How old is Ida?
This question has two answers, or you might say it is actually two questions: How old was Ida when she died? And how old is the fossil?
How old was Ida when she died?
The teeth of fossils can tell us a lot. They can tell us which species the fossilized animal was, whether it was an herbivore or a carnivore, and they can often tell us a good deal about the animal’s age.
Mammals have milk teeth – i.e., teeth which fall out (exfoliate) when young animals grow up. The milk teeth are replaced by teeth which should last for life.
Since these permanent teeth are present inside the jaw from birth, we can see them in X-ray images. Below we see what the teeth of a seven-year old human child look like, compared with an X-ray of Ida’s teeth. In both, we see some teeth which have other teeth below and behind them. These are milk teeth which will be pushed out. We can also see that both Ida and the child have molars which have not emerged yet.
By comparing Ida with other primates, we can determine that she had reached the age when tooth exfoliation had started.
X-ray of Ida’s teeth. Teeth which have not emerged yet are coloured pink, the milk teeth yellow. Photo: PloS One
Teeth of a seven-year old human child. Milk teeth are coloured blue. We can recognize them in the X-ray.
When we study the skeleton, we find yet another clue: Ida has cartilage plates at the bottom edge of her thigh bone and in the pelvic region. This means that she was not yet fully grown. This is another trait we also know from all mammals and also human children.
However, Ida was not seven years old. Her species did not have as long a lifespan as human beings do. To find out how old she was, we compare her to the South-American squirrel monkey. It has been studied in detail, and is the same size as Ida. It takes three years for a squirrel monkey to reach adult size. In the figure below, we see how quickly various body parts develop. We can also see when specific teeth emerge (at the bottom of the figure).
When we compare Ida with species living today and which look like her, it seems probable that she was 9-10 months old when she died.
How old is the fossil?
The Messel quarry, where Ida was found, was a lake when she was alive. The lake was formed 47,8 million years ago in the crater of an exploded volcano. In this explosion, a type of rock called breccia was created, and this particular volcanic breccia is dated to be 47,8 million years old.
We can also compare pollen, plants and other fossils from Messel with fossil finds of known age from other European locations. Based on this, we can determine that Ida is 47 million years old. This is called the geological age.