Science Snippets: Why Humans Dominate

Draft script:

It’s difficult to argue with the idea that humans dominate the planet. Consider what we have done, collectively. We are the apex predator, despite our having no great speed and no morphological features that give us advanced ability to kill. At the local and regional level, we have modified landscapes sufficiently to remove habitat for many species. At the global scale, have put such an enormous volume of greenhouse gases into the atmosphere that we threaten the continued existence of many species, including ours. Our planetary home is amid abrupt, irreversible climate change, an event unprecedented in planetary history.

Yet, the question persists: Why do humans dominate?

The lede for an article published at Phys.Org on 7 November 2024 poses the question: “Why is human culture—the shared body of knowledge passed down across generations—so much more powerful than animal cultures?” The title of the article is Evolutionary anthropologist presents hypothesis about why humans are dominating the world over other animals.

I appreciate the title’s recognition that humans are animals: “why humans are dominating the world over other animals.” The second paragraph of the article at Phys.Org , after the question about human culture, asks the relevant question: “What’s special about our species?”

The question is addressed by two scholars in a peer-reviewed paper published in the renowned Nature series of publications. Published 7 November 2024, the paper is titled Human culture is uniquely open-ended rather than uniquely cumulative. It was published in Nature Human Behaviour. I recognize that the language used in this peer-reviewed paper can be a little daunting to those of us who are not anthropologists. Still, I believe it is worth the effort to persevere.

The Abstract tells much of the story: “Theories of how humans came to be so ecologically dominant increasingly centre on the adaptive abilities of human culture and its capacity for cumulative change and high-fidelity transmission. Here we revisit this hypothesis by comparing human culture with animal cultures and cases of epigenetic inheritance and parental effects. We first conclude that cumulative change and high transmission fidelity are not unique to human culture as previously thought, and so they are unlikely to explain its adaptive qualities. We then evaluate the evidence for seven alternative explanations: the inheritance of acquired characters, the pathways of inheritance, the non-random generation of variation, the scope of heritable variation, effects on organismal fitness, effects on genetic fitness and effects on evolutionary dynamics. From these, we identify the open-ended scope of human cultural variation as a key, but generally neglected, phenomenon. We end by articulating a hypothesis for the cognitive basis of this open-endedness.”

Unfortunately, this Abstract fails to reveal the hypothesis derived in this peer-reviewed paper, which is not open-access. My Emeritus status allows access, and I quote from the Discussion, which relies on many other peer-reviewed papers:

“We have reviewed a wide range of factors to identify how human culture differs from animal cultures and from other modes of non-genetic inheritance. Specifically, we first argued that cumulative change and transmission stability are not as characteristic of human culture as previously suggested and so are unlikely to explain the adaptive propensity of human culture. We then considered a range of other hypotheses. In most cases, human culture exhibits similar properties to animal cultures and other forms of non-genetic inheritance. However, the most distinctive feature of human culture, which has hitherto been relatively unexplored, is its comparatively open-ended capacity for generating heritable variation. That is, many modes of inheritance involve the stable transmission of non-random, acquired variation that accumulates over time and flows along non-vertical pathways. However, these modes are typically limited to particular kinds of variation, such as songs or antibodies, or those that are supported by the regulation of existing genetic variation. Indeed, the scopes of epigenetic inheritance and parental effects are sufficiently dependent on available genetic variation that novel heritable traits usually depend on genetic change. For instance, epigenetic factors typically switch between previously evolved alternatives, be they desert locust morphs, Daphnia helmets or wild radish trichome density. By contrast, human culture is much less constrained and can be characterized as open-ended.”

The paper then goes on to provide supporting information from many other published, peer-reviewed papers:

“Although we suggest that the broad scope of human culture has been underexplored, some studies have identified open-endedness as an important factor. For instance, it has been suggested that although animal cultures can cumulatively increase the efficiency with which they exploit natural phenomena, only human cumulative culture is able to expand to exploit new phenomena. Other work has explored how human culture is enhanced by recombination and the ability to avoid getting stuck at local optima. Although such work does not evaluate alternative hypotheses or draw on literature concerning epigenetic inheritance or parental effects, it is nonetheless consistent with our position that open-endedness, as opposed to cumulativity or stability, is the key distinction between human and animal cultures, as well as other modes of non-genetic inheritance.

…

How best to assess cultural open-endedness across species? One approach is to map out the scope of inheritance systems through long-term observations and by testing what can or cannot be transmitted. However, such a top-down approach is likely to be extremely time consuming given the variety of cultural traits and the potentially slow pace of cultural change. Instead, we suggest a bottom-up approach in which how inheritance systems store information is used to understand and predict the kinds of variation they can support. That is, we suggest drawing on cognitive science to inform our understanding of the limits cultural inheritance. For instance, differences between the scope of human culture and that of animal cultures and other modes of non-genetic inheritance may result from how individuals mentally represent actions and their goals. It is well established that complex action sequences are represented hierarchically; that is, an overarching goal is broken into a series of sub-goals, each of which can consist of its own series of steps, sub-steps and so on. Such a system uses working memory resources more efficiently than non-hierarchical representations. This approach has been used to describe dust bathing in birds, nettle folding in gorilla foraging, human memory of short stories and how humans sort items. In particular, the songs of humpback whales, which evolve cumulatively in their complexity, are hierarchically composed of themes and smaller phrases. Such cumulatively evolving animal vocalizations may prove a useful model system for the role of hierarchical representation in cultural change.”

The Discussion section goes on provide additional supporting information and a few examples:

“A system for the hierarchical representation of actions can differ across species in a multitude of different ways that can be identified both behaviourally and neurologically. For instance, given that they support working memory, effective representations can be detected by improved performance when hierarchical representations are possible. Cross-species work could also assess the number of nested levels that can be represented, the maximum number of steps at any level, the ability to adjust plans when a particular step fails, how long representations can be held in working memory and the accuracy of representations constructed through observation. In particular, the open-ended, cumulative evolution of hierarchical songs, but not other behaviours, in whales and songbirds may prove a useful entry. All this work would contribute to a cognitive basis for the causes of culture in humans and other species, as well as why our species is uniquely cultural and our culture so adaptive. Nonetheless, such considerations have been hindered by strong emphases on cumulative change and transmission stability as the distinctive features of human culture. Whether our particular hypothesis is correct or not, we suggest that such emphases may no longer be warranted and that greater insight may be gained by considering alternative explanations for the unique adaptiveness of human culture.”

I greatly appreciate the humility in this final sentence: “Whether our particular hypothesis is correct or not …” Pointing the way to greater insight is also a good idea. Why humans dominate other organisms is a critical issue. However, I doubt we have time to determine the definitive answer to this and many other important questions.

Neurological evidence supporting this hypothesis has identified a rostro-caudal axis of the frontal lobe involved in the representation of actions. Critically, the patterning of activity along this axis matches the hierarchical arrangement of sub-goals, with caudal activity associated with concrete steps and action execution, while rostral activity corresponds to abstract, higher-level goals. Further support comes from the observation that damage to points along this axis leads to impairments at the corresponding level of abstraction. Of particular importance to theories of cultural evolution, the same system appears to represent the individual’s own actions and goals as well as those of others that they observe. It turns goals into a hierarchy of steps and actions that can be executed, as well as inferring the hierarchy and goals behind another individual’s actions. It is thus well positioned to explain the differing scopes of human and animal cultures.

This limited scope of epigenetic, parental and animal-cultural variation masks the capacity of these systems for cumulative change and has contributed to the perception that, among non-genetic biological processes, cumulative change is unique to our species’ culture. The distinction between open-ended cumulative change being rare, and any capacity for cumulative change being rare, is important because broad acceptance of the latter hypothesis led to theories focusing on specific features of human cultural transmission that enable cumulative change. However, as discussed above, such theories remain contested. Indeed, we suggest that such theories will probably never fully succeed because human culture is not unique in its mere capacity for cumulative change. Instead, we suggest that it may be more fruitful to explore why human culture can accumulate in a uniquely open-ended fashion.