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Cats Climb New Family Tree

Maryann Mott
for National Geographic News
January 11, 2006
 
Researchers say they have solved an 11-million-year-old puzzle: how a
single feline-like ancestor in Asia spread throughout the world and
developed into all modern cat species.

Cats are native to all continents except Australia and Antarctica, and the 37 wild and domestic species living today belong to one of the world's most successful carnivore families.

The details of this evolution success story proved elusive for biologists, however, in part because of incomplete fossil records and the few distinguishing dental and skeletal characteristics among ancient cats.

But after nearly ten years of genetic research, a team led by Stephen O'Brien and Warren Johnson of the National Cancer Institute in Frederick, Maryland, has constructed a feline family tree that better charts the journey of cat evolution.

Analyzing genetic material from all living cat species, scientists read traces of the Felidae family history, such as divergences and booms and busts in populations, by the patterns of gene organization and diversity within and between species.

The scientists found that when sea levels fell, exposing land bridges between continents ten to three million years ago, ancient felines moved around the globe to areas where they had less competition.

Cats ended up in their current homes through at least ten of these intercontinental migrations.

During this time, felines did more than just roam. They also developed into new species when sea levels rose to cover land bridges and cut off cat species from their original groups.

"When you have two groups of populations that become isolated, they begin to drift apart," Johnson said. "They adapt to different things to the point where—when they have the chance of coming back together—they no longer can breed and, at that point, are distinct species."

This process happened rapidly and occurred numerous times over the past 11 million years, he said.

The radiation of modern felines began with the divergence of the Panthera lineage. From this common ancestor all the big cats emerged 6.4 million years ago—lions, jaguars, leopards, tigers, and snow and clouded leopards.

They were quickly followed by three Asian species (bay cat, marbled cat, and Asian golden cat) and three African species (caracal, serval, and African golden cat).

The final feline to appear was the household cat, which emerged between 6.2 and 6.7 million years ago.

The house cat either came from European ancestors that remained in Asia or from North American cats that migrated from across the Bering land bridge, which spanned present-day Siberia and Alaska.

"It's really just the last blink of an eye in evolutionary time when domestication created … house cats like Garfield," O'Brien said.

Experts believe domestication occurred several thousand years ago in the Middle East, probably in Egypt, where some of the first archeological evidence of domestic cats was discovered. Hieroglyphics from the region depict cats, and their bones have been found buried with humans.

Johnson says the genetic evidence his team discovered does not contradict that common belief. However, he said, domestication may have occurred more than once and in more than one place.

Skin Samples

In their study, described in the current issue of the journal Science, the team took skin samples from all 37 living cat species.

This was a difficult task, Johnson says, because many are rare and live in remote locations.

The team then looked at data from the species' X and Y chromosomes (the genetic determinants of gender) and mitochondrial DNA.

"Genomics is giving us a tool that goes beyond just simply understanding the basis of genetic diseases," O'Brien said.

"It's also giving us a compass, or a map, where we may actually begin to try to reconstruct the history of some of the species that live on Earth, including ourselves."

The researcher says he next plans to investigate where and when the household cat was domesticated.

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