A spectacular pink type of Galapagos iguana promises to rewrite the family's evolutionary history in the islands.
Rosada was missed by Charles Darwin during his 1835 visit, but appears to indicate the earliest known divergence of land animals in the archipelago.
Writing in Proceedings of the National Academy of Sciences (PNAS), researchers say rosada split from other land iguanas about 5.7 million years ago.
The scientists suggest that less than 100 of the animals still exist.
Park rangers first noted the presence of a pink variety of iguana on the slopes of Volcano Wolf on the island of Isabela in 1986, but it was not until 2000 that scientists began to examine it.
Darwin's studies in the Galapagos provided key evidence underpinning the theory of evolution by natural selection, which he set out in the 1859 tome On the Origin of Species.
Animals such as finches and tortoises showed subtle changes of form between the islands, leading him to theorise that they had evolved along different paths in different environments.
His travels bypassed Volcano Wolf, and so he missed the characteristic pink iguana, which has not been found outside this single, relatively young volcano.
Separate lives
The researchers have now produced several strands of evidence suggesting that rather than being one form of the main land species Conolophus subcristatus, rosada should be considered a separate species.
Iguanas typically bob and duck when they meet each other - a behaviour thought to be important for marking territory and courtship - and rosada does so in a more complex fashion than the yellow-coloured subcristatus or the other Galapagos species, Conolophus pallidus.
It also has a different shape of crest. There is little sign of cross-breeding between pink and yellow.
And the DNA analysis shows it is far more distinct from all the other land iguanas than they are from each other.
That means the line that led to subcristatus and pallidus must have diverged from that leading to rosada long ago, with the split between subcristatus and pallidus coming much later.
The DNA work puts the date of divergence between rosada and the rest at about 5.7 million years ago, raising a new set of questions.
"At 5.7 million years ago, all of the western islands of the archipelago did not exist," said Gabriele Gentile from the University of Rome Tor Vergata, who led the new analysis.
"That's a conundrum, because it's now only inhabiting one part of Isabela that formed less than half a million years ago," he told BBC News.
Even the oldest parts of the archipelago may be less than five million years old.
The explanation may be that some volcanoes that are now under water were above it at the time when the first iguanas arrived, and this allowed some of the creatures to climb onto land and begin their separate evolution.
Earlier DNA analysis suggests that land-based iguanas split from their marine counterparts about 10 million years ago.
Small world
Whatever the history, Dr Gentile's team believes rosada's single population is so tiny as to put its survival in danger.
"Our studies would indicate that the population size is very small," he said.
"We only collected 36 in two years; and last year a large research team hiked up Wolf and only found 10, and most of those were ones that we'd marked earlier."
These numbers are low enough to make rosada a Critically Endangered species - if, indeed, it is a separate species from its yellow relatives.
Dr Gentile's team is now preparing a formal description of the animal, and will be asking the International Commission on Zoological Nomenclature (ICZN), the body that adjudicates on such matters, to rule that it is separate and distinct.
Pink iguana discovered on Galapagos
A new species of iguana with unusual pink skin has been discovered by scientists on the island where Charles Darwin first developed his theory of evolution.
Louise Gray, The Telegraph 5 Jan 09;
The large lizard, which is at risk of extinction, has rose-coloured scales and only lives on one volcano in the Galapagos Islands.
Darwin, whose 200th birthday will be celebrated on February 12, observed both marine and land iguanas when he visited the archipelago in 1835.
The way the creatures adapted themselves to their surroundings helped to develop his revolutionary ideas about natural selection.
But he never explored the Volcan Wolf volcano on the island of Isabela and therefore did not see the pink iguana.
The creature was first spotted in 1986 by park rangers but was dismissed as a curiosity and soon forgotten.
It is only now that scientists have discovered that the "rosada" – or pink – iguana is a species in its own right after comparing its genes with other land iguanas on the Galapagos.
There were also physical differences, besides the striking pink and black-striped colouring. Pink iguanas had flat head scales, unlike other land iguanas, and a thick fatty crest on the back of the neck with small conical scales.
Action is now needed to prevent this scientifically valuable creature becoming extinct, say the researchers writing in the journal Proceedings of the National Academy of Sciences.
The discovery comes as seventeen previously unknown species of reptiles and amphibians were found in the threatened rainforests of eastern Tanzania.
The new species, which include chameleons, tree frogs and snakes, were discovered by scientists from the Natural Science Museum of Trento in Italy.
The discovery shows the rich biodiversity of the area which is under threat from fire, logging, collection of wood for fuel and land clearance for cultivation.