As I mentioned here there is a new article out concerning Neanderthal brain size and life history. In some ways, this is the continuance of a debate started by Trinkaus a number of years ago.
Trinkaus had noticed that the Neanderthal pubic rami seemed to be longer than those found in anatomically modern Homo sapiens and, hence, had a wider birth canal. This led him to suggest that Neanderthal gestation length was a year long (this search gets you most of the relevant papers). This created a bit of controversy and two main theories were proposed to explan the pelvic differences. The first, by Stringer, argued that Neanderthals had their growth and life history accelerated compared to modern humans. The second, put forth by Karen Rosenberg, suggested that the pelvic differences were simply a matter of Neanderthal body size. The debate ended when the analysis of the male Kebara pelvis was published. This analysis indicated that the size of the pelvic inlet in Neanderthals is not different from that of humans. So, the year gestation hypothesis was dead, but the idea that Neanderthals had an accelerated growth schedule which, may or may not have had something to do with size remained.
Which is where the new paper comes in. Neanderthals do have larger brains than anatomically modern humans and there are a number of explanations for it. First, this could be due to rapid growth in utero resulting in larger neonates. Second, there could be a higher growth rate during infancy. Third, there could be a longer period of growth. Each of these possibilities is accompanied by certain costs and benefits, which I will get to below. The point of the de Leon paper was to test some of them.
The authors virtually reconstructed the skull of a Neanderthal neonate found at Mezmaiskaya and estimated the endocranial volume. The estimate puts it in the range of similarly aged recent modern human neonates (so it is not explanation one). The authors also virtually reconstructed the fragmentary Tabun 1 female Neanderthal pelvis and were able to dertermine that, like humans, Neanderthals had rotational birth. They also included some cool stereoscopic pictures in the paper and supplemental material:
Then they reconstructed the skulls of two Neanderthal infants (Dederiyeh 1 and 2) and conclude that the two were at the upper end of the range for modern humans. This suggests that:
Comparison of endocranial growth trajectories from birth to adulthood further indicates that on average, Neanderthal brains expanded at a higher rate and attained larger adult volumes than those of rAMHS … However, Neanderthal and rAMHS trajectories coincide when related to taxon-specific adult mean ECVs …, indicating that both taxa reached adult sizes within the same time period, and along equivalent trajectories. Together, these graphs show that compared with rAMHS, Neanderthals attained their larger adult brains via higher growth rates rather than via an extended growth period. The graphs [not show – afarensis] also show that higher growth rates do not imply faster completion of brain growth.
This is where things get interesting. Back in 2004 Steve Leigh published a paper that examined brain growth and maturation in primates. Building on earlier work by RD Martin, he came to a couple of interesting conclusions. First, brain size and reproductive maturation are not directly associated in primates. Second, there seems to be two alternate life history strategies that are followed by primates. In the first strategy, females mature late, are larger, and produce larger brained infants. As Leigh explains:
Late maternal maturation may confer the ability to the meet metabolic costs of very early (fetal or infantile) brain growth through larger size and other sequelae of later maturation, such as better social skills, higher rank, and greater foraging proficiency. The advantages of larger maternal size in terms of metabolism may contribute to large brain size. In addition, an early maternal investment in brain growth may result in offspring that are precocious in terms of cognition, locomotion, foraging, and social behavior.
In the second strategy, females mature early and infant’s brains grow for a longer period postnatally. The advantages of this are:
Early maturation confers major demographic advantages to these species by enabling significantly increased intrinsic rates of increase (mediated by early reproductive maturation) … Thus, these species may realize the benefits of early maturation by ”distributing” the costs of offspring brain growth to the offspring itself or to other group members. Furthermore, shifting costs to offspring may enable shorter interbirth intervals in these species.
This makes me think that de Leon et al were a little confused in their claims to National Geographic (claims which do not appear in the PNAS paper). To refresh your memory:
Young Neanderthals’ rapid growth required lots of energy, experts say.
“Neanderthals must have had a rich diet in protein and fat for children to fuel rapid growth in [their] brains,” said Holly Smith of the Museum of Anthropology at the University of Michigan, who was not involved in the research.
Mothers also likely had to consume vast quantities of calories to produce enough breast milk.
This energy-intensive child rearing may have caused “somewhat longer interbirth intervals, or somewhat older mothers,” study co-author Zollikofer said.
As Mike Shupp pointed out in comments to the previous post, breastfeeding can restrict fertility – something we also know from studies of hunter-gatherers so at least that portion of the above quote is accurate. My point here is that there seems to be to conflicting processes at work between high rate of growth, birth spacing and breastfeeding. From reading both Leigh and de Leon et al at least part of the problem is that Neanderthal growth does not seem to fit Leigh’s model. In Neanderthals there is a high rate of growth for the same amount of time as in anatomically modern humans. At any rate, breastfeeding confers additional benefits. According to this recent Science Daily article research indicates that breastfeeding is linked to better physical and cognitive development. Something that would apply to both Neanderthals and moderns. An interesting side note in the Science Daily article is that eating fish provides the same type of benefits as breastfeeding. I mention this because of the recent news about giant clams indicates that the amount of marine exploitation by both neanderthals and early modern humans may have been underestimated.
There is some other interesting stuff in the de Leon paper, but that is for a future post…
de Leon et al 2008 Neanderthal brain size at birth provides insights into the evolution of human life history, PNAS 105(37):13764-13768
Leigh (2004) Brain Growth, Life History, and Cognition in Primate and
Human Evolution, American Journal of Primatology 62:139-164