Selam
On December 10, 2000, the Dikika Research Project (DRP), led by Dr. Alemseged, found the first piece of a major paleoanthropological discovery. The team, which was then composed of only Alemseged and three Ethiopian assistants, found the skull of a fossilized child that year and over the course of five successive field seasons between 2000 and 2005, after an intensive process of screening and excavation, the team recovered the partial skeleton of Selam: the earliest and most complete juvenile human ancestor ever found. She is a member of the species Australopithecus afarensis, she was 3 years old when she died and she predated Lucy by 150,000 years.
The discovery’s significance lay not only in Selam’s antiquity, but also in her age at death. Although relatively complete infant skeletons have been recovered for more recent human species, not a single juvenile skeleton has been found for any of the species in the preceding millions of years. Most of these early finds consist of nothing more than a skull, a piece of jaw or some isolated teeth. In contrast to these relatively sparse finds, not only was the DRP team able to recover Selam’s complete skull, but also a sandstone impression of her brain and even the hyoid bone. Due to the fragility of the hyoid, such a discovery in a species of Selam’s antiquity is almost completely unprecedented. The team was able to recover a significant portion of Selam’s bones below the neck as well, including most of the spinal column, the ribs, both collar bones and both shoulder blades. These bones are almost completely absent in the fossil record, except for fragmentary pieces from Lucy. Both knee caps and large portions of the thigh and shin bones from each leg were recovered, as well as an almost complete foot.
The bones show no indications of cuts or abrasions, nor do they show the type of damage associated with scavenging carnivores; this suggests that she was buried rapidly, perhaps by a flood, shortly after her death. It is also possible that it was this flood event which killed her. As the sediment pressed down on her through the years, Selam’s bones became basically glued together in a highly compressed sandstone block. Usually paleoanthropologists struggle to reassemble fragmentary skeletal finds so as to place them back together, but Alemseged faced the exact opposite situation with Selam. He worked painstakingly to extricate her impacted skeleton, using dental tools and removing the soil from her ribs and twisted spinal column virtually grain by grain. The process took 6 years before it announced in 2006 and is still ongoing.
Selam’s skull was CT scanned and it was this method that allowed her sex and age at death to be determined. Further analyses were able to establish the size of Selam’s brain which, at approximately 330 cubic centimeters, would not have been very different from that of a 3-year-old chimpanzee. Whereas chimpanzees at this age have already formed over 90% of their brains, Selam had formed less than 90% of the adult brain size of her species when she died. This might point to a relatively slow brain growth in Australopithecus afarensis, similar to the brain growth pattern of modern humans, rather than that of chimps; this may point to a possible behavioral shift in Selam’s species 3.5 million years ago and the emergence of the delayed pattern of brain development and maturity that we know of as human “childhood”.
The post-cranial skeleton also yielded several important lines of data regarding the locomotion (movement) and height of Australopithecus afarensis. The femur (thigh bone), tibia (shin bone), and foot indicate that Selam (and hence the species she represents) walked fully upright, even at 3 years old, while the shoulder bones are more similar to those of gorillas. Selam’s fingers, as well as those of other members of Australopithecus afarensis, are long and curved. This suggests that while the species was an effective biped while on the ground, it retained the ability to climb, which would have been a beneficial adaptation for avoiding predators, especially at night and especially for the smaller or younger members of the species.
The rare presence of the hyoid bone also yielded some significant data pertaining to Selam’s species. In this bone Selam is more similar to the African great apes than she is to modern humans. Along with a single Neanderthal example, Selam’s hyoid is one of only two extinct hominin hyoids preserved in the fossil record and is the only example from a species of her antiquity. This bone is presumed to have played an important role in the development of human speech and its recovery gives us some clues towards understanding the nature and evolution of the human voicebox. This extraordinary ancient skeleton preserves a mosaic of features shared by both humans and the apes and clearly shows that both the anatomy and behavior of our ancestors was changing, slowly but progressively. In other words, evolution was in the making.
Read more about this topic: Zeresenay Alemseged