Is the Ida fossil the missing link?

Analyzing Darwinius Masillae

Sketches by Bogdan Bocianowski show what Darwinius masillae may have looked like in life.
Sketches by Bogdan Bocianowski show what Darwinius masillae may have looked like in life.
Illustration courtesy PLoS ONE

The Darwinius masillae fossil known as Ida is so complete and well-preserved that it has allowed researchers to piece together lots of information about the animal's life and death. It still had some of its baby -- or deciduous -- teeth, so it was young when it died. Radiographic images of its skull revealed the formation of molars and adult teeth. Researchers noted which teeth had come to the surface and compared that information to other animals. Based on which teeth had emerged and which hadn't, researchers concluded that D. masillae was a fast-growing animal.

The specimen in question was also probably female. Most mammals have a penis bone known as a baculum, although it's not a trait particular to humans. The D. masillae fossil has no baculum, and considering how well the rest of the specimen is preserved, it's unlikely that the animal had one in life.

Other evidence points to D. masillae's behavior. Its eye sockets are large, meaning it may have been nocturnal, using big eyes to gather more light. Since the fossil has long, agile fingers and possibly prehensile big toes, D. masillae probably spent its life living in the rainforest canopy. Based on its size and the brain cavity of the skull, researchers estimate that D. masillae's adult weight would have been 650 to 900 grams, or less than 2 pounds, as an adult.

That's a lot of information from one fossil, and while this information is useful in learning about past life on Earth, it's only part of the analysis. Part of the research process involves comparing D. masillae to animals that lived at the same time and animals that live today. Animals that come from a shared ancestor typically share particular traits, known as synapomorphies. Establishing and comparing synapomorphies helps researchers understand how animals are related and how they have changed over time.

D. masillae has been compared to lemurs, which are part of the primate suborder Strepsirrhini. But unlike lemurs, it doesn't have special teeth and claws adapted to grooming. Because of this and other traits, the researchers who have studied it claim that it could belong in the suborder Haplorhini, which includes tarsiers, simians and anthropoids -- including humans. This is where the idea of the fossil being a missing link comes from: According to the researchers' analysis, its traits bridge the two suborders.

But not all scientists agree. Yale paleoanthropologist Chris Gilbert called the evidence "less than convincing" [source: Moskowitz]. Discover blogger Carl Zimmer quotes two primatologists as characterizing the paper's data as "old news" [source: Zimmer]. Rather than answering questions about the origin of humanity, the D. masillae research will probably continue to fuel ongoing debate on when and how primates diversified and how humans originated.

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