I might point out that Singer's little book on Hegel is an absolute beauty (I admit this the enthusiastic recommendation of a non-philosopher, but the clarity of Singer's prose is to be seen to be believed - a bit of an advantage when discussing Hegel ... ).
Anyway, you quote Singer thusly:
For Marx, it is the "ensemble of social relations" which makes us the people we are, and so, as Singer points out, "It follows from this belief that if you can change the ‘ensemble of social relation’, you can totally change human nature."
Well, you can get that impression from *The German Ideology*, but you can get a very different one from the brilliant young (25ish) bloke who wrote the *Economic and Philosophical Manuscripts*. I know it's a bit unacceptably western-marxist of me, but Erich Fromm's *Marx's Concept of Man*, which follows the Frankfurters in going back to the young Marx, presents us (most compellingly) with a Marx quite unlike the one Singer feeds us. I certainly do not believe you have to throw out decisive lumps of the older Marx on grounds of assuming an essentially humanistic Marx. For instance, in *C*1, the 'older' Marx is pretty unambiguous on this:
'To know what is useful for a dog, one must study dog nature. This nature itself is not to be deduced from the principle of utility. Applying this to man, he that would criticise all human acts, movements, relations, etc, by the principle of utility, must first deal with human nature in general, and then with human nature as modified in each historical epoch.' (page 668 of the Charles Kerr Chicago 1906 edition)
Recalcitrant humanists (like yours truly) can, I think, happily have historical materialism and human essence residing in tandem. And it all sits well enough with Singer's proposed linkages.
I think the following Singer quotelet is pretty important stuff, hence its gratuitous repeat.
'First of all, we have evolved not to be ruthless proto-capitalists, but to "enter into mutually beneficial forms of co-operation." It is the evolutionary psychologist’s work in explaining how ‘survival of the fittest’ translates into co-operative behaviour which has been, arguably, its greatest success. Secondly, there is the "is/ought" gap. To say a certain type of behaviour has evolved is not to say it is morally right. To accept a need to understand how our minds evolved is not to endorse every human trait with an evolutionary origin.'
Cheers, Rob.