Sunday, October 11, 2009

Pleased to Meet You, Ardi

The Discovery Channel special on Ardipithecus ramidus aired tonight. (For any of you who missed it, don't worry, you have a few additional opportunities to catch it if you're interested.)  Nicknamed Ardi, this early hominid fossil was discovered in the Middle Awash region of what is present-day Ethiopia in the early 90s. The media has been buzzing with news of her recently—and after the Darwinius flurry earlier this year, I was interested in seeing how she'd be treated by the media and science audiences. But Ardi appears to have been methodologically studied by anthropologist Tim White and team, who uncovered her in the early 90s and have only just permitted their findings to be published, and as a result Ardi has been well received. And the research her bones have permitted have revealed findings that have addressed questions put forth by Darwin himself.

Here's the rundown on Ardi: She lived about 4.4 million years ago (mya), and with that number she officially unseats Lucy as the oldest complete hominid fossil known to science. Of course, Lucy, had to have known she wouldn't hold her throne forever—scientists have known about Ardipithecus for a few decades, and they have bone fragments from at least two species who are even older than Ardi: Orrorin lived at least 6 mya, and Sahelanthropus lived in Africa between 6 and 7 mya. Nonetheless, Ardi gets to enjoy her 15 minutes of archaeological fame because work with the fossil record is never guaranteed— and good work with the fossil record requires careful analysis, which means time. But Ardi is more than just "complete"; she gives us answers.

Ardi's bones were found in the Middle Awash and her placement in the geological record allows us to understand her relative to Lucy (Australopithecus afarensis) as she appears in a distinct fossil layer from Australopithecus indicating that they did not coexist as a species. Lucy was a fully bipedal, although not without some difficulties. We know this from studying Lucy's feet which lacked flexible bones and had stiff, forward pointing toes. Around 4.4 mya, Ethiopia was a woodland, and Ardi's feet and hands indicate that she likely moved on all fours in the branches of the trees using her hands to pull her along and bear most of her weight (i.e., she was not swinging from branches). And she wasn't limited to the trees: Ardi was bipedal on the ground—and she wasn't a knuckle-walker either. Our closest cousins, the chimps (Pongo) are more specially adapted for life in the trees, which includes knuckle-walking, than even Ardi herself. She gives us evidence that a shift did indeed take place—we left the trees, perhaps due to a shift in social structure, and found our way to the form of Austral. afar. within a relatively short period of time. Research is currently underway on a species that falls between Ardi and Lucy geologically speaking. In time, Australopithecus anamensis may help us understand the (seemingly) rapid progression from Ardipithecus to Australopithecus to Homo.

Another major point from Ardi's fossil comes from her canines, which are blunted like Lucy's, and unlike present-day and primitive chimps. Scientists speculated that blunted canines evolved in our ancestors in relation to decreased mate competition, which occurs and has occurred in primate social organizations. Even today, some primates sport large, sharp teeth that they use to fight for females. But Ardi's teeth were blunted, and she still spent some of her time in the trees—Ardi still had ape-like qualities, which means that blunted teeth possibly evolved separate from mate competition. The human evolutionary line developed blunted canines, while pronounced canines continued to develop in other primate lines, such as Pongo. C. Owen Lovejoy proposes that bipedality and the reduction in canines arose as a result of dietary and environmental factors that supported male-male cooperation, and increased reproductive successes. (After all, sex remains the reason we do most things, no?)

I'm deeply excited by Ardi and I have no doubt she will continue to spark discussion and debate. For me, the following quotation from White et. al. (2009) sums up the strength this research has added to evolutionary studies:
Perhaps the most critical single implication of Ar. ramidus is its reaffirmation of Darwin’s appreciation: Humans did not evolve from chimpanzees but rather through a series of progenitors starting from a distant common ancestor that once occupied the ancient forests of the African Miocene.
Prior to Ardi, researchers proposed that our evolutionary ancestor from the point of divergence with other primates was chimp-like, and I have heard enough people denounce evolution because they don't believe they "descended from a chimp." Chimp-like was never meant to mean "chimp," and now we know that while our evolutionary branch with Pongo is linked, if the break happened well before Ardi then we may share an ancestor far more "primitive" than either Ardi or whatever "chimp-like" may entail.

Ardi gives us another piece to the amazing puzzle that is evolution. For anyone interested in learning more about what the fossil record can teach us, I recommend Your Inner Fish: A Hourney into the 3.5-Billion Year History of the Human Body by Neil Shubin.  It's good science writing and will help you understand how scientists work with clues from the fossil record to construct an understanding of our evolutionary history.

If anyone caught the special on Ardi, I'd welcome your comments—or any comment you may have on this fossil find.

Articles Reviewed:
Tim D. White, Berhane Asfaw, Yonas Beyene, Yohannes Haile-Selassie, C. Owen Lovejoy, Gen Suwa, and Giday Wolde Gabriel. Science 2 October 2009: 64, 75-86.

C. Owen Lovejoy. Science 2 October 2009: 74, 74e1-74e8. 

**Photo reconstructions © J. H. Matternes; Bone photo spread © Tim White.

Additional Ardi Resources:
Special Edition of Science: Issue devoted to Ardi findings. A free account will give you full access to all articles on this find. 

Discovering Ardi: Interactive guides to support the Discovery channel special.

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