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

Evolution of human fingers and toes: The two go foot in hand

A really cool study was just published in the journal Evolution, and what with getting my apartment ready for a New Year’s party on the 31st, and my being completely incapacitated yesterday, I didn’t get to read through it until today. Campbell Rolian and colleagues (in press) address the question: In human evolution, were hand and foot digital proportions each the targets of direct selection, or could hand/foot proportions have evolved as a byproduct of selection on only the hand or only the foot?

This is an interesting question. In your standard Anthropology 101 class, you learn about how humans (and hominins) are unique relative to apes. Two unique things about us are: a robust, adducted big toe for bipedalsim, and a hand adapted for tasks requiring a fairly high degree of dexterity, such as tool use. But something to keep in mind–indeed the authors of this study did–is that the hand and foot are serially homologous, each is a variant on a common theme. Because the developmental architecture behind the hand and foot are largely similar, an intuitive question is whether selection on the hand or foot only would effect the evolution of the element that wasn’t under selection. Could developmental integration of the hominin hand and foot have led to evolutionary integration, do/did the hand and foot co-evolve?

Turns out this may well be the case. Authors looked at lengths and widths of hand and foot phalanges (finger bones) in a sample of humans and chimpanzees. Generally, in both Pan and Homo, homologous traits in the hand and foot are more highly correlated than expected by chance, even compared to correlations between traits within the hand and foot. Cool, and none too unexpected.

But then the authors did some crazy simulations, to see what kinds of selection regimes on the hand and foot may have led from a chimp-like morphology to the morphology we humans enjoy today. I’ll need to reread this section a couple times, but it looks like selection on the big toe is one of the most important aspects of hominin hand/foot evolution. And it would not be impossible for evolutionary changes in the human hand to be largely by-products of selection on the foot, due to the nature of covariation (integration) of the hand and foot. Whoa!

The implication, which the authors seem to like, is this: given a chimp-like ancestral morphology for the hand and foot, it seems that the two major hominin/human traits given above (bipedalism and tool-use/manual dexterity) are largely due to selection simply on the foot. That is, because of the developmental integration of the hand and foot, selection for a bipedally capable foot indirectly induced the evolution of a hand conducive to manipulation. Ha, the hand was just along for the ride! Get it, because the feet move the body, and so the hand… but also evolutionarily… Dammit.

Anyway, that’s nuts! Of course, another very interesting thing about the first digits of the human hand and foot, aside from the fact that the first digit on both is relatively large and robust, is that the mobility of these digits is just about opposite what it is in the apes. Whereas the big toe is very mobile/opposable in apes (and the 4.4 million year hold putative hominin, Ardipithecus ramidus), it is completely adducted in humans (and fossil hominins that aren’t Ar. ramidus). Less extreme, the human thumb joint is allegedly more mobile than apes’ thumbs. So this is the next step, I guess: what is the developmental basis for the wild evolution of the human hallux and pollex joints?

Rolian C, Lieberman DE, and Hallgrimsson B. Coevolution of human hands and feet. Evolution: in press.