most of Guillaumin's work relates to Europe, and unfortunately not widely translated into English from French. there's an essay of hers in a volume on scientific racism -eugenics - but I can't find the ref unless I go sorting through boxes. but, from memory, Gullaimin argues that modern racism had its origins in the challenge to caste: that in the eighteenth and nineteenth centuries, it forms the response to challenges to hereditary nobility by designating social hierarchies as given in nature (as distinct from given by god), and it is linked with the emergence of nationalism (as in the emergence of an ethnicised definition of national belonging and identity), and finally the contradictions of liberal capitalism.
Balibar, referring to guillaumin's work, says:
"The aristocracy did not initially conceive and present itself in terms of the category of 'race': this is a discourse which developed at a late stage, the function of which is clearly defensive (as can be seen from the example of France with the myth of 'blue blood' and the 'Frankish' or 'Germanic' origin of hereditary nobility), and which developed when the absolute monarchy centralized the state at the expense of the feudal lords and began to create within its bosom a new administrative and financial aristocracy which was bourgeois in origin, thus marking a decisive step in the formation of the nation-state. ... [and, for instance] the persecution of the Jews after the Reconquista [in Spain], one of the indispensable mechanisms in the establishment of Catholicism as state religion, is also the trace of the 'multinational' culture against which Hispanization (or rather Castillianization) was carried out. It is therefore intimately linked to the formation of this prototype of European nationalism. yet it took on an even more ambivalent meaning when it gave rise to the 'statutes of the purity of blood' (limpienza de sangre) which the whole discourse of European and American racism was to inherit: a product of the disavowal of the original interbreeding with the Moors and the Jews, the hereditary definition of the raza (and the corresponding procedures for establishing who could be accorded a certificate of purity) serves in effect both to isolate an internal aristocracy and to confer upon the whole of the 'Spanish people' a fictive nobility, to make it a 'people of masters' at the point when, by terror, genocide, slavery and enforced Christianization, it was conquering and dominating the largest of the colonial empires. In this line of development, class racism was already transformed into nationalist racism, though it did not, in the process disappear.
... Aristocratic racism ... is already indirectly related to the primitive accumulation of capital, if only by its functions in the colonizing nation. The industrial revolution, at the same time as it creates specifically capitalist relations of production, gives rise t the new racism if the bourgeois era... : the one which has as its target the proletariat in its dual status as exploited population... and politically threatening population. ... It is at this point, with regard to the 'race of labourers' that the notion of race becomes detached from its historico-theological connotations to enter the field of equivalences between sociology, psychology, imaginary biology and the anthropology of the 'social body'. ... For the first time those aspects typical of every procedure of racialisation of a social group right down to our own day are condensed in a single discourse: material and spiritual poverty, criminality, congenital vice (alcoholism, drugs), physical and moral defects, dirtiness, sexual promiscuity and the specific diseases which threaten humanity with 'degenracy'. ... Through these themes, there forms the phantasmatic equation of 'labouring classes' with 'dangerous classes', the fusion of a socio-economic category with an anthropological and moral category...
... The 'bourgeois revolutions' - and in particular, the French revolution, by its juridical egalitarianism - had raised the question of the political rights of the masses in an irreversible manner. This was to be the object of one and a half centuries of social struggles. The idea of a difference in nature between individuals had become juridically and morally contradictory, if not inconceivable. It was, however, politically indispensable, so long as the 'dangerous classes' ... had to be excluded by force and by legal means from political 'competence' and was confined to the margins of the polity - so long as it was important to deny then citizenship by showing, and by being oneself persuaded, that they constitutionally 'lacked' the qualities of fully fledged or normal humanity. Two anthropologies clashed here: that of equality of birth and that of hereditary inequality which made it possible to re-naturalise social antagonisms."
Etienne Balibar, "Class Racism" (pp208-10) in Balibar and Wallerstein _Race, Nation, Class_.