Ancient Lucy Spent Some Of Her Time In Trees
October 26, 2012

Researchers Find The Lucy And Selam Did Indeed Climb Trees

April Flowers for - Your Universe Online

Scientists have known that Australopithecus afarensis — the species of the well-known Lucy specimen — was an upright walking species, but they debate whether or not A. afarensis spent much of its time in trees. A comprehensive answer to this question has been unavailable because a complete set of A. afarensis shoulder blades has never been available for study.

Professor David Green of Midwestern University and Zeresenay Alemseged, Curator of Anthropology at the California Academy of Sciences, have thoroughly examined the two complete shoulder blades of the fossil "Selam" for the first time. Discovered in 2000 by Dr. Alemseged, Selam is an exceptionally well-preserved specimen of an A. afarensis child from Dikika, Ethiopia, just a few kilometers from where her famous cousin, Lucy, was excavated. Extensive analysis showed the bones to be quite apelike. This suggests that the species not only walked bipedally, but was adapted to climbing trees.

Selam, Lucy and others of their species were "very human-like from the waist down -- the hip bone, the knee and the foot -- but looked ape-like above the waist -- the torso, long arms, gorilla-like scapula, jutting snout, small brain and a skull with no forehead," Alemseged told Discovery News. "A sketchy depiction of it would be an upright walking ape."

A study published in 2011 asserted that A. afarensis's foot was ill-suited to grasping branches, as they had a permanent arch in their feet. This arch allowed them to walk upright, but prevented the use of the big toe to grasp. By this time in human evolution, according to the study, our ancestors had already abandoned climbing behavior. The new study suggests that the transition from an arboreal lifestyle to a terrestrial one occurred later than previously thought.

"The question as to whether Australopithecus afarensis was strictly bipedal or if they also climbed trees has been intensely debated for more than thirty years," said Dr. Green in a prepared statement. "These remarkable fossils provide strong evidence that these individuals were still climbing at this stage in human evolution."

Dr. Alemseged and his team spent 11 years extracting the shoulder blades from the rest of the skeleton, which had been encased in a sandstone block.

"Because shoulder blades are paper-thin, they rarely fossilize--and when they do, they are almost always fragmentary," said Dr. Alemseged. "So finding both shoulder blades completely intact and attached to a skeleton of a known and pivotal species was like hitting the jackpot. This study moves us a step closer toward answering the question 'When did our ancestors abandon climbing behavior?' It appears that this happened much later than many researchers have previously suggested."

Selam was a three-year-old female A. afarensis who lived approximately 3.3 million years ago, reports Nick Collins for The Telegraph. She is the most complete skeleton of her kind found to date. Green and Alemseged digitized the shoulder blades after freeing them from the sandstone with an instrument called a Microscribe. They then took measurements to characterize their shape and function by comparing them to the rare shoulder fossils of other early human relatives. Selam's shoulders were compared to Homo ergaster ("Turkana Boy"), Homo floresiensis ("The Hobbit"), A. africanus, and two adult specimens of A. afarensis, along with an extensive modern sample of adult and juvenile chimpanzee, gorilla, orangutan and human specimens.

A. afarensis lived across East Africa in a wide range of habitats, including forests. The study revealed that apelike features in our ancestors were not evolutionary baggage, but rather they reflected adaptations to habitat.

"Within this range of environments, Selam and her kin walked upright to move from one place to the other, but also had an arboreal lifestyle that enabled them to nest in trees, evade predators, and provision themselves," Green told Discovery News. "This was a significant adaptation that enabled this short-statured hominin, with no sophisticated tools, to survive in a dangerous landscape filled with large felines and other carnivores."

A. afarensis shoulder blade shape and function were revealed to be apelike, indicating at least a partial arboreal lifestyle, reports Kate Wong of Scientific American. The team concluded that, like modern apes, the shoulder anatomy of both juvenile and adult A. afarensis were quite similar.

"Human scapulae change shape throughout ontogeny in a significantly different manner than closely related apes," Dr. Green wrote in the prepared statement. "When we compared Selam's scapula with adult members of Australopithecus afarensis, it was clear that the pattern of growth was more consistent with that of apes than humans."

Dr Susan Larson of Stony Brook University in New York told The Telegraph that the A. afarensis shoulder blades "do not closely resemble those of any specific living ape, reflecting the fact that they were habitual bipeds on the ground", but suggest that climbing in trees "remained part of their overall survival strategy".

Most scientists agree that the structure of the A. afarensis hip bone, lower limb and foot are unquestionably humanlike and adapted for walking upright. According to Jennifer Viegas of Discovery News, walking upright led to a domino effect of changes that lead to modern humans.

"This new find confirms the pivotal place that Lucy and Selam's species occupies in human evolution," said Dr. Alemseged in the statement. "While bipedal like humans, A. afarensis was still a capable climber. Though not fully human, A. afarensis was clearly on its way."

"If we look at later hominins, the proliferation of stone tools and the increased exploitation of animal proteins, it is clear that bipedalism enabled hominins to use their forelimbs to fashion and use tools and exploit this new niche (the ground environment)," Green explained to Viegas. "Environmental change may have prompted this transition, and bipedal exploration of the terrestrial environment likely paved the way for stone tool use, manufacture and hunting."

"It is also plausible that this enabled subsequent expansions in brain volume, which is another hallmark of our lineage," he added. "Other primates may have been able to continue to exploit arboreal or other food sources, which did not require such a dramatic behavioral transition. The ability of early hominins to exploit new resources is probably why bipedalism may be viewed as a successful evolutionary experiment."

The findings of this study were recently published in the journal Science.