Is the Ida fossil the missing link?

Although Ida doesn't quite fit the bill for the way most people use the term "the missing link," it's still an important find. First, it appears to document a previously unknown species,Darwinius masillae.It's the onlyD. masillaespecimen discovered so far, and it's one of a very few primatefossilsfound in the Messel pit fossil beds.

The fossil is also almost entirely complete. Most animal fossils are highly fragmented -- they'rebones, pieces of bones or small portions of skeletons. TheD. masillaefossil, on the other hand, is about 95 percent complete [source: Franzen et al]. Even its tiny, delicate bones, like the tips of its toes and tail, are still intact. Only the lower part of one leg and possibly a few tail vertebrae are missing. This means researchers can get a complete look at an animal's entire skeleton instead of having to use knowledge of similar animals to try to fill in gaps.

On top of that, theD. masillaefossil shows the animal's hair and soft tissues, which doesn't happen often. The fossilization process works best on hard tissues, like bone. Hair, skin and other soft tissues usually rot away or are eaten by scavengers before they can harden into stone. But in the case of theD. masillaefossil, the animal most likely fell into a lake created by avolcanocrater and was quickly buried in sediment at the bottom, where it was protected from destructive bacteria and predators.

As the material surroundingD. masillae's body hardened, bacteria left a dark stain. This dark stain essentially paints a picture of the animal's soft tissue and hair in theoil shalethat surrounds the fossilized bones today. This gives researchers a much clearer idea of what the animal actually looked like, how itsmuscleswere situated and how it moved. The level of detail even extends to the animal's stomach, which contains remnants of its last meal -- fruit and leaves.

Much like thedinosaurfossil known asLeonardo,D. masillaefossil gives scientists a lot of information to work with -- and some research challenges. Both fossils are embedded in stone, and removing them from the rock would destroy them. That's why researchers used numerous imaging techniques to get clear pictures ofD. masillaeto study. Some of the technologies include digitalX-rays,CAT scansand mammography.

Together with analysis of both halves of the fossil and comparisons to other fossil specimens, these images have allowed scientists to draw several conclusions about the fossil. And those conclusions -- while they don't quite makeD. masillaethe missing link -- have an importance all their own.

TheDarwinius masillaefossilknown 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 thatD. masillaewas 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. TheD. masillaefossil 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 toD. masillae's behavior. Itseyesockets 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. masillaeprobably spent its life living in the rainforest canopy. Based on its size and thebraincavity of the skull, researchers estimate thatD. 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 onEarth, it's only part of the analysis. Part of the research process involves comparingD. masillaeto animals that lived at the same time and animals that live today. Animals that come from a shared ancestor typically share particular traits, known assynapomorphies. Establishing and comparing synapomorphies helps researchers understand how animals are related and how they have changed over time.

D. masillaehas 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, theD. masillaeresearch will probably continue to fuel ongoing debate on when and how primates diversified and how humans originated.