PCSY-D-24-00168

Environmental variability promotes the evolution of cooperation among geographically dispersed groups on dynamic networks

PLOS Complex Systems

Dear Dr. Inaba,

Thank you for submitting your manuscript to PLOS Complex Systems. After careful consideration, we feel that it has merit but does not fully meet PLOS Complex Systems's publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

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We look forward to receiving your revised manuscript.

Kind regards,

Travis A Whetsell, Ph.D.

Academic Editor

PLOS Complex Systems

Travis Whetsell

Academic Editor

PLOS Complex Systems

Hocine Cherifi

Editor-in-Chief

PLOS Complex Systems

Journal Requirements:

1. We ask that a manuscript source file is provided at Revision. Please upload your manuscript file as a .doc, .docx, .rtf or .tex.

Additional Editor Comments (if provided):

Dear Dr. Masaaki Inaba,

I have received two reviews (copy-pasted below) for your manuscript submission entitled "Environmental variability promotes the evolution of cooperation among geographically dispersed groups on dynamic networks" PCSY-D-24-00168. The reviews suggest a minor revision is appropriate at this point. Reviewer 1 requests some minor revisions to the manuscript that are listed numerically as 1-5 below. Reviewer 2 has some thoughts for the development of your future models discussed in their fourth paragraph which could be mentioned in the manuscript as you discuss potential future work.

Thank You,

Travis Whetsell

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Reviewers' Comments:

Reviewer's Responses to Questions

Comments to the Author

1. Does this manuscript meet PLOS Complex Systems’s publication criteria? Is the manuscript technically sound, and do the data support the conclusions? The manuscript must describe methodologically and ethically rigorous research with conclusions that are appropriately drawn based on the data presented.

Reviewer #1: Yes

Reviewer #2: Yes

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2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

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3. Have the authors made all data underlying the findings in their manuscript fully available (please refer to the Data Availability Statement at the start of the manuscript PDF file)?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception. The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

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4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS Complex Systems does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

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5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: This manuscript provides a very detailed background of human evolution. To explore the connection between intensified environmental variability (EV) and the evolution of intergroup cooperation, the authors analyzed three stochastic models of EV: Regional Variability (RV), Universal Variability (UV), and Combined Variability (CV). Through the lens of evolutionary game theory, they have constructed a streamlined mathematical model to simulate environmental variability. Each step of model simplification is accompanied by a robust and credible rationale, showcasing the rigor of the study. By simulation analysis of the cooperator frequency and some theoretical derivations, they find that RV strongly promotes cooperation, while UV has a comparatively weaker effect. An intuitive explanation is provided through an analytical solution: RV does not directly affect the cooperation rate but instead promote fluctuations in it, therefore agents in poorer regions frequently undergo reformations and mutations. In conclusion, this is a novel and detailed work, and I am happy to recommend the acceptance. The following comments are provided to improve the paper further:

1. “Agents in poorer regions frequently undergo reformations and mutations” (line 265) shows that frequent updating promotes cooperation. Existing studies have also explored update rates (Dynamics of collective cooperation under personalised strategy updates. Nat. Commun. 15, 3125 (2024)), what is the difference between them? A brief discussion is needed to clarify the difference.

2. In the Reformation subsection, the neighbor selection process is proportional to resources. A valuable extension of this approach is to contrast it with random selection mechanisms. Such a comparison could offer valuable insights into the effects of these distinct selection strategies on the evolution of cooperation, thereby enhancing the comprehensiveness of the study. Furthermore, the manuscript highlights that the network structure changes over time. Could a dynamically changing network be substituted with a temporal network? For this point, you might refer "Evolution of cooperation on temporal networks. Nat. Commun. 11, 2259 (2020)".

3. The manuscript employs a two-player public goods game, which is typically utilized to illustrate interactions among multiple agents. What led to the decision to use this model over the classic two-player games such as the Prisoner's Dilemma and the Snowdrift game? Would the conclusions drawn still stand if these classic two-player games were employed?

4. “This creates a positive feedback loop” (line291) How does this positive feedback loop arise? Why can an increase in the rate of cooperation promote network heterogeneity? A plausible explanation is lacking here.

5. In the parameter table, the elements that appear should be explained, for example “(0.1 steps)”. And there is an error in the value of $\sigma_{\theta}$ in the table. There seems to be an error in the labeling of parameters in Fig. 4.

Reviewer #2: The model described in the paper offers a test of the Variability Selection hypothesis (VHS), which was forwarded and refined by this reviewer as a potential selective process that can favor the evolution of adaptability. Although the VHS has testable implications for other lineages of organisms, the hypothesis was originally proposed as a framework for understanding the ecological (resource) drivers of behaviors that emerged in the genus Homo enabling adaptive flexibility in highly variable environments. The model significantly extends the original hypothesis by illustrating how environmental variability (as posited by the VHS) can be an especially potent driver of evolution in the context of enhanced cognition and social cooperation. That is, the results of Inaba and Akiyama's model show how the VHS, the Cognitive Buffering hypothesis, and the Social Brain hypothesis could well have been interconnected during the later evolution of the genus Homo and the emergence of Homo sapiens. Importantly, the model and interpretations offered in the manuscript break down a false dichotomy between ecology (e.g., resource variability) and sociality as separate and alternative factors in the evolution of complex behavior in the human genus. The manuscript thus offers a valid corrective to this dichotomy.

The model further extends and refines the VHS in its description of how social cooperation is likely to emerge and be sustained when resource distributions across space (different geographic regions) vary over time. The model thus improves on how the VSH initially sought to explain the evolution of complex hominin behavior, which was largely taking the temporal dimension into account.

Previous studies of how environment may impact the evolution of cooperation are concisely and clearly summarized (manuscript p. 2). The modeling assumptions are well explained and appear to be well justified, as are the simplifications of the model -- e.g., creation of an abstract geographic distribution of hominin groups (p. 2). The model (Game) parameters are well-defined and justifiable (p. 3). The Game conditions are clearly explained and fairly easy to follow for even a non-modeler.

The Game posits several realistic conditions: E.g., only those agents (groups in different places) that have surplus resources can contribute to another group (i.e., can establish or heighten a cooperative relationship between groups). The Game entails switching of resource partners probabilistically toward an agent based on a criterion of "the higher the

resource". While this is a reasonable modeling criterion, Homo sapiens foragers (for energy and other resources), as evident across living hunting-gathering peoples from the equator to high latitudes, on the basis of memory of past exchanges and alliances -- i.e., reciprocity. That is, groups/agents possess a memory that establishes a real-world positive bias toward contributing resources to those agents that had previously offered resources in difficult times. This memory bias (reciprocity) is not reflected in the current Game. While this is not an important flaw in this first version model, I encourage the authors to take such "resource strategy memory" into account in a later iteration.

The paper's results (discussed on pp. 9-10) are impressive and innovative: Cooperation evolves when the spatial/agent distribution of resource strategies is disrupted - that is, the abundance of resources shifts spatially due to environmental variability (i.e., EV shifts not only temporally but also spatially). This disruptive effect is much in line with what the VSH posits; yet the model shows how the spatial dimension of variable resource regimes is critical for social cooperation to develop and be sustained. The result is significant in that the model has the potential to explore and explain how human forager exchange networks have developed and were intermittently reactivated up to the present day. The result is also in line with published archeological data in East Africa (Brooks et al., 2018, Science 360: 90-94; Potts et al., 2020, Science Advances 6: eabc8975) indicating how strong environmental variability was associated with a middle Pleistocene archeological location interpreted as a node in a social network. In this latter example, different places (and presumably agents) were involved in transferring a functionally valuable (and spatially unevenly distributed) resource (obsidian rock used in toolmaking) as an element in a resource exchange network. In this particular empirical example, one node drew upon resources located in up to eight distant places. The authors' model appears, then, to be concordant with this example dated to the onset of the Middle Stone Age in East Africa. The paper further suggests the potential for asymmetries to develop in such a multiplex exchange network, as previously explored by models presented by the authors (Inaba and Akiyama 2023, Sci. Reports).

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Reviewer #1: No

Reviewer #2: Yes: Richard Potts

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Environmental variability promotes the evolution of cooperation among geographically dispersed groups on dynamic networks

PCSY-D-24-00168R1

Dear Mr. Inaba,

We are pleased to inform you that your manuscript 'Environmental variability promotes the evolution of cooperation among geographically dispersed groups on dynamic networks' has been provisionally accepted for publication in PLOS Complex Systems.

Before your manuscript can be formally accepted you will need to complete some formatting changes, which you will receive in a follow-up email from a member of our team. 

Please note that your manuscript will not be scheduled for publication until you have made the required changes, so a swift response is appreciated.

IMPORTANT: The editorial review process is now complete. PLOS will only permit corrections to spelling, formatting or significant scientific errors from this point onwards. Requests for major changes, or any which affect the scientific understanding of your work, will cause delays to the publication date of your manuscript.

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Thank you again for supporting Open Access publishing; we are looking forward to publishing your work in PLOS Complex Systems.

Best regards,

Luca Maria Aiello

Section Editor

PLOS Complex Systems

Hocine Cherifi

Editor-in-Chief

PLOS Complex Systems

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Reviewer Comments (if any, and for reference):

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

Reviewer #2: All comments have been addressed

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2. Does this manuscript meet PLOS Complex Systems's publication criteria? Is the manuscript technically sound, and do the data support the conclusions? The manuscript must describe methodologically and ethically rigorous research with conclusions that are appropriately drawn based on the data presented.

Reviewer #1: Yes

Reviewer #2: Yes

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3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available (please refer to the Data Availability Statement at the start of the manuscript PDF file)?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception. The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

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5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS Complex Systems does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: (No Response)

Reviewer #2: The authors have organized and written an effective and complete response to my review comments - and they appear to have addressed effectively the comments of the additional reviewer.

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7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

Do you want your identity to be public for this peer review? If you choose “no”, your identity will remain anonymous but your review may still be made public.

For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: Yes: Richard Potts

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