The Child from Dikika
It is an axiom in biology that the evolution of species is intimately linked to the development of organisms. Long before the 20th century discoveries of genes as units of heredity, and of the three-dimensional structure of DNA, naturalists, anatomists, and embryologists appreciated that Darwinian "descent with modification" (phylogeny) could be detected in how changes in the pathways of growth and development (ontogeny) affected the form of adult organisms. In a general sense, the divergence of developmental trajectories in part explains how it is that chimpanzees and humans are so similar in the "structural" genes that code for the production of proteins yet differ in so many aspects of anatomy and behavior.
One of the insights from recent research on the skeletal development of hominoids (apes and humans) is that the foundations of adult skull anatomy diagnostic of the different great ape species are established very early in ontogeny. So, for example, the distinctive anatomies of chimpanzee and gorilla skulls are already established by 2 or 3 years of age (signifying that their skull's developmental paths have diverged by this age). This is useful information for the paleoanthropologist to have when attempting to sort out the taxonomy of fossil remains of individuals who died prior to the onset of adulthood.
One example is the skeleton of an early hominin recently discovered at the Dikika site in Ethiopia. The Dikika project is directed by Dr. Zeresenay ("Zeray") Alemseged, a young Ethiopian paleoanthropologist who spent several years as a postdoctoral scholar with the Institute of Human Origins at Arizona State and now works in the Max Planck Institute for Evolutionary Anthropology in Leipzig. In December 2000, Zeray's team recovered the skull and partial skeleton of a very young australopith in a block of sandstone that had eroded out of a hillside of ca. 3.3-myr-old sediments of the Hadar Formation, the same sequence of rocks that has yielded numerous remains (including "Lucy") of Australopithecus afarensis at the neighboring Hadar site. Little of the Dikika specimen was actually visible poking through the block, and Zeray has spent several years painstakingly picking away at the sandstone, literally grain by grain, to reveal the bones of the juvenile (this preparation task is still incomplete, and will take a couple more years to finish). Preliminary description and interpretations of the skeleton were published in the September 21 2006 issue of Nature (Zeray was the lead author, and I was a co-author, along with Fred Spoor, Rene Bobe, Denis Geraads, Denne Reed, and Jonathan Wynn).
The skeleton is astonishingly intact. The cranium and mandible, which are still in articulation, hold the full set of milk teeth. The broken brain case houses a natural cast of its interior, which mirrors details of the brain's surface topography. Extending below the base of the skull, the vertebral column is nearly complete, and on the back of the rib cage both scapulae (shoulder blades) are preserved, a rarity for a fossil hominin of such antiquity. The hyoid bone, which supports the voicebox in the throat, is partly revealed - no other australopith hyoid bone is known, and the Dikika child's appears to be apelike. Also present are parts of the arms, legs, hands, and feet (portions of which were found in separate clumps of sandstone during thorough cleaning of the hillside). CT scans of the skull reveal clear images of the unerupted adult dentition, still developing inside the jaws at the time of death.
Beyond its completeness, the importance of the Dikika child's skeleton is that it contains a mine of information on growth and development. As shown by my colleagues Chris Dean (University College London) and Gary Schwartz (IHO/ASU), among others, early hominins developed their teeth with apelike rapidity, contrasting with the slow rate of human dental development. (Ape and australopith teeth show a relatively low number of daily incremental growth lines laid down by enamel-secreting cells over a given expanse of tooth enamel, meaning that an ape's or australopith's tooth crown formed more quickly than a human's, despite the teeth being much larger.) The Dikika child's state of dental development corresponds to that of a 3-year-old chimpanzee. Comparisons of the Dikika child with the partial remains of juveniles and adults in the Hadar collection reveal architectural features of the face and jaw that, among early hominins, are unique to A. afarensis, reaffirming that, as in modern hominoids, immature australopith skull form closely foreshadowed that of adults. We estimated the brain size of the Dikika child at ca. 275-330 cc, firmly, and unsurprisingly, within the juvenile great ape range. The bones that comprise the hip, knee, and ankle joints (femur, tibia, talus, and calcaneus) are consistent with a striding mode of bipedal walking, seen only in species in the lineage leading to humans. However, the shape of the scapula is a mixture of humanlike and gorillalike features (for example, the slightly upward orientation of the shoulder joint). While the gorillalike aspects of the scapula could indicate some climbing ability in the Dikika child's shoulder (or in its ancestors'), the gorilla's scapula looks quite different from that of the more frequently arboreal chimpanzee (and a bit more like that of humans) at all stages of growth. At present it is unclear what the Dikika shoulder implies about the development of locomotion in A. afarensis, and more research on this intriguing question is in order. Understanding whether and how the locomotor skeleton changed with growth in australopiths could shed new light on the contentious debate about whether these terrestrial bipeds were also partly arboreal.
This and a host of related questions about growth, development, and evolution in our early ancestors remain to be investigated. The greatest importance of the Dikika skeleton is the promise it holds for helping to answer fundamental questions about the relationship between growth and development of the dentition, brain, and locomotor skeleton-the life history profile-of a 3.3-myr-old ancestor of humans
William H. Kimbel PhD
Science Director, IHO