Category: Prehistoric Families

Looking for Quantitative Data on Children's Productivity in Domestic Work

11/3/2015

After having set computer modeling aside (out of necessity) for the past year, I'm ready to start getting back to that component of my research. I'm hoping to get things ramped back up this semester and be doing some fairly hardcore computational work by next semester. I've got some technical and logistical issues to address, and I'll surely have to spend some time getting back up to speed with Java and Repast. But it shouldn't be too bad

One of the things I'd like to do when I get up and running again is take another look at how the children's labor might affect the dependency ratio and household-level calculations about family size, etc. (see this paper for my first attempt to address the issue). In the models I'm currently using (e.g., the ForagerNet series), I represent a child as either a "producer" or a "non-producer" by comparing the child's age to the value of a parameter that specifies the age at which children become producers. Clearly that's a significant simplification of reality: children do not magically become "producers" overnight on their 8th birthdays or their 12th birthdays, etc.

I was watching my two boys (ages 4.7 years and 2.2 years) shell beans at the kitchen counter over the weekend, and it got me wondering what kind (if any) quantitative data are available about age-based changes in the proficiency of children in doing various kinds of productive tasks. Watching my two kids, there was a huge difference in the proficiency (and interest level) of the older and younger boys. The older one stuck with the task until we ran out of beans and, while not as fast as an adult, was really pretty good. Despite an equal contribution of energy (at least in the beginning), the productivity of the younger one was much lower. Part of that had to do with the desire to do a victory dance for each single bean that he managed to pull from a pod.

What would the age-based "proficiency curve" of bean processing look like? A linear progression? A rapid increase in proficiency between ages 2 and 6? At what age do individuals reach "full proficiency" in processing beans?

Are there ethnographic data available that would allow me to understand how proficiency at processing various kinds of plant foods changes with age? How about tasks related to gathering? Or planting?

There has been an increase in interest in the lives of children in hunting-gathering societies (see this paper by Nurit Bird-David), and I'm hoping that some quantitative data are available from recent ethnographic studies (e.g., see this 1994 paper by Blurton Jones et al., this 2004 paper by Raymond Hames and Patricia Draper, this 2009 volume edited by Barry Hewlett and Michael Lamb). I'm less familiar with possible sources for quantitative data relevant to modeling age-based changes in children's work proficiency in agricultural societies. Maybe there are government agencies or NGOs that monitor that sort of thing (the site of the International Labour Organization discusses children's domestic work, for example).

Anyway, this blog post is just a placeholder. I'm interested in tracking down some data if they're available (if you know of any, please let me know!).

If there are not suitable data already out there, I might have to generate some of my own through some controlled experimentation. Let me know if you like beans. We may soon be producing a surplus at my house. And I may soon ask to borrow the participation of children of various ages.

The Dependency Ratio in Human Evolution

5/15/2015

As far as I know, humans are unique among animals in having an extended period between weaning and being able to subsist on their own. We call this “childhood.” The long period of post-weaning dependence provides our large brains with a lot of time to mature. It also requires a lot of parental investment (in terms of time, energy, calories, etc.) and means that we would have to wait a long time between offspring if each one had to independent before the mother could have another.   We don’t do that, tending to have a shorter interval between subsequent births (the inter-birth interval, or IBI) than other great apes. The long period of childhood dependence and the short IBI mean that, as a species, humans tend to have multiple, dependent offspring of different ages at the same time. Speaking as a parent of multiple, dependent offspring of different ages, I can tell you that this is often no walk in the park. This peculiar human strategy has a lot of costs.

Understanding when, how, and why this distinctly human reproductive strategy developed is a great evolutionary question. Reducing the IBI increases the potential fertility of human populations, but also creates new demands on the energies of parents and families. Human families today often offset those extra energy demands by getting help (evolutionary anthropologists call it “cooperative breeding”). A new paper in the Journal of Human Evolution titled “When Mothers Need Others” by Karen Kramer and Erik Otárola-Castillo tries to further our understanding of where cooperative breeding comes from, using an “exploratory model” to try to understand the selective pressures associated with the evolution of human-like patterns of reproduction and child-rearing. The goal of the paper “is to develop a model to predict those life history transitions where selective pressure would have been strongest for cooperative childrearing” (pg. 5).

Kramer and Otárola-Castillo call their model the “Force of Dependence Model.” Their model is a simple one, calculating “the net cost of offspring as a function of dispersal age, birth intervals and juvenile dependence” as a 3-dimensional surface (Supplementary Online Material from Kramer and Otárola-Castillo 2015). The authors use several different combinations of settings to represent a range of conditions from “ancestral” (juvenile independence at age 10, IBI of 6 years, and a dispersal age of 14) to “most derived” (juvenile independence at age 20, IBI of 3 years, and a dispersal age of 20). Their graphs show that “net costs” within a domestic group (a mother and her offspring) are lower when offspring are spaced further apart and become independent at a younger age. When offspring hang around past the age of juvenile independence, there is a net benefit to the domestic group as their productive capacities can be used to offset the drain of their younger siblings. The authors find that the strain points – where selective pressures for assistance would be greatest – occur in domestic groups with the most derived set of characteristics: late juvenile independence and a low IBI (lots of children who remain dependent for a long time).

As I understand it, the “net cost” in this model more-or-less mirrors the dependency ratio (the ratio of consumers to producers) of a domestic group or family, something anthropologists have been interested in understanding for a long time. The higher the ratio of consumers to producers, the higher the dependency ratio, and the higher the “cost” to each producer supporting the family. The dependency ratio changes through the lifespan of a family in a patterned way: every domestic group that has children goes through a “pinch” period when the dependency ratio is highest, and the pinch period logically corresponds to the time when there are a lot of dependent offspring. As I wrote in my 2013 paper in the Journal of Anthropological Archaeology (“Subsistence Economics, Family Size, and the Emergence of Social Complexity in Hunter-Gatherer Systems in Eastern North America,” available here):

“the duration and amplitude of the ‘pinch’ is affected by the rapidity of the addition of offspring and how quickly those offspring turn from consumers into producers. The rapidity of addition of offspring will depend on factors such as fertility, infant and childhood mortality, and the number of wives. The productive potential of children will be affected by the presence and distribution of resources that can be procured by children and the foraging strategies that are employed to exploit those resources” (White 2013:128).

The main part of that paper used an agent-based model (ABM) to try to understand how the distribution of family size changes when the age at which children become producers (the “age of juvenile independence” in Kramer and Otárola-Castillo’s model) decreases and there is an incentive for polygynous marriage. In addition to the ABM, I used a simple spreadsheet model to show how the dependency ratio changed through the course of the developmental cycle of an individual family in cases where the age at production was low (8 years old) and where it was high (14 years old). In this simple model, I used an IBI of 3, a dispersal age of 16 for females and 20 for males, and a female reproductive period spanning ages 20-35 years (giving a total fertility of 6 offspring).

The figure below compares Kramer and Otárola-Castillo’s graphs from their cases with early and late juvenile independence (holding IBI at 3 and dispersal age at 14) with my data on changes in dependency ratio through the developmental cycle in cases with a single reproducing female and an age at production of 8 (top) and 14 (bottom).   My model data are the same as in my 2013 paper (Figure 5), but I have re-graphed them to make comparison with Kramer and Otárola-Castillo’s figure easier. I have redrawn the graphs from Kramer and Otárola’s Figure 1 (third graphs from the left, top and bottom rows). The dotted lines on the graphs of my results indicate a dependency ratio of 1.75, which is what I have generally used in my modeling efforts as a “typical” dependency ratio among hunter-gatherers (following Binford 2001:230).

My results showed the same pattern as Kramer and Otárola-Castillo’s: the peak of the “pinch” comes earlier and is less severe when children become producers at an earlier age. Even though our models have some differences (and some of the values of the parameters were different), the correspondence in results is notable. Compare, for example, when the amplitude of the “pinch” (peak dependency ratio in my results, greatest net cost in Kramer and Otárola-Castillo’s results; marked by stars) is greatest and the differences in amplitude between the early and late ages of juvenile contributions to subsistence.

The correspondence between my results and Kramer and Otárola-Castillo’s is unsurprising. The idea that the dependency ratio of a domestic group changes through the course of its developmental cycle in a somewhat predictable way is not new (and the idea that the “pinch” comes when you have lots of little kids running around at the same time won’t come as a revelation to anyone who has multiple children). This is a phenomenon that has been studied for decades (e.g., Chayanov 1966; Donham 1999; Fortes 1958; Goody 1958) and recognized as a key aspect of how hunter-gatherers organize themselves (Binford 2001:229).

So where does this kind of work put us in terms of understanding the evolution of human reproduction, society, and family life? I think it primarily puts us in a spot where we’re asking some good questions. Going back to the issue of the origins of monogamous pair-bonding (which I touched on briefly in this post about birth assistance and this post about australopithecine sexual dimorphism), having a two person (male-female) unit forming the core of a domestic group would have a mitigating effect on the strain caused by a decrease in IBI (i.e., you’d be adding another producer into the equation). If the appearance of male-female pair-bonding was associated with a sexual division of labor (which is I think what most of us would hypothesize), males and females would presumably be focused on procuring somewhat different sets of subsistence resources. Offspring could be largely “independent” with regards to some of those resources but not to others – think about the difference between collecting berries and running down large game. A sexual division of labor and an environment where relatively young children could make some contribution to their own subsistence (even if that contribution does not include the full range of resources that are exploited) would go a long way toward easing the “pinch” that comes from having more children spaced closer together.

When does this happen in human evolution? Of course that’s a tough thing to get at directly. I think if you took a poll, the winner would probably be “around the time our genus emerges” or “with Homo erectus.” An increase in total fertility (coincident with a lowering of the IBI) would help explain the population growth that must have been part of the dispersal of our species out of Africa prior to 1.8 million years ago. It would also fit nicely with the evidence for an increased exploitation of animal resources around that same time. Maybe Glynn Isaac was right all along to propose the emergence of human-like central place foraging with home bases and a sexual division of labor at the beginning of the Lower Paleolithic?

But what if monogamous pair-bonding and a sexual division of labor appeared much earlier – with australopithecines or even some pre-australopithecine like Ardipithecus? If those things came along with bipedal locomotion, would a decreased IBI and increased fertility have followed automatically? Maybe not. Perhaps those earlier hominids just didn’t have the wherewithal to exploit their environments like later hominids did – perhaps the diversity of the resource base they could exploit wasn’t great enough to really leverage a sexual division of labor until animal products became readily attainable. That may have required a suite of anatomical adaptations for daytime exhaustion hunting (loss of body hair, skin pigmentation, greater body size, stiffer foot) and cognitive/behavioral adaptations for making and using stone tools to process carcasses. The date of the “earliest” proposed use of stone tools continues to be pushed back (now it’s at 3.3. million years ago), but as far as I know the density of stone tools and butchered animal bones that appears at about 1.8 million years ago is unlike anything that precedes it.

More modeling work will be required to really understand how changes in the dependency ratio might have articulated with changes in reproductive, social, and technological behaviors deep in human prehistory. In order to understand what changes in reproduction might have meant in terms of social interactions, however, we’ll need a different grade of model than that used by Kramer and Otárola-Castillo. Of course I’m going to say that complex systems modeling is the way to go on this: it will let us get past the limitations of deterministic inputs and help us understand how constraints, costs, and interactions would have played out within a society. In order for “others” to help with raising and provisioning multiple dependents, those others had to have existed within these small-scale hominid societies and (again, speaking as someone involved in raising multiple small kids) there wouldn't have been some inexhaustible Plio-Pleistocene babysitting pool of “others” out there just waiting to step in and provide extra calories for a few years. A different kind of modeling effort with broader scope will let us get at the group- and society-level contexts in which family-level changes in child-bearing and child-rearing would have played out. Stay tuned.

References

Binford, Lewis R. 2001. Constructing Frames of Reference: An Analytical Method for Archaeological Theory Building Using Hunter-Gatherer and Environmental Data Sets. University of California Press, Berkeley.

Chayanov, A. V. 1966. A. V. Chayanov on the Theory of Peasant Economy. University of Wisconsin Press, Madison.

Donham, Donald L. 1999. History, power, ideology: Central issues in Marxism and anthropology. University of California Press, Berkeley.

Fortes, Meyer. 1958. Introduction. In The Developmental Cycle in Domestic Groups, edited by Jack Goody, pp. 1-14. Cambridge Papers in Social Anthropology. Cambridge University Press, London.

Goody, Jack. 1958. The Fission of Domestic Groups among the LoDagaba. In The Developmental Cycle in Domestic Groups, edited by Jack Goody, pp. 53-91. Cambridge Papers in Social Anthropology. Cambridge University Press, London.

Kramer, K., & Otárola-Castillo, E. (2015). When mothers need others: The impact of hominin life history evolution on cooperative breeding Journal of Human Evolution DOI: 10.1016/j.jhevol.2015.01.009

Was Birth Assistance Among Early Hominids a Gendered Activity?

4/25/2015

Depiction of male and female Australopithecines at the American Museum of Natural History.

I've always like Karen Rosenberg and Wenda Trevathan's (2002) paper "Birth, Obstetrics, and Human Evolution" (available here). It does a nice job of exploring the social implications of anatomical changes related to the emergence of bipedalism, focusing on how the shape of the pelvis in australopithecines would have complicated birth and transformed delivery from a solitary experience into one that often required assistance. I used a condensed version of the article (Scientific American 13:80-85, 2003) in my 200-level Human Origins class: it really helps students see how you can make connections between fossils and behaviors and how one aspect of human evolution can have implications for others.

While I was reading one of the student papers from that class this semester, I started to question the model I had in my head of australopithecine females assisting each other in birth. The student wrote something that seemed to imply (intentionally or not - I'm not sure) that males were the ones giving assistance at birth. It struck me as odd, which made me ask myself why it should strike me as odd: why did I assume females and not males?

I don't think Rosenberg and Trevathan ever specify that females would have been the ones providing assistance to other females, but they use the term "midwifery" several times in their papers. Though not technically defined today as excluding males, the term "midwife" carries a lot of history that associates it very closely with females. This is the etymology as provided by Wikipedia:

"The term midwife is derived from Middle English: midwyf literally "with-woman", i.e. "the woman with (the mother at birth), the woman assisting" (in Middle English and Old English, mid = "with", wīf = "woman")."

Based on a quick perusal of some of the web resources that pop up first (e.g.,this, this, and this) the idea of the ancient origins of the association between females and birth assistance is widespread. I have no reason to doubt that this is correct: I would presume a strong female bias in birth assistance could be amply demonstrated both historically and ethnographically.

So should we assume that females were also providing birth assistance among early hominids?

Maybe not.

It seems clear that our modern conceptions of who should provide assistance at birth are culture-bound. By "culture-bound" I mean connected to other shared, leaned aspects of human societies, behaviors, and symbolic systems. I don't think I'll get an argument if I state that there is zero evidence for anything like human culture among australopithecines. So what happens to our gendered conceptions of birth assistance if you remove all or most of their cultural underpinnings? Would birth assistance still be performed primarily by females?

For the sake of argument, consider how selection might act to reinforce the behavior of males assisting females during birth. If labor and delivery are dangerous for both the female and the neonate, any assistance a male mate can provide increases the chances his genes will be passed on. This would be true in a situation of stable male-female pair bonding, as pair-bonding decreases the uncertainty of paternity. The inclination of males to provide effective assistance during birth would be selected for if pair-bonding was present and birth was dangerous.

If we assume that pair-bonding among australopithecines would not have been the same thing as "marriage" (a human cultural behavior), why do we assume that australopithecine birth assistance would be the same thing as "midwifery"? Our historical and ethnographic record of what humans do now really only gets us so far in addressing questions like this: the universality of a cultural practice among modern humans does not necessarily mean it always existed in the same form deep into the past.

Rosenberg, K., & Trevathan, W. (2002). Birth, obstetrics and human evolution BJOG: An International Journal of Obstetrics and Gynaecology, 109 (11), 1199-1206 DOI: 10.1046/j.1471-0528.2002.00010.x

Update (5/3/2015): The response of one of my friends on Twitter to this post was that "Sexual selection says they should've been too busy getting busy to care." I wrote this quick post taking an informal look at ideas about sexual dimorphism and sexual selection among australopithecines. It looks to me like the jury is still out, as there's a lot of conflicting information about the degree of sexual dimorphism in the two criteria we look at most often in primates: canine size and body size.

Fetal Head Molding and Obstetrics in Late Pleistocene Humans

3/15/2015

Preface: This post presents some work I did as a graduate student at the University of Michigan in 2007. It was a poster for a class called "Evolution of the Genus Homo," taught by Milford Wolpoff. I chose the topic because of my interest in the culture, biology, and social organization of Middle/Late Paleolithic humans (see discussion of my 2014 SAA paper here and here, and my 2015 AJPA paper here). I had hoped to develop this into a paper either alone or with a collaborator, but I have never found the time to follow through. As the information I collected ages and I begin to focus on moving to a new job that will require a lot of attention up front to the archaeology of the southeastern United States, it seems less and less likely that I'll ever get around to turning this into a paper. So I'm going to put my analysis from the poster out there "as is" and hope it useful to someone. If you read this and think it's an interesting idea or one that you'd like to pursue, let me know!

I apologize for the state of the bibliography: there are some formatting errors that I will correct when I have the time.

Fetal Head Molding and Obstetrics in Late Pleistocene Humans

Figure 1. Illustration of changes in head shape that occur during birth (from A.D.A.M).

Introduction

This study compares available data from fetal and neonatal crania from the Late Pleistocene to the mechanics of fetal head molding during birth in recent humans.   The small number of fetal and neonatal remains dating to the Late Pleistocene offer an opportunity to simultaneously explore issues of obstetrics, selection, and early brain growth. Most treatments of birth and obstetrics in Pleistocene humans have focused on pelvic anatomy (e.g., Rak and Arensburg 1987; Rosenberg 1998; Rosenberg and Trevathan 2002; Trinkhaus 1984). Studies of childhood growth and development after birth are limited mainly by the dearth of sub-adult skeletons, particularly those that pre-date Neandertals (see Anton 2002; Dean et al. 1986; Minugh-Purvis 1988, 2002; Nelson and Thompson 2002; Stringer et al. 1990; Tellier 1998; Trinkhaus and Tompkins 1990).   Fetal and neonatal remains, from Neandertals and other Late Pleistocene humans, have been described but have not been the subject of detailed, hypothesis-based research.

Deformation (molding) of the fetal cranium is an important part of successful birth in recent humans. This study examines the hypothesis that thickness of fetal cranial bone would have been an impediment to successful birth in Neandertals and other Late Pleistocene humans. Investigating the possible role of fetal head molding in Pleistocene obstetrics may help shed light on both anatomical trends in human cranium (i.e., the emergence of "modern" cranial morphology) and demographic variables and population genetics that may underlay the spread of anatomical "modernity."   Mortality and trauma during childbirth, acting on the mother and/or the fetus, would be an important selective force.

Two aspects of fetal head molding are emphasized: cranial vault thickness and head dimensions.    Vault thickness affects the response of the cranium to pressure during birth. Head size and shape affect both the degree of molding that is required for the fetal head to pass through the birth canal and the distribution of forces on the fetal cranium.

Hypothesis: Thick fetal cranial bone in Late Pleistocene archaic humans would have caused difficulties during childbirth (relative to recent humans) by inhibiting head molding during delivery.

Assuming uniformity in the size of the birth canal between archaic and recent humans, this hypothesis has two test implications:

1) the increased thickness of Late Pleistocene fetal cranial vaults would have a significant effect on elasticity of the cranium

2) the dimensions of the fetal cranium are such that significant molding is required for delivery

In other words, fetal head molding must be shown to be both necessary (by the dimensions of the cranium) and significantly impeded (by the in-elasticity of the vault) in order to fail to reject the hypothesis. If either one of these test implications is rejected, then the hypothesis can be rejected.

Pleistocene Obstetrics: Previous Research

Much research focused on questions of obstetrics in Pleistocene humans have emphasized the selective constraints between locomation and birth mechanics in the pelvis (Rak and Arensburg 1987; Rosenberg 1998; Rosenberg and Trevathan 2002; Ruff 1995; Trinkhaus 1984). Based on pelvis remains, most researchers conclude that birth in Pleistocene humans was much like birth in recent humans (Rosenberg 1998; Rosenberg and Trevathan 2002). Subsequent to the description of the Kebara 2 pelvis (Rak and Arensburg 1987), most ideas about an unusually long gestation periods (Trinkhaus 1984) and rapid in utero brain growth (Dean et al. 1986) in Neandertals have been rejected (see Stringer et al. 1990:148).

While pelvic inlet size during the Pleistocene appears to be, overall, similar to modern humans, cranial capacity increased. during the Middle and Late Pleistocene. Stasis in pelvic inlet size and increase in head size produces an "obstetric dilemma" where the fetal head is larger than the birth canal. Rosenberg and Trevathan (2002:1205) state that

"Two changes could have allowed an increase in adult brain size to occur: human infants could have been born with a smaller percentage of adult brain size (resulting in greater infant helplessness) and/or there could have been an alteration of the shape of the pelvis concomitant with a change in the mechanism of birth."

There is a third possibility: fetal head molding. The possible importance of fetal head molding in Neandertals is raised by similarities in both pelvic inlet size and adult cranial capacity to recent humans. Minugh-Purvis (1988:260) speculated that the thicker vault bone observed in Neandertal fetal remains would have posed a problem if delivery required a "considerable degree of head molding." The possibility was also discussed by Friedlander and Jorndan (1994).

Figure 2.

Fetal Head Molding in Recent Human Birth

In recent humans, the fetal cranium is a flexible structure that deforms during birth because of pressures between the fetal head and the cervical walls (Lapeer and Prager 2001; McPherson and Kriewall 1980a, 1980b) (Figure 2). Pressures and deformation are greatest at the sub-occipito bregmatic plane (Lapeer and Prager 2001; Rosenberg and Trevthan 2002).

During a normal labor, the parietal bones undergo the most significant changes in shape, being compressed towards each other and elongating in the axial plane (Lapeer and Prager 2001; McPherson and Kriewall 1980b). The occipital bone is relatively rigid and undergoes little change during molding (McPherson and Kriewall 1980b:18; Rosenberg and Trevathan 2002:1201). The frontal, occipital, and parietal bones interlock at the sutures after a certain limit of deformation occurs, preventing excessive molding and protecting the brain within a more rigid structure (McPherson and Kriewall 1980a:15).

The risk of excessive molding is greater in pre-term deliveries, where cranial bone is not sufficiently thick to prevent excessive molding (McPherson and Kriewall 1980a). Clinical studies have shown that excessive molding during birth (i.e., where too much deformation occurs) may be linked to psycho-neurological disorders, mental retardation, cerebral palsy, and death (see McPherson and Kriewall 1980b).

Fetal cranial bone must be thin enough to allow sufficient deformation of the cranium, but thick enough to form a rigid structure to protect the brain. Optimal thickness values would vary for different portions of the fetal cranium depending on the pressures that are exerted and the required responses to those pressures.

Parietal Thickness, Span, and Deformation Under Load

Parietal bones grow outward from a center of ossification that later becomes the parietal eminence (Ohtsuki 1980). The bones are thickest at the eminence, thinning towards their margins (McPherson and Kriewall 1980b). Ohtsuki (1977) reported a mean thickness of 0.54 +/- 0.13 mm for term (9-10 month) fetal parietal bones at the center of ossification and a thickness of 0.40 +/- 0.10 for term fetal frontal bones at the center of ossification (n = 10). McPherson and Kriewall (1980a:10) reported term fetal parietal bones that varied in mean thickness from 0.71-0.86 mm.

In their analysis of the mechanical properties of fetal parietal bone, McPherson and Kriewall (1980a:11) found that differences in thickness and the orientation of the bone fibers affected the elastic modulus (the resistance to deformation when a load is applied). Thicker cranial bone requires more force to deform. Figure 3 shows the relationship between thickness and elastic modulus in the data supplied by McPherson and Kriewall (1980a:10,13), using only the parietal bones with fibers orientated parallel.

Using the formulae provided by McPherson and Kriewall (1980:11), we can use the estimates of elastic modulus to estimate the loads that would be required to bend segments of bone of varying length and thicknesses (Figures 4 and 5). Other things being equal, longer "beams" of bone require less force to bend, while thicker "beams" require more. To have the same resistance to bending force, a longer "beam" must be thicker.

Figure 3. Thickness of fetal cranial bone plotted against elastic modulus (data from McPherson and Kriewall 1980a). The regression (R2 = 0.78) is: 4.13 + 2.86(log of thickness in mm)

Figure 4. Plot of force required to cause a deflection of 1 mm in "beams" of bone of varying thickness (assuming a beam length of 75 mm - approximately that of a modern human term fetus). While the absolute values of these calculations may not be accurate (parietal bones vary in thickness in cross-section and do not behave simply as "beams") the calculations show that the resistance to force changes dramatically when thickness increases from 1 mm to 2 mm.

Figure 5. Plot of force required to cause a deflection of 1 mm in "beams" of bone of varying length (assuming a beam thickness of 1 mm).

Fetal Vault Thickness, Dimensions, and Molding in Late Pleistocene Homo

The Late Pleistocene fossil record contains numerous remains from sub-adult specimens. Of interest here are those remains that preserve portions of the cranial vault, particularly the frontal and parietal bones. Fetal and neonatal remains of Neandertals have been recovered from La Ferrassie (Heim 1982) and Hortus (Lumley-Woodyear 1973). Rremains of two Neandertals less than about a year old have been reported from Shanidar (Trinkhaus 1983) and Krapina (Minugh-Purvis 1988). Neonatal remains attributed to anatomically modern Homo sapiens have been reported from Cro-Magnon (Minugh-Purvis 1988), Qafzeh (Tillier 1999), and Abri Patuad (Minugh-Purvis 1988). Krapina is the earliest site, dating to the late Riss/early Wurm (Wolpoff 1999). La Ferrassie, Shanidar, Qafzeh, and Hortus date to Wurm I/Wurm II. Abri Pataud and Cro-Magnon date to Wurm III/IV (Wolpoff 1999).

Vault Thickness

Data on vault thickness at the parietal and frontal eminences are available for six fetal/neonate skeletons and three young (<1 year) infants from the Late Pleistocene.

Figure 6. Top: Drawing of Neandertal neonatal parietal from Hortus 1b (adapted from Lumley-Woodyear 1973). Bottom: Neandertal fetal and newborn frontal bone fragments from La Ferrassie compared to frontals from recent humans (adapted from Heim 1982).

Figure 7.

The thickness of the frontal and parietal bones in this sample contrasts with the data from the modern sample provided by Ohtsuki (1977) (Figure 7), outside the 2 sigma range of his means for term fetuses. If the Late Pleistocene fetal and neonate remains are aged accurately, fetal parietal bone was generally 1.5 to 2 times thicker than that of modern humans.

Considerably more force would be required to deform these bones, assuming the geometry of the bones was otherwise equivalent (see below).

Test prediction 1 is supported: differences in fetal cranial vault thickness are sufficient to affect molding of the cranium.

Fetal Head Dimensions

The limited data available on very young Late Pleistocene individuals suggests that some aspects of fetal head geometry may have differed from that of more recent humans (Minugh-Purvis 2002; Stringer et al. 1990). Bregma-lambda distance appears to have been shorter in Neandertals than in recent humans throughout life (see Minugh-Purvis 2002:488-489; Gunz and Havarti 2007; Harvarti 2003; Trinkhaus 1983:371). In mature Neandertals, the shorter distance is associated with a lower position of bregma (see Harvarti 2003)

Figure 8.

A shorter bregma-lambda distance would reduce the cross-section of the fetal cranium in a dimension that is key to the necessity for fetal head molding. It appears that this distance may have been about 10 mm less in Neandertals at the time of birth relative to recent humans: perhaps 80 mm rather than 90 mm (see Minugh-Purvis 2002). A difference of ca. 10-12% in the bregma-lambda chord would be sufficient to account for a 3-7% reduction in the sub-occipito-bregmatic diameter (SOBD). Assuming equivalence in other dimensions of the head and pelvic inlet, this difference alone would significantly lessen the degree of fetal head molding that would be required for successful delivery.

Test prediction 2 is not supported: the dimensions of the fetal cranium are such that significant molding was probably not required for delivery.

Conclusions

Significant fetal head molding was probably not critical to successful Neanderthal birth. While thicker cranial bone would have reduced elasticity, a smaller SOBD would have negated or lessened the need for molding during birth.

Reduction in fetal cranial thickness may not have been a reproductive advantage for "modern" humans. Rather, cranial thinness associated with an increase in the SOBD may have increased the risks to the fetus during birth (i.e., though excessive molding) while reducing or maintaining the risk to both mother and fetus (i.e., through arrested labor). In the absence of selection for thinner bone associated with a flexibility requirement, thick fetal cranial bone would have offered protection to the fetal brain during delivery. Apparent stasis in pelvic anatomy suggests that smaller, thicker fetal crania may be the ancestral condition. An increase in SOBD, perhaps reflecting some difference in fetal brain growth, would have preceded selection for thinner cranial bone in this scenario.

The fetal cranium is a complex mechanical structure. Constructing a simulation model (similar to that of Lapeer and Prager 2001) of delivery in Neanderthals is possible with the available data. This model could be used to test hypotheses about obstetrics in a more sophisticated way than is possible by calculating simple ratios of head and pelvic size.

References Cited

Anton, Susan C. 2002. Cranial growth in Homo erectus. In Human evolution through developmental change, edited by Nancy Minugh-Purvis and Kenneth J. McNamara, pp. 349-380. Baltimore: Johns Hopkins University Press.

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"Slicing the Pie" of Neandertal Family Life

4/11/2014

Part two of my foray into the Middle Paleolithic . . .

The phrase slicing the pie refers to a tactical method of systematically clearing an area hidden by an obstacle: you move around the obstacle and take care of one slice at a time. That’s often a better option than just jumping right past the obstacle and exposing oneself to whatever unknown terrible things might lurk around the corner. Slicing the pie is a method of breaking a big problem up into several smaller problems with the added stipulation that the problems must be addressed in a specific sequence in order for the method to be successful.

My 2014 SAA presentation is my attempt to work through the first slice of the pie of Neandertal family life (and take a peek around the corner to see what the next couple of slices might look like). As I discussed a little bit in this post, I'm using an agent-based model to explore how the high adult mortality regimes suggested by the Atapuerca-SH and Krapina assemblages might have affected the behavioral conditions under which hunter-gatherer populations were demographically viable. Agent-based modeling lets you create representations of plausible human systems unlike those we can observe ethnographically. It lets you understand how those systems are structured and work, and it provides a basis for developing expectations that can be compared to archaeological and fossil data. We could, of course, jump right past those kinds of nuts and bolts questions and argue about whether or not the symbolic contractual aspects of Neandertal male-female pair bonds were like those of “modern” humans. That’s a great piece of the pie to argue about, and I like those arguments as much as the next person. But I think that’s pretty far around the corner. Developing a basic understanding of the structure, organization, and behaviors of Neandertal domestic groups is a better piece of pie to start with.

I’ve still got some work to do on the presentation, but I thought I’d go ahead and post it here. [Edit: I've removed the draft version of the presentation - the version that I presented at the SAAs is here]. Some of the organization and a few details might change before the meetings, but the basic content and ideas will remain the same. I’m hoping that “pre-posting” this helps me do the things I go to conferences to do: learn something, exchange ideas, and meet people who are interested in similar topics or approaches. Maybe it will mitigate the downsides of both posters (does anyone actually read them?) and talks (how much can you get across in 15 minutes?). I’m betting I can generate more interest in my work by posting it and giving a 15 minute presentation than I can by just giving the 15 minute presentation. Or maybe the real benefit will be that I won’t be sitting in my hotel room the night before still trying to organize my Powerpoint. Even if that’s the only benefit there is . . . I’ll still take it.

I'm not done with this question, and I don't claim to have "solved" anything. But I’m generally happy with what I’ve managed to do so far: getting the presentation in shape has helped me clarify my thinking a bit, and working part of this into a publication will be on my summer agenda. I’m going to try to make the case (by showing rather than assertion) that a complex systems approach gives you a fighting chance to understand the structure and organization of domestic life during the Paleolithic. Paleolithic domestic life is, of course, a really big pie. Understanding the implications of high adult mortality in terms of population viability and family-level behaviors during the Middle Paleolithic is just the first slice. To cover in depth all the ideas that are in this presentation is potentially a dissertation- or book-level project: there's a lot of room here. I’ve already written my dissertation, so that’s out. We’ll see where the rest of this goes. Please let me know if you’re interested in thinking about ways to address domestic life during the Paleolithic -- there may be a conference symposium and/or an edited volume in the future.

Looking for Quantitative Data on Children's Productivity in Domestic Work

The Dependency Ratio in Human Evolution

Was Birth Assistance Among Early Hominids a Gendered Activity?

Fetal Head Molding and Obstetrics in Late Pleistocene Humans

"Slicing the Pie" of Neandertal Family Life

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