Ian spoke for himself, not Marx. I beg to differ that we discard the concept of nature or biology in understanding humans. Humans have both unique biological features ( bipedalism, high level of sociality e.g.) and features shared with other species ( instinct of self-preservation).
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Well, since we're playing at channeling dead people, I'll post some writing from a neglected Marxist who had an interesting proposition or two about the issue at hand. My posting of the writing by no means constitutes a product endorsement of all the terms brought to bear on the issue:
What do we mean by saying that existence precedes essence? We mean that man first of all exists, encounters himself, surges up in the world – and defines himself afterwards. If man as the existentialist sees him is not definable, it is because to begin with he is nothing. He will not be anything until later, and then he will be what he makes of himself. Thus, there is no human nature, because there is no God to have a conception of it. Man simply is. Not that he is simply what he conceives himself to be, but he is what he wills, and as he conceives himself after already existing – as he wills to be after that leap towards existence. Man is nothing else but that which he makes of himself.
[snip]
Furthermore, although it is impossible to find in each and every man a universal essence that can be called human nature, there is nevertheless a human universality of condition. It is not by chance that the thinkers of today are so much more ready to speak of the condition than of the nature of man. By his condition they understand, with more or less clarity, all the limitations which a priori define man’s fundamental situation in the universe. His historical situations are variable: man may be born a slave in a pagan society or may be a feudal baron, or a proletarian. But what never vary are the necessities of being in the world, of having to labor and to die there. These limitations are neither subjective nor objective, or rather there is both a subjective and an objective aspect of them. Objective, because we meet with them everywhere and they are everywhere recognisable: and subjective because they are lived and are nothing if man does not live them – if, that is to say, he does not freely determine himself and his existence in relation to them. And, diverse though man’s purpose may be, at least none of them is wholly foreign to me, since every human purpose presents itself as an attempt either to surpass these limitations, or to widen them, or else to deny or to accommodate oneself to them. Consequently every purpose, however individual it may be, is of universal value. Every purpose, even that of a Chinese, an Indian or a Negro, can be understood by a European. To say it can be understood, means that the European of 1945 may be striving out of a certain situation towards the same limitations in the same way, and that he may reconceive in himself the purpose of the Chinese, of the Indian or the African. In every purpose there is universality, in this sense that every purpose is comprehensible to every man. Not that this or that purpose defines man for ever, but that it may be entertained again and again. There is always some way of understanding an idiot, a child, a primitive man or a foreigner if one has sufficient information. In this sense we may say that there is a human universality, but it is not something given; it is being perpetually made. I make this universality in choosing myself; I also make it by understanding the purpose of any other man, of whatever epoch. This absoluteness of the act of choice does not alter the relativity of each epoch.
What is at the very heart and center of existentialism, is the absolute character of the free commitment, by which every man realises himself in realising a type of humanity – a commitment always understandable, to no matter whom in no matter what epoch – and its bearing upon the relativity of the cultural pattern which may result from such absolute commitment. One must observe equally the relativity of Cartesianism and the absolute character of the Cartesian commitment. In this sense you may say, if you like, that every one of us makes the absolute by breathing, by eating, by sleeping or by behaving in any fashion whatsoever. There is no difference between free being – being as self-committal, as existence choosing its essence – and absolute being. And there is no difference whatever between being as an absolute, temporarily localised that is, localised in history – and universally intelligible being.
<http://www.marxists.org/reference/archive/sartre/works/exist/sartre.htm>
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I have no problem at all with humans doing biology to understand themselves. I'd assert that as the concepts used in the disciplines of biology, ecology, neuroscience, medicine etc. more thoroughly permeate human societies, people will no longer bother with futile arguments over what is and is not human nature.