Technical Comments

Response to Comment on “The growth pattern of Neandertals, reconstructed from a juvenile skeleton from El Sidrón (Spain)”

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Science  09 Mar 2018:
Vol. 359, Issue 6380, eaar3820
DOI: 10.1126/science.aar3820

Abstract

The comment by DeSilva challenges our suggestion that brain growth of the El Sidrón J1 Neandertal was still incomplete at 7.7 years of age. Evidence suggests that endocranial volume is likely to represent less than 90% adult size at El Sidrón as well as Neandertal male plus Krapina samples, in line with further evidence from endocranial surface histology and dural sinus groove size.

DeSilva (1) challenges one of several conclusions we drew from the analysis of the El Sidrón J1 skeleton (2)—namely, the suggestion that the brain of this juvenile Neandertal was still growing at the time of his death (7.7 years old). The main objective of our research on the El Sidrón J1 skeleton was to present a study of growth and maturation of a Neandertal juvenile from an organismic perspective, rather than focusing on one specific region or system such as the dentition or cranium. DeSilva (1) seems to agree that we presented a “rich, whole-body treatment of El Sidrón J1,” although he then writes that “an emphasis was made in the paper—and widely reported by the science media (2)—that at 7.7 years of age, this individual had only achieved 87.5% of its total brain volume, and was therefore still growing its brain.”

We would prefer to limit our response to the findings and interpretations that we presented in the report, without reference to the remarks expressed and emphasized in the media. Basically, we do not agree that undue emphasis was made in the paper regarding an extended period of brain growth in Neandertals based on the estimation of the endocranial volume (EV), which is the variable studied by DeSilva. We studied the pattern of growth and maturation of the teeth, postcranium, spine, body, and cranium. For the latter, we considered three types of information: surface histology, the size of the dural sinus grooves, and the EV. What we really emphasized were the following points: (i) The growth and maturation of the dentition and postcranium fell well within the modern human range, and thus we did not observe a fundamental difference in the overall pace of growth in comparison with modern humans. (ii) One divergent aspect of ontogeny is the timing of maturation of the spine. (iii) On the basis of the three types of aforementioned observations, the brain of El Sidrón J1 could still be growing. (iv) The maturation of the spine, together with ongoing brain growth, could point to an extended period of growth and maturation of the neuraxis.

The review of EV estimations in Neandertals by DeSilva contributes to the ongoing debate about the rate and duration of EV growth in Neandertal ontogeny (3, 4). Variation in sample size, accuracy, and comparability of different measurement techniques of EV remains an issue in all comparative studies of Neandertals. For the sake of clarity, we address the questions raised by DeSilva using only the values of EV provided by him (1). DeSilva presents successive different combinations of specimens in order to provide Neandertal averages. It is interesting to note that as we restrict the initial Neandertal sample accordingly to fit the characteristics of the El Sidrón sample, the percentage of adult EV attained by El Sidrón J1 decreases, as shown in Table 1. When the five male Würm Neandertal specimens are considered, the percentage of adult EV attained by El Sidrón J1 would be 81.79%. According to DeSilva, to restrict the comparison to adult male Neandertals is questionable because of problems with sex estimation, and although we agree with this general concern in paleoanthropology, we offer this comparison in order to present the whole range of possible estimations. We also agree with DeSilva that the Krapina fossils could be included in the reference sample for methodological reasons, and in this case (Würm males and Krapina adults), the percentage of adult EV attained by El Sidrón J1 would be 87.7%. If we consider all the comparisons included in Table 1, the average of percentage adult size attained by El Sidrón J1 is 90.67%.

Table 1 Neandertal endocranial volumes and percentage of adult size attained by El Sidrón J1.

All values come from DeSilva (1).

View this table:

In addition to this debate focused on direct estimations of EV, in our report we also included a second method for estimating the EV derived from the isolated occipital bone, which we briefly summarize here. A significant lineal relationship was found between the size of the occipital bone (geometric morphometric centroid size) and the endocranial volume in modern humans (n = 20; y = 104.8581x – 243.6349; P = 0.0016; r = 0.6736; r2 = 0.4537). The fitted lineal function in seven mostly male Würm Neandertals (y = 153.56x – 953.5652; P = 0.0598; r = 0.7350; r2 = 0.5403; table S33 in our report) yields a Neandertal adult mean ± 2SD of 1499 ± 270 cm3. The predicted value of 1253.2 cm3 for El Sidrón J1 (specimen SD-2300) lies at the lower end of this interval (1228.6 to 1769.4 cm3), whereas the value of 1448 cm3 for El Sidrón adult occipital SD-1219 is close to the mean. The EV of El Sidrón J1 estimated by this method represents 86.5% of the adult occipital from El Sidrón (SD-1219). With the data used, the predicted value for J1 would be unlikely for an adult Neandertal male, although it is clear that predictions derived from this method must be considered as relative estimations of EV.

As mentioned, beyond the discussion on EV, we supported our suggestion of ongoing brain growth with other observations. The presence in El Sidrón J1 of inner occipital resorption areas, the smallest widths of the dural sinuses in a large hominin sample, and extremely fresh neural relieves are not conclusive by themselves, but they support our interpretation of potential continued brain growth. When considered together with the observations on spine maturation, we suggested that Neandertal neural growth and maturation might be extended in comparison with modern humans.

We agree that using isolated specimens and cross-sectional data is not the best methodology to infer growth and maturation—a problem that pervades paleoanthropology. Our study surely is a first step toward a more comprehensive understanding of the absence or presence of ontogenetic differences between two Homo species that successfully interbred. Detecting any potential differences with certainty will probably require the complete analysis of many Neandertal subadult skeletons, both known and yet to be discovered.

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