Zachary Cofran has been dissertation blogging about his work on dental development in robust australopithecines: "Data, development and diets". An interesting look at how the skimpy pattern of data in the fossil record can guide how research is developed.
The face, mandible and endocast from Taung, South Africa, was the first australopithecine fossil to be discovered. We now know that the fossil dates to the period between 2.5 and 3.0 million years ago, but at the time of its discovery, the precise date was not known; only that it was likely earlier than fossil evidence for human evolution outside Africa.
The morphology of the specimen was therefore the strongest evidence about its relationships to humans and living apes. Interpreting the morphology means coming to terms with the developmental age of the skull.
Assess the age and morphology of the individual:
The fossil record is not made up only of adults. We have abundant skeletal evidence from juvenile individuals of a broad range of ages. At this station you will find model mandibles and maxillae from human children of a range of ages. These provide a comparison for the casts at the station, each of which represents a fossil hominin specimen from Africa, between 3.6 million and 1.5 million years ago.
The mandibles represent several different species. They include:
A selection of other mandibles, including some adult mandibles of the same species, is also available. Examine these in comparison with the modern dental models. Which teeth are present in the fossil specimens? What teeth are in the process of eruption? What do they tell you about the ages of the individuals?
Teeth have a close association with longevity. Enamel is the hardest substance in the body, but it does break, wear out, and is sometimes attacked by microbes. In Westernized contexts, we are all familiar with cavities, caused by acid-emitting bacteria in the mouth. But in many natural human societies, cavities (called caries) are rare. Instead, a lifetime of eating abrasive natural foods usually causes the teeth to wear down, a process called attrition.
Dental attrition is very important in the anthropology of ancient peoples. It helps us to understand the food processing techniques — for example, the use of abrasive grinding stones to process grain in early agriculturalists. Dental wear also provides a way of understanding the ages at death of ancient skeletons. In Western societies, excessive tooth wear may be indicative of habitual behaviors such as grinding the teeth, or may result from biases in the chewing pattern to one side or part of the mouth.
Differential wear describes a dentition in which one tooth is worn significantly more than its neighbors. A normal process of tooth wear results in differential wear, as first molars erupt at age 5 and develop many years of wear before the third molars erupt in the mid- to late teens.
What to do: Examine the teeth at this station. How are they worn? Is there anything complicating their wear pattern, such as the presence of caries? Which individuals have differential wear? Which are worn the most?
The long bones grow in parts. Early in fetal development, the bones are formed from cartilage. Bone tissue forms as special cells (called osteoblasts) lay down mineralized channels into the cartilage. Initially, the shafts, or diaphyses of the long bones begin to ossify. Later, the articular ends of the bone form their own centers of ossification, called epiphyses. Between the diaphysis and epiphyses remains a thin plate of cartilage, called the metaphysis.
As the bone grows, the metaphysis constantly adds new cartilage, and the diaphysis continues to ossify into this cartilage. So the bone can grow even as parts of it have already become mineralized tissue.
During the course of development, the bone tissue is recycled, gradually altering its shape. The hard cortical tissue can be invaded by cells that destroy the bone, called osteoclasts, only to have new bone laid down by secondary osteoblasts. The surface of the bone can be altered by having bone gradually removed, a process called resorption. Thus, bones remain living organs that can change their shape gradually, heal themselves, and adapt to new habits and needs.
What to do: This station has many juvenile bones, including a model skeleton of a young child. Try to identify the shafts of the long bones.
Like most mammals, humans have two sets of teeth. The first set is called the deciduous dentition, but you probably know these as "baby teeth."
The human deciduous dentition includes two incisors, one canine, and two molars in each quadrant. When people lose their deciduous molars, these are replaced by permanent premolars. The permanent molars do not have deciduous teeth in their places before them.
Deciduous teeth are abbreviated with a "d" and the tooth type and number in lowercase. For example, the deciduous lower first molar is a dm1; the upper left deciduous canine is luc.
What to do: Consider the series of models at this station. They represent the mandibular dentitions of children at different ages during their development. Can you determine the order that the permanent teeth erupt and replace the deciduous teeth? For example, are the permanent incisors the first to erupt? The permanent molars?
Part 2
There are several kinds of primate represented at this station. These primates have different adult body sizes, and grow at very different rates. Nevertheless, their teeth erupt in sequences that are very much like the human dental eruption sequence.
Yet, there are exceptions. Many primates erupt their canine teeth relatively late in their eruption sequence. In humans, the upper canine typically erupts before the second molars. In many primates, the canine is delayed in development compared to the second molars.
What to do: Examine the primate dentitions at this station. Identify the deciduous and permanent teeth that you see in each. Try to think about what age a human would likely be, with the same teeth present. Can you find aspects of tooth eruption that differ between humans and these primates?
The Wall Street Journal reported on Chet Sherwood's work late last month: "Brain Shrinkage: It's Only Human".
The human brain normally can shrink up to 15% as it ages, a change linked to dementia, poor memory and depression. Until now, researchers had assumed this gradual brain loss in later years was universal among primates.
But in the first direct comparison of humans to chimpanzees, a brain-scanning team led by George Washington University anthropologist Chet Sherwood found that chimpanzees don't experience such brain loss. From that, researchers concluded that only people are afflicted by this oddity of longevity.
The paper is in PNAS [1]. The press article doesn't really explain the findings of the paper very well. Sherwood and colleagues found that the age effect in their sample of humans was limited to ages older than any chimpanzee in their samples. So there's no evidence that humans and chimpanzees differ across the same ages. Now, whether we expect chimpanzees to shrink their brains at a younger age (because they develop and senesce faster) is an open question; I can see arguments both ways. Anyway, I think the study goes as far as gross morphological comparisons can take this question, and more detail will have to wait for us to understand the cellular mechanisms that influence brain size senescence.
Greg Mayer has a post on preformationism and epigenesis on the Why Evolution Is True blog:"Development is epigenetic".
He later quotes Richard Dawkins in a similar light, but I'm linking because of Mayer's own useful synopsis of the blueprint analogy versus the recipe analogy for development.
Preformationism, though wrong, is frequently reinforced by the common (though badly mistaken) practice of referring to DNA or the genome as a “blueprint” for the organism. It is of course no such thing. A blueprint is a two dimensional representation of a three dimensional object. There is, in a blueprint, a scaled representation of all the parts of the object. We can tell, for example, that the window on the second floor is 4 m above and 2m to the left of the door. There is nothing like that in your DNA: there isn’t a gene for your left eye, which is a scaled distance away from the gene for your right eye. Your DNA (and your development) is much more akin to a recipe. In a raisin cake recipe, there isn’t a line in the recipe that says place a raisin 2 cm in from the upper left hand corner (there would be, if we had a blueprint for the cake). Rather, if you combine the right ingredients, in the right sequence, in the right environment, the result is a cake with raisins distributed through it at a certain density.
In the end both these analogies entail some mechanism. A blueprint needs some past mechanism capable of producing an iconic representation of the final object. A recipe needs some mechanism capable of recording a sequence of steps. Neither of those is impossible to evolve (Mayer briefly mentions the iconic nature of the arrangement of Hox genes), but it's pretty clear that the blueprint analogy does not apply to most developmental processes.
I was thinking about this issue in light of the nativist and learning theoretic views of language development. In that problem, the question is about the locus of the recipe -- did evolution lay down special instructions for language learning, or does the language environment contain most of the structure necessary for children to learn without special instructions beyond those used for learning many other kinds of behavior? Chomsky argued that language environments cannot in principle supply the necessary structure, so biology must have done so ("Language and spandrels"). But he was essentially preformationist in this position, even to the extent of denying that language could have evolved. He instead preferred to see language as a side-effect of other evolutionary processes, or emerging as a physical principle from humanlike brains.
Anyway, I'll return to this later, I just wanted to register a note on preformationism and epigenesis in relation to the issue.
Scicurious has been blogging from the Experimental Biology 2011 meeting. This morning she writes about some of Lynn Copes' work: "Experimental Biology Blogging: On Thick Skulls and...Chewing." Copes has kept a crew of mice on hard foods in a cold room, to get them to chew more. Would it give them thicker skulls?
Copes found that the mice who had the soft diet had weaker jaw muscles (masseters) than those eating normal chow or chewing more in the cold, but it wasn't by much, and the skull (cranial vault) thickness did not significantly vary in any of the conditions. While this may seem like negative data, this actually suggests that, rather than the activity varying skull thickness, the thickness of our skulls may be genetically determined. Copes hopes to eventually address this question by looking at the skulls of various modern and ancient human groups. By looking at the thickness of adult skulls compared to those of children, she hopes to determine whether skull thickness is genetically determined, and if so, when, and why, our skulls got so thin.
Another point in favor of Homo erectus as stone age pachycephalosaurs.
Following on after yesterday's post about hunter-gatherer population structure, I ended with the proposal that cooperation may be a "cognitive technology" in the same way suggested for numbers ("Number as cognitive technology").
The technology perspective attracts me. It seems a productive way to examine the interaction between innate and extrinsic factors leading to human behaviors. We learn about numbers. Without a development of the brain within a cultural setting with widespread counting and training in number use, people don't develop the habits of mind that allow rapid comparison of cardinal values. They can still operate on sets of objects and compare their quantities, but they are missing a shorthand, a symbolic shortcut, that comes with learning and practice. Numerical concepts, invented and repeatedly used by human societies, give learners access to this symbolic method of problem-solving.
Cooperation and other prosocial behaviors are similar in some respects. Whether you share with another person or not in a particular concept depends on the rules about sharing that you learned as a member of your society. What's interesting is that these rules change with age in various ways. So I went looking in the developmental psychology literature for some data about how kids share. My notes here are just a start -- and I'm pretty sure they're rough to read near the end -- but I found it interesting how the data seem to illuminate the issue of cooperation in the archaeological record.
Toddlers can, in some circumstances, exhibit a surprising degree of understanding about the intentions of others. They can also be surprisingly helpful -- that is, they can see when another individual wants something, and can actively help that other person to get it. A paper last fall by Kristen Dunfield and colleagues [1] gives a nice review of this kind of helping behavior in toddlers aged 18 and 24 months.
Replicating previous work by Warneken and Tomasello (2006, 2007), we found that by 18 months, infants are beginning to identify the situations in which helping behavior is required; that is, they will aid instrumentally by retrieving an item that is out of a person’s reach, thus fulfilling another’s unmet goal. Further, the present study found a similar frequency of helping behavior to Warneken and Tomasello (2006), even though in the current study participants only received one experimental helping trial as opposed to the three trials they received in the previous paradigm. In light of previous studies, helping behavior may also be seen as young as 14 months, though the contexts in which it occurs are less flexible, owing perhaps to an emerging understanding of goal-directed activities (Warneken & Tomasello, 2007), recognition of the means by which certain unmet goals can be fulfilled, and the physical ability to mediate the completion of the goal.
However, as I well remember from my own toddlers, the "prosocial" characteristics of infants can be temperamental, to say the least. Dunsfield and colleagues considered 18 and 24-month-olds, finding substantial heterogeneity among individuals in the kind of helping or sharing behavior they exhibited.
While acknowledging the dangers of arguing from a null effect, it is the case that although the majority of the participants engaged in at least some prosocial behavior, there were no correlations between the various prosocial behaviors. Further, the most common pattern of response was to engage in only one type of prosocial behavior (helping or sharing). Although the tendency to engage in prosocial behavior in general tended to increase across our two timepoints, the increase was not the result of systematic development within or between the various subtypes of prosocial behavior. Thus, we have no evidence in the present study for “across the board” prosocial behavior within individuals in these two age groups. With future research that explores the consistency both within and between the multiple specific types of behavior, and that considers enduring behavior over time in a longitudinal manner (Eisenberg et al., 1999), it may be the case that helping, comforting, and sharing do not cluster together within an individual’s repertoire and perhaps should not be grouped together as one general category of unified behavior in infancy.
A natural question is, what does it take to manage any kind of sharing at all among children this young? By this age most children have experienced thousands of times when an adult or another caregiver has performed the opposite role, giving the child what she cannot reach herself. This long history of positive exemplars for sharing and cooperative behavior nevertheless leaves substantial variation among children in how they actually behave in a similar context.
The first article by Warneken and Tomasello cited above [2] compared human children with chimpanzee juveniles of a similar age. They showed that the human children did show these prosocial tendencies by 18 months, but that so do chimpanzees -- at least to a certain extent. The chimpanzee juveniles handled the most indexical of the tasks relatively well -- the case where a person is reaching for something but needs help to reach it. Other tasks didn't bring out the cooperative nature in chimpanzee juveniles:
However, the chimpanzees did not help the human reliably in the other types of tasks—that is, in those involving physical obstacles, wrong results, or wrong means. In a follow-up study, we gave them two additional tasks of these types—designed to make the human's problem especially salient and with more time for a response—and they still did not help in these tasks (14). Presumably, when someone is reaching with an outstretched arm toward an object, the goal is in principle easier to understand and the kind of intervention follows straightforwardly. This could explain why out-of-reach tasks (in contrast to the other scenarios) elicited more helping by children and the only instances of helping by chimpanzees. Children and chimpanzees are both willing to help, but they appear to differ in their ability to interpret the other's need for help in different situations.
This goes some distance toward explaining what children need to make them potential helpers. They need some way of figuring out the goal of the person who needs help, and they need to have no goal of their own that directly conflicts. Before Warneken and Tomasello's work, chimpanzee juveniles had not shown signs of such prosocial behaviors in other experimental contexts. Those authors attribute the difference to food: Most chimpanzee experiments had involved food treats, attempting to get individuals to share food with each other. The chimpanzee's own desire for the food may directly interfere with the goals of other individuals -- a conflict that is hardly likely to lead to sharing, even in human toddlers.
There is little sense in calling the chimpanzee behavioral pattern "rudimentary", as psychologists sometimes do. The human pattern here is rudimentary compared to the extent of helping and sharing that occur later in childhood. The human children in this context seem to have an ability to diagnose the intentions of another individual more than do the chimpanzees. They also seem to have more patience for helping, in some sense. Warneken and Tomasello returned to the topic in a 2009 review [3] that puts forward the situation with respect to sharing, helping, and information transfer. They note that human language depends on cooperation in a way that chimpanzee vocalizations do not. It may not be coincidental that language is learned across the same ages as cooperative behaviors.
Olson and Spelke [4] reported on a slightly more intricate study with 3.5-year-old children. They assessed sharing behavior in which children had to divide a pool of items among a number of recipients. These potential recipients sometimes included both relatives and strangers. In other instances, the potential recipients varied in terms of whether they had interacted with the children by sharing with them. Olson and Spelke intended to find whether children of this age would engage in direct and indirect reciprocity, and whether they would skew their distribution of the resource toward relatives as opposed to strangers.
What they found is that kids of this age typically divy things up fairly:
Children may have distributed resources equally on the four-resource trials for either of two reasons. First, it is possible that children will resort to equal sharing whenever resources are plentiful and will favor family, friends, reciprocators, and generous others only under conditions of scarcity. Such a possibility is consistent with the finding that social conflicts among older children and adults arise primarily when resources are limited ([Jackson, 1993] and [Sherif et al., 1961]). Alternatively, the equality response may be driven by a predisposition to distribute resources in a one-to-one correspondence with recipients whenever such a distribution is possible. That predisposition, in turn, could arise either spontaneously or through the internalization of an explicit rule children are taught by parents and other adults.
As soon as they can manage matching objects with people, they are parceling out things one to a person. That's obviously an integral part of most children's experience -- everything from passing out parts in a game, to passing out food at dinner. So the behavior itself is highly reinforced if not explicitly taught, and it may well be explicitly taught to most children.
The children in Olson and Spelke's trials also tended to share more with people who had previously been generous in the past, either directly or indirectly to the child. By rewarding past generosity, the children were fulfilling their end of a reciprocity arrangement. This seems pretty relevant to the dynamics in ancient human groups; if a 3-year-old can manage the basics of reciprocity, it may not have taken much to push people into a stable hunting and gathering economy, which is based on reciprocity.
Here's what interested me the most. Kids at 3.5 years already get the idea of sharing equally and fairly. So you might think this would be deeply ingrained in older children. But instead what we see is that older children start to reason more and more like adults, which ironically makes them share less evenly. They just get more clever about how to rationalize their choice to be unfair.
For example, a nice study by Gummerum and colleagues [5] compared students age 9 to age 17 for their performance in the "dictator game."
The "dictator game" is an experimental model that has been repeatedly employed in adults to study the themes of cooperation and altruism. An individual is given control over how to divide a single sum between herself and another anonymous person. The individual can choose any division down all for himself and zero for the anonymous player.
Gummerum and colleagues added a twist, making individuals work in groups of three to decide on their offers. The offers then reflected not only the preferences of individuals going into the study but also their moral reasoning with each other after discussing the offers in small groups. This yielded an interesting, almost ethnographic picture of how the children came to make their decisions about appropriate offers.
They found that the offers made by groups were strongly influenced by the level of moral reasoning employed by group members. When a student who favored a low offer was arguing at a higher level of sophistication, the group was more likely to adopt a low offer. And vice-versa -- when the clever student was arguing for a more equitable offer at a higher level, the group was more likely to give more. Girls gave higher offers than boys in the experiment as well.
In a game like this, the sharing and reciprocity aspects of prosocial behavior are transformed into moral questions. No punishment befalls students who choose to make low offers in the dictator game; yet there is the consideration of self-regard. And others have heard the arguments that a student makes, affecting her reputation. Moral reasoning is, in other words, public.
What I find so interesting about comparing children of different ages, is not about cooperation but instead about how the rules are shifted to higher levels of description. Sharing and reciprocity are quite simple, and children can manage them young, although irregularly. Kids can learn about sharing and helping in a rather unsophisticated way, and their performance reflects very simple expectations. Equal division, turn-taking, and punishment of defectors are all integral parts of early childhood.
Obviously, any humans living in foraging societies in the recent past have grappled similarly with the moral aspects of cooperation and altruism. But that moral reasoning comes at an age far past when children are taught about the importance of fairness, sharing and helping. The kind of dynamic that concerns many anthropologists -- how do foraging peoples maintain the rules that underlie reciprocity and altruistic behavior -- is simply at a different level than the dynamic that actually inculcates cooperation. Yet with children who learn systematically to help and cooperate, such behaviors have a much higher chance of existing stably, even in small societies. If there is any cognitive invention that a human society would not want to lose, I think some conception of fairness may be it.
