Bones, it is said, tell tales, but in the case of the bones of hominids or human ancestors the story has become complex and convoluted, as paleoanthropologists try to decipher relationships between a host of similar species with subtly shifting characteristics in a spotty fossil record.
A novel method has been developed that may prove to be a valuable tool in clarifying the picture. A form of three-dimensional mathematical analysis is used to quantify bone characteristics of humans and eight species and subspecies of great apes and to arrive at a clear family tree that defines ancestral relationships down to the subspecies level. The analysis relies solely on bone shape but its results agree with genetic analyses.
Charles A. Lockwood of the Department of Anthropology at University College London (formerly of Arizona State University), William H. Kimbel of the Institute of Human Origins and Department of Anthropology at ASU, and John M. Lynch of ASU's Barrett Honors College apply a methodology, known as geometric morphometric analysis, to shape data from the temporal bones of humans and eight other species, including chimpanzees, gorillas and orangutans.
"In this work we aimed to link modern quantitative methods, which are undergoing something of a revolution, with the analysis of a part of the skull we were interested in for a variety of reasons," said Lockwood.
Kimbel adds, "The temporal bone has long been thought to have taxonomic and phylogenetic significance and seemed like an ideal target. Because of its unique place in the skull, its shape says a lot about a species.” The complex shape of the temporal bone is influenced by many other anatomical features, including brain size, jaw size, hearing and posture.
Using the method to compare data involving 22 “landmarks” - essentially a surface map - on this complex bone, the group was able to do a statistical analysis on shape differences between species and to arrive at precise ancestral relationships based on shape. Because the ancestral tree of humans and great apes is already well understood for genetic data, the group used these species to test the accuracy of the method. The morphometric results closely matched the known genetically derived trees.
“We were actually able to replicate with the temporal bone data the molecular phylogeny of the hominoids down to the subspecies level,” Kimbel said. “There is a strong correlation between the landmark data and the DNA data.
“This is of interest because it has become commonplace in paleoanthropology to claim that morphological data from great apes and monkeys are not faithful guides to phylogeny, because it has been very difficult to derive the molecularly derived phylogenies,” Kimbel said. “But here we've done that in a very quantifiable way.”
The method, Kimbel points out, is particularly important because it may provide a reliable way to analyze relationships between species when no DNA evidence is available, as is the case with early fossil hominids.
Though numerous species of fossil hominids have been discovered in the last few decades, the discoveries have not fully clarified the pathway of human ancestry. What was once envisioned as a line of descent or even a simple tree, in fact now looks more like a tangled thicket of species over the past six million years - Ardipithicus ramidus, kadabba; Sahelanthropus tchadensis; Orrorin tugenensis; Australopithicus anamensis, afarensis, africanus, aethiopicus, robustus, boisei; Homo rudolfensis, habilis, ergaster, erectus, heidelbergensis, neanderthalensis, and sapiens.
Though the fossil record is still fragmentary and incomplete, the fossils are diverse and plentiful enough to present a very complicated picture of hominid evolution. Modern genetic techniques, which are very useful in reconstructing the phylogeny or kinship of the ancestors of living species, are useless here because these species have no living ancestors besides Homo sapiens and (with the exception of neanderthalensis and early sapiens) are too old to have left any DNA for analysis.
“Like it or not, morphology matters,” said Lynch, a biomorphologist who helped developed the mathematical analysis. “For some problems you can't use genetics, and clearly the fossil hominid record is one of them.”
The problem with morphology is that past methods of comparing bone forms have been largely subjective, and very hard to rigorously quantify. "There has for a long time been a gap between the anatomy you can see on a skull and what you can actually measure,” Lockwood observes. “These methods -- 3D geometric morphometrics -- help to fill that gap because they go to such a great level of detail. And in this case, not only do they capture the anatomy, they show that the anatomy is a good guide to evolutionary relationships."
“One of the great things about this kind of technique is that you know if you do the analysis right and you choose the right landmarks, you are exhaustively capturing all the data that exists for that bone,” said Lynch. “Intuitively, you can tell apart the temporal bone of a human and the temporal bone of a gorilla, but the beauty for us is that we found that our mathematical formulations were mapping those common-sense intuitive differences in a beautiful, quantitative way.”
“The fact that you can quantify the topography of the bones gives you the ability then to approach the differences both within and among groups statistically,” said Kimbel.
“This is the big leap,” he said. “It's finding a quantitative way to express the variation that we see and then having the ability both to analyze that variation with statistical robustness, as well as deriving phylogenetic information from it. Our next challenge will be to apply the method to the fossils themselves, and we're anticipating some fascinating results."