Marx changed his mind about a lot of stuff, but not this, as a quick perusal of the final part of Capital Vol. 1 should make pretty obvious. To be clear, Marx does not claim that slavery and colonialism are sufficient to explain the development of capitalism, but he does seem to be saying that they played an essential role in its concrete historical emergence.

Here’s what Pakistani Marxist scholar Hamza Alavi has to say about this division between “early” and “late”:

Wood is led away from that key definition in Marx’s thinking. Instead she mistakenly posits the existence of ‘two different narratives’ in Marx. The first of these she attributes to the German Ideology and The Communist Manifesto. In that ‘conventional model’, (as she puts it), history is a succession of stages in the division of labour, with a transhistorical (sic) process of technological progress and the leading role assigned to burgher classes who seem to bring about capitalism just by being liberated from feudal chains’. This rendering of Marx’s ideas is unrecognisable. (Wood, 1997: 10; emphasis added). The second ‘narrative’ in Marx, she writes, is to be found in the Grundrisse and Capital. That, she writes, ‘has more to do with changing property relations’. We can take this notion of ‘changing property relations’ as a euphemism (that obscures rather than clarifies) for the separation of the producer from the means of production. Further on Wood writes: ‘What Marx is trying to explain is the accumulation of wealth’ (ibid:13) Wood must know that there is a fundamental conceptual difference between the idea of accumulation of ‘wealth’ (which could include such ‘wealth’ as palaces or jewels etc. which are unproductive) and that of the ‘accumulation of capital’ that provides a basis of ever rising circuits of production. Accumulation of capital refers to the conversion of surplus value into productive capital, which sets in train a process of reproduction on a progressively increasing scale. It was the accumulation of capital that Marx’s work was all about. One should not have to point out such elementary distinctions to someone whose work has been celebrated so generously in Historical Materialism.

full: http://ourworld.compuserve.com/homepages/sangat/Colonial.htm

“Direct slavery is just as much the pivot of bourgeois industry as machinery, credits, etc. Without slavery you have no cotton; without cotton you have no modern industry. It is slavery that gave the colonies their value; it is the colonies that created world trade, and it is world trade that is the precondition of large-scale industry. Thus slavery is an economic category of the greatest importance.” –The Poverty of Philosophy (1847)