It is normal for humans to consider themselves beyond or above nature. It isn't that humans oppose 'culture' to 'nature' in a neat binary; it's simply that they don't tend to think of their societies as being part of the natural world, no matter where they live or what society they live in.
This is a tendency that has continued in studies of human beings: anthropology is widely considered to be something other than 'a science' because its subject matter is humans, beings apparently unlike other animals. There is a subtle implication that social science is completely different to natural science, and in some respects incommensurate with it, because humans are sentient beings who perform actions in a network of social and cultural meaning, as opposed to brute apes who blindly attempt to maximise procreation.
In some cases, this idea is made explicit, as it was by R. G. Collingwood, an early-20th century Oxford philosopher. Collingwood claimed that the difference between science and history - and by extension all social sciences - could be found in approach. Science depends on analysing things from the outside: to find out about birds or minerals, a scientist has to look at their outward properties and watch their behaviour without interpolating anything about their internal qualities without good reason for it. You don't begin with the beliefs and desires of a pile of ore. You proceed from the outside-in.
By contrast, Collingwood claimed, history proceeds from the inside-out: only by beginning with the beliefs and thoughts of humans can history be understood. We don't think about Caesar crossing the Rubicon as a simple, visible, outward action. It was only meaningful and, well, historical, because of Caesar's thoughts and the act's cultural meaning. In order to understand history, we have to proceed from the inside (emotion, culture, and thought) outward to action, because humans are special creatures living in a parallel social world that is different from the natural one.
But this perspective is wrong. In history or social science, we don't have to begin with unwarranted humanistic leaps of imagination. We begin with some kind of evidence, whether it be statements or other actions made by humans, and on the basis of other evidence - say, from the mass of experience we've had of living with humans - make inferences about mental states that would causally explain the behaviour. It's not the same as peering into the minds of others (which is in any case an impossibility). We really don't know much about the mental states of Gaius Iulius Caesar as he was crossing the Rubicon except by what was written down by him and others, and any inferences we make have to be cautious inferences made on the basis of a scientific appraisal of human emotion and cognition. We don't have to think of human rule-following (or, in the case of Caesar, rule-breaking) as something unnatural or 'social' in contrast to 'natural'. Social stuff just is natural stuff.
There is nothing about Caesar crossing the Rubicon that is unnatural. Nothing about it is beyond nature and no investigation of it should proceed from the 'inside-out'. It should begin from the outside-in, with evidence, and with the understanding that all human action is determined by natural mental causes, rather than beginning in a parallel social world. It's all natural, and all investigations of the world - the single, natural, world - should be commensurate with naturalism. This includes investigations of beliefs and ideas held by groups of people, including those beliefs that 'give meaning' to life or that relate to one another in loose systems. Beliefs are perfectly natural and are not 'inside' in contrast to the basic facts of existence, considered to be 'outside'.
Anthropologists - the socio-cultural kind, the ones who need to do ethnographic fieldwork to obtain a doctorate - tend to focus on these systems of belief. It is comparatively rare to find discussions of prehistory, language family affiliation, subsistence, settlement hierarchy, or ordinary material culture in the work of social anthropologists, who are often more interested in systems of belief and idea, and in the workings of local social-ideological systems, than in the basics of life. This type of investigation is useful and it can certainly help inform us about human life, and I wouldn't want to give the impression that I find it valueless or non-anthropological. We can infer from such studies that no matter who they are or where they live, humans have complex and interesting mental lives full of 'meaning'. That's important. Figuring out how these systems work, if they really are systems at all, is also important.
But that's not all there is to it. If you only focus on the existing belief systems - which aren't exactly concrete, and which are much more likely to be misinterpreted than other products of human existence - then you'll miss out on a bunch of other interesting stories.
I wrote about headhunting for my master's thesis. It's not an especially good thesis - a bit muddled, honestly, and I'd write it very differently now - but I did end up reading about a number of different headhunting theories, especially with regard to maritime southeast Asia. The main theory now is that headhunting spread through island southeast Asia as a practice of Austronesian-speaking people.
A form of headhunting was once to be found in almost all societies in the Indo-Malaysian-Philippine archipelago, as well as in Taiwan (the Urheimat of Austronesian) and large swathes of Melanesia, especially the Austronesian-speaking areas (it was probably introduced to non-Austronesian-speaking populations on New Guinea by Austronesian-speaking interlopers). The fact that almost identical practices were once to be found in Melanesia demonstrates that headhunting must have spread with Austronesian languages, because any other mode of transmission in southeast Asia - say, the diffusion of metallurgy in the archipelago in the late second-millennium BCE, or the diffusion of iron-working around 7-500 BCE - was missed by Melanesians.
So the tradition has to be 5,000 years old or so. It spread in the Neolithic. Here's the thing: when ethnographers in the nineteenth and twentieth centuries asked local people why they took heads, they received different answers. Often wildly different answers, in fact.
Some people claimed that they took heads because the ancestors (generally known by a term derived from proto-Austronesian *qanicu, IIRC) were angry: they had to be placated with heads. (Why the ancestors wanted heads was explained differently in different communities, too.) Others noted a connection between fertility and the taking of heads - taking heads to ensure that there will be enough food for the community, with the head connected to metaphors of fruit and growth. Yet others found a connection between taking heads and personal grief - that's the famous conclusion of research by Michelle and Renato Rosaldo in twentieth-century Ilongot communities. One researcher, Andrew Vayda, who restricted himself to headhunting in Borneo, believed that headhunting had begun there due to payments for the heads of enemies by Malay pirates operating in Bornean waters, which we now know to be false.
These reasons are all restricted in distribution. The practice itself covers a much wider area than any of the reasons for it. What this means is that if we look for a reason for the practice in any system of beliefs of the kind social anthropologists usually concern themselves with, we won't have a real explanation. We might believe that we have explained Ilongot headhunting, but we won't have found out why Ilongot people actually took heads. It is clearly closely related to other practices in the Indo-Malaysian archipelago which were apparently undertaken for different reasons, and its explanation must have something to do with this relationship rather than with a set of abstract beliefs about the practice in any single population.
In order to explain the practice, we'd have to look at the social psychology of headhunting in communities with clans and lineages and a belief in ancestral spirits, all of which can be reconstructed to proto-Austronesian. Coming-of-age is clearly connected to headhunting; in almost all communities, men needed to take heads to be considered marriageable or masculine. Men who didn't take heads wouldn't be able to settle down and have children, or realistically do anything in the community. Whatever the justification of the practice in terms of a belief system, I think we'd actually find that headhunters were motivated more by the desire for advancement in a community that valued headhunting than by the religious reasons for the practice. We'd certainly need to look at the diversity of traditions, including the belief systems and metaphorical associations that ethnographers like to focus on - but we would need to understand from the outset that these systems could actually be post-hoc rationalisations for existing practices instead of the root reason.
The idea of researching language family affiliation and prehistory is relegated to the status of a minor concern in anthropology departments despite its clear importance for understanding what people do and why people do it, and researchers who focus on these questions - among them notably Jared Diamond - are sometimes accused of bringing a naturalist's eye to what should be humanistic work (in some cases, as in Diamond's, this may sometimes be warranted; see Alex Golub's review of Diamond's The World Until Yesterday here, and the short discussion here).
We should be looking inside the heads of others, they say, and the things that should interest us should be the things that interest our subjects. A plough is interesting if the users of it invest it with 'meaning', not because it might show ancient connections between diffuse populations. Look at the system of ideas and the connections between different cultural phenomena: that's where humanisation and interest are to be found, not in the origins of or reasons behind certain practices. There's the implication that researching the prehistory or basic cultural history of a population is demeaning or, somehow, colonialist, instead of something necessary when it comes to explaining why people do what they do.
I think research on belief systems is vital and interesting stuff. But it doesn't tell us all that much about what people do and why they do it. A total study of humankind shouldn't begin with the premise that 'meaning' is the most important thing in human life or the most important determinant of human history, because it tends not to be. People like to have meaning to what they do, but they also don't seem to mind justifying what they already do with convenient reasons or mythological meaning. 'Meaning' is only one part of the picture.
It's also perfectly natural, and there's nothing about anthropology, any branch of it, that makes it separate from nature or naturalism.