Technical Comments

Response to Comment on "The Brain of LB1, Homo floresiensis"

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Science  19 May 2006:
Vol. 312, Issue 5776, pp. 999
DOI: 10.1126/science.1124972

Abstract

Martin et al. claim that they have two endocasts from microcephalics that appear similar to that of LB1, Homo floresiensis. However, the line drawings they present as evidence lack details about the transverse sinuses, cerebellum, and cerebral poles. Comparative measurements, actual photographs, and sketches that identify key features are needed to draw meaningful conclusions about Martin et al.'s assertions.

Martin et al.'s (1) efforts to show that LB1's cranial capacity cannot be attributed to intraspecific dwarfing in Homo erectus suggest that Falk et al. (2) claimed otherwise. However, Falk et al. documented that LB1's cranial capacity is too small to be attributed to normal dwarfing of H. erectus and further showed that its relative brain size is consistent with those of apes/australopithecines (Fig. 1). It would therefore be surprising if LB1's relative brain size scaled like a dwarfed elephant, for example, leading us to question the aptness of table 1 and figure 1 presented by Martin et al.

Fig. 1.

Cranial capacity in cubic centimeters as a percentage of body weight in grams (RBS, relative brain size) plotted against body weight (kg) for humans and apes (10). Indices (i) describe apelike RBS (i = 1) and RBS that are twice (i = 2) and three times (i = 3) those expected for apes of equivalent body weights. Congo pygmies are placed on the human curve at their mean body weights of 42 kg and 48 kg for 319 women and 405 men, respectively (11). The curve for H. erectus is hypothetical because it is based on data showing that i = 2 from only one available skeleton (KNM-WT 15000). Estimated juvenile and adult weights for WT 15000 are 48 kg and 68 kg, respectively, and juvenile and adult cranial capacities are 880 and 909 cm3, respectively (12). LB1's cranial capacity of 417 cm3 places it on or near the ape curve at its minimum, mean, and maximum body weight estimates (16 to 36 kg, mean 26 kg) (9), which is consistent with data from australopithecines. Figure and legend from figure S1 in (2).

As noted in (2), the virtual endocast of the microcephalic in question was prepared from a skull cast reposited in the American Museum of Natural History (AMNH), and Falk et al. scaled the virtual endocasts of all comparative specimens to match LB1's volume (417 cm3) to facilitate shape comparisons. We have since reanalyzed the CT data and determined a capacity of 276 cm3 for the microcephalic (3). Although the AMNH was unable to provide a recorded age for the microcephalic, its anomalous teeth suggested to us that that it might be a juvenile, and we are happy to learn that it is, in fact, from a 10-year-old male.

Contrary to Martin et al., we did not assume that there is only one type of primary microcephaly, as detailed in the text of our supporting online material (2). Martin et al. claim that there are more than 400 genetic syndromes associated with primary microcephaly that “typically have autosomal recessive inheritance.” This conflicts with our reading of the literature. Rather than being associated with primary microcephaly, small head size “defined as an occipitofrontal circumference that is at or below–2 standard deviations (SD) at birth...constitutes a feature found in more than 400 genetic syndromes” (4), and these 400 syndromes are not portrayed as generally uniform nor of known genetic inheritance (4). Our more restrictive definition of microcephaly (2) is therefore warranted, especially in discussions pertaining to LB1, because adult primary microcephalics typically have a brain volume of about 400 cm3 (4).

Martin et al. further argue that because LB1 was an adult, comparison with microcephalics should focus on high-functioning types (which can survive to adulthood) and that Jakob Moegele's early death renders comparison inappropriate. On the contrary, “early death” in low-functioning microcephalics is defined as “death that typically occurs within the first several years of life” [table 1 in (4)]. Because Jakob Moegele lived to be 10, one cannot rule out that he may have been a high-functioning primary microcephalic. Martin et al. were indeed fortunate to locate a 32-year-old female microcephalic, because by that age 78% of female microcephalics are estimated to have died (5). Because of brain shrinkage, one would also not expect to obtain a highly convoluted endocast (like LB1's) from such a specimen (5, 6), and we gather from the lack of detail on Martin et al.'s line drawings that neither of their microcephalics reproduced endocasts that are highly convoluted. As illustrated in (6), Jakob Moegele's virtual endocast is shaped remarkably like that of endocasts from other microcephalics, rather than like that from LB1 as claimed (7), and we are glad to have a microcephalic specimen of its size and age in the sample of microcephalics that we are currently investigating.

The weight of Martin et al.'s comment is their assertion that they have identified one hemicast and one complete endocast from microcephalics that appear similar to LB1's. However, the line drawings they provide lack crucial details about the transverse sinuses and frontal and occipital poles [figure 2 in (1)]. Although the authors provide a line drawing for LB1's skull, no image is provided for its endocast. A line drawing is provided for an endocast of a microcephalic from the Field Museum, but not for its skull, which, as described (and illustrated in actual photographs) in the reference cited by Martin et al., “is long-headed and narrow, with a lowly vault, the face narrow, with apelike protrusion of the jaws” (8). This skull differs starkly from LB1's, which is extremely brachycephalic (2, 9). Providing drawings of one hemicast and one endocast from microcephalics and asserting that they look like the virtual endocast from LB1 without illustrating the latter is therefore inadequate. Comparative measurements along with actual photographs of the microcephalic hemicast and endocast should be compared with published images of LB1's endocast in frontal, occipital, lateral, and dorsal views (2) and accompanied by corresponding sketches that identify key features, such as the transverse sinuses and cerebellum, similar to Falk et al. (2, 6). Without this evidence, the assertions of Martin et al. remain unsubstantiated and difficult to address in further detail.

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