Older it may seem, the discussion about the use of racial categorization in our species is ongoing. Such is, to a great extent, due to the lack of objectivity of race as a concept and the need for a holistic approach to examine it. Even if no definitive answer may be provided on whether its appliance is legitimate or not, an alternative approach can be advanced: seek to better inform about the fascinating diversity existent within our species, and the difficulties our cognitive system faces when processing it. The race for Humanity is a series of articles in which the reader is invited to take this path and formulate an opinion even beyond that expressed by the author. Its second part focuses on race as a biological construct.
Between-groups prejudice is, and will always be, a subject of great importance, because we are a species whose natural history and behaviour has been deeply influenced by migration. Otherwise, how could we have spread to a virtually global distribution? It hardly matters where one exactly lives: lay back on a comfortable chair and think, throughout history, on how many nomadic groups encountered each other in your region of origin, making war and alliances; on how many empires dominated them before falling; on how many emigration crises have occurred among your close ancestors… Plus, in the case of the colonizing countries, it can be added: how many people from different societies living far away were forced to move to your hometown?
In fact, it should be accounted that the concept of nation-state, with well-defined borders, is very recent . Migration itself had an important role on the World’s politics development, functioning as a vehicle of exchange of ideas and technologies, being the pre-modern history dominated by poly-ethnic empires, such as the Roman and the Greek ones – but, of course, it also spread epidemics in the past, dramatic enough to provoke population replacements .
Despite our tendency to define static and well-defined categories of Human phenotypes, each individual’s genome and inherited culture is always to a great extension an admixture of influences from the most unexpected parts of the World! Focusing on the difficulty of clearly define the concept of race as a biological construct, some challenges are met. Because it is not a formal taxonomic category, both its exact meaning and application are quite arbitrary. This being stated, it must be concluded that it’s theoretically impossible to provide a concluding answer about the legitimacy of its use within our species. Nevertheless, in order to be possible to provide a biological analysis of Human diversity, race will be here treated as a synonym of subspecies.
Why don’t we start with palaeoanthropology?
Some paleoanthropologists advert that, when examining fossil records from our genus, it is a common mistake to misinterpret intraspecific variation as species diversification . This indicates that our evolutionary history predicts a high within-species diversity. In fact, despite Neanderthals being estimated to have diverged from the Homo sapiens lineage roughly 350,000 years ago, when they had contact with modern shaped Humans, around 270.000 years after, admixture still occurred – with an estimated contribution of 1-4% to the actual genome of non-African populations .
The same happened with a sister group of Neanderthals, the Denisovans: traces of their posterior contact with our species were found in around 4-6% of the genome of actual Melanesians . Hence, on the one hand, our species should have the potential to exhibit a great morphologic diversity; on the other, it seems to be able to maintain a certain genetic unity, which allowed it to successfully hybridize with other groups within the genus Homo.
How committed is our development?
Hand in hand with morphological diversity goes an ontogenetic versatility. This is shown by a notable variation of life-history traits – such as body growth, and age of puberty and menarche – detected across several small-scale societies . Moreover, there are evidences that some traits can change by a standard deviation in a given population over timescales of 2 to 6 decades . Developmental shifts have also been detected in the migration context. Comparing with non-migrating countrymen, females from developing countries being adopted in Europe, had their puberty earlier . However, to understand which factors underlie this effect is very complex. Still, this research supports the words of Wells and Stock , that we are “the ape that doesn’t commit”, a species with notable biological plasticity. Such is expected to be favoured in organisms adapted to unstable environments or prone to regular migration.
Considering our global colonization, it becomes clear that both factors must be taken into account. Again, the idea that Human diversity is not necessarily embedded in genetic differences remains. Although biological plasticity may enhance differentiation, it doesn’t seem to imply, at the genetic level, neither a marked between-population heterogeneity, nor within-population homogeneity, as one commonly tends to assume.
Are forensic anthropologists about to lose their job?
Sauer  wrote a paper with the provocative title “Forensic anthropology and the concept of race: if races don’t exist, why are forensic anthropologists so good at identifying them?”. According to it, forensic anthropologists are capable of attributing to a corpse, with a remarkable success rate, one of the traditional racial categories (White, Black, Mongoloid, and Native American) recurring to a series of cranial measurements. Despite the fact that the method tends to fail when admixture is prominent, it turns out that, at least at the continental level, the internal homogeneity of native populations within each group admits some kind of categorization to be applied – based on a restricted and arbitrary set of characteristics. Nevertheless, the author didn’t defend that these categories correctly represent Human diversity as its whole – instead, he treats them as an artificial classification merely useful to help to identify corpses.
It can be posited that it would be only a matter of perfecting technologies and methodologies so to allow origins to be tracked down to the regional, or country-level; though the probability of errors due to admixture would increase more and more. This scenario was raised for the sake of arguing that the borders of our categorization, the point in which we decide to stop “zooming”, will be quite based on preconceived ideas, rather than objective criteria.
Does it run in the genes?
Having discussed phenotypical accounts, the focus shall now be laid on our genes. A relevant initial remark is that our species shows a particularly high degree of genetic unity when compared with other ape species, especially given our vast geographical distribution. On the one hand, some bono and chimpanzee clades show more mitochondrial variability than our entire species ; on the other, chimpanzees seem to have more genetic adaptations influenced by directional selection , whereas our adaptive evolution appears to occur mostly through subtle alterations of the frequency of the existing alleles . Additionally, many of our traits have a polygenetic basis , which contributes to the relaxation of selective pressures on the genome and constrains genetic differentiation, thus partially explaining the maintenance of our genetic unity across so many diverse ecological environments.
How is Human diversity distributed?
In a classical study from Lewontin , the taxonomic concept of race was dismissed because most diversity was harboured within populations, an average of 85.4%, and between-races differences had a mean of 6.3%. Later, some researchers considered this approach statistically fallacious, as being a unidimensional, arbitrary selection of a small number of loci . Rosenberg et al. , seeking to address this issue, ended up increasing the debate. On one side, they gave further support to their predecessor, as 93-95% of the diversity was due to within-population differences, and only 3-5% was exclusively due to between-population contrasts. On the other, the genetic data could be clustered with no a priori information into 6 main groups, 5 of them corresponding to the main geographic regions – just like forensic anthropologists could assign origins by examining craniums.
It is tempting to close the case recurring to the conventionalized standards of differentiation (themselves arbitrary to some extension): values superior to 15% imply a significant differentiation . However, one study adding a chimpanzee sample into a similar analysis came to challenge all the previous results, as the between-group portion of genetic variability increased only slightly, from 11.9% to 18.3% . Hence, something had to be wrong, and it turned out that the statistic being used (FST) was improper, because it assumed equal within-groups divergence, a principle that our species violates . It turns out that we are diversely diverse.
This paramount piece of information, nonetheless, seemed to be ignored; probably because it was seen as an inconvenient refutation of one of the strongest arguments against the existence of Human races. But, assuming so is ignoring the basic functioning of science: evidence is collected to seek support to reject a given null hypothesis, in favour of alternative ones. When discussing the creation of some kind of hierarchical divisions to better describe a species’ diversity, the null hypothesis is obviously that those categories are meaningless. Now, if the statistical tests that would be able to produce such tests do not work, the null hypothesis prevails – and the existence of racial categories remains unconvincing.
Can you hand me the map of Human diversity?
Our pattern of genetic diversity can provide further clues about our evolutionary history. Hunley et al.  gathered DNA samples of Human populations from every habited continent and ran several simulations testing assumptions from the main models trying to explain our gene identity variation. Only after allowing migration between contiguous populations after each region was colonized could the data be accurately comprehended. Conclusion: “the observed pattern of global gene identity variation was produced by a combination of serial population ﬁssions, bottlenecks and long-range migrations associated with the peopling of major geographic regions, and subsequent gene ﬂow between local populations”. According to them, were there to be any possible taxonomic divisions, the traditional geographic groups which have been used to determine races would be assigned in different levels of hierarchical classification, as represented in Figure 1.
Non-African populations significantly differ from the sub-Saharian ones, but as the former is nested within the latter, which is the root of Human diversity, they cannot be assigned within the same taxonomic unit. The same would happen with the East Asian and Native American populations. Because they are nested within the non-African populations, they should be hypothetically classified as a sub-race. As our species is thought to be originated in Africa, and from there colonized the rest of the World serially, Human diversity can be pictured as a matryoshka.
Later, Hunley et al.  advanced with a study, which was possibly the first identifying a root for the tree of Human populations without recurring to a different species as an outer-group. According to it, the Suruí from Brazil, despite being the least diverse population examined, still accounted for 60% of the global species’ diversity. From this, they concluded “that the biological race concept fails (…) because no matter how much variation might exist among human populations under a given model of evolution, human populations are not genetically homogeneous within”.
Notably, there are reports that diversity within certain African populations is greater than between this group as a whole and Eurasians . Still, some disagree that a great intra-group diversity implies the inexistence of races, were there to be a marked genetic discontinuity between the populations . However, Serre and Pääbo  provided evidence that Human variation is mostly clinal; and they argued that methods used in some previous studies were inflating genetic discontinuities.
What about the future?
Ramachandran et al.  stated: “There clearly has not been time to reach equilibrium between the extremes of man’s inhabited range, or even within continents, in the very short evolutionary history of modern humans”. But: will it ever happen?
The main intercontinental migration fluxes of nowadays are represented in Figure 2, though it should be accounted that the greatest flux registered is between Asian countries . 257.7 million people, 3.4% of the World’s population, have been registered as living as international migrants in 2017 . With data from only 11 countries, though gathered in different years, it was possible to very roughly estimate the existence of at least 35 million of illegal, non-accounted migrants. It also should be noticed that there are 700 million people registered as migrating internally in their country of origin . And, not only all these numbers are highly underestimated – due to lack of accuracy of the methods for data collection, and because many countries simply do not gather enough information on their census – as potential genetic inputs of tourists and short-term exchanges in the labour sphere are not being taken into account.
According to Hartl and Clark , genetic drift is one of the major forces increasing divergence between populations. They argue that, as an evolutionary process, migration is qualitatively similar to mutation, integrating new alleles into a population; whereas, quantitatively speaking, the migration rate among populations is always greater than the rate of mutation of a gene. In their mathematical analysis, both considering a one-way migration model and an island model of migration, migration turned out to have a remarkable homogenising effect. Nonetheless, migration does not always lead to population admixture.
In fact, Leonardi et al. (2018) , after giving examples of genetic isolates with long-term genealogical continuity, such as Basques, Ogliastra, and Casentino, documented a case of matrilineal continuity across 5000 years in Lucca, Italy. Notably, there are no geographical barriers or any known cultural trait which could justify such a degree of reproductive isolation through the considered centuries. They further suggested that some of the major migration events could consist essentially of males; and that military invasions may not have a remarkable genetic impact in the autochthonous populations, especially in the female lineages.
Nevertheless, with the advance of technology, travelling is getting more easy and accessible. If those changes are to be accompanied by a decrease of nepotism, and inter-marriage becomes more common, it is as if we were shaking the Human matryoshka to prepare a cocktail of Humanity. If examining race as a biological construct means studying the possibility of creating subspecies of Homo sapiens, then increasing the admixture between populations would definitely undermine the possibility of establishing reasonable criteria to guide further taxonomical divisions. Though the future cannot be predicted, it may be that our genetic unity is to be continued.
Rather than unique genetic features, it is the differences in alleles’ frequency that changes from population to population . Furthermore, our characteristics frequently have a polygenetic basis , which contributes to maintain our genetic unity. Thus, the great biological plasticity our species shows [5,6,7,8], may be less rooted in between-populations contrasts, than in a general ability to respond to changing environments. However, both recurring to phenotypical  and genetic  data, the origins of an individual can be assigned with a high success rate, at least at the continental level. Sill, it seems impossible to find reasonable criteria to evidence a coherent pattern within these continental groups. Because, within each cluster, populations are neither genetically homogeneous , nor diverging homogeneously ; Human variation is clinal, tendentially presenting no marked discontinuities ; and the model of Human diversity is that of a matryoshka, product of many serial migration events . Additionally, the more technology increases our mobility capacity, greater the admixture of populations – which prevents differentiation from crystalizing . Therefore, the present work argues that there is no ground to establish Human subspecies. Notably, not due to lack of Human diversity; on the contrary, since the existing diversity is overwhelming for the task of defining rational categories below the species level.
References in Part II: Race as a biological construct
 Koslowski, R. (2002). Human migration and the conceptualization of pre-modern world politics. International Studies Quarterly, 46(3), 375-399.
 Lordkipanidze, D., de León, M. S. P., Margvelashvili, A., Rak, Y., Rightmire, G. P., Vekua, A., & Zollikofer, C. P. (2013). A complete skull from Dmanisi, Georgia, and the evolutionary biology of early Homo. Science, 342(6156), 326-331.
 Green, R. E., Krause, J., Briggs, A. W., Maricic, T., Stenzel, U., Kircher, M., … & Hansen, N. F. (2010). A draft sequence of the Neandertal genome. Science, 328(5979), 710-722.
 Reich, D., Green, R. E., Kircher, M., Krause, J., Patterson, N., Durand, E. Y., … & Maricic, T. (2010). Genetic history of an archaic hominin group from Denisova Cave in Siberia. Nature, 468(7327), 1053.
 Walker, R., Gurven, M., Hill, K., Migliano, A., Chagnon, N., De Souza, R., … & Kramer, K. (2006). Growth rates and life histories in twenty‐two small‐scale societies. American Journal of Human Biology: The Official Journal of the Human Biology Association, 18(3), 295-311.
 Wells, J. C., & Stock, J. T. (2011). Re-examining heritability: genetics, life history and plasticity. Trends in Endocrinology & Metabolism, 22(10), 421-428.
 Parent, A. S., Teilmann, G., Juul, A., Skakkebaek, N. E., Toppari, J., & Bourguignon, J. P. (2003). The timing of normal puberty and the age limits of sexual precocity: variations around the world, secular trends, and changes after migration. Endocrine reviews, 24(5), 668-693.
 Wells, J. C., & Stock, J. T. (2012). The biology of human migration: the ape that won’t commit (pp. 21-44). In Crawford, M. H., & Campbell, B. C. (Eds.) (2012). Causes and consequences of human migration: An evolutionary perspective. Cambridge: Cambridge University Press.
 Sauer, N. J. (1992). Forensic anthropology and the concept of race: if races don’t exist, why are forensic anthropologists so good at identifying them?. Social Science & Medicine, 34(2), 107-111.
 Gagneux, P., Wills, C., Gerloff, U., Tautz, D., Morin, P. A., Boesch, C., … & Woodruff, D. S. (1999). Mitochondrial sequences show diverse evolutionary histories of African hominoids. Proceedings of the National Academy of Sciences, 96(9), 5077-5082.
 Bakewell, M. A., Shi, P., & Zhang, J. (2007). More genes underwent positive selection in chimpanzee evolution than in human evolution. Proceedings of the National Academy of Sciences, 104(18), 7489-7494.
 Hancock, A. M., Witonsky, D. B., Ehler, E., Alkorta-Aranburu, G., Beall, C., Gebremedhin, A., … & Di Rienzo, A. (2010). Human adaptations to diet, subsistence, and ecoregion are due to subtle shifts in allele frequency. Proceedings of the National Academy of Sciences, 200914625.
 Yang, J., Benyamin, B., McEvoy, B. P., Gordon, S., Henders, A. K., Nyholt, D. R., … & Goddard, M. E. (2010). Common SNPs explain a large proportion of the heritability for human height. Nature genetics, 42(7), 565.
 Lewontin, R. C. (1972). The apportionment of human diversity. In Evolutionary biology (pp. 381-398). Springer, Boston, MA.
 Sesardic, N. (2010). Race: a social destruction of a biological concept. Biology & Philosophy, 25(2), 143-162.
 Rosenberg, N. A., Pritchard, J. K., Weber, J. L., Cann, H. M., Kidd, K. K., Zhivotovsky, L. A., & Feldman, M. W. (2002). Genetic structure of human populations. science, 298(5602), 2381-2385
 Frankham, R., Ballou, S. E. J. D., Briscoe, D. A., & Ballou, J. D. (2002). Introduction to conservation genetics (p. 326). Cambridge university press.
 Long, J. C., & Kittles, R. A. (2009). Human genetic diversity and the nonexistence of biological races. Human biology, 81(5/6), 777-798.
 Long, J. C., Li, J., & Healy, M. E. (2009). Human DNA sequences: more variation and less race. American Journal of Physical Anthropology, 139(1), 23-34.
 Hunley, K. L., Healy, M. E., & Long, J. C. (2009). The global pattern of gene identity variation reveals a history of long‐range migrations, bottlenecks, and local mate exchange: implications for biological race. American Journal of Physical Anthropology, 139(1), 35-46.
 Hunley, K. L., Cabana, G. S., & Long, J. C. (2016). The apportionment of human diversity revisited. American journal of physical anthropology, 160(4), 561-569.
 Serre, D., & Pääbo, S. (2004). Evidence for gradients of human genetic diversity within and among continents. Genome research, 14(9), 1679-1685.
 Ramachandran, S., Deshpande, O., Roseman, C. C., Rosenberg, N. A., Feldman, M. W., & Cavalli-Sforza, L. L. (2005). Support from the relationship of genetic and geographic distance in human populations for a serial founder effect originating in Africa. Proceedings of the National Academy of Sciences, 102(44), 15942-15947.
 International Organization for Migration World Migration Report 2018 (https://publications.iom.int/system/files/pdf/wmr_2018_en.pdf)
 IOM’s Migration data portal (https://migrationdataportal.org/data?i=stock_abs_&t=2017)
 Hartl, D. L., & Clark, A. G. (1997). Principles of population genetics (pp. 194-196). 3rd ed. Sunderland: Sinauer associates.
 Leonardi, M., Sandionigi, A., Conzato, A., Vai, S., Lari, M., Tassi, F., … & Barbujani, G. (2018). The female ancestor’s tale: Long‐term matrilineal continuity in a nonisolated region of Tuscany. American journal of physical anthropology, 167(3), 497-506.