion of Danish people and Swedish people may not be as diverse as a society that is composed of equal fractions of Danish and Japanese. The other is that ethnic and linguistic groups are not internally homogenous. A nation composed entirely of Japanese people would not necessarily be as diverse as a nation composed entirely of Danish people. In fact, diversity within an ethnic group is typically an order of magnitude larger than the diversity between groups.

 A comprehensive measure of the overall diversity of a national population ought therefore to capture at least two additional aspects of diversity. First, diversity within each ethnic or subnational group, such as within the Irish or the Scottish population in the US. Second, the degree of diversity between any pair of ethnic or subnational groups, capturing, for example, the relative cultural proximity of the Irish and Scottish populations of the US in comparison to its Irish and Mexican populations.

In view of the tight negative correlation between migratory distance from East Africa and diversity in observable traits, these migratory distances can be used as a proxy for the historical level of diversity in each geographical location on Planet Earth. We can therefore construct an index of predicted overall diversity for each national population today, based on the migratory distance from Africa of their ancestral populations, taking into account (the relative size of each ancestral subgroup within the country;  the diversity of each of these subgroups as predicted by the distance that their ancestors traveled over the course of their migration from East Africa; and (iii) the degree of pairwise diversity between each of these subgroups, as predicted by the migratory distances between the geographical homelands of the ancestral populations of each pair. 

This statistical measure of predicted diversity has two major virtues. First, prehistorical migratory distance from East Africa is clearly independent of current levels of economic prosperity and thus the measure permits the estimation of the causal effect of diversity on living standards. Second, as highlighted above, mounting evidence from the fields of physical and cognitive anthropology suggests that migratory distance from Africa has had an important effect on diversity in a range of traits that are expressed physically and behaviourally; reassuringly, therefore, the kind of diversity our measure predicts could affect societal outcomes. Moreover, if the index measures diversity inaccurately (in a random fashion) – because of a failure to properly account for internal migration within each of the continents, for example – statistical theory suggests that this would tend to lead us to reject, rather than to confirm, the hypothesized impact of diversity on economic prosperity. In other words, if we are erring, we are erring on the side of caution.

Finally, it is important to clarify that our measure of diversity is a societal characteristic. It measures the breadth of variety of human traits within society regardless of what those traits are or how they may differ between societies. It, therefore, does not and cannot be used to imply that some traits are more conducive than others for economic success. Rather, it captures the potential impact of the diversity in human traits, within a society, on economic prosperity. In fact, accounting for confounding geographical and historical factors, it appears that migratory distance from Africa per se has no impact on the average level of traits such as height and weight across the globe. It predominantly affects the extent of the deviation of individuals in the population from this average level.

Armed with this powerful measure of the overall diversity of each population, we can, at last, explore whether the exodus out of Africa that occurred tens of thousands of years ago and its impact on human diversity, might have had an astoundingly long-lasting effect on current living standards across the globe.


Diversity and Prosperity

Living conditions in the course of history have indeed been influenced significantly by levels of diversity and therefore by the migration of Homo sapiens out of Africa. Migratory distances of the ancestral populations of each country, or ethnic group, from the cradle of humankind in East Africa, have generated a persistent ‘hump-shaped’ influence on development outcomes, reflecting a fundamental trade-off between the beneficial and the detrimental effects of diversity on productivity at the societal level.

This ‘hump-shaped’ effect of diversity on economic productivity, whether captured by past levels of population density or urbanization rates, or current levels of per capita income or night-light intensity (based on satellite imagery), is both stark and consistent across countries and ethnic groups. Moreover, these hump-shaped patterns remain qualitatively unchanged over the 12,000 years since the Neolithic Revolution. Thus, in the absence of policies that mitigate the cost of diversity in heterogeneous nations and enhance the level of diversity in homogenous ones, intermediate levels of diversity have been most conducive to economic prosperity. 

In fact, this hump-shaped effect is unique to the impact of ancestral migratory distance from Africa. Alternative distances, unrelated to the exodus of Homo sapiens from Africa and to human diversity, do not generate similar hump-shaped patterns. In particular, aerial distance from East Africa, as opposed to migratory distance, is uncorrelated with economic prosperity, which is reassuring, since prehistoric humans migrated out of Africa by foot rather than by airplane. Furthermore, migratory distances from ‘placebo origins’ – other focal points on Planet Earth from which Homo sapiens has clearly not emerged – London, Tokyo, or Mexico City – do not have any effect on economic prosperity. Nor is this relationship-driven by geographic proximity to leading technological frontiers in the distant past, such as the Fertile Crescent.

Separate bodies of evidence confirm the proposed mechanism behind this intriguing result: namely, that societal diversity has indeed exerted conflicting effects on economic well-being. On the one hand, by widening the spectrum of individual values, beliefs, and preferences in social interactions, the findings suggest that diversity has diminished interpersonal trust, eroded social cohesion, increased the incidence of civil conflicts, and introduced inefficiencies in the provision of public goods, thus adversely affecting economic performance.[16] On the other hand, greater societal diversity has fostered economic development by widening the spectrum of individual traits, such as skills and approaches to problem-solving, thus fostering specialization, stimulating the cross-fertilization of ideas in innovative activities, and facilitating more rapid adaptation to changing technological environments.

The top panels depict the impact of predicted population homogeneity on economic development as reflected by either population density (Panel A) or urbanization rate (Panel B). The bottom panels depict the impact of predicted ancestry-adjusted homogeneity on economic development in the contemporary era, as reflected by either income per capita during the 2010-18 time period (Panel C) or luminosity per capita during the 1992–2013 time period (Panel D).

This figure depicts the impact of observed population homogeneity of geographically indigenous ethnic groups, as predicted by migratory distance from Africa, on long-run historical economic development, as reflected by population density in 5000 BCE (Panel A), 3000 BCE (Panel B), 1000 BCE (Panel C) and 100 ce (Panel D).

Furthermore, the ‘sweet spot’ level at which diversity is most conducive for economic prosperity has increased in the past centuries. This pattern is consistent with the hypothesis that diversity is increasingly beneficial in the rapidly changing technological environments that have been characteristic of advanced stages of development.[20] This growing importance of diversity in the development process sheds new light on the causes of China’s and Europe’s reversal of fortunes. In the year 1500 CE, the level of diversity most conducive to development existed among nations such as Japan, Korea, and China. Evidently, their relative homogeneity fostered social cohesion more than it stifled innovation and was ideal in the pre-1500 era when technological progress was slower and the benefits of diversity, therefore, more limited. Indeed, China prospered greatly in the pre-industrial era. But as technological progress accelerated over the subsequent five centuries, the relative homogeneity of China appears to have delayed its transition to the modern era of economic growth, transferring economic dominance to the more diverse societies of Europe and subsequently North America. The level of diversity most advantageous for economic development in the modern era is now closer to the current level of diversity in the United States.

Human diversity is of course just one of the factors that have affected economic fortunes, and proximity to the ‘sweet spot’ of population diversity does not ensure prosperity. Nevertheless, accounting for geographical, institutional, and cultural characteristics, diversity retains a considerable effect on the economic development of countries, regions, and ethnic groups in the present, just as in the past. The significance of these effects is particularly extraordinary given the eons that have passed since Homo sapiens first stepped out of Africa – and it can be quantified. About a quarter of the unexplained variation in prosperity between nations, as reflected in average income per capita during 2010–18, can be attributed to societal diversity. By comparison, using the same methods, geo-climatic characteristics account for about two-fifths of the variation, the disease environment for about one-seventh, ethnocultural factors account for one-fifth, and political institutions account for about one-tenth.

Yet, despite human diversity being such a powerful determinant of prosperity, the fate of nations is not carved in stone. Quite the contrary: by understanding the nature of that power we can design appropriate policies to foster the benefits of diversity while mitigating its adverse effects. If Bolivia – which has one of the least diverse populations – would foster cultural diversity, its per capita income could increase as much as five-fold. In contrast, if Ethiopia – one of the world’s most diverse countries – were to adopt policies to enhance social cohesion and tolerance of difference, it could double its current income per capita. 

More generally, much could be achieved through education policies aimed at making the best use of the levels of diversity that already exist, with highly diverse societies seeking to promote tolerance and respect for difference, and highly homogeneous ones encouraging openness to new ideas, skepticism, and a willingness to challenge the status quo. In fact, any measures that successfully enhanced pluralism, tolerance, and respect for difference would further elevate the level of diversity that is conducive to national productivity. And given the likelihood that technological progress will intensify in the coming decades, the advantages of diversity in societies that are able to foster social cohesiveness and mitigate its costs are only set to grow. 


The Grip of the Past

The impact of human diversity on economic development may be the most striking example of how modern variations in the wealth of nations are rooted in complex factors originating in the ancient past. In fact, readers in urban pockets of the developed world with large migrant populations might find it surprising that the distribution of human diversity has persisted for quite so long across large segments of the planet. Institutional and cultural differences between countries have diminished in the modern era, as developing countries have tended to adopt the advantageous political and economic institutions of developed nations, and individuals have sought to emulate beneficial cultural norms. Likewise, some adverse effects of geography, such as disease prevalence or lack of access to the sea, have been mitigated by technological progress. And yet, largely due to the inherent attachment of individuals to their homelands and their native cultures, as well as the presence of legal barriers to international migration, human diversity in some regions in the modern era has changed at a much slower pace. 

Thus, in the absence of proper inducements – educational, institutional, or cultural – highly diverse societies are likely to struggle to achieve the trust and social cohesion levels needed for economic prosperity, while homogeneous ones will fail to benefit sufficiently from the intellectual cross-pollination on which technological and commercial progress relies. The income gap between nations may therefore endure despite the convergence of institutional and cultural traits across them. Such is the grip of the past. 

Since the first bands of Homo sapiens walked out of Africa millennia ago, their societal characteristics and the distinct natural environments they settled have been dissimilar, and the effects of those dissimilarities have persisted over time. Some were blessed at the outset with human diversity levels and geographical attributes that were conducive for economic development, while others faced less favorable initial conditions that have been detrimental to their growth process ever since. Favorable initial conditions contributed to technological progress and led to the adoption of growth-enhancing institutional and cultural characteristics – inclusive political institutions, social capital, and future-oriented mindset – further stimulating technological progress and the pace of the transition from stagnation to growth. Unfavorable endowments, by contrast, dictated slower trajectories, reinforced by the adoption of institutions and cultural characteristics that hindered growth. Although throughout our history institutions and cultures have been greatly influenced by geographic characteristics and human diversity, they have also remained susceptible to abrupt historical fluctuations that occasionally sway the fates of nations. As in the case of North and South Korea, living standards may differ sharply even between countries that share both their geography and population diversity. In these infrequent instances, cultures and institutions could be the leading forces behind the observed gaps across some nations. 

However, the long arc of human history reveals that geographical characteristics and population diversity, formed partly during the migration of Homo sapiens from Africa tens of thousands of years ago, are predominantly the deepest factors behind global inequalities, while cultural and institutional adaptation has often dictated the speed at which development progressed in societies across the globe. In some regions, growth-enhancing geography and diversity led to the rapid adaptation of cultural traits and institutional features to their surroundings, and the acceleration of technological progress. Centuries later, this process triggered an outburst of demand for human capital, a sudden drop in birth rates, and thus an earlier transition to the modern era of growth. Elsewhere, this interaction set societies on a slower journey, delaying their escape from the jaws of the Malthusian beast. Thus emerged the extreme global disparities of the modern world.



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