News and ViewsBrain ontogeny and life history in Homo erectus
Introduction
Coqueugniot et al.'s (2004) recent study of the Mojokerto fossil (Perning 1), an important juvenile Homo erectus specimen from Indonesia, provides valuable insights into the evolution of human ontogeny, life history, and cognition. These authors used computed tomography to study the subarcuate fossa and the bregmatic region in Mojokerto, modern humans, and chimpanzees (Pan troglodytes). These analyses enabled assessment of the age at death and endocranial volume (EV) in the fossil, which suggested that the Mojokerto juvenile died at about one year of age (range: 0.5–1.5 years, and possibly younger) and had an endocranial volume of about 663 cc. Coqueugniot et al. used these estimates to investigate age-related changes in proportional endocranial volume in H. erectus. Specifically, they calculated proportional EVs by dividing each juvenile's EV by average adult EV within each species. Mojokerto's proportional EV was measured at 72–84% of adult volume, indicating that the fossil fell into the range for one-year-old chimpanzees, but possibly above the range for comparably aged Homo sapiens. Based on these results, Coqueugniot et al. proposed that, like some primates, H. erectus gave birth to offspring with proportionately large brains. The authors argued that this result suggests restricted postnatal brain growth in H. erectus, and possibly the absence of secondary altriciality. They also suggested that H. erectus brains matured soon after birth, constraining the development of language and cognitive skills.
Despite the high quality and innovative design of Coqueugniot et al.'s study, their analyses leave several important questions unresolved. The most important question concerns the absolute brain size of the Mojokerto specimen in relation to the new age estimate. Specifically, proportional analyses may obscure biologically significant developmental patterns. Questions about absolute growth rates, size, and scaling cannot be addressed using proportions. Moreover, Coqueugniot et al.'s measure of proportional EV depends on adult brain size. Given the brain size difference between H. erectus and H. sapiens (which differ by a factor of nearly 1.5), it is possible that both species could show identical absolute brain sizes during some phases of growth, but have very different proportional brain sizes. This is particularly important because previous studies usually estimate an older age for Mojokerto. For example, Antón (1997), concentrating mainly on temporal bone features, argued for an age of 4–6 years, but her thorough review documented previous estimates ranging from one year of age to more than eight years of age. In general, absolute brain size should be evaluated because growth rates have significant implications for cognition and life history. For example, brain growth rates and absolute size are important with regard to both individual metabolic costs (Aiello and Wheeler, 1995, Leonard et al., 2003) and maternal metabolic costs (Martin, 1983, Martin, 1996). Finally, Rightmire (2004) documented a wide range of variation in H. erectus adult brain size (727–1251 cc), and by including Dmanisi hominins, Coqueugniot et al. (2004) presented adult values as low as 600 cc. This range raises questions about precisely what should be considered an adult value for H. erectus.
The analysis presented here compared brain size growth patterns in chimpanzees, H. erectus, and H. sapiens in order to test hypotheses regarding the evolution of human brain growth in light of Coqueugniot et al.'s analysis. The most general hypothesis predicts close similarities in absolute brain growth patterns between H. erectus and H. sapiens, given the new age estimate. In effect, Coqueugniot et al.'s focus on proportional EV may have obscured brain growth similarities between H. erectus and H. sapiens. The general hypothesis of similarity between hominins is evaluated here by testing three more specific hypotheses. First, Mojokerto's absolute brain size is expected to lie within the range of variation for 0.5–1.5 year-old H. sapiens. Second, comparable absolute brain size growth curves for H. erectus and H. sapiens are expected. Third, analyses of proportional brain sizes are expected to reveal differences between H. erectus and H. sapiens, following Coqueugniot et al.'s (2004) results. An important and obvious alternative general hypothesis is that the age estimate is inaccurate.
Studies of absolute brain size complement Coqueugniot et al.'s (2004) study by providing insights into the relations between development and evolution of the human brain. These comparisons also enable an understanding of the evolutionary history of secondary altriciality in the human lineage (Portmann, 1941, Martin, 1983).
Section snippets
Materials
Comparative samples for humans and chimpanzees were derived from literature sources. Brain weight and age data for H. sapiens represent 19th Century German autopsy cases (Marchand, 1902), which have been analyzed previously in comparative studies (Jolicouer et al., 1988, Leigh, 2004). It is important to note that brain weight and EV are different variables (see Blinkov and Glezer, 1968, Tobias, 1971, Hofman, 1991, Leigh, 1992). Endocranial volume includes the brain and associated soft tissues
Results
Mojokerto's brain size falls within the bivariate scatter for H. sapiens, but outside the chimpanzee distribution (Fig. 1). Several H. sapiens brains between 0.5–1.5 years of age are smaller than that of Mojokerto. If Mojokerto was only 6 months of age at death, then its brain weight would actually exceed the predicted value for a male modern human. Moreover, Mojokerto falls within the lower bounds of the 95% human regression prediction interval (Fig. 2). The unadjusted EV also falls inside
Discussion
Comparative analyses indicate that proportional EV offers an incomplete picture of hominin brain growth. On the other hand, absolute brain size analyses show that brain size growth curves may be very similar between H. erectus and H. sapiens. This result depends entirely on the accuracy of the new age estimate. If this age estimate is accurate, then the distinctive pattern of rapid early human brain growth may have evolved more than 1.8 million years ago, an interpretation that contrasts
Conclusions
Coqueugniot et al.'s (2004) new endocranial volume and developmental age estimates for the Mojokerto fossil enhance our understanding human ontogeny and life history evolution. However, their decision to restrict analyses to proportional brain size obscures biologically significant similarities between H. erectus and H. sapiens. Analyses of absolute brain size growth reveal comparable early growth patterns in Homo, contingent on the accuracy of the new developmental age estimate. Mojokerto's
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2015, Journal of Human EvolutionHomo erectus and middle pleistocene hominins: Brain size, skull form, and species recognition
2013, Journal of Human EvolutionCitation Excerpt :This implies alterations in the growth process. For example, the H. erectus brain may have matured over a relatively short time period, as is true for apes (Coqueugniot et al., 2004; Hublin and Coqueugniot, 2006; but see also; DeSilva and Lesnik, 2006; Leigh, 2006; Zollikofer and Ponce de León, 2010). Supraorbital tori, which are so prominent in this species, must have begun to enlarge early in ontogeny, before the cessation of brain growth.
Mojokerto revisited: Evidence for an intermediate pattern of brain growth in homo erectus
2013, Journal of Human EvolutionCitation Excerpt :An ape-like brain ontogeny for Mojokerto would indicate that H. erectus experienced less of its brain maturation during the toddler years and therefore this species may not have been capable of acquiring the complex cognitive skills characteristic of humans, such as symbolic language (Coqueugniot et al., 2004). Leigh (2006) countered that H. erectus possessed a more human-like growth pattern based on the absolute brain size of Mojokerto falling within the range of modern humans. Hublin and Coqueugniot (2006) responded that the proportional brain size was the crucial variable in understanding brain ontogeny in this extinct human species.
Endocranial volume of Australopithecus africanus: New CT-based estimates and the effects of missing data and small sample size
2012, Journal of Human EvolutionCitation Excerpt :Relating Taung's EV to Sts 71's, Taung would have achieved 96–100% of its adult brain size. According to this, the brain growth pattern in A. africanus would be more similar to that of chimpanzees than of humans (see Coqueugniot et al., 2004; Leigh, 2004, 2006; Hublin and Coqueugniot, 2006; Neubauer and Hublin, 2012). Using state-of-the-art methods, we could confirm several previous estimates of EVs in A. africanus.