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THE great ape family is about to welcome a new member. A comparison of genomes has found signs that a previously unknown species of chimpanzee once lived in the forests of central Africa.

As far as we know, there are no physical remains of the ancient ape. We only know about it because it mated and had offspring with bonobos 鈥� a chimp species 鈥� roughly 400,000 years ago, and its genes persist in living apes today. It is what is known as a ghost species.

Traditionally, it was thought that species were groups of organisms that would not produce 鈥渧iable鈥� offspring 鈥� ones capable of having babies 鈥� with any other group. But we now know that is not the case. Grizzly bears and polar bears, for instance, have begun mating as climate change squeezes their ranges together. Many other species have mated over the years. Genetic studies are revealing that 鈥渋mpossible鈥� relations once happened with previously unknown extinct animals.

When researchers compared the genomes of common chimps and bonobos, they found evidence that the two species had once interbred (see 鈥�Friends with benefits鈥�), much like humans and Neanderthals once did.

Martin Kuhlwilm at the Institute of Evolutionary Biology in Barcelona, Spain, went one step further by comparing the genomes of 59 wild chimpanzees and 10 wild bonobos. In each species, he looked for unusual DNA: fragments that could neither be explained by ancient matings with the other species, nor by random mutations.

鈥淭his means there was once a third species of chimpanzee living in the forests of central Africa鈥�

That DNA, he reasoned, had to have another origin altogether, a ghost source. This statistical method has previously been used to identify extinct human species.

Kuhlwilm found no ghost DNA in common chimps. But 鈥渂onobos have regions that are unusually special compared to chimps鈥�, he said at the AsiaEvo conference in . On average, roughly 1 per cent of each bonobo genome came from the ghost.

This means there was once a third species of chimpanzee living in the forests of central Africa. From his genetic data, Kuhlwilm was able to determine that it probably split from the common ancestor of chimps and bonobos 3 to 4 million years ago.

The three species would have lived in roughly the same region for millions of years. Chimpanzees and bonobos mated when the rivers that had separated them dried up, allowing them to cross these otherwise impenetrable barriers. Perhaps the ghost ape was also mostly kept apart by the landscape.

But at some point the ghosts and the bonobos met and mated, producing viable offspring. Kuhlwilm estimates that this happened around 400,000 years ago 鈥� 100,000 years before Homo sapiens first evolved elsewhere on the continent.

Now that the ghost ape has been spotted from its genes, there are two logical next steps. If we are lucky, the ghost DNA might teach us something about the newest member of our great ape family, such as how much it differed from chimps and bonobos.

The fragments recovered from bonobos together represent 3 per cent of the archaic ape鈥檚 genome, although what can be said about it from that DNA remains to be seen.

Far better would be to look for fossilised bones and teeth. The trouble is that such ancient chimp fossils are rare. One 400,000-year-old fossil was discovered in the Great Rift valley a few years ago. But as for finding new ones? 鈥淭he odds are low,鈥� says Kuhlwilm.

This article appeared in print under the headline 鈥淢ystery ghost ape discovered鈥�