There are two developmental phases in which a theory contributes to scientific knowledge (a good example is Freud’s psychoanalytical theory of psychosocial development): in the first phase, the theory initiates empirical research in order to test its key assumptions; in the second phase, the fallacies and limitations of the theory become apparent, and the empirical research and further theorization serve more to criticize the original theory. Both phases are useful for science; in which phase is life history “theory”?

Even if the second phase has started, there comes a moment when criticism needs to be replaced with new constructive proposals or, in extreme cases, with the rejection of the whole conceptual framework. It seems that the manuscript written by Ellis, Reid, and Kramer (i.e., ERK) leads in the former direction. I believe their manuscript offers four significant contributions to LHT, which I will briefly summarize below. Firstly, there have been various criticisms of the fast-slow continuum in LHT in previous years (e.g., Međedović, Reference Međedović2020; Sear, Reference Sear2020). ERK’s paper, in my opinion, provides an additional strike against the continuum by dividing at least some of the life history indicators as functionally unrelated. Hence, there is no continuum – we need a more complex understanding of LH. Secondly, ERK’s paper provides an integration of the long-standing and interesting debate on “internal” and “external” predictions in the development of life history trajectories – a debate we enjoyed reading in Rickard, Frankenhuis, and Nettle (Reference Rickard, Frankenhuis and Nettle2014) and Belsky (Reference Belsky2014). Still, an interesting question remains: both internal (first-tier) and external (second-tier) predictions may depend on the congruence between childhood and adult environments. It is quite reasonable that such congruence exists (leaving extrinsic mortality aside in this specific matter and focusing on the very plausible, positive, and significant correlations between childhood and adult SES), but more importantly, the magnitude of this congruence is variable. There are populations in which this congruence may be stronger (e.g., populations with higher status inequality) as well as those in which it is weaker, i.e., where ecological conditions are more flexible. Further theoretical and empirical research is needed to answer how this influences development through the first and second tiers. Thirdly, ERK’s proposal is much more aligned with the pace-of-life syndrome (POLS) framework (Montiglio et al., Reference Montiglio, Dammhahn, Dubuc Messier and Réale2018). Animal evolutionary ecology researchers seem to have done a more thorough job compared to evolutionary social scientists, recognizing that life histories, physiology, and behavior are intertwined in potentially adaptive dynamics in certain environments, labeled as POLS. By highlighting the role of energetics (first tier), ERK efficiently enter the POLS framework, which is much more heuristically beneficial for understanding human evolution than the basic LHT proposal. However, this seems to be only the beginning: ERK mention only energetic constraints or life history allocations to somatic growth as a reaction to the environment. We need to move toward a more detailed model: does human POLS also include metabolism, immune functions, temperature, or other endocrinological processes involved in sexual development? This awaits us in the future.

However, ERK’s most important contribution is bringing us to demography. Demography is the key to evolution because major demographic processes – fertility, mortality, and migration – are crucial evolutionary processes as well: the former two as core fitness components and the latter as a source of gene flow. ERK skillfully analyze various data, both from contemporary and preindustrial human populations, to evaluate the two-tier model. I find their analysis convincing, and their proposal that reproductive outcomes (fertility and age of first reproduction – AFR) may be separated from maturation is certainly plausible. Since this is, in my opinion, the most important aspect of their contribution, I would like to add some caveats. Most importantly, I believe we should take an even bigger step back to demography in order to develop our understanding of LH. At the core of LH and evolution itself are tradeoffs. But tradeoffs are not ubiquitous; they are variable and, sometimes, may not even exist in certain populations. Moreover, there are empirical phenotypic (Haave-Audet et al., Reference Haave-Audet, Besson, Nakagawa and Mathot2022) and genetic (Chang et al., Reference Chang, Moiron, Sánchez-Tójar, Niemelä and Laskowski2024) positive correlations between longevity and fertility, despite LHT predicting negative associations. These data are by far the greatest challenge for LHT, as the fertility-mortality tradeoff is crucial in generating life histories; hence, we face the difficult task of reconciling them with our LH theorizing. Other data seem to be incongruent with ERK’s theorizing as well: for example, there are gene alleles associated with both earlier menarche and AFR (Dvornyk, Reference Dvornyk2012), although ERK believe that these two LH outcomes are unrelated. Also, we should be cautious when making generalizations from data obtained in existing traditional populations to human ancestors. The demography of historical hunter-gatherers differs significantly from the demography of contemporary small-scale societies (French & Chamberlain, Reference French and Chamberlain2021), which casts doubt on whether the existing preindustrial, traditional, and natural fertility populations can be representative of ancestral human societies.

To summarize: there are certainly benefits to ERK’s proposal and it should be welcomed. This proposal should be seen as an initial step toward more complex LH trajectories: we should further explore fundamental LH dynamics by revisiting basic processes and searching for tradeoffs. ERK did not analyze the quantity–quality tradeoff, which is elusive itself, and whose repercussions for offspring fitness in contemporary humans remain unclear. The AFR tradeoff is also questionable: low AFR is detrimental to fitness only when it occurs during adolescence (Pirkle et al., Reference Pirkle, de Albuquerque Sousa, Alvarado, Zunzunegui and Research Group2014): in every other case, lower AFR is associated with higher completed fertility (i.e., it is adaptive). Yet, humans continue to delay AFR – a puzzle with high heuristic significance for both demography and evolution. Perhaps our first task is to refrain from calling LH a theory because it is not one – this is not merely a question of semantics, but a conceptual issue in itself. We need to think about LH as a spatiotemporal populational dynamic of ecological conditions and fitness outcomes that constitute evolutionary tradeoffs; LH exhibits characteristics of dynamical systems that show significant variability between and within populations (Međedović et al., Reference Međedović, Karić, Kostić and Kovačević2025). Yes, we began by acknowledging Occam’s razor reflected in the fast-slow continuum, but it is time to move forward and develop more complex formal models and nuanced empirical research. In doing so, we should pay additional attention to (perhaps still undiscovered) socio-ecological conditions that may affect the strength, or even the sign of associations between fitness components that constitute tradeoffs, as these conditions may vary across populations. If ERK’s proposal initiates this, it will secure a valuable place in the development of the LH framework, and, I hope, in the much-needed unification of LH models in evolutionary social sciences and evolutionary animal ecology.