The Phylogenetic Roots of Human Lethal Violence José María Gómez1,2, Miguel Verdú3, Adela González-Megías4 & Marcos Méndez5

Home , Allactaga, Euarchonta, Nesokia, Pardofelis, Primatomorpha, Rusa (genus)

Letter doi:10.1038/nature19758

The phylogenetic roots of human lethal violence José María Gómez1,2, Miguel Verdú3, Adela González-Megías4 & Marcos Méndez5

The psychological, sociological and evolutionary roots of 600 human populations, ranging from the Palaeolithic era to the present conspecific violence in humans are still debated, despite attracting (Supplementary Information section 9c). The level of lethal violence the attention of intellectuals for over two millennia1–11. Here we was defined as the probability of dying from intraspecific violence propose a conceptual approach towards understanding these roots compared to all other causes. More specifically, we calculated the level based on the assumption that aggression in mammals, including of lethal violence as the percentage, with respect to all documented humans, has a significant phylogenetic component. By compiling sources of mortality, of total deaths due to conspecifics (these sources of mortality from a comprehensive sample of mammals, were infanticide, cannibalism, inter-group aggression and any other we assessed the percentage of deaths due to conspecifics and, type of intraspecific killings in non-human mammals; war, homicide, using phylogenetic comparative tools, predicted this value for infanticide, execution, and any other kind of intentional conspecific humans. The proportion of human deaths phylogenetically killing in humans). predicted to be caused by interpersonal violence stood at 2%. Lethal violence is reported for almost 40% of the studied mammal This value was similar to the one phylogenetically inferred for species (Supplementary Information section 9a). This is probably the evolutionary ancestor of primates and apes, indicating that a an underestimation, because information is not available for many certain level of lethal violence arises owing to our position within species. Overall, including species with and without lethal violence, the phylogeny of mammals. It was also similar to the percentage we found that the percentage of deaths due to conspecifics was seen in prehistoric bands and tribes, indicating that we were as 0.30 ± 0.19% of all deaths (phylogenetically corrected mean ± s.e.m). lethally violent then as common mammalian evolutionary history This level was not affected by the number of individuals sampled would predict. However, the level of lethal violence has changed per species (Supplementary Information section 1). These findings through human history and can be associated with changes in suggest that lethal violence, although infrequent, is widespread among the socio-political organization of human populations. Our study mammals19–21. provides a detailed phylogenetic and historical context against We determined whether related species tended to have similar levels which to compare levels of lethal violence observed throughout of lethal violence by calculating the phylogenetic signal. We used the our history. most recently updated mammalian phylogenies, including 5,020 Debate on the nature of human violence has been ongoing since extant mammals22 and 5,747 extant and recently extinct mammals 23. before the publication of Leviathan by Thomas Hobbes in 1651. We found a significant phylogenetic signal for lethal violence, even Lethal violence is considered by some to be mostly a cultural trait5,6,12; after combining disparate causes of intraspecific killings (λ > 0.60, however, aggression in mammals, including humans13,14, also has a P < 0.0001; Supplementary Information section 2). While lethal genetic component with high heritability. Consequently, it is widely violence was uncommon in certain clades such as bats, whales and acknowledged that evolution has also shaped human violence2–4. lagomorphs, it was frequent in others, such as primates (Fig. 1). The From this perspective, violence can be seen as an adaptive strategy, phylogenetic signal was also significantly lower than one (P < 0.0001), favouring the perpetrator’s reproductive success in terms of mates, indicating that lethal violence exhibits certain evolutionary flex- status or resources15,16. Yet this does not mean that violence is invariant ibility (Fig. 1). For example, the level of lethal violence strongly or even adaptive in all situations15. In fact, given that the conditions differs between chimpanzees (Pan troglodytes) and bonobos under which violence benefits evolutionary fitness depend on the (Pan paniscus)10,17,20. This outcome suggests that additional factors ecological and cultural context, levels of violence tend to vary among may subsequently modify the level of lethal violence in related species. human populations 12,13,15,16. Disentangling the relative importance Territoriality and social behaviour mediate conspecific aggression in of cultural and non-cultural components of human violence is mammals20,24. We scored these two traits for every mammal in our challenging 3,5 owing to the complex interactions between ecological, study and statistically related them to the level of lethal violence using social, behavioural and genetic factors. phylogenetic generalized linear models. Using this method, we found Conspecific violence is not exclusive to humans. Many primates that the level of lethal violence was higher in social and territorial exhibit high levels of intergroup aggression and infanticide4,10. Social species than in solitary and non-territorial species (Fig. 2; Extended carnivores sometimes kill members of other groups and commit Data Table 1). infanticide when supplanting older members of the same group17,18. The occurrence of a phylogenetic signal for lethal violence in Even seemingly peaceful mammals such as hamsters and horses mammals enables the phylogenetic inference of lethal violence in sometimes kill individuals of their own species19,20. The prevalence humans. We used ancestral-state estimation methods that infer the of aggression throughout Mammalia raises the question of the extent value of a trait in any extant species according to its position in the to which levels of lethal violence observed in humans are as expected, phylogenetic tree25. The level of human lethal violence was estimated given our position in the phylogenetic tree of mammals. In this study, both with and without considering the territoriality and sociability of we quantified the level of lethal violence in 1,024 mammalian species mammals. Because phylogenetic inferences are much more accurate from 137 families (Supplementary Information section 9a) and in over and reliable when including information from close relatives26

1Estación Experimental de Zonas Áridas (EEZA-CSIC), E-04120 Almería, Spain. 2Dpto de Ecología, Universidad de Granada, E-18071 Granada, Spain. 3Centro de Investigaciones sobre Desertificación (CSIC-UV-GV), E-46113 Valencia, Spain. 4Dpto de Zoología, Universidad de Granada, E-18071 Granada, Spain. 5Área de Biodiversidad y Conservación, Universidad Rey Juan Carlos, E-28933 Madrid, Spain.

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Figure 1 | Evolution of lethal aggression in non-human mammals. http://www.phylopic.org) illustrate the main mammalian clades. Tree showing the phylogenetic estimation of the level of lethal They are licenced for use in the Public Domain without copyright, aggression in mammals (n = 1,024 species) using stochastic mapping. except for the silhouettes of Murinae (D. Liao), Jaculus (M. Karaka), Lethal aggression increases with the intensity of the colour, from Philander (S. Werning), Rattus (R. Groom), Molossus (Zimices), yellow to dark red. Light grey indicates the absence of lethal aggression. Balaenoptera (C. Hoh), Rousettus (O. Peles), Connochaetes, Redunca, Mammalian ancestral nodes compared with human lethal violence are and Kobus (J. A. Venter, H. H. T. Prins, D. A. Balfour and R. Slotow), shown in red, whereas main placental lineages are marked with black that are licenced under a Creative Commons 3.0 license nodes. The red triangle indicates the phylogenetic position of humans. (http://creativecommons.org/licenses/by/3.0). The silhouettes of representative mammals (downloaded from and fossils23, information on Homo neanderthalensis was included across all models, was 2.0 ± 0.02% of all deaths (Fig. 3a). These estimates when estimating the level of human lethal violence (Supplementary seem to be robust to many potential biases, such as phylogenetic Information section 9b). In addition, because the level of violence uncertainty, phylogenetic depth, sampling effort, and phylogeny size varies among populations of the same species10,20,21, all models (Supplementary Information sections 3–6). Territoriality and sociability include intraspecific variation in the level of mammalian lethal affect the phylogenetic inference of the level of lethal violence, as it was violence. The phylogenetically inferred level of lethal violence, averaging 1.9 ± 0.01% in the models without these two variables but 2.1 ± 0.02%

1.0 because we have included more populations in our study and weighted all the analyses by the number of individuals per sample. The level n = 313 of lethal violence during most historic periods was higher than the phylogenetic predictions for both humans (Fig. 3c and Supplementary Information section 7) and the ancestral Hominoidea (Fig. 3b). 0.8 However, on entering the Modern and Contemporary ages (defined in Methods), the level of lethal violence decreased markedly, as previously reported11 (Fig. 3c). Several potential biases may affect these results. The level of lethal violence inferred from skeletal remains could be underestimated because many deadly injuries do not damage the Territorial species bones8. Nevertheless, no underestimation was detected for the periods 0.6 n = 221 in which both skeletal remains and statistical yearbooks are available (Supplementary Information section 7). Similarly, the presence of n = 195 battlefields may artificially overestimate the level of lethal violence. However, the periods with highest level of lethal violence were not those with more organized intergroup conflicts (Supplementary 0.4 Information section 8). Thus, the temporal pattern in the level of lethal violence seems to hold even after considering these potential biases. Concomitant changes in the cultural and ecological human

Deaths owing to conspecic killings (%) environment may have caused this pattern. Notably, population density, a common ecological driver of lethal aggression in mammals18,21, was 0.2 lower in periods with high levels of lethal violence than in the less violent Modern and Contemporary ages. High population density is therefore probably a consequence of successful pacification, rather than 7 n = 295 a cause of strife . Non-territorial species Socio-political organization is a factor widely invoked to explain changes in violence5,7,11. To assess this effect, we classified human 0 populations into four types28: bands, tribes, chiefdoms and states. Levels of lethal violence in prehistoric bands and tribes did not differ Social species Solitary species from the phylogenetic inferences (Fig. 3d). However, lethal violence is common in present-day bands and tribes (Fig. 3d), possibly because there are more detailed data on mortality from living people than from Figure 2 | Social behaviour and territoriality influence lethal archaeological records. Nevertheless, some authors suggest that the aggression in mammals. The figure shows the phylogenetically corrected level of lethal violence has increased in hunter–gatherers because they level of lethal aggression per group (mean ± s.e.m) and the number of now live in denser populations in which intergroup conflicts are more mammalian species included in each group. We used a phylogenetic 3 generalized linear model (PGLS) to test the effect of territoriality likely , or because they have contacted colonial societies where warfare 29 (yes or no) and social behaviour (social or solitary) on lethal aggression. or interpersonal violence is frequent . The level of lethal violence in The level of lethal aggression was more intense in social and territorial chiefdoms was also higher than the phylogenetic inferences (Fig. 3d). species (PGLS, P < 0.05 in all cases and mammal phylogenies; Extended Severe violence has been frequently reported in chiefdoms30, mostly Data Table 1), with no interaction between these two terms (Extended caused by territorial disputes, population and resource pressures, and Data Table 1). competition for political status30. Finally, the level of lethal violence in state societies was lower than the phylogenetic inferences (Fig. 3d). It is widely acknowledged that monopolization of the legitimate use of violence by the state significantly decreases violence in state in the models including them (Fig. 3a). This is a consequence of societies11,30. H. sapiens being both social and territorial, two characteristics In this study, we have explored the origin and evolution of human associated with a stronger tendency towards lethal violence in lethal violence by integrating a phylogenetic approach with an empirical mammals (Fig. 2). analysis of lethal violence in human populations. The phylogenetic We subsequently explored how the level of lethal violence has analysis suggests that a certain level of lethal violence in humans changed during our evolutionary history by comparing it with the arises from the occupation of a position within a particularly violent phylogenetically inferred level of lethal violence in relevant ancestral mammalian clade, in which violence seems to have been ancestrally nodes that describe the course of human evolution (Fig. 1). The level of present. This means that humans have phylogenetically inherited lethal violence was low in the most basal nodes, increasing to 2.3 ± 0.1% their propensity for violence. We believe that this phylogenetic effect of all deaths in the two nodes closely related with the origin of primates entails more than a mere genetic inclination to violence. In fact, and slightly decreasing to 1.8 ± 0.1% of all deaths in the ancestral ape social behaviour and territoriality, two behavioural traits shared with (Fig. 3b). These results suggest that lethal violence is deeply rooted in relatives of H. sapiens, seem to have also contributed to the level of the primate lineage. lethal violence phylogenetically inherited in humans. Our analysis of We then compared whether the phylogenetically inferred level of human lethal violence shows that lethal violence in prehistoric humans lethal violence differed from the level empirically observed in human matches the level inferred by our phylogenetic analyses, suggesting populations. The samples were categorized according to their age, using that we were, at the dawn of humankind, as violent as expected the standard periods from the New and Old World chronologies27. considering the common mammalian evolutionary history. This pre- These data must be interpreted cautiously, because there was extensive historic level of lethal violence has not remained invariant but has intra-period variation in lethal violence. Nevertheless, a clear temporal changed as our history has progressed, mostly associated with changes pattern emerged (Fig. 3c). The level of lethal violence during human in the socio-political organization of human populations. This suggests prehistory did not differ from the phylogenetic predictions (Fig. 3c). that culture can modulate the phylogenetically inherited lethal violence This result contrasts with some previous observations9,11, probably in humans.

3 3

2 2

1 1 Human lethal violence (%) Mammal lethal aggression (%)

0 0 n = 100 100 100 100 100 100 100 100 n = 800 800 800 800 800 800

s Models including Models including Models including Models including e ia only intercept territoriality and sociability only intercept territoriality and sociability ir chonta Primates Without H. neanderthalensis With H. neanderthalensis Mammalia Euar Placental Hominoidea chontogl c d Euar 65 65 Old World New World Whole planet 60 60

55 55

50 50

45 45 )

40 40

35 35

30 30

25 25 Human lethal violence (%) Human lethal violence (% 20 20

15 15

10 10

5 5

0 0 n = 80 46 101 72 52 72 22 21 44 23 61 84 n = 99 201 136 96 34 13 9

e nd tate a lithic s trib chaic tribe . p. age c . b on age Classic p nze age Ir Ar p Neolithic o m Paleo Mesolithic Formative Chiefdom Br te Post-classic Modern age n Medieval age ehistori Historic Contem ehistoric band Contem Co Contemp. state Pr Pr Figure 3 | Lethal violence in humans. a–d, Box plots showing a, different temporal periods of human history, according to the Old World the phylogenetic inferences of human lethal violence assessed as the and New World chronologies27. d, Human lethal violence in different percentage of human deaths caused by conspecifics. These estimates were socio-political organizations28. In all cases the boxplots show median achieved through phylogenetic generalized linear models and correspond values, 50th percentile values (box outline), 95th percentile values to the ancestral node of the tree rooted at the node separating H. sapiens (whiskers), and outlier values (circles). We tested whether the level of from the rest of the mammals. All models were performed after logit- lethal violence observed in each ancestral node, human period and human transforming the dependent variable and considering the intraspecific socio-political organization differed significantly from the phylogenetic variation in mammal lethal aggression. Phylogenetic uncertainty was inferences in a. Colour indicates whether the observed lethal violence incorporated by using the tree provided by Fritz et al.22 (grey colour) and was statistically similar (white), higher (red), or lower (blue) than the a set of 100 randomly sampled trees from Faurby and Svenning23 (white phylogenetic inferences (Extended Data Tables 2, 3). In a and b, n indicates colour). b, The lethal aggression inferred for six important ancestral the number of iterations and in c and d it indicates the number of human nodes of human evolution (apes, primates, Euarchonta, Euarchontoglires, populations (see Supplementary Information sections 7, 9c for the number placental mammals, and all mammals). c, Human lethal violence during of deaths).

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Methods societies). Lethal violence was determined for each source using the criteria of No statistical methods were used to predetermine sample size. The investigators the researchers. Ethnographic records, statistical yearbooks and verbal autopsies were not blinded to allocation during experiments and outcome assessment. commonly included the casualties of the interpersonal violence. The death Lethal aggression in mammals. To estimate lethal aggression in mammals toll owing to interpersonal violence in bioarchaeological studies was found by (defined as the percentage of deaths caused by conspecifics) we compiled a database following the most widely used criterion in this type of study; that is, the presence including the amount of conspecific killing observed in many species of mammals. of perimortem and blade injuries as an indication of death caused by interpersonal We conducted computer searches including the words (alone or in combination): violence8,37. This means that we did not include antemortem and healed injuries ‘mammal’, ‘mortality factors’, ‘causes of mortality’, ’infanticide’, ‘death’, ‘conspecific in our calculation of lethal interpersonal violence37. Nevertheless, skeletal trauma mortality’, ‘conspecific fighting’, ‘intraspecific aggression’ and ‘conspecific aggression’, should be viewed as minimal estimates, since many injuries caused by conspecifics as well as some other words related to relevant mortality factors in some mammal do not damage the bones8,38. species, such as ‘bushmeat’, ‘road killing’ and ‘overhunting’. We pooled all sources The samples were categorized according to their age and socio-political of conspecific mortality (active and passive infanticide, intergroup aggression, organization. To assign the age to each sample, we considered the periods used cannibalism and intraspecific predation, male–male fighting during mating to divide human history according to both the New World and Old World period, territorial defensive behaviour, maternal abandonment, accidental injury). chronologies27. Old World human societies were grouped into Paleolithic We considered only lethal conspecific interactions, ignoring non-lethal aggression, (~50,000–12,000 bp), Mesolithic (~12,000–10,200 bp), Neolithic/Calcolithic because the recording of aggressive interactions ending in the death of any of the (~10,200–5,000 bp), Bronze Age (~5,300–3,200 bp), Iron Age (~3,200–1,300 bp) interacting organisms, both in humans and non-human mammals, is more precise8. and Medieval periods (~1,300–500 bp). New World human societies were grouped We found information about more than four million deaths in the 1,024 mammal in Archaic (~12,000–3,000 bp), Formative (~3,000–1,500 bp), Classic (~1,500– species (~20% of the total species) from 137 families (~80% of total families) and 800 bp) and Post-Classic periods (~800–500 bp). From then on, we considered the three main extant mammalian clades (Prototheria, Metatheria and Eutheria) two further periods affecting human societies throughout the entire world, the (Supplementary Information section 9a). We obtained information from several Modern Age (~500–100 bp) and the Contemporary Age (~100 bp–present day). studies in order to incorporate the intraspecific variability in lethal aggression for We followed the widely accepted socio-political classification28,39, according to each mammal species. For each mammal included in our database, we recorded its which human societies can be classified into four types: bands (small, nomadic, territoriality (yes or no) and social behaviour (social or solitary) using information egalitarian groups of people, usually hunter–gatherers), tribes (small, mostly compiled in the Animal Diversity Web (http://www.animaldiversity.org). egalitarian, groups with limited social rank usually resident in permanent villages Mammal phylogeny. The phylogenetic relationship between the mammals as hunter–horticulturalists), chiefdoms (stratified, hierarchical non-industrial included in the database was built using Fritz et al.22 and Faurby and Svenning23 societies usually based on kinship) and states (politically organized complex phylogenies, which are updated phylogenies of the supertree of Bininda-Emonds societies). To assign each sample to different socio-political and temporal et al.31, to account for the more recent mammalian taxonomy of Wilson and categories, we relied on the information from each original source (Supplementary Reeder32. First, we used the phylogeny provided by Fritz et al.22 including 5,020 Information section 8c). The use of standard statistics to summarize information extant mammals. Afterwards, we used a set of 100 phylogenies provided by Faurby coming from disparate sources with extremely different sample sizes and time and Svenning23 that contains 5,747 extant and extinct mammals (including species coverage is problematic, as has been reported40. To avoid such issues, we pooled with dated records from the Late Pleistocene, defined as the last 130,000 years). Using all the samples (skeletal remains, dead individuals and so on) found during each this set of phylogenies, we were able to incorporate phylogenetic uncertainty in all period (see Supplementary Information section 8c for an exhaustive list of cases, our analyses. In each phylogeny we pruned all species not included in the database samples and studies) and depicted them using box plots. and, in the few cases in which a species was missing in the supertree, we selected the Phylogenetic signal of mammal lethal aggression. The phylogenetic signal for closest relative (usually, a congeneric species, see Supplementary Information section lethal aggression was calculated using Pagel’s lambda41 that compares the similarity 9a). Mortality data about subspecies were pooled at the species level. of the covariances among species with the covariances expected under Brownian We performed additional analyses with the inclusion of H. neanderthalensis evolution. Significant phylogenetic signal occurs when λ > 0 and may take values because: i) close relatives of modern humans can be very informative to estimate of either 0 < λ < 1 (indicating that close relatives resemble each other less than their phylogenetically shared traits, and ii) including fossils in the phylogeny results expected under Brownian evolution) or λ = 1 (indicating that close relatives are as in more reliable ancestral state reconstructions33. The Faurby and Svenning22 similar as would be expected under Brownian motion). Values of λ > 1 (indicating phylogeny includes H. neanderthalensis. However, the Fritz et al.23 phylogeny that close relatives are more similar than expected by Brownian evolution) cannot only contains extant species. For this reason, we grafted H. neanderthalensis be reached because the off-diagonal elements in the variance–covariance matrix into this latter phylogeny, indicating an evolutionary divergence from H. sapiens cannot be larger than the diagonal elements42. To account for the possibility of 0.43 million years ago (Mya)34 and extinction 0.028 Mya35. Although these a phylogenetic signal higher than expected under Brownian motion, we also dates are contested36, variations of a few thousand years did not significantly calculated Blomberg’s K (that is, the ratio between the observed phylogenetic signal alter the phylogenetic prediction of human lethal violence. For example, when and that expected under a Brownian evolution model)43. This phylogenetic signal time of divergence was changed to 0.23 Mya, the mean prediction remained the metric is not restricted in its upper limit, and ranges from 0 (no phylogenetic signal) same but with a slightly higher confidence interval. The level of lethal violence to infinity, with K = 1 indicating Brownian evolution. Statistical significance of in H. neanderthalensis was obtained from multiple sources (see Supplementary Pagel’s λ was calculated through a likelihood ratio test, comparing the likelihood Information section 9b). of the model that was fitted to the data to that of a model in which λ was fixed to 0. Lethal violence in humans. To estimate lethal violence in humans (defined as Significance of Blomberg’s K was calculated through a randomization test from a the percentage of people that died owing to interpersonal violence) we compiled null model constructed with 1,000 random permutations of the data across the tips information from almost 600 human populations and societies spanning from of the mammal tree. Both tests were performed using the R package ‘phytools’44. the Palaeolithic to the present (Supplementary Information section 9c). Because The level of phylogenetic signal of lethal aggression in mammals measured as of the extremely wide temporal range, we obtained information derived from Blomberg’s K (K = 0.09) was significantly higher than 0 (P = 0.013) and lower than 1 very disparate sources, namely bioarchaeological and palaeo-osteological (P ≪ 0.001). This indicates that close relatives tend to have similar values of lethal reports, ethnographic records, statistical yearbooks and verbal autopsies violence but at a level lower than would be expected under Brownian evolution. (a method to determine probable causes of death when no medical record or This evolutionary pattern is consistent with that shown by Pagel’s lambda (λ = 0.60) formal medical attention is available; they are performed by non-medical field and therefore only this metric is shown in the main text. The evolution of lethal workers, recording written narratives from reliable informants in local languages aggression throughout the phylogeny of mammals was estimated using stochastic that describe the events that preceded the death). Owing to this heterogeneity, mapping as implemented in the R package ‘phytools’44. Lethal aggression was and because our goal was to compare the level of lethal violence in humans with logit-transformed before all analyses. the level of lethal aggression in mammals, we did not differentiate the specific Effect of territoriality and sociability on mammal lethal aggression. To examine causes of intraspecific mortality. Rather, we pooled together the deaths caused which factors explained the level of lethal aggression in mammals, we performed by war, homicide, manslaughter, infanticide, sacrifice, cannibalism and so on, a phylogenetic generalized-least-squares (PGLS) model45, with lethal aggression without differentiating whether lethal events involved only one perpetrator or (logit-transformed) as the dependent variable and territoriality and sociability as were coalitional and collective killings. Although it is worth investigating how independent variables. PGLS takes into account the phylogenetic signal in the specific types of violence have evolved in humans, we could not explore this residuals of the model fitted to the data45. To account for the intraspecific variability issue because some types of violence have been insufficiently studied, both in in lethal aggression, for each of the 1,024 mammal species, we generated a normal non-human mammals (for example, inter-group aggression in social mammals distribution of lethal aggression values with their empirically observed means other than chimpanzees) and humans (for example, infanticide in historical and standard errors. To control for potential biases produced by between-study

© 2016 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. Letter RESEARCH differences in sample size, the means and standard errors that were used to generate by the size of the phylogeny, we repeated these analyses with the progressive the random distributions were first weighted by the number of individuals included inclusion of more species in the phylogenies. Specifically, we estimated human in each study. We then ran the analysis 100 times, randomly sampling each time a lethal violence and its 95% confidence interval in 50 randomly generated value from each of the 1,024 normal distributions. When a species was represented phylogenies with 100, 200, 300, 400, 500, 800, 900 and 1,000 spp., using the PGLS by a single value, we used as its standard error the across-species average of model without covariates. Afterwards, we contrasted these values with the level of standard errors. The analyses were run with the help of the PGLS command in human lethal violence obtained using the empirical phylogeny, checking whether the R package ‘caper’46. smaller phylogenies departed from empirical results more strongly than larger Phylogenetic estimation of human lethal violence. Phylogenetic trait estimation phylogenies (Supplementary Information section 6). techniques were used to obtain the lethal violence level for H. sapiens as a Statistical difference between phylogenetically estimated lethal violence in function of its position in the mammal phylogeny. These techniques take advantage humans and ancestral nodes. We have checked whether the level of lethal violence of ancestral state estimation methods to predict traits of extant species25,47. phylogenetically inferred in humans is different from the lethal aggression inferred The trait value of the focal species can be estimated as the ancestral node of the for the main ancestral nodes using t-tests. The phylogenetic estimates of both lethal tree rerooted at the most recent common ancestor of the focal species and the rest violence in humans and lethal aggression in ancestral mammals were obtained by of the tree48,49. The trait value estimated with this ancestral estimation method is joining the 100 values obtained for each of the four PGLS models (with and without the same as that provided by the intercept of a PGLS performed on the same tree. covariates and with and without H. neanderthalensis) and the two mammalian However, PGLS allows us to simultaneously include the level of the phylogenetic phylogenies used (Fritz et al.22 and Faurby and Svenning23 phylogenies). signal and other traits as covariates to improve the phylogenetic estimation of the We subsequently tested, by means of t-tests, whether these two distributions study trait25. Following this approach, we also estimated human lethal violence with differed. Because we repeated the same test six times (once per ancestral node), the help of a PGLS approach with territoriality and sociability as covariates and the we corrected all P values by means of sequential Bonferroni corrections. phylogenetic information of the mammal tree rooted in the node where H. sapiens Statistical difference between observed and phylogenetically estimated lethal diverged from the rest of the mammals. The target species must be excluded from violence. For each temporal period and socio-political organization, we randomly the analysis to estimate the PGLS parameters. Four PGLS models were fitted to our sampled a given value of observed mortalities from a normal distribution with data: (i) without covariates and without H. neanderthalensis; (ii) with territoriality the same mean and standard error and compared it with a randomly sampled, and sociability as factorial covariates but without H. neanderthalensis; (iii) without phylogenetically estimated value. The phylogenetically estimated values were covariates and with H. neanderthalensis; and (iv) with territoriality and sociability obtained by joining the 100 values obtained for each of the four PGLS models (with as factorial covariates and with H. neanderthalensis. In all models, the dependent and without covariates and with and without H. neanderthalensis) and the two variable was logit-transformed and its variance was included using the approach mammalian phylogenies (Fritz et al.22 and Faurby and Svenning23 phylogenies). explained in the previous section. We repeated these paired comparisons 800 times, and recorded the proportion of Lethal aggression in main ancestral nodes of the human lineage. We estimated times where the observed values were higher or lower than the phylogenetically levels of lethal aggression in the most recent common ancestor of six important estimated values. We subsequently tested, by means of binomial tests, whether this clades defining the course of the evolutionary history of humans: the class proportion differed from the randomly expected deviation. We ran each binomial Mammalia, the infraclass Placentalia (placental mammals), the superorder test 1,000 times and retained the average P values and deviance from the expected Euarchontoglires or Supraprimates (primates, tree-shrews, colugos, rodents and value. All P values shown underwent sequential Bonferroni correction. hares), the grandorder Euarchonta (primates, colugos and tree-shrews), the order Primates (primates) and the superfamily Hominoidea (apes). Lethal aggression in 31. Bininda-Emonds, O. R. P. et al. The delayed rise of present-day mammals. these ancestral nodes was inferred using the same analytical approach as that used Nature 446, 507–512 (2007); Corrigendum 456, 274 (2008). to estimate lethal violence in humans. 32. Wilson, D. E. & Reeder, D. M. Mammal Species of the World: a Taxonomic and Geographic Reference, 2nd–3rd edn. (Smithsonian Institution Press / John Accuracy of the estimation of mammal lethal aggression from the PGLS. The Hopkins Univ. Press, 1993–2005). accuracy of trait-estimation in a particular species increases with the level of 33. Finarelli, J. A. & Flynn, J. J. Ancestral state reconstruction of body size in the phylogenetic signal of the study trait25. To test for the accuracy of our models Caniformia (Carnivora, Mammalia): the effects of incorporating data from the under the observed phylogenetic signal, we used leave-one-out cross-validations fossil record. Syst. Biol. 55, 301–313 (2006). with the whole mammalian data set in Supplementary Information section 9a. 34. Finlayson, C. et al. Late survival of Neanderthals at the southernmost extreme We inferred the level of lethal violence (logit-transformed) for each mammal of Europe. Nature 443, 850–853 (2006). 35. Arsuaga, J. L. et al. Neandertal roots: Cranial and chronological evidence from species with the PGLS procedure and compared it with its actual value. We first Sima de los Huesos. Science 344, 1358–1363 (2014). examined the relationship between the estimated and observed lethal violence 36. Hublin, J. J. The origin of Neandertals. Proc. Natl Acad. Sci. USA 106, values50 and subsequently calculated the proportion of species for which the actual 16022–16027 (2009). value fell inside the 95% confidence interval of the estimated trait (Supplementary 37. Mays, S. The Archaeology of Human Bones (Routledge, 2010). Information section 2). 38. Milner, G. R. Nineteenth-century arrow wounds and perceptions of prehistoric warfare. Am. Antiq. 70, 144–156 (2005). Effect of sampling effort on the estimation of human lethal violence. To check 39. Service, E. R. Profiles in Ethnology (Harpercollins College Div., 1963). whether the estimates of conspecific-mediated human mortality were influenced 40. 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Extended Data Table 1 | Outcome of the phylogenetic generalized linear model testing the effect of territoriality and social behaviour on the magnitude of lethal aggression in mammal species (n = 1,024 species)

We performed this analysis using the mammalian phylogeny provided by Fritz et al.22 and 100 mammalian phylogenies provided by Faurby and Svenning23. In this latter case, we show the across-phylogeny mean of each statistical parameter. Lethal aggression was logit-transformed before all analyses.

Extended Data Table 2 | Outcome of the t-tests assessing difference between the inferred value of lethal violence at each of the chosen ancestral nodes in the mammalian phylogeny and the phylogenetic estimates of human lethal violence

We compared the lethal aggression of the ancestral nodes with the magnitudes of lethal violence obtained according to the four PGLS models (with and without covariates and with and without H. neanderthalensis) and the two mammalian phylogenies (Fritz et al.22 and Faurby and Svenning23 phylogenies) using a t-test. Significance after sequential Bonferroni correction atα = 0.05.

Extended Data Table 3 | Outcome of the binomial tests assessing difference between the observed lethal violence in human societies and the inferred lethal violence according to the phylogenetic analysis

We compared the observed lethal violence of each type of human society with the magnitudes of lethal violence obtained according to the four PGLS models (with and without covariates and with and without H. neanderthalensis) and the two mammalian phylogenies (Fritz et al.22 and Faurby and Svenning23 phylogenies). Each binomial test was run 1000 times. Significance after sequential Bonferroni correction at α = 0.05.