Which it might well be if that was the quote. Marx says : 'The anatomy of man is a key to the anatomy of the ape.' The addition 'but not vice versa' would indeed be a bridge too far.
What Marx means is that it is easier to uncover the course of development from the starting point of the higher form, to see the important features in the lower (not that modern day Darwinians would use those terms). It wasn't actually a discussion of biology, but a biological analogy to explain the development of forms of property relations.
Far from being bullshit, it is rather good, as long as it is understood to describe the process of research, not the actual development itself.