Elizabeth Culotta reports from the Vertebrate Paleontology meetings about an analysis of the hominid femur from Galili, Ethiopia.
In the view of Viola and his Vienna colleague Horst Seidler, the bone is more primitive than Lucy's femur and resembles that of a much earlier hominin, Orrorin tugenensis, thought to be about 6 million years old. They suspect that it came from Au. anamensis, a species that lived about 4 million years ago and is widely considered to be Lucy's ancestor.
Details of the femur's anatomy, such as a long neck of bone leading to a large femoral head (the "ball" of the hip's ball and socket joint), suggest that its owner--whatever its name--was bipedal, Viola said. But other clues imply that it may also have climbed trees, he added. For example, a thick layer of dense cortical bone is evenly distributed around the femoral neck. In upright walkers like us, that cortical bone is unevenly distributed. "Both Orrorin and this femur seem to show several traits which indicate bipedalism but also retain signs of arboreal behavior," Viola says. That suggests that our ancestors' move out of the trees was a long process.
Culotta has quotes from other experts either way. I have no strong opinion – the even distribution of cortical bone is likely a sign of locomotor flexibility, but that might mean arboreality, a less stereotyped pattern of bipedality, or some other pattern of activity.
I tend to be more convinced by ecological arguments in favor of the continued use of trees/ For example, pre-afarensis hominids are always found in wooded environs. The dental pattern of later hominids emerged very slowly, plausibly reflecting a shift from fruits toward the products of grasses and other savanna foods, but this pattern may be post-Ardipithecus. And early hominids never attain the body size of later Homo, despite the advantages of larger size in a terrestrial habitat. Plausibly, this is because climbing remained important, and large body size made climbing harder.
That’s a very circumstantial argument. From that perspective, the femur doesn’t conflict with the idea that the locomotor requirements included climbing. But it’s not very strong evidence in favor of the hypothesis, either.
The article is short, and doesn’t mention the history of the Galili field site, including the conflicts over excavation rights and territorial boundaries. Those details are reviewed in Ann Gibbons’ book, a great introduction to many of the field sites and current personalities (I reviewed it here).
There have been few publications on the Galili site. Roberto Macchiarelli and colleagues (2004) described the first hominid tooth found at the site, and this year, Ottmar Kullmer together with Seidler and others described the paleoecology and chronology of the site. Their paleoecological reconstruction places Galili as open woodland to bushland, and is perhaps the most open of the pre-4.0-myr sites. They suggested that the field site represents a transitional time period from A. anamensis to A. afarensis, which would be chronologically just after the Asa Issie sample at 4.1-4.2 myr, but possibly overlapping with Allia Bay.
Culotta E. 2008. Two legs good. Science 322:670-671. doi:10.1126/science.322.5902.670b
Kullmer O, Sandrock O, Viola TB, Hujer W, Said H, Seidler H. 2008. Suids, elephantoids, paleochronology, and paleoecology of the Pliocene hominid site Galili, Somali region, Ethiopia. Palaios 23:452-464. doi:10.2110/palo.2007.p07-028r
Machiarelli R, Bondioli L, Falk D, Faupl P, Illerhaus B, Kullmer O, Richter W, Said H, Sandrock O, Schäfer K, Urbanek C, Viola TB, Weber GW, Seidler H. 2004. Eaerly Pliocene hominid tooth from Galili, Somali region, Ethiopica. Collegium Anthropologium 28:65-76.