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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, eaar3611
DOI: 10.1126/science.aar3611

Abstract

Rosas et al. (Reports, 22 September 2017, p. 1282) calculate El Sidrón J1 to have reached only 87.5% of its adult brain size. This finding is based on an overestimation of Neandertal brain size. Pairwise comparisons with a larger sample of Neandertal fossils reveal that it is unlikely that the brain of El Sidrón would have grown appreciably larger.

Rosas et al. (1) are to be congratulated for their discovery and whole-body analysis of El Sidrón J1—an important addition to our understanding of Neandertal paleobiology, growth, and development. Although Rosas et al. present a rich, whole-body treatment of El Sidrón J1, 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. This finding would be quite extraordinary, given that 95% of brain growth is achieved in modern human children by the age of 6 to 7 years (3, 4).

The cranial capacity estimated for El Sidrón J1 is 1330 cc (1). Careful and clever analysis of bone surface remodeling on the J1 occipital demonstrates that this region of the skull was still osteogenic, consistent with the juvenile status of the individual. The relationship between occipital remodeling and the magnitude of remaining brain growth is unknown, however. Future work examining whether evidence for occipital remodeling is present in juvenile modern humans or Neandertals who are >8 years old (e.g., Teshik-Tash, Le Moustier 1) and have reached adult brain volume will be informative. The authors propose that only 87.5% of full brain capacity had been reached in El Sidrón J1 on the basis of a Neandertal adult brain volume of 1520 cc (1). This adult value is an overestimate.

The use of 1520 cc for adult Neandertal brain size is explained as an average of five different studies [table S32 of (1)]. The largest adult cranial capacity average used to create this consensus value (5) is an average of only six adult Neandertal crania (Amud, Shanidar 5, La Ferrassie, La Chapelle, Spy 2, Tabun C1), which also happen to possess some of the largest brain sizes of the known Neandertals (Table 1). The lowest of the average Neandertal cranial capacities (1494 cc) used to create the consensus average derives from a source that restricts its analysis to nine Würm Neandertals (6). However, there is a math error in that paper, and the average of the listed cranial capacities should have been 1468.7 cc, not 1494 cc. Additionally, using values from Holloway et al. (7) for those Neandertals would result in an average cranial capacity of 1438.3 cc. A third value of 1498 cc, based on Holloway et al., is used to generate the consensus average (1). In Holloway et al., an average for Homo sapiens neanderthalensis is reported as 1487.5 ml in appendix I and 1427.2 ml in appendix II. The 1427.2-ml value is almost identical (1428 ml) to the average of reported adult values in the text of Holloway et al. However, Holloway et al. also attribute fossils typically assigned to Homo sapiens [Jebel Ihroud (n = 2) and Skhul (n = 4)] to Neandertals. When these are removed, the average Neandertal adult cranial capacity in Holloway et al. is 1414.8 ml. Given these problems with the values used to generate the consensus average of cranial capacity in adult Neandertals, it is necessary to recalculate the likely percentage of adult brain size achieved by the El Sidrón juvenile.

Table 1 Neandertal cranial capacities.

Würm Neandertals (living <115,000 years ago) are in italics. Cranial capacities are from (1, 7, 1113).

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When all adult crania assigned to Neandertals (n = 26) are used (Table 1), the average adult brain size is 1388 cc, and the El Sidrón juvenile had reached 97.4 ± 12.6% of its full growth. When those crania listed specifically in table S32 of (1) are used, the average adult Neandertal brain is 1438 cc and the El Sidrón juvenile had reached 94 ± 12.6% of its adult cranial capacity. To account for temporal changes in Neandertal cranial capacity (Fig. 1), it is reasonable to only use crania from the later (<115,000 years ago) Würm Neandertals, which average 1459 cc. Using this value, the El Sidrón juvenile had achieved 92.5 ± 12.0% of its full growth. However, because the dimensions of Krapina 1 are used to help reconstruct the El Sidrón J1 cranium (1), it would make sense to include the Krapina fossils as well, in which case the average Neandertal adult brain was 1437 cc and El Sidrón J1 had reached 93.9 ± 11.8% of its brain growth.

Fig. 1 Neandertal brain size evolution.

Adult is known for 26 Neandertals (blue circles). Notice that there is a slight but statistically significant (r = 0.49, P = 0.006) increase in brain size over the course of Neandertal evolution. El Sidrón J1 (red ×) falls comfortably within the range of adult cranial capacities in late Würm Neandertals.

Note that in every case, El Sidrón J1 is within the 95% confidence interval of the mean for 100% brain growth (Fig. 2). The only statistically supported way in which the El Sidrón juvenile could be used as evidence for continued brain growth in a 7.7-year-old Neandertal is if his cranial capacity (1330 cc) is compared exclusively with the largest half of the Würm Neandertals. Accurately identifying the sex of Neandertal crania is fraught with challenges, and using just the largest-brained Neandertals as a comparative male sample biases the results and borders on circular reasoning. Given both spatial and temporal variation in Neandertal brain size, I would recommend caution in inferring Neandertal brain ontogeny from El Sidrón J1 until the average adult brain size in this specific population of Neandertals at El Sidrón is better understood.

Fig. 2 El Sidrón J1 relative brain size.

The cranial capacity of El Sidrón (1330 cc) was divided by all adult Neandertal cranial capacities to calculate a percentage of adult brain size achieved by the age of the individual’s death (7.7 years). The mean value (black square) is plotted along with the 95% confidence interval of the mean (gray bars). Note that the percentage of brain size achieved at death in a similarly aged modern human (95 to 100%; green highlighted range) either encompasses the mean (for all Neandertals) or very nearly does. These data suggest that El Sidrón J1 had achieved significantly more of its adult cranial capacity than the 87.5% reported by Rosas et al. (1), and likely had little remaining brain growth.

There is inherent error in using isolated specimens and cross-sectional data to infer longitudinal patterns. Therefore, attempts to reconstruct the pattern of Neandertal brain growth should incorporate all of the available data. Consider, for instance, that the ~3-year-olds (8) Roc de Marsal (1325 cc) and Engis 2 (1362 cc) have brain sizes similar to, or slightly larger than, El Sidrón J1. The 4.6-year-old Devil’s Tower child (8) has a cranial capacity of 1400 cc, exceeding that found in the developmentally older El Sidrón J1. These specimens should not be used to argue for early cessation of Neandertal brain growth any more than El Sidrón J1 can be used for extended brain growth. Instead, when all of the currently available data are plotted (Fig. 3), Neandertals and modern humans present very similar brain growth curves. Neandertals therefore probably did experience human-like extended brain ontogeny [(9), but see (10)]. On the basis of the evidence currently available, it is quite likely that El Sidrón J1 had very nearly reached his full brain volume.

Fig. 3 Neandertal brain ontogeny.

El Sidrón J1 is one of 13 known juvenile Neandertal crania (Table 1). Developmental ages of these individuals were based on data in (1, 8, 13, 14). These juvenile cranial capacities were each divided by the 18 Würm + Krapina adult cranial capacities to calculate a Neandertal brain growth curve. Error bars indicate 95% confidence interval of the mean of the percentage of brain growth achieved by that chronological age in Neandertals and modern humans. Human data were drawn from (15). Notice the similarities between human and Neandertal brain development [see also (9)]. Although El Sidrón J1 is slightly smaller than expected, in this broader context it appears best interpreted as normal variation in Neandertals. Accepting the premise that El Sidrón J1 (at age 7.7 years) is evidence for extended brain growth in the Neandertals would also necessitate accepting that Neandertals essentially ceased brain growth between the ages of 3 and 7.7 years, only to resume again.

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