Tribal History

Kevin Lewis

March 02, 2024

Super-additive cooperation
Charles Efferson et al.
Nature, 29 February 2024, Pages 1034-1041 


Repeated interactions provide an evolutionary explanation for one-shot human cooperation that is counterintuitive but orthodox. Intergroup competition provides an explanation that is intuitive but heterodox. Here, using models and a behavioural experiment, we show that neither mechanism reliably supports cooperation. Ambiguous reciprocity, a class of strategies that is generally ignored in models of reciprocal altruism, undermines cooperation under repeated interactions. This finding challenges repeated interactions as an evolutionary explanation for cooperation in general, which further challenges the claim that repeated interactions in the past can explain one-shot cooperation in the present. Intergroup competitions also do not reliably support cooperation because groups quickly become extremely similar, which limits scope for group selection. Moreover, even if groups vary, group competitions may generate little group selection for multiple reasons. Cooperative groups, for example, may tend to compete against each other. Whereas repeated interactions and group competitions do not support cooperation by themselves, combining them triggers powerful synergies because group competitions constrain the corrosive effect of ambiguous reciprocity. Evolved strategies often consist of cooperative reciprocity with ingroup partners and uncooperative reciprocity with outgroup partners. Results from a behavioural experiment in Papua New Guinea fit exactly this pattern. They thus suggest neither an evolutionary history of repeated interactions without group competition nor a history of group competition without repeated interactions. Instead, our results suggest social motives that evolved under the joint influence of both mechanisms.

Concern for Animals among Hunter-Gatherers
Barton Thompson
Cross-Cultural Research, forthcoming 


This study examined the degree to which hunter-gatherers exhibited concern for animals. Six types of concern (sympathy, consequential, identity-based, respectful, protective, and indifference) were assessed in twenty-eight hunting-gathering groups from around the world using eHRAF World Cultures. Findings demonstrated that sympathy for animal agents was low, and indifference was high. High levels of consequential concern and moderate levels of identity and respectful concern were evident. Protective concern was found in about a third of the groups. Higher levels of concern were experienced when the animals were perceived as either pseudo-humans or pseudo-spirits. The transition from hunter-gatherer to agro-industrial lifestyle has led to fewer interactions and less overall concern for animals, but it has not produced a decrease in sympathy for animal agents, which was already relatively low among foragers. These findings lend support to the theory that humans did not evolve to experience high levels of sympathetic concern for animals as animal agents, and it helps to explain the widespread present exploitation of animals. It also demonstrates that concern can and does arise when animals take on pseudo-human agency.

The contribution of gene flow, selection, and genetic drift to five thousand years of human allele frequency change
Alexis Simon & Graham Coop
Proceedings of the National Academy of Sciences, 27 February 2024 


Genomic time series from experimental evolution studies and ancient DNA datasets offer us a chance to directly observe the interplay of various evolutionary forces. We show how the genome-wide variance in allele frequency change between two time points can be decomposed into the contributions of gene flow, genetic drift, and linked selection. In closed populations, the contribution of linked selection is identifiable because it creates covariances between time intervals, and genetic drift does not. However, repeated gene flow between populations can also produce directionality in allele frequency change, creating covariances. We show how to accurately separate the fraction of variance in allele frequency change due to admixture and linked selection in a population receiving gene flow. We use two human ancient DNA datasets, spanning around 5,000 y, as time transects to quantify the contributions to the genome-wide variance in allele frequency change. We find that a large fraction of genome-wide change is due to gene flow. In both cases, after correcting for known major gene flow events, we do not observe a signal of genome-wide linked selection. Thus despite the known role of selection in shaping long-term polymorphism levels, and an increasing number of examples of strong selection on single loci and polygenic scores from ancient DNA, it appears to be gene flow and drift, and not selection, that are the main determinants of recent genome-wide allele frequency change. Our approach should be applicable to the growing number of contemporary and ancient temporal population genomics datasets.

Trypillia mega-sites: A social levelling concept?
Robert Hofmann, Nils Müller-Scheeßel & Johannes Müller
Antiquity, forthcoming 


Explanations for the emergence and abandonment of the Chalcolithic Trypillia mega-sites have long been debated. Here, the authors use Gini coefficients based on the sizes of approximately 7000 houses at 38 Trypillia sites to assess inequality between households as a factor in the rise and/or demise of these settlements. The results indicate temporarily reduced social inequality at mega-sites. It was only after several generations that increased social differentiation re-emerged and this may explain the subsequent abandonment of the mega-sites. The results indicate that increases in social complexity need not be associated with greater social stratification and that large aggregations of population can, for a time at least, find mechanisms to reduce inequality.

On the genetic basis of tail-loss evolution in humans and apes
Bo Xia et al.
Nature, 29 February 2024, Pages 1042-1048


The loss of the tail is among the most notable anatomical changes to have occurred along the evolutionary lineage leading to humans and to the ‘anthropomorphous apes’, with a proposed role in contributing to human bipedalism. Yet, the genetic mechanism that facilitated tail-loss evolution in hominoids remains unknown. Here we present evidence that an individual insertion of an Alu element in the genome of the hominoid ancestor may have contributed to tail-loss evolution. We demonstrate that this Alu element -- inserted into an intron of the TBXT gene -- pairs with a neighbouring ancestral Alu element encoded in the reverse genomic orientation and leads to a hominoid-specific alternative splicing event. To study the effect of this splicing event, we generated multiple mouse models that express both full-length and exon-skipped isoforms of Tbxt, mimicking the expression pattern of its hominoid orthologue TBXT. Mice expressing both Tbxt isoforms exhibit a complete absence of the tail or a shortened tail depending on the relative abundance of Tbxt isoforms expressed at the embryonic tail bud. These results support the notion that the exon-skipped transcript is sufficient to induce a tail-loss phenotype. Moreover, mice expressing the exon-skipped Tbxt isoform develop neural tube defects, a condition that affects approximately 1 in 1,000 neonates in humans. Thus, tail-loss evolution may have been associated with an adaptive cost of the potential for neural tube defects, which continue to affect human health today.

Mediterranean Early Iron Age chronology: Assessing radiocarbon dates from a stratified Geometric period deposit at Zagora (Andros), Greece
Rudolph Alagich et al.
Antiquity, forthcoming 


In this article, the authors present an analysis of radiocarbon dates from a stratified deposit at the Greek Geometric period settlement of Zagora on the island of Andros, which are among the few absolute dates measured from the period in Greece. The dates assigned to Greek Geometric ceramics are based on historical and literary evidence and are found to contradict absolute dates from the central Mediterranean which suggest that the traditional dates are too young. The results indicate the final period at Zagora, the Late Geometric, should be seen as starting at least a century earlier than the traditional date of 760 BC.


from the


A weekly newsletter with free essays from past issues of National Affairs and The Public Interest that shed light on the week's pressing issues.


to your National Affairs subscriber account.

Already a subscriber? Activate your account.


Unlimited access to intelligent essays on the nation’s affairs.

Subscribe to National Affairs.