Results of the toe-tally easy lab activity

Alternate title: Dorsal canting in primate PPP4s

Earlier this year I suggested a classroom activity in which students can scrutinize the evidence used to argue that the >5 million year old (mya) Ardipithecus kadabba was bipedal. To recap: Ar. kadabba is represented by some teeth, a broken lower jaw, and some fragmentary postcrania. The main piece of evidence that it is a human ancestor and not just any old ape is from a single toe bone, and the orientation of its proximal joint. In Ar. kadabba and animals that hyperdorxiflex their toes (i.e., humans and other bipeds when walking), this joint faces upward, whereas it points backward or even downward in apes. This “dorsal canting” of the proximal toe joint has also been used as evidence that the 4.4 mya Ardipithecus ramidus and 3.5 mya owner of the mystery foot from Burtele are bipedal hominins. A question remains, though – does this anatomy really distinguish locomotor groups such as bipeds from quadrupeds?

Use ImageJ to measure the canting angle between the proximal joint and plantar surface. Proximal to the right, distal to the left.

STUDENT SCIENTISTS TO THE RESCUE! Use ImageJ to measure the canting angle between the proximal joint and plantar surface, as I’ve done on this Japanese macaque monkey (they are not bipedal). Proximal to the right, distal to the left Note I changed the measured angle from the March post.

I sicked my students in Ant 364 (Human Evolutionary Developmental Biology) here at NU on this task. I had students look at only 11 modern primates from the awesome KUPRI database. Most groups are only represented by 1 (Homo sapiens, Hylobates lar and Macaca fuscata) or two (Pongo species and Gorilla gorilla) specimens, all adults. For chimpanzees (Pan troglodytes) there is one infant and four adults. The database has more individuals, and it would be better to include more specimens to get better ideas of species’ ranges of variation, but this is a good training sample for a class assignment. The fossil group includes one Ardipithecus ramidus, one Ar. kadabba, one Australopithecus afarensis, and the PPP4 of the mystery foot from Burtele. The human and all fossils except Ar. kadabba are based off of lateral photographs and not CT scans like for the living primates, meaning there may be some error in their measurements, but we’ll assume for the assignment this is not a problem. Here are their results:

Dorsal canting angle of the fourth proximal pedal phalanx in primates.

Dorsal canting angle of the fourth proximal pedal phalanx in primates. The lower the angle, the more dorsally canted the proximal joint surface. The “Fossil” group includes specimens attributed to ArdipithecusAustralopithecus and something unknown.

Great apes have fairly high angles, meaning generally not dorsally canted proximal joint surfaces. The two gorillas fall right in the adult chimpanzee (adult) range of variation, while chimp infant and orangutans have much higher angles (≥90º means they’re actually angled downward or plantarly). The gibbon (Hylobates) is slightly lower than the chimpanzee range. The macaque has an even more dorsally canted joint, and the human even more so. The fossils, except the measurement for Ar. ramidus (see note above), have lower angles than living apes, but higher than the human and the monkey. If dorsal canting really is really a bony adaptation to forces experienced during life, then the fossil angles suggest these animals’ toes were dorsiflexed more so than living great apes (but not as much as the single monkey and human).

This lab helps students become familiar with CT data, the fossil record, taking measurements (students also measure maximum length of the toe bones and look at the relationship between length and canting), analyzing data, and hypothesis testing. You can also have fun exploring inter-observer error by comparing students’ measurements.

Here’s the full lab handout if you want to use or modify it for your own class: Lab 5-Toe instructions and report

Molar? I hardly even know her!

I was recently at the State Zoology Museum of Munich, studying their amazing plethora of orangutan bones. Jaw bones are especially useful skeletal remains when you study growth, because different teeth come in at different points in one’s life. Remember when your 1st permanent molar teeth came in? You were probably 5 or 6 years old at the time. It was a big deal, your first permanent teeth! What about your 4th permanent molars, after your wisdom teeth, remember those?

An adult male orangutan mandible, with bilateral supernumerary molars. Or more simply, “an extra molar on both sides of the jaw.”
I hope not. As a good eutherian, you should never have more than 3 molars in each half of each jaw. And as a modern human, there’s a good chance you’ve only got 2 in each half (but that’s a whole other story). So when I was looking at orangutan skulls to get an idea of individuals’ ages, I was shocked to find specimen after specimen with at least one extra molar. So far as I could tell, 27 out of 181 (14.9%) adult orangutans in this collection had extra molars.
Supernumerary (fancy word for “extra”) molars manifest a number of ways in this collection. Sometimes there’s only one extra tooth. Sometimes there are extra teeth in both upper and lower jaws but only on one side. Sometimes there’s a full set (4). Et cetera. One poor bastard even had a 5th molar lurking behind one of his four 4ths! Deplorable.

An adult male with a fairly normal 4th (blue arrow) and even a weird, unerupted 5th (red arrow) molar. Gross!
This is rather strange, such a regular occurrence of supernumerary teeth – what gives? A starting clue is the fact that all specimens with extra molars are of the species Pongo pygmaeus from Borneo (173 of 181 specimens). The remaining eight specimens, with a normal dental formula, are Pongo abelii from the island of Sumatra. But how much of this difference in frequency is due to the fact we’re looking at 181 Bornean, vs. only eight Sumatran orangutans?
Resampling to the rescue! Is it weird that 27/181 (15%) Bornean orangutans have extra teeth, while 0/8 Sumatran orangs do not? Another way to ask the question is, what are the chances of sampling 8 Bornean orangs, none of which have extra molars? This is very easy to program and test in R:
Set up a vector (basically, a string of numbers) to represent your Bornean orangs, each entry representing an individual, assigning “0” for no extra teeth and “1” for at least one (this admittedly oversimplifies the nature of extra teeth). Then simply randomly sample – lots and lots of times –  eight individuals from this Bornean vector, to see how often you get a set in which 0/8 have extra molars.
“b” is our vector of Bornean orangutans, consisting of 0s and 1s for whether there are extra teeth. “n” tells us how many individuals had extra teeth in that subsample. The “(i in 1:10000)” means for each of 10,000 resamplings.
Following this resampling procedure, there’s about a 25.5% chance that none of them will have extra molars. That means the remaining 74.5% of the time, a random subsample of the Bornean orangutans will contain at least one individual with at least one extra tooth.
A number of interesting questions arise from this – if we were to examine more Sumatran orangutans, would we eventually find one with an extra molar? After all, the 25.5% chance of sampling 0/8 suggests maybe we just missed some Sumatrans with extra molars. Regardless, within the Bornean orangs, why is the frequency so high? Does one pattern of extra teeth (say, just in the lower jaw, or on both sides, etc.) predominate? Are there differences between the sexes? These are questions for another day….