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?
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:
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