This article was contributed by Amanda Nottke

“Oldest ‘Human’ Skeleton Found – Disproves ‘Missing Link’”, says a recent headline at www.nationalgeographic.com, one of many introducing us to Ardi, a fossil of the 4.4 million year old early hominid Ardipithecus ramidus. She is being described as the next Lucy in terms of her importance to the paleoanthropological field, and in fact was discovered only miles away from the Australopithecus afarensis fossil known as Lucy. So Lucy and Ardi lived (or at least died) nearly side-by-side geographically, but 3.2 million year old Lucy now finds herself dramatically upstaged by her much older neighbor.

Paleoanthropology in the News

Due to the coordination between the media and the scientists involved in Ardi’s discovery and description, we are seeing the development of two stories in parallel: the story of Ardi as reported in the news, and the public release of the scientific data behind the sound bites. What are the data, and how well do they support the headlines? To use the National Geographic headline as an example: Is Ardi the oldest human skeleton ever discovered, and does her existence disprove the idea of the “Missing Link”?

Naturally, the answer to the first question depends on our definition of “human”, but it seems clear that Ardi’s fossil is the most ancient and complete specimen on the human side of the human-chimp divergence, which genetic and fossil evidence suggests occurred some time between 8 and 6 million years ago. This doesn’t necessarily mean that members of Ardi’s species were direct human ancestors (although they may have been), just that based on comparative anatomy, Ar. ramidus was definitely more closely related to the human ancestor species than the chimp ancestor species.

But what “Missing Link” is the headline describing? The authors were most likely referencing long-standing predictions made about the characteristics of the earliest hominids (the “Missing Links” between the human ancestors and other ape lineages). Ardi’s discovery doesn’t disprove their existence, but rather suggests that their qualities were not what we originally thought – they were not necessarily bipedal chimp-like savannah-dwellers like Lucy.

Simply put, the commonly-accepted model for early hominid evolution was that some chimps remained in the forests and stayed chimps, whereas others (the “Missing Links”) moved out of the trees into the savannah, developing bipedalism in response to their new environment and evolving into human ancestors. So how does the discovery of Ardi influence this theory?

Ardi: Conclusions and Caveats

Ardi’s skeleton was discovered in 1992 and includes the skull and teeth, lower arms and hands, the feet and left leg, and parts of the pelvis. Other fragmentary Ar. ramidus remains of at least 36 individuals were discovered over the course of the excavation, allowing for some inferences about general traits of the species. This includes the discovery of smaller-sized canine teeth in the Ar. ramidus males relative to male chimps, which has been (somewhat controversially) extrapolated to suggest that Ar. ramidus may have had more human-like pair bonding behaviors, as opposed to the harem-like chimp societies. This interpretation is based on the believed function of canine display in chimp social behaviors including male competition and male-female intimidation.

Ardi and the other Ar. ramidus remains were found within a large intact geological layer that includes both forest and savannah environments, allowing a detailed survey of the other animal and plant life present in Ardi’s environment. One key finding was that Ar. ramidus remains were only found in the forests, never in the open grassland. This argues against leaving the trees being the first stage in human evolution.

Why did it take over 15 years for this discovery to be published? The major reason is the fragmentary nature of Ardi’s remains, especially her skull and pelvis. It took years just to remove the fragile fossils from the surrounding material, and as the skull and pelvis are arguably the most informative bones in terms of early hominid evolution, the scientists had to be very careful that their reconstruction was as scientifically sound as possible. They used CT scanning and the latest computer technology to create 3-D models of the original bones, but it is important to remember that both the skull and pelvis are reconstructions (unlike Lucy’s remains, which were largely intact).

Ardi’s reconstructed pelvis is intermediate in morphology between modern chimps and Lucy, and therefore does not suggest an obligate mode of locomotion – meaning that Ardi may not have been completely quadrupedal like a chimp, or bipedal like Lucy and modern humans. She is being referred to as a facultative biped, meaning she was quadrupedal in the trees, bipedal on the ground. Furthermore, Ardi’s foot was very ape-like, with a grasping big toe, as opposed to the later Australopithecine foot (as indicated by human-like fossil footprints dated to Lucy’s era).

Ardi’s foot and some other aspects of her anatomy were not particularly human-like, but surprisingly they were not particularly chimp-like either; they instead appear to be a mix of primitive ape-like and later australopithecus-like characteristics. These findings are key in predicting the qualities of the earliest human ancestors – they were not necessarily chimp-like at all.

How do these new findings change field of hominid evolution?

ardi imageThe well-accepted former theory was that a chimp-like common ancestor species divided sometime around 6-8 million years ago, with the forest-dwelling chimps staying quadrupedal and the savannah-dwelling pre-humans becoming bipedal (followed by a much later expansion in brain size). Ardi, however, was already becoming bipedal despite her forest environment. This is a surprising finding, and overturns the commonly accepted theory that bipedalism was an adaptation to the savannah environment. If we conclude that hominid ancestors were beginning to walk on two legs before they left the forest, new theories will need to be developed to explain why bipedalism developed then and there.

Another informative aspect of Ardi’s discovery is that she wasn’t as chimp-like as expected for the earliest hominid, and that she had traits that were neither chimp-like nor human-like. This argues that the last common ancestor was something different from both chimps and humans. If nothing else, this should serve as a reminder that chimps have been evolving just as long as hominids, and shouldn’t be considered a substitute early human ancestor in behavioral and locomotion studies.

It is unfortunate that few fossils have been found from the proposed time of the split between human and chimp ancestors, but Ardi’s discovery has already given us valuable and unexpected insight into what that ancestor may have been like, allowing us to further refine theories about the early evolution of human ancestors.

Further Reading:

For more information on Lucy, please see our previous article, “Meeting Lucy”

For more coverage of Ardi:

www.sciencemag.org/ardipithecus

http://news.nationalgeographic.com/news/2009/10/091001-oldest-human-skeleton-ardi-missing-link-chimps-ardipithecus-ramidus.html

http://news.bbc.co.uk/2/hi/science/nature/8285180.stm

http://www.nytimes.com/2009/10/02/science/02fossil.html

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