Creationists frequently like to dismiss fossils by saying they are only fragments, with the implicit idea being that nothing can be learned from them. Yet, this is frequently not the case and the idea derives from a lack of knowledge of skeletal anatomy. So what can one learn from a fragment? Since the White et al paper published in Nature dealt with skeletal fragments let’s look at that and see what we can learn. In particular let’s look at the femur and the intermediate phalanx (which is complete, but interesting none the less).
The above is a picture of the femur with various important points labeled. Note the linea aspera, the gluteal ridge (which serves as the attachment of the gluteus maximus), the greater trochanter (which serves as the attachment for the gluteus medius and gluteus minimus muscles) and the supracondylar ridges.
The above picture is a comparison of the Australopithecus anamensis femur (left) with a comparable portion of the AL288-1 (Lucy) femur. How can we tell it’s a femur since only the shaft was found? This, in part, relies on experience with skeletal pieces. The shaft of the femur could be mistaken for the tibia or the humerus. The tibia has a somwhat triangular shape while the humerus is much smaller. Additionally, part of the greater trochanter (yellow arrow) is present as well as the medial and lateral supracondylar lines (extensions of the linea aspera) which are only present on the femur. To determine what side the femur is locate the nutrient foramen – whis is on the posterior surface and points towards the head of the femur. This allows you to put it in anatomical position at which point it is easy to determine the side. So far we have detemined that the shaft is a femur and which side it is from. What else can we learn?
I mentioned the gluteus maximus muscle above (see PZ isn’t the only person who can talk about stuff that arouses prurient interest!). In humans this is one large muscle that inserts onto the gluteal ridge of the femur and the illiotibial tract (a large band of tissue that insert onto the tibia). In African apes the gluteal maximus is composed of two parts. The gluteus maximus proprius (inserts on the illiotibial tract) and the ischiofemoralis (inserts on the femur from the gluteal line to lateral epicondyle). The human gluteus maximus is equivalent to the African ape gluteus maximus proprius and humans lack the ischiofemoralis. In orangutans the gluteus maximus proprius and the ischiofemoralis are two separate muscles. The same could be said about the gluteus medius and gluteus minimus. In humans these muscles are relatively small (compared to the gluteus maximus) and insert on the greater trochanter. In apes they are larger than the gluteus maximus (incidently, in some African apes the gluteus minimus is partially or complete divided into a muscle called the scansorius – which is not present in humans). So by examining the muscle markings we can learn something about the soft tissue anatomy of the specimen (and hence about locomotion). The next step would be to put it into evolutionary perspective by comparing it to other fossils (such as Ardipithecus ramidus or Australopithecus afarensis – White et al compares the <Au. anamensis femur to Au. afarensis). The femur is broken so we can examine cortical thickness and determine polar moments of area (ala Ruffs’ article Biomechanical Analysis of Archaeological Human Skeletal Samples) which would also provide clues to patterns of mobility.
This brings us to the intermediate phalanx. As I mentioned above, it is actually a complete specimen and not a fragment. But it is interesting nonetheless. White et al state:
The last shows slight dorsal longitudinal curvature, accentuated distally. The proximal half of the palmar surface shows deeply excavated attachment sites for m. flexor digitorum superficialis encroaching on a prominent, raised central ridge. The specimen is morphologically similar to those from Hadar, but is longer relative to its breadth.
In Au. afarensis this morphology is taken to indicate some aboreal component to it’s behavior. Although some have argued that this morphology is a primitive retention which doesn’t say much about behavior. Since the older species, Au. anamensis has this morphology as well the argument gains a little more force (although the White et al article does not address this issue).