Ida (pronounced EE-duh) is the most perfectly preserved primate fossil in the world. Paleontologists, scientists who study fossils, estimate that Ida died 47 million years ago. Ninety-five percent complete, she is the most complete primate fossil ever found. By comparison, the famous “Lucy” fossil, from the species Australopithecus afarensis, is only 40 percent complete.

Ida was a small primate, about nine months old when she died. From end to end she is only 58 centimeters (23 inches) long, about the size of a small house cat. Her body is 24 centimeters (nine inches) long. Ida's legs were longer than her arms, indicating she was a leaper. X-ray scans show she was a female. Ida’s remains also show she had a broken right wrist. She didn’t die of a broken wrist, but it almost certainly contributed to her early death.

Ida had large eye sockets, which suggest she was nocturnal. Nocturnal animals are active mainly at night. The shape of Ida’s teeth suggests she was a vegetarian. However, scientists didn’t have to guess what she ate. Her last meal—fruit—was still preserved in her gut millions of years after she ate it.

Ida had long fingers and toes and opposable thumbs. Her hands show she had rounded fingertips with nails, not claws. Rounded fingertips with nails are classic primate features.

The scientific team that introduced Ida to the world was led by Dr. Jørn Hurum, a paleontologist at the Natural History Museum in Oslo, Norway. Hurum persuaded the museum to purchase Ida.

A private collector discovered Ida near Messel, Germany, in 1983. Until 2000, Ida's remains were split into two pieces. The main part (“Slab A”) remained with the collector, while the other part ("Slab B") was sold to the Wyoming Dinosaur Center in Thermopolis, Wyoming, in the United States. Slab A and B were reunited in 2007. The identity of the person who dug up Ida remains unknown.

One of the discoveries Hurum and other paleontologists made when they x-rayed Ida was that she had many more teeth than the average primate. When the scientists looked closer, they discovered Ida was in the process of losing her baby teeth.

Unerupted molars—adult teeth that were pushing out her baby teeth—could still be seen in her jaw. From this evidence, the paleontologists determined Ida was a juvenile primate—not a baby, but not fully adult, either. Developmentally, she was about the same age as Hurum’s daughter, Ida, who was also losing her baby teeth. They decided to name the fossil after her.

Darwinius masillae

Ida's scientific name is Darwinius masillae. The genus Darwinius was named in honor of Charles Darwin's 200th birthday. The species name, masillae, commemorates the Messel Pit in Germany, where Ida was found.

The Messel Pit is an abandoned quarry about 35 kilometers (22 miles) southeast of Frankfurt, Germany, near the village of Messel. The pit was formed millions of years ago when hot magma bubbling from under the earth came too close to the underground water table. When the magma hit the water table, it instantly turned to steam. The pressure of the steam caused a massive explosion as it tore into the earth. The explosion created a type of volcanic lake known as a maar.

The explosion that formed Messel Lake happened about 50 million years ago, during the early Middle Eocene epoch. Because the maar had no rivers running into or out of it, the water at the bottom of the lake received very little oxygen. Anything that fell into the lake was remarkably well-preserved. They didn’t go through the same decomposition process that other living things do when they die.

Another characteristic of maars is that they sometimes spit out toxic gas. Maars have volcanoes under them, and from time to time they emit carbon dioxide gas. The pure CO2 travels up from the lake and can kill any living creature that breathes in the gas.

Scientists speculate this is what killed Ida. Her broken wrist meant she couldn’t leap and cling to high tree branches. Lower to the ground, she encountered the toxic gas in Messel Lake, lost consciousness, and drowned.

In 2001, a hole was drilled into the center of the Messel Pit. Scientists extracted volcanic rocks that formed the ancient lake. Dating the rocks in the Messel Pit showed that Ida was about 47 million years old.

The Nose Knows

Ida lived her short life during the Eocene. The Eocene lasted from 55 million to about 34 million years ago. The Eocene is an important period in human evolution, because it was during this time that the first primates were evolving. About 40 million years ago, there were two distinct primate groups: prosimians and anthropoids.

One way taxonomists separate prosimians and anthropoids is by their noses. Prosimians, or strepsirrhini, have dog-like, wet noses. Extant, or living, representatives of strepsirrhini include lemurs, lorises, and bush babies. Anthropoids, or haplorhini, have dry noses. Extant representatives of haplorhini include monkeys and apes. Humans are also dry-nosed primates. At some point during the Eocene, primates evolved into these two different branches.

So was Ida a wet-nosed or a dry-nosed primate? She contains qualities of both, making her a truly remarkable specimen.

The team from the Natural History Museum in Oslo believes her physical size and diet were similar to those of the eastern woolly lemur, a wet-nosed primate native to Madagascar. However, Ida doesn’t possess two key lemur traits: a toothcomb or a grooming claw. A toothcomb is a set of fused, forward-angled teeth in the lower jaw that lemurs use to groom their fur. A grooming claw is a long claw on the second toe that lemurs use to groom fur they can't reach with their toothcomb. Ida also has a tarsus bone in her ankle that is shaped like a dry-nosed primate ancestor’s.

The Oslo team believes Ida comes from a time when primates were still evolving into these two distinct groups. Because Ida has characteristics of both, they consider her a transitional species—a link between prosimian and anthropoid primates.

Other scientists who have read the published paper disagree with its conclusions. They say Ida is an ancestor of lemurs and lorises, not a “missing link.”