Delayed increase in stone tool cutting-edge productivity at the Middle-Upper Paleolithic transition in southern Jordan.

Although the lithic cutting-edge productivity has long been recognized as a quantifiable aspect of prehistoric human technological evolution, there remains uncertainty how the productivity changed during the Middle-to-Upper Paleolithic transition. Here we present the cutting-edge productivity of eight lithic assemblages in the eastern Mediterranean region that represent a chrono-cultural sequence including the Late Middle Paleolithic, Initial Upper Paleolithic, the Early Upper Paleolithic, and the Epipaleolithic. The results show that a major increase in the cutting-edge productivity does not coincide with the conventional Middle-Upper Paleolithic boundary characterized by the increase in blades in the Initial Upper Paleolithic, but it occurs later in association with the development of bladelet technology in the Early Upper Paleolithic. Given increasing discussions on the complexity of Middle-Upper Paleolithic cultural changes, it may be fruitful to have a long-term perspective and employ consistent criteria for diachronic comparisons to make objective assessment of how cultural changes proceeded across conventional chrono-cultural boundaries.

Evolution of dispersal under spatio-temporal heterogeneity.

Theoretical studies over the past decades have revealed various factors that favor or disfavor the evolution of dispersal. Among these, environmental heterogeneity is one driving force that can impact dispersal traits, because dispersing individuals can obtain a fitness benefit through finding better environments. Despite this potential benefit, some previous works have shown that the existence of spatial heterogeneity hinders evolution of dispersal. On the other hand, temporal heterogeneity has been shown to promote dispersal through a bet-hedging mechanism. When they are combined in a patch-structured population in which the quality of each patch varies over time independently of the others, it has been shown that spatiotemporal heterogeneity can favor evolution of dispersal. When individuals can use patch quality information so that dispersal decision is conditional, the evolutionary outcome can be different since individuals have options to disperse more/less offspring from bad/good patches. In this paper, we generalize the model and results of previous studies. We find richer dynamics including bistable evolutionary dynamics when there is arrival bias towards high-productivity patches. Then we study the evolution of conditional dispersal strategy in this generalized model. We find a surprising result that no offspring will disperse from a patch whose productivity was low when these offspring were born. In addition to mathematical proofs, we also provide intuition behind this initially counter-intuitive result based on reproductive-value arguments. Dispersal from high-productivity patches can evolve, and its parameter dependence behaves similarly, but not identically, to the case of unconditional dispersal. Our results unveil an importance of whether or not individuals can use patch quality information in dispersal evolution.

Infectious diseases may have arrested the southward advance of microblades in Upper Palaeolithic East Asia.

An unsolved archaeological puzzle of the East Asian Upper Palaeolithic is why the southward expansion of an innovative lithic technology represented by microblades stalled at the Qinling-Huaihe Line. It has been suggested that the southward migration of foragers with microblades stopped there, which is consistent with ancient DNA studies showing that populations to the north and south of this line had differentiated genetically by 19 000 years ago. Many infectious pathogens are believed to have been associated with hominins since the Palaeolithic, and zoonotic pathogens in particular are prevalent at lower latitudes, which may have produced a disease barrier. We propose a mathematical model to argue that mortality due to infectious diseases may have arrested the wave-of-advance of the technologically advantaged foragers from the north.

Hominin forager technology, food sharing, and diet breadth.

We propose a predator-prey model to explain diachronic changes in Palaeolithic diet breadth. The fraction of rapidly-reproducing hard-to-catch hares and birds among small animals in the hominin diet shows a significant increase between the Middle and Upper Palaeolithic in the Levant, with an associated decrease in slowly-reproducing easily-caught tortoises. Our model interprets this fraction in terms of foraging effort allocated to, and foraging efficiency for each of these two classes of resource, in addition to their abundances. We focus on evolutionary adjustments in the allocation of foraging effort. The convergence stable strategy (CSS) of foraging effort and the dietary fraction of hares/birds are both highly sensitive to variation in the foraging efficiencies, which may have been upgraded by advanced technology introduced from Africa or developed locally. A positive correlation (not necessarily a cause and effect relationship) is observed between this fraction and forager population when the foraging efficiency for hares/birds is varied. Overexploitation can however result in a reduction of both diet breadth and forager population, as can food sharing within the forager group. Food sharing is routine among recent (and perhaps also Palaeolithic) foragers. We speculate that some controversial issues regarding this public goods problem might be resolved if we could incorporate sexual selection into our model.

Coevolutionary dynamics of genetic traits and their long-term extended effects under non-random interactions.

Organisms continuously modify their living conditions via extended genetic effects on their environment, microbiome, and in some species culture. These effects can impact the fitness of current but also future conspecifics due to non-genetic transmission via ecological or cultural inheritance. In this case, selection on a gene with extended effects depends on the degree to which current and future genetic relatives are exposed to modified conditions. Here, we detail the selection gradient on a quantitative trait with extended effects in a patch-structured population, when gene flow between patches is limited and ecological inheritance within patches can be biased towards offspring. Such a situation is relevant to understand evolutionary driven changes in individual condition that can be preferentially transmitted from parent to offspring, such as cellular state, micro-environments (e.g., nests), pathogens, microbiome, or culture. Our analysis quantifies how the interaction between limited gene flow and biased ecological inheritance influences the joint evolutionary dynamics of traits together with the conditions they modify, helping understand adaptation via non-genetic modifications. As an illustration, we apply our analysis to a gene-culture coevolution scenario in which genetically-determined learning strategies coevolve with adaptive knowledge. In particular, we show that when social learning is synergistic, selection can favour strategies that generate remarkable levels of knowledge under intermediate levels of both vertical cultural transmission and limited dispersal. More broadly, our theory yields insights into the interplay between genetic and non-genetic inheritance, with implications for how organisms evolve to transform their environments.

Time to extinction of a cultural trait in an overlapping generation model.

How long a newly emerging trait will stay in a population is a fundamental but rarely asked question in cultural evolution. To tackle this question, the distribution and mean of the time to extinction of a discrete cultural trait are derived for models with overlapping generations, in which trait transmission occurs from multiple role models to a single newborn and may fail with a certain probability. We explore two models. The first is a Moran-type model, which allows us to derive the exact analytical formula for the mean time to extinction of a trait in a finite population. The second is a branching process, which assumes an infinitely large population and allows us to derive approximate analytical formulae for the distribution and mean of the time to extinction in the first model under a large population size. We show that in the first model, the mean time to extinction apparently diverges (becomes so large that even numerical computation is impractical) under a certain parameter condition as the population size tends to infinity. Using the second model, we explain the underlying mechanism of the apparent divergence found in the first model and derive the mathematical condition for this divergence in terms of transmission efficiency and the number of role models per newborn. When this mathematical condition is satisfied in the second model, the probability of extinction is less than 1, and the mean extinction time does not exist. In addition, we find that in both models, the time to extinction of the trait becomes longer as the number of role models per individual increases and as cultural transmission becomes more efficient.

The components of directional and disruptive selection in heterogeneous group-structured populations.

We derive how directional and disruptive selection operate on scalar traits in a heterogeneous group-structured population for a general class of models. In particular, we assume that each group in the population can be in one of a finite number of states, where states can affect group size and/or other environmental variables, at a given time. Using up to second-order perturbation expansions of the invasion fitness of a mutant allele, we derive expressions for the directional and disruptive selection coefficients, which are sufficient to classify the singular strategies of adaptive dynamics. These expressions include first- and second-order perturbations of individual fitness (expected number of settled offspring produced by an individual, possibly including self through survival); the first-order perturbation of the stationary distribution of mutants (derived here explicitly for the first time); the first-order perturbation of pairwise relatedness; and reproductive values, pairwise and three-way relatedness, and stationary distribution of mutants, each evaluated under neutrality. We introduce the concept of individual k-fitness (defined as the expected number of settled offspring of an individual for which k-1 randomly chosen neighbors are lineage members) and show its usefulness for calculating relatedness and its perturbation. We then demonstrate that the directional and disruptive selection coefficients can be expressed in terms individual k-fitnesses with k=1,2,3 only. This representation has two important benefits. First, it allows for a significant reduction in the dimensions of the system of equations describing the mutant dynamics that needs to be solved to evaluate explicitly the two selection coefficients. Second, it leads to a biologically meaningful interpretation of their components. As an application of our methodology, we analyze directional and disruptive selection in a lottery model with either hard or soft selection and show that many previous results about selection in group-structured populations can be reproduced as special cases of our model.

Evolution of dispersal in a spatially heterogeneous population with finite patch sizes.

Dispersal is one of the fundamental life-history strategies of organisms, so understanding the selective forces shaping the dispersal traits is important. In the Wright's island model, dispersal evolves due to kin competition even when dispersal is costly, and it has traditionally been assumed that the living conditions are the same everywhere. To study the effect of spatial heterogeneity, we extend the model so that patches may receive different amounts of immigrants, foster different numbers of individuals, and give different reproduction efficiency to individuals therein. We obtain an analytical expression for the fitness gradient, which shows that directional selection consists of three components: As in the homogeneous case, the direct cost of dispersal selects against dispersal and kin competition promotes dispersal. The additional component, spatial heterogeneity, more precisely the variance of so-called relative reproductive potential, tends to select against dispersal. We also obtain an expression for the second derivative of fitness, which can be used to determine whether there is disruptive selection: Unlike the homogeneous case, we found that divergence of traits through evolutionary branching is possible in the heterogeneous case. Our numerical explorations suggest that evolutionary branching is promoted more by differences in patch size than by reproduction efficiency. Our results show the importance of the existing spatial heterogeneity in the real world as a key determinant in dispersal evolution.

The popularity spectrum applied to a cross-cultural question.

We investigate a new approach for identifying the contribution of horizontal transmission between groups to cross-cultural similarity. This method can be applied to datasets that record the presence or absence of artefacts, or attributes thereof, in archaeological and ethnographic assemblages, from which popularity spectra can be constructed. Based on analytical and simulation models, we show that the form of such spectra is sensitive to horizontal transmission between groups. We then fit the analytical model to existing datasets by Bayesian MCMC and obtain evidence for strong horizontal transmission in oceanic as opposed to continental datasets. We check the validity of our statistical method by using individual-based models, and show that the vertical transmission rate tends to be underestimated if the datasets are obtained from lattice-structured rather than island-structured meta-populations. We also suggest that there may be more borrowing of functional than stylistic traits, although the evidence for this is currently ambiguous.

Lithic technology, chronology, and marine shells from Wadi Aghar, southern Jordan, and Initial Upper Paleolithic behaviors in the southern inland Levant.

The Initial Upper Paleolithic (IUP) temporally overlaps with the range expansion of Homo sapiens populations in various parts of Eurasia and is often considered a key archaeological phase for investigating behavioral changes from the Middle Paleolithic. This paper reports upon new data from IUP occupations at Wadi Aghar, a rock shelter site in the southern Levant. In combining the results of radiometric dates and lithic analyses, we clarify the chronological and cultural position of Wadi Aghar assemblages in the Levantine IUP. As for the records about mobility, on-site activities, and resource procurement behaviors, we present analyses of lithic use-wear, tool-type composition, soil micromorphology, and marine shells. The lithic analyses and the optically stimulated luminescence (and subsidiary radiocarbon) dating of the Wadi Aghar materials suggest their chronocultural position in the IUP (45-40 ka for Layers C-D1; 39-36 ka for Layer B; possibly 50 ka for Layer D2), providing the southernmost location for the IUP in Eurasia. In the Levant, Wadi Aghar represents one of the few IUP sites in the inland areas. The results also indicate that the timing and technological sequences from the IUP to the following bladelet industries differed between the inland and coastal zones, likely reflecting geographically variable adaptive behaviors and/or cultural transmissions. One of the behavioral characteristics of IUP foragers at Wadi Aghar is the procurement of remote resources, represented by the transportation of marine shells from the Red Sea: Canarium fusiforme and Canarium cf. mutabile. Whether it was a direct procurement with increased mobility or a result of intergroup exchanges, it was not part of behavioral repertoires during the late MP in the same area. This can be understood as the expansion of resource procurement range, functioning as additional buffers from risk in the semiarid environments in the inland Levant.

Evolution of cumulative culture for niche construction.

A mathematical model of the joint evolution of learning and niche construction in a spatially subdivided population is described, in which culture is used to practice niche construction and can evolve by accumulating small improvements over generations. Individuals allocate their lifetimes to social learning, individual learning, niche construction to improve the environment, and exploitation of resources according to their genetically determined strategies. The coordinated optimal strategy (COS) is defined as the allocation strategy which maximizes the equilibrium fecundity of the population, as opposed to the convergence stable strategy (CSS), which is the strategy favored by natural selection. Both the COS and CSS are analytically derived and compared. It turns out that, although the levels of the CSS in terms of culture and the environmental quality can be high in a highly viscous population, they are in general much lower than those of the COS. It is argued that the discrepancy between the CSS and COS stems from the producer-scrounger structure inherent in the model. Analysis of transient dynamics reveals that the level of culture and the environmental quality may temporarily undergo drastic increases after sudden changes in parameter values, although they eventually drop down to low values due to the genetic adaptation of the time allocation strategy to the new cultural and environmental backgrounds. Implications of the results for human evolution are discussed.

Spatial heterogeneity and evolution of fecundity-affecting traits.

It is widely recognized that spatial structure in a population has some, and occasionally great, impacts on ecological and evolutionary dynamics. However, it has been observed that in the homogeneous Wright's island model with a certain standard demographic assumption, spatial structure does not affect the fitness gradient of a fecundity-affecting trait. The location and convergence stability of singular strategies thus remain unchanged. Furthermore, evolutionary branching is impossible for small dispersal rates, and for a wide class of fecundity functions, evolutionary branching is impossible for any dispersal rate if branching does not occur in the corresponding well-mixed model. Spatially homogeneous structure thus often inhibits evolutionary branching. Here we study the impact of spatial heterogeneity on evolutionary dynamics. We consider an infinite Wright's island model, where different islands have different capacity and fecundity consequences, and therefore the population is spatially heterogeneous. Through the analysis of metapopulation fitness, we derive its first-order and second-order derivatives with respect to mutant's trait, which are explicitly represented in terms of fecundity derivatives. The selection gradient turns out to be a biased average of local selection pressures in different patch types. We find that evolutionary branching is generally favored in the presence of spatial heterogeneity. We also find a simple condition under which evolutionary branching is particularly favored. Applications to public-goods cooperation and emergent evolutionary branching to cooperators and defectors are discussed.

Genealogies and ages of cultural traits: An application of the theory of duality to the research on cultural evolution.

A finite-population, discrete-generation model of cultural evolution is described, in which multiple discrete traits are transmitted independently. In this model, each newborn may inherit a trait from multiple cultural parents. Transmission fails with a positive probability unlike in population genetics. An ancestral process simulating the cultural genealogy of a sample of individuals is derived for this model. This ancestral process, denoted by M-, is shown to be dual to a process M+ describing the change in the frequency of a trait. The age-frequency spectrum is defined as a two-dimensional array whose (i,k) element is the expected number of distinct cultural traits introduced k generations ago and now carried by i individuals in a sample of a particular size n. Numerical calculations reveal that the age-frequency spectrum and related metrics undergo a critical transition from a phase with a moderate number of young, rare traits to a phase with numerous very old, common traits when the expected number of cultural parents per individual exceeds one. It is shown that M+ and M- converge to branching or deterministic processes, depending on the way population size tends to infinity, and these limiting processes bear some duality relationships. The critical behavior of the original processes M+ and M- is explained in terms of a phase transition of the branching processes. Using the results of the limiting processes in combination, we derive analytical formulae that well approximate the age-frequency spectrum and also other metrics.

Ecocultural range-expansion scenarios for the replacement or assimilation of Neanderthals by modern humans.

Recent archaeological records no longer support a simple dichotomous characterization of the cultures/behaviors of Neanderthals and modern humans, but indicate much cultural/behavioral variability over time and space. Thus, in modeling the replacement or assimilation of Neanderthals by modern humans, it is of interest to consider cultural dynamics and their relation to demographic change. The ecocultural framework for the competition between hominid species allows their carrying capacities to depend on some measure of the levels of culture they possess. In the present study both population densities and the densities of skilled individuals in Neanderthals and modern humans are spatially distributed and subject to change by spatial diffusion, ecological competition, and cultural transmission within each species. We analyze the resulting range expansions in terms of the demographic, ecological and cultural parameters that determine how the carrying capacities relate to the local densities of skilled individuals in each species. Of special interest is the case of cognitive and intrinsic-demographic equivalence of the two species. The range expansion dynamics may consist of multiple wave fronts of different speeds, each of which originates from a traveling wave solution. Properties of these traveling wave solutions are mathematically derived. Depending on the parameters, these traveling waves can result in replacement of Neanderthals by modern humans, or assimilation of the former by the latter. In both the replacement and assimilation scenarios, the first wave of intrusive modern humans is characterized by a low population density and a low density of skilled individuals, with implications for archaeological visibility. The first invasion is due to weak interspecific competition. A second wave of invasion may be induced by cultural differences between moderns and Neanderthals. Spatially and temporally extended coexistence of the two species, which would have facilitated the transfer of genes from Neanderthal into modern humans and vice versa, is observed in the traveling waves, except when niche overlap between the two species is extremely high. Archaeological findings on the spatial and temporal distributions of the Initial Upper Palaeolithic and the Early Upper Palaeolithic and of the coexistence of Neanderthals and modern humans are discussed.

Inclusive fitness analysis of cumulative cultural evolution in an island-structured population.

The success of humans on the globe is largely supported by our cultural excellence. Our culture is cumulative, meaning that it is improved from generation to generation. Previous works have revealed that two modes of learning, individual learning and social learning, play pivotal roles in the accumulation of culture. However, under the trade-off between learning and reproduction, one's investment into learning is easily exploited by those who copy the knowledge of skillful individuals and selfishly invest more efforts in reproduction. It has been shown that in order to prevent such a breakdown, the rate of vertical transmission (i.e. transmission from parents to their offspring) of culture must be unrealistically close to one. Here we investigate what if the population is spatially structured. In particular, we hypothesize that spatial structure should favor highly cumulative culture through endogenously arising high kinship. We employ Wright's island model and assume that cultural transmission occurs within a local island. Our inclusive fitness analysis reveals combined effects of direct fitness of the actor, indirect fitness through relatives in the current generation, and indirect fitness through relatives in future generations. The magnitude of those indirect benefits is measured by intergenerational coefficients of genetic relatedness. Our result suggests that the introduction of spatial structure raises the stationary level of culture in the population, but that the extent of its improvement compared with a well-mixed population is marginal unless spatial localization is extreme. Overall, our model implies that we need an alternative mechanism to explain highly cumulative culture of modern humans.

The effect of fecundity derivatives on the condition of evolutionary branching in spatial models.

By investigating metapopulation fitness, we present analytical expressions for the selection gradient and conditions for convergence stability and evolutionary stability in Wright's island model in terms of fecundity function. Coefficients of each derivative of fecundity function appearing in these conditions have fixed signs. This illustrates which kind of interaction promotes or inhibits evolutionary branching in spatial models. We observe that Taylor's cancellation result holds for any fecundity function: Not only singular strategies but also their convergence stability is identical to that in the corresponding well-mixed model. We show that evolutionary branching never occurs when the dispersal rate is close to zero. Furthermore, for a wide class of fecundity functions (including those determined by any pairwise game), evolutionary branching is impossible for any dispersal rate if branching does not occur in the corresponding well-mixed model. Spatial structure thus often inhibits evolutionary branching, although we can construct a fecundity function for which evolutionary branching only occurs for intermediate dispersal rates.

The Driving Forces of Cultural Complexity : Neanderthals, Modern Humans, and the Question of Population Size.

The forces driving cultural accumulation in human populations, both modern and ancient, are hotly debated. Did genetic, demographic, or cognitive features of behaviorally modern humans (as opposed to, say, early modern humans or Neanderthals) allow culture to accumulate to its current, unprecedented levels of complexity? Theoretical explanations for patterns of accumulation often invoke demographic factors such as population size or density, whereas statistical analyses of variation in cultural complexity often point to the importance of environmental factors such as food stability, in determining cultural complexity. Here we use both an analytical model and an agent-based simulation model to show that a full understanding of the emergence of behavioral modernity, and the cultural evolution that has followed, depends on understanding and untangling the complex relationships among culture, genetically determined cognitive ability, and demographic history. For example, we show that a small but growing population could have a different number of cultural traits from a shrinking population with the same absolute number of individuals in some circumstances.

A paradox of cumulative culture.

Culture can grow cumulatively if socially learnt behaviors are improved by individual learning before being passed on to the next generation. Previous authors showed that this kind of learning strategy is unlikely to be evolutionarily stable in the presence of a trade-off between learning and reproduction. This is because culture is a public good that is freely exploited by any member of the population in their model (cultural social dilemma). In this paper, we investigate the effect of vertical transmission (transmission from parents to offspring), which decreases the publicness of culture, on the evolution of cumulative culture in both infinite and finite population models. In the infinite population model, we confirm that culture accumulates largely as long as transmission is purely vertical. It turns out, however, that introduction of even slight oblique transmission drastically reduces the equilibrium level of culture. Even more surprisingly, if the population size is finite, culture hardly accumulates even under purely vertical transmission. This occurs because stochastic extinction due to random genetic drift prevents a learning strategy from accumulating enough culture. Overall, our theoretical results suggest that introducing vertical transmission alone does not really help solve the cultural social dilemma problem.

Evolutionary branching in deme-structured populations.

Adaptive dynamics shows that a continuous trait under frequency dependent selection may first converge to a singular point followed by spontaneous transition from a unimodal trait distribution into a bimodal one, which is called "evolutionary branching". Here, we study evolutionary branching in a deme-structured population by constructing a quantitative genetic model for the trait variance dynamics, which allows us to obtain an analytic condition for evolutionary branching. This is first shown to agree with previous conditions for branching expressed in terms of relatedness between interacting individuals within demes and obtained from mutant-resident systems. We then show this branching condition can be markedly simplified when the evolving trait affect fecundity and/or survival, as opposed to affecting population structure, which would occur in the case of the evolution of dispersal. As an application of our model, we evaluate the threshold migration rate below which evolutionary branching cannot occur in a pairwise interaction game. This agrees very well with the individual-based simulation results.

Trade-off between learning and exploitation: the Pareto-optimal versus evolutionarily stable learning schedule in cumulative cultural evolution.

Inheritance of culture is achieved by social learning and improvement is achieved by individual learning. To realize cumulative cultural evolution, social and individual learning should be performed in this order in one's life. However, it is not clear whether such a learning schedule can evolve by the maximization of individual fitness. Here we study optimal allocation of lifetime to learning and exploitation in a two-stage life history model under a constant environment. We show that the learning schedule by which high cultural level is achieved through cumulative cultural evolution is unlikely to evolve as a result of the maximization of individual fitness, if there exists a trade-off between the time spent in learning and the time spent in exploiting the knowledge that has been learned in earlier stages of one's life. Collapse of a fully developed culture is predicted by a game-theoretical analysis where individuals behave selfishly, e.g., less learning and more exploiting. The present study suggests that such factors as group selection, the ability of learning-while-working ("on the job training"), or environmental fluctuation might be important in the realization of rapid and cumulative cultural evolution that is observed in humans.