Oldest infant skeleton helps solidify human timeline

Discovery of a nearly intact, 3.3 million-year-old juvenile skeleton is filling an important gap in understanding the evolution of a species thought to be among the earliest direct ancestors to humans, says William Kimbel, a paleoanthropologist with ASU’s Institute of Human Origins.

Kimbel is part of the team that studied the skeleton of an approximately 3-year-old female Australopithecus afarensis, the same species as the well-known “Lucy,” from Dikika, Ethiopia.

“It’s extraordinarily rare to have such a complete skeleton,” says Kimbel, a professor and science director at the Institute of Human Origins. “It’s unprecedented to have such a complete skeleton of a young child.”

The researchers describe their discovery and initial analysis of it in “A juvenile early hominin skeleton from Dikika, Ethiopia,” in the current issue of Nature (Sept. 21). The skeleton was discovered by lead author Zeresenay Alemseged, director of the Dikika Research Project and a former postdoctoral researcher at ASU’s Institute of Human Origins. Alemseged is at the Max Planck Institute for Evolutionary Anthropology, located in Leipzig, Germany.

Other authors of the paper are Fred Spoor, University College London; Rene Bobe, State University of New York at Buffalo; Denis Geraads, National Center for Scientific Research, Paris; Denne Reed, University of Texas at Austin; and Jonathan G. Wynn, University of South Florida.

Alemseged has been carefully preparing the skeleton for the last five years by chipping away sandstone from the fragile bone fragments. He still has several years of work to complete it. The skeletal remains include the skull and jaws with teeth, and parts of the shoulders, spinal column, ribs, right arm, fingers, legs and left foot.

Analysis of the skeleton by Alemseged and his team has shown that its lower body is adapted for bipedal locomotion, like that of adult Australopithecus, while the upper body – especially the scapula, or shoulder blade – exhibits some gorilla-like features, Kimbel says. However, there continues to be debate about the interpretation of these features and what was the preferred mode of locomotion for this species.

Some researchers have argued that the ape-like features of the upper limb are just retained from a common ancestor but are not functional, while others argue the features are functional and indicate that the species spent some time climbing trees.

“I don’t think a 3-year-old adapted to bipedality is going to spend much time climbing in trees, but the mix of features in this skeleton is going to stir up the debate about locomotion in early Australopithecus,” Kimbel says.

Alemseged uncovered a hyoid bone in the skeletal remains, which is the first time that bone has been discovered in the early part of the hominin fossil record. The hyoid bone is located in the larynx, or “voice box,” and supports muscles of the throat and tongue involved in speech. The one discovered in the infant appeared to be primitive and is more similar to hyoids found in apes than humans, according to Spoor, who has studied the evolution of this region in humans and other primates.

The findings are expected to provide insights into the growth and development cycle of Australopithecus afarensis since the infant remains will be compared to Lucy, a remarkably complete adult female skeleton which was among the first Australopithecus afarensis to be discovered. Lucy dates back 3.18 million years and was discovered in 1974 in the Hadar region of Ethiopia by Donald Johanson, who directs ASU’s Institute of Human Origins.

Understanding growth and development and how it has changed in human ancestry is central to the study of human evolution, Kimbel says, adding that information about growth and development can help answer questions about the mechanisms that drove changes in body form that we see in the fossil record.

“Most differences between humans and their ancestors can be associated with developmental changes,” Kimbel says.

The genetic makeup of humans is so similar to that of our close relatives, such as the chimpanzee, that most anatomical differences are explained by changes in the pattern of growth from infant to adult.

The infant Dikika skeleton is expected to provide an important reference point that will help researchers explain how changes in growth and development contributed to the evolution of humans.

Alemseged announced the publication of the research on the Dikika skeleton at a Sept. 20 press conference in Addis Ababa, Ethiopia.