Student or Learner
This might seem a technical problem, not an English question, but when you say biological make-up, does it mean "your inherited traits" such as physical or psychological inheritance from your ancestors?
ex)Whereas general anthropology combines both nature and culture in its conceptual framework, social and cultural anthropologists are concerned with human belief and behavior that are not explicable in terms of biology. The basic assumption in sociocultural anthropology is that the range of variation in human belief and behavior cannot be explained or reduced to biology. This does not mean that social and cultural anthropologists deny that biology has any influence on social life;to do so would be ridiculous. What it does mean is that they treat human biological make-up as a constant, and attempt to explain the range of social and cultural variation that is not reducible to biology. Actually, this merely confirms the importance of a fundamental principle of logic: a constant cannot explain variation, the constant in this case being biology.
Does "biology" in this context mean, for example, sexual difference between man and woman? It could be an example that men are naturally more active or extroverted while women are more introverted or internal,(don't get me wrong, I might have used an inproper term), so it can explain how social composition is affected by these biological traits. Does the constant(=biology) mean such things?
So the biology can affect cultural phenomenon to a fixed point as it's a constant. Is this what the write is trying to say? I'm sorry to bring up too much a technical issue.
I'm not sure I understand what the problem is but, for what its worth, here's how I understand the paragraph.
Human beings are nearly identical biologically everywhere. There is no biological variation -- the biology of a human being is constant. Yet, there is variety of sociocultural phenomena around the world. Sociocultural traits are variable. A constant cannot explain variation. Therefore human biology cannot explain human culture.
Suppose culture is a function of (depends solely on) biology. Then, given the fact that everyone has the same biology, everyone should have the same culture.