Variation: a blessing and a curse

Trying to start on finishing my dissertation, I’m thinking about the issue dental development and how it relates to skeletal growth. Specifically I’m trying to decide whether I want to analyze my human and Australopithecus robustus samples based on estimates of “dental age,” or if I want to be a bit more cavalier and divide the sample into rougher age categories.

To avoid copyright issues, here’s a crappy picture I drew a few years ago, of the youngest A. robustus jaws. The youngest, “SK 438” is erupting its last baby tooth (bottom right), while the others have their full set of baby teeth, and none of them has its first adult tooth yet. I don’t think I can estimate ages accurately enough to capture the true chronological difference between SK 438 and the rest. Would I be better off just dividing the group into “younger” (SK 438) and “older” (the rest) infants, or even lumping them all together as simply “infants”?

On the one hand, I could assign individuals a chronological age based on a modern referent of known age, at similar stages of dental development. This could allow me to get more fine-scale glimpses into patterns of growth in my samples, but that’s assuming I’ve accurately estimated their ages. Individuals vary in the ages and sizes at which their teeth erupt; a person’s first molar, for example, may erupt at anywhere from 4-8 years of age. How can I estimate an individual’s age in light of such variation? And what if I’m as poor a judge of ages as Dennis Duffy?! Conceivably I could program my analysis to account for error estimation (which in itself could be educational and interesting, but is it worth the trouble?), but this would also add a further source of uncertainty. And it’s like Dwight Schrute said (Michael Scott said), “K-I-S-S: keep it simple, stupid. Great advice, hurts my feelings every time.”

On the other hand, I could divide my sample into coarse age categories – say, putting specimens who’ve attained a given level of dental development in the same group, such as ‘infant, child, juvenile, adolescent, and young adult.’ This method loses the temporal resolution of the first method, but also avoids the possible errors of assigning strict ages I’m pretty sure I would not infer accurately. But, tooth development does not show a clean 1-to-1 relationship with other systems in the body, such as hormonal axes or the bony skeleton. It’s uncertain how accurately kids can be put in any of the above categories (based on general life history variables; Bogin 1999) based on dental development.

Choices, choices.

Variation is a problem for biologists. The theory of evolution was conceived as a way to explain the conundrum of why there is such remarkable variation in the forms of life that Earth is lucky to have harbored. The problem of within-species variation in the relative timing of skeletal and dental development isn’t just a bug-bear for paleoanthropologists. It’s important to medical doctors and pathologists investigating genetically-based developmental disorders, and to epidemiologists looking at aspects of population health, such as the prevalence of growth stunting. It’s also important for forensics specialists who need to use biological clues about the age and identity of crime victims and defendants. I mean, how else would we know whether Jon Voight bit both Kramer and this pencil?

The silver lining, I suppose, on this storm-cloud of biological of variation is that without variation there cannot be evolution. And stasis is boring. If nothing changed since the Cambrian, none of us would be here today. We’d probably be some gross stupid monstrous thing, like this Hallucigenia to the right. It’s the quirks and weird variants that arise randomly, that make evolution possible. If individuals all developed exactly the same, then all organisms through all time would be the exact same, and probably all would have gone extinct as they succumbed to some sinister fate, no new variants would have arisen that may have been able to survive the devastation.
So variation is a blessing and a curse. Individual and population variation make it difficult to state norms such as what is “average” or “healthy,” and nothing to be concerned about. Variation is also the magic ingredient of adaptation, without which Life could not survive the randomness inherent in any environment.

Things I cited
Bogin, B. (1999). Evolutionary perspective on human growth Annual Review of Anthropology, 28 (1), 109-153 DOI: 10.1146/annurev.anthro.28.1.109

Also 30 Rock, The Office and Seinfeld. Well done, NBC.

100,000 year old child skeleton on National Geographic

National Geographic aired a special tonight about a recently-excavated child’s skeleton (they focused on the skull) from Grotte des Contrebandiers in Morocco, dated to around 108,000 years ago. So far as I know this material has not been fully published (aside from a brief blurb in Science). Hmm, a highly publicized TV special on a big hominid fossil discovery around/before the scientific publication, sounds familiar…

The program presented work of archaeologists, paleontologists, reconstruction artists, taphonomists, and lots of other people, hoping to figure out who the kid was and such. All in all it was pretty cool, I’d recommend checking it out if you didn’t see it. Or again if you did see it.

While I think it was a great program and the researchers involved are doing a terrific job, I had two main concerns: first, I wish they’d treated the topic of growth-n-development a little more. They noted that the child (5-6 years old possibly) looked really “modern” because of its flat face. But looking at it, it didn’t really have that flat of a face, especially for a child. They talked about how human-like (rather than Neandertal-like) the kid was, but they only compared it with adults – children tend to have relatively smaller faces and larger brain-cases than adults (right), so it’s no wonder it looked more like an adult human than the adult Neandertal from Amud (Israel) that they compared it with. It would’ve been great to see more comparisons with other late Pleistocene hominid kids, such as from Skhul/Qafzeh or La Quina. A future program, perhaps.
Second, they kept asking whether the kid was “a Homo sapien.” I know it’s counterintuitive for English-speakers, but “H. sapiens” is the singular and plural of humans’ scientific name. Silly, right, cuz it doesn’t even get paid twice as much. But you’ll have take that up with C. Linnaeus. I am a Homo sapiens. You are a Homo sapiens. Fifty people are a gaggle of Homo sapiens. I fail my students if they say “sapien” when referring to humans. Because it’s not very sapient of them.
Anyway it was a cool show. Check it out, dammit!
Figure credit: Fig. 2 from Bogin. 2003. The human pattern of growth and development in paleontological perspective. In Patterns of Growth and Development in the Genus Homo, eds. Thompson JL, Krovitz GE and Nelson AJ. New York: Cambridge University Press: 15-44.

Community on NBC: Anthropology is cool dammit

I try not to watch too much TV these days, but one show I do enjoy is Community. It’s about a diverse group of students at a small community college in some town called Greendale. Last season, we followed the exploits of their Spanish 101 & 102 study group. This year, the group is brought together because they’re taking ANTHROPOLOGY, and Betty White plays their professor. Awesome (I liked anthropology before it was cool again). If you like to laugh and be happy, do yourself a favor and follow the link above to watch Community on Hulu.

"Spring" Break TV

Spring break, even if it takes place in February, is not a time for being productive. I took this to heart last week as I caught up on zoning out in front of the TV. But it wasn’t all Sabrina the Teenage Witch reruns for this girl: I also caught bits and pieces of two fairly interesting Nova episodes on PBS, “The Four-Winged Dinosaur” and “Ape Genius”.

“The Four-Winged Dinosaur”
I’ll admit, I’m not as fascinated by dinosaurs as some in this lab, so this episode did not hold my interest for the entire hour. However, I came in toward the end and was fascinated to discover that amidst scenes from “Jurassic Park”, they were interviewing a real scientist who actually is trying to breed dinosaurs! The entire episode (or so I gathered) looked at how birds evolved from dinosaurs. They took a bird embryo, and injected it with a virus that would attack the DNA and turn on previously turned off genes, including one that apparently coded for the bird to grow teeth in its beak. But the bird embryo with a beak-full of teeth was not the coolest part. Next they tried to create an emu-asaurus! They didn’t actually manage this, but the Nova voice-over talked us through the technique of how it could be possible to turn an emu egg into an emu-like dinosaur. To quote a friend, “haven’t they seen Jurassic Park? Don’t they know how that turns out??” All I know, is that if they ever manage to recreate dinosaurs, they should avoid breeding raptors – those guys in the movie are SCARY!

“Ape Genius”
This episode aired a while ago, but unfortunately I missed it the first time around. Thank god for reruns! This episode, which I actually watched most of, looked at what apes were and were not capable of doing in terms of communicating, learning, and working together (all those traits people try to describe as culture). I don’t have any profound conclusions about this episode, but I found a number of the experiments they did interesting and will describe a few of them here.

  • Will two chimps work together to get and then share food? The animals had to both pull a string at the same time to get a long plate of food close enough to their cages to eat. Results: if the food was split already, the chimps would work together and each take their share. If the food was not split up, the chimps would usually start fighting and not get the food close enough to reach. If the apes were bonobos, they worked together and shared their food without problems.
  • Do chimps understand the value of an M&M? A trainer puts 7 M&Ms in one bowl and 2 in another, and whichever bowl the chimp points to, a second chimp gets those M&Ms. The first chimp gets the bowl he doesn’t point to. Results: Even after multiple trials, the chimp still points first to the bowl with the most M&Ms, and thus does not get them.
  • Do human children understand the value of a gummy bear? Similar to the above test, in this one an adult explains to a 4-year-old that the one gummy bear in front of them is theirs to eat, but if they wait until the adult leaves and comes back, then they get the entire package of gummy bears. In most cases, the child ate the one gummy bear and did not get the package (these tapes were adorable to watch, by the way).
  • Do chimps understand numbers? A chimp is taught, using dots, the numbers 1-9. Then the previous chimp experiment (the one with the M&Ms) is repeated using numbers instead of M&Ms (introducing a symbolic element). In this case, the chimp learns to point to the lower number, and thus receives more M&Ms. However, the voice-over points out that chimps in the wild do not develop symbols on their own, they are just able to understand some of them when humans teach them.
  • Will bonobos protect or share outside of their own family group? A dead bonobo that is a stranger to a group of live bonobos is placed in the live bonobos’ habitat. Humans with long poles use the sticks to “attack” the dead bonobo. Result: the live bonobos shriek and try to protect the dead bonobo from the offending poles, even though they did not know the bonobo in life.

These were just a few of the interesting experiments compiled in this episode. If you ever catch the rerun of either of these, they might be worth checking out! Next up, a post about all the crappy TV I also watched over break… or maybe I’ll just keep that to myself.