In the midafternoon today, the communications line rang in the Control Center tent. It was excavation team member Becca Peixotto calling from the fossil chamber to get us to look at the monitor. Thirty meters underground, she held a fossil tag up to the camera. When the lighting was right, expedition leader Lee Berger saw what it read: Fossil Number 100.
Lest you picture a skeleton army rapidly amassing in the tents of our camp, note that Fossil Number 100, like a good number of the fossils recovered, is a fragmentary piece of a larger bone.
It’s not all skulls and femurs in the land of paleoanthropology. Thankfully, there are also taluses.
Over in the science tent, Peter Schmid was gleefully unwrapping his presents. Every 30 minutes to an hour the highly regarded experts in this part of the camp celebrate a little mini Christmas, emptying the sack they were handed by the runner and removing the protective pink bubble-wrap from each piece in turn with care.
“Oh ho! This is a good one!” he called out. Caver/scientist Lindsay Eaves was looking on through the window chuckling at his enthusiasm. “Hey,” he said defending himself. “For hours, all we had was these little fragments!”
The piece that he then revealed matched perfectly with one sent up earlier. Together they made the characteristic shape of a talus, or ankle bone. With him saying as much, Lindsay, the other cavers looking on, the camera crew, and I all perked up and paid attention.
While the ankle bone alone isn’t likely to indicate what species you’ve found, it does begin to tell you something about how the creature moved. Different sizes and shapes of different parts can indicate the overall architecture of the foot, and in the context of hominids, that can help tell you whether the creature walked upright.
In modern humans, explained Bernard Zipfel, who is a master of hominid feet, the foot’s default position is flat against the ground, and there is an arch both front to back and side to side. The big toe shoots out straight from the line of the ankle and absorbs the stresses from both arches.
In ancient hominids like Australopithecus, when upright the foot appears to default in a pronated position with the outside of the foot pointing down, as seen by the deep depression on the outside of the famous footprints at Laetoli. Certain features of the talus can give an indication of the direction of pressure and the shape and position of the big toe, and hence start to give an indication of how the creature walked.
Zipfel was clear that you should never read too much into a single bone though. When studying Australopithecus sediba, discovered nearby, he and his collaborators realized that the ankle bone looked similar to ours, but the heel bone, or calcaneus, looked more like that of other australopiths. If you found either one independently, you’d not likely suspect they came from the same creature.
So the ankle will hopefully tell us something about the species of hominid found in this cave, but the heel or other foot bones could end up looking completely different. The catch is that there are few heel bones found generally.
The talus, Bernard explained more fully, is very dense because it supports the weight of the body pushing down on it, and the compression against the ground as we walk, run, jump, and so on. The heel on the other hand (or should I say foot?) bears weight but also serves as an attachment point for tendons and other soft tissue, requiring more blood supply, and as such is more lightly constructed with only a dense core.
That causes ankle bones to be some of the more common hominid foot bones to be preserved, and heels to be some of the rarest.
With only four and a half days of fossil recovery under the team’s belt, it’s hard to say whether we’ll get a heel that will help reveal how these creatures walked.
But piecing together two pieces of the ankle bone is certainly a step in the right direction.