After two years of examining Ida in secret, the team of scientists who have described the fossil presented her to the world in May 2009. The timing couldn't be more appropriate - Ida's full scientific name, Darwinius masillae, is a tribute to the English naturalist Charles Darwin as it's the 200th anniversary of his birth, and the 150th anniversary of the publication of his most important work.
It was Darwin who first proposed and gathered evidence for the theory of evolution by natural selection. He laid out the idea that species change over time through the selective retention of beneficial traits, gradually evolving into new species. In this way, all species alive today are descended from common ancestors.
When Darwin published his groundbreaking theory in On the Origin of Species in November 1859, there were major gaps in the fossil record. As a rigorous scientist, he acknowledged that the lack of intermediate transitional fossils between species posed a problem, noting that this was "the most obvious and gravest objection which can be urged against my theory".
Nevertheless, all the available evidence was consistent with his theory of descent with modification, and just two years later, in 1861, the first classic transitional fossil, Archaeopteryx was discovered. This beautifully illustrated an intermediate form between dinosaurs and birds, supporting Darwin's ideas. Subsequently, extensive evidence has been collected demonstrating the relationship between the major groups of animals.
Now, for the first time, an incredibly complete early primate fossil has been discovered which provides us with direct evidence of an intermediate link between the human primate lineage and earlier mammals. Ida is an example of a transitional fossil between primitive primates and the prosimian and anthropoid branches, the latter of which eventually led to humans. It's not correct to describe her as "the missing link" - there are many transitional links in our evolutionary story, but she is the earliest, and one of the most significant links, ever found.
On the 150th anniversary of its publication, Darwin's theory is being celebrated all over the world for the incredible understanding it gives us of life on Earth. As Jørn Hurum explains, "Darwin said a lot about transitional species and how they were missing from the geological record. And he said that if a transitional species is never found, his whole theory will be wrong. For the last fifty years, we've gained a deeper and deeper understanding of species that show transitions between large groups of mammals. Ida is very comparable to some of the most significant fossils that have been described like Lucy, the Neanderthals, Tyrannosaurus rex, and Archaeopteryx. It's a really important specimen that will become an icon of evolution. So I think Darwin would be really happy about this specimen."
A note on the branching tree of human evolution
The story of human evolution is a branching tree, with several different human species alive at the same time for the vast majority of our evolutionary history. Only since around 24,000 years ago, when the Neanderthals died out, have Homo sapiens been the only species of human living on the planet. The species described here are all potential human ancestors in different time periods but, due to the imperfections of fossil preservation, we may never find or be able to definitively identify the species that did evolve into modern humans. Nevertheless, these species are at least closely related to the human lineage and provide valuable information about the different stages of human evolution.
A common misconception is that humans are the "most evolved" species on Earth, and that biological evolution is linear and progressive. This is misunderstanding Darwin’s central theory; as every species alive today has an equally long evolutionary history, and every primate, like humans, has been evolving for 47 million years since Ida. As Jørn Hurum explains, "When we say that Ida is related to all the monkeys and apes that live today, at the same time we’re saying that they all have separate lines of evolution. Chimpanzees, gorillas, orang-utans and lemurs all have the same amount of evolution as humans do. We just evolved in different directions. The beauty about the theory of evolution is that we all have a starting point somewhere, and then the ancestors go in different directions. What we see with specimens like Ida is something about the deep evolution of all primates - we have a common ancestor somewhere down the timeline. All of us."