Decision letter | New footprints from Laetoli (Tanzania) provide evidence for marked body size variation in early hominins

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New footprints from Laetoli (Tanzania) provide evidence for marked body size variation in early hominins

Decision letter

Affiliation details

University of Dar es Salaam, Tanzania; Università di Perugia, Italy; Sapienza Università di Roma, Italy; Università di Pisa, Italy; Studio Associato Grassi, Italy; Università di Firenze, Italy
George H Perry, Pennsylvania State University, United States

In the interests of transparency, eLife includes the editorial decision letter and accompanying author responses. A lightly edited version of the letter sent to the authors after peer review is shown, indicating the most substantive concerns; minor comments are not usually included.

Thank you for submitting your article "New footprints from Laetoli (Tanzania) provide evidence for marked body size variation in early hominins" for consideration by eLife. Your article has been favorably evaluated by Ian Baldwin (Senior Editor) and three reviewers, one of whom, George H Perry (Reviewer #1), is a member of our Board of Reviewing Editors. The following individuals involved in review of your submission have agreed to reveal their identity: William Jungers (Reviewer #2) and Matthew Bennett (Reviewer #3).

The reviewers have discussed the reviews with one another and the Reviewing Editor has drafted this decision to help you prepare a revised submission.

Summary:

The reviewers agree that this paper provides both the description of a major discovery of new footprints of early bipedal hominins from Laetoli, Tanzania at 3.6 MYA and a valuable analytical result concerning early hominin body size variation. Laetoli is already famous for its Site G fossil footprints of (presumably) Australopithecus afarensis individuals. The new Site S footprints reported here represent very important additions to the Pliocene record of hominin behavior and morphology. A dozen new footprints from the S1 (N=11) and S2 (N=1) trails are sufficiently complete to estimate the body sizes of their makers (again, presumably A. afarensis) as well as approximate walking speeds. The new stature estimate for one of the individuals greatly exceeds those previously reconstructed from fossilized skeletal material or footprint data for any A. afarensis individual, consistent with substantial body size variation and likely sexual dimorphism within a single species of sexually dimorphic australopithecines.

Essential revisions:

The reviewers raise a several concerns that must be adequately addressed before the paper can be accepted. These points include the need to focus on the most appropriate body size estimation methods, to provide appropriate analysis and visualization of the comparisons of body size estimates among the different australopithecine individuals, and to revise the presentation of the manuscript to better reach a general, non-specialist audience.

1) The authors provide stature and mass estimates from the footprints using both modern human and australopithecine reference samples. The use of the modern human reference samples for stature estimates are inappropriate because they assume body proportions akin to modern humans, which are not possessed by australopithecines. Unfortunately, the australopith-based estimates are based on one individual, the tiny iconic female "Lucy" (A.L.288-1), and foot length in this individual is itself estimated. The authors should be more circumspect in reporting their results, acknowledging that the stature estimates from modern humans are likely exaggerations, and focus their interpretations on the more appropriate (but still tenuous) australopithecine-based predictions – still with the caveat about the limitations of the data from which the predictor is derived. Regardless, these footprints do expand the upper limit of size in (presumably) A. afarensis, and offer strong support for arguments favoring strong sexual size dimorphism in this extinct species.

2) The manuscript should provide additional data and analysis on the footprints and on the comparisons to previous australopithecine body size estimates. Table 3 should provide the range and standard deviation for print length, width, and index in addition to the average for S1, G1 and G3 trails. An ANOVA or other statistical test for significant differences in these parameters among the 3 trails should also be provided.

3) While there are 14 figures in the current version of the manuscript that contain images of the footprints themselves (see Essential Revisions comment #5 below), there are currently no figures depicting the key analytical result of the paper; that is, the large stature estimate of the S1 individual compared to estimates available for other australopithecines. This needs to be remedied. One option could be to depict stature estimates for all available australopithecines on the Y axis with MYA on the X axis, using different symbols for estimates derived from skeletal elements vs. footprints (and including the error estimates on the plot where available). Another part of the figure could illustrate the geographical location of each fossil/ footprint, with notation making it possible to reference each datapoint between the two panels.

4) The manuscript presentation requires rework, especially for general audience suitability. In the Introduction and Discussion, for non-specialist readers the significance of this result for interpretations of hominin behavior and evolutionary biology needs to be much more clear. There is not a need to list every theory; rather, focus on one or two of the main/ strongest implications (and the logic behind them, based on previous work) for early hominin behavior/ evolution based on the results of this study, laying out the case very clearly in the Discussion (after presenting possibilities of both sides in the Introduction). In the Results, the main results need more attention. For example, it is insufficient to simply (and briefly) refer the reader to the Tables 2 and 3 for the main results of the study. Also, some brief explanation for how body mass and stature are calculated should be provided in this section (Essential Revisions comment #1 above is also relevant here).

5) Many of the 14 figures are gorgeous, and the images and descriptions of the find are an important part of the paper. However, the key findings and results would be more apparent to the reader if there were some streamlining of the figures. Some of the existing figures could be integrated as panels into others (e.g., there are many figures with images of the S1 track and different footprints that could potentially be combined) or using the eLife feature for embedded supplementary figures.

6) Please ensure that the scans of the footprints and images (at the full resolution available and used for analyses in the study) are made publicly available through an appropriate data repository, and report the database accession details in the revised manuscript.

DOI: http://dx.doi.org/10.7554/eLife.19568.027