Matt Walker, BBC News 12 Feb 10;
The tiger may be more ancient and distinct than we thought.
Tigers are less closely related to lions, leopards and jaguars than these other big cats are to each other, according to a new comprehensive study.
The genetic analysis also reveals the tiger began evolving 3.2 million years ago, and its closest living relative is the equally endangered snow leopard.
The discovery comes as the BBC launches a collection of intimate videos of wild tigers and the threats they face.
Despite the popularity and endangered status of tigers, much remains to be discovered about them, including how they evolved.
It has long been known that the five species of big cat, the tiger, lion, leopard, jaguar and snow leopard, which belong to the Panthera genus, and the two species of clouded leopard, are more closely related to each other than to other smaller cats.
But it has been difficult to pin down the exact relationships between them.
So to find out more, scientists Mr Brian Davis, Dr Gang Li and Professor William Murphy conducted an analysis of the DNA of all these species.
By looking at similarities in DNA held in mitochondria and within the sex chromosomes among other places, the researchers found that the five big cat species are related to each other in a different way to previously thought.
Their data strongly suggests that lions, leopards and jaguars are most closely related to each other.
Their ancestor split from other cats around 4.3 to 3.8 million years ago.
About 3.6 to 2.5 million years ago, the jaguar began to evolve, while lions and leopards split from one other about 3.1 to 1.95 million years ago.
But the tiger had already emerged by this point.
The ancestor of tigers and snow leopards also branched off around 3.9 million years ago.
The tiger then began to evolve into a unique species toward the end of the Pliocene epoch, about 3.2 million years ago.
That makes the tiger and snow leopard "sister species", the researchers report in the journal Molecular Phylogenetics and Evolution.
Both tigers and snow leopards are among the world's most endangered big cats.
Fewer than 3500 tigers are thought to survive in the wild.
One subspecies, the Sumatran tiger, is so enigmatic that the first film of a wild individual was only recorded this year, and Indonesia is considering entrusting them to private individuals for safe-keeping.
Last year, a study revealed that the largest sub species, the Amur tiger, may be on the genetic brink, as so few individuals remain.
