Lack Of Genetic Diversity May Have Doomed Tasmanian Tiger, Scientists Say

When it comes to the extinction of modern animal species, humans usually end up taking the rap.

The traditional view of the disappearance of the Tasmanian tiger is no different. It follows a well-worn indictment: After the first humans began arriving in Australia (by recent estimates) some 18,000 years ago, the dog-like predatory marsupial began to disappear.

By about 3,000 years ago, the Tasmanian tiger, also known as thylacine (Thaylacinus cynocephalus), vanished on the Australian mainland, leaving only an isolated population on the island of Tasmania.

While no one doubts that human hunting was the proximate cause of thylacine's demise, according to lead researcher Andrew Pask of the University of Melbourne, its fate may have been sealed tens of thousands of years before the last known animal died at the Hobart Zoo in 1936.

(Although it was officially declared extinct in 1982, there have been occasional unconfirmed sightings of the Tasmanian tiger, such as this one, as reported by NPR's Merrit Kennedy in March.)

Gene sequencing using a piece of DNA extracted from soft tissue of a 108-year-old Tasmanian tiger specimen that had been stored at Museums Victoria, Australia, showed that the species apparently underwent a "steep decline in diversity beginning around [70,000 to 120,000 years ago]," according to the study, published in the journal Nature.

"The population decline appears to have begun before the human colonization of Australia and overlaps with climate changes associated with the beginning of the penultimate glacial cycle," the study says. In other words, the last ice age may have doomed the Tasmanian tiger long before "European settlers deemed the thylacine a threat to the Tasmanian sheep industry and the government aggressively targeted it for eradication by offering a £1.00 bounty for each animal killed," the Nature study notes.

"Even if we hadn't hunted it to extinction, our analysis showed that the thylacine was in very poor [genetic] health," Pask said. "The population today would be very susceptible to diseases, and would not be very healthy."

But the thylacine's genome sequence revealed another interesting thing: Despite a 160-million-year separation between metatheria, the subbranch (or clade) to which mammals such as the Tasmanian tiger (and other marsupials) belong, and eutherian, the clade that spawned the more familiar placental mammals (including canines), from a purely physical standpoint, it would be hard to deny that thylacine bears a remarkable resemblance to dogs.

It is an exceptional example of what scientists call "convergent evolution" — species that are not closely related by genetics nonetheless independently evolve similar traits to exploit similar evolutionary niches. Another such example might be sharks, which are cartilaginous fish, and dolphins, which are mammals.

The similarities between Tasmanian tigers and dogs "are absolutely astounding because they haven't shared a common ancestor since the Jurassic period," Pask was quoted by The Guardian as saying.

"The appearance of the thylacine is almost a dingo with a pouch. And when we looked at the basis for this convergent evolution, we found that it wasn't actually the genes themselves that produced the same skull and body shape, but the control regions around them that turn genes 'on and off' at different stages of growth," he said.

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