A new study of the genes of a long-dead Tasmanian tiger suggest the animal’s numbers were dwindling since about 70,000 years ago. But this doesn’t change the fact that humans drove it into extinction.
The Tasmanian tiger – or thylacine – looked like a dog, had stripes like a tiger and carried its young in a pouch like a kangaroo. This strange creature went extinct in the early 20th century. The last thylacine died in captivity at Hobart Zoo, Tasmania, in 1936, just a few years after humans had hunted it to extinction in the wild.
But now, researchers sequenced the DNA of a century-old preserved specimen and found the thylacines suffered from poor genetic diversity. This, it seems, left them vulnerable to many diseases long before humans even arrived in Australia.
The thylacine was a top marsupial predator found across Australia and Tasmania. Weighing about 30 kg, it looked very similar to a wolf or the Australian dingo. Populations started dwindling on mainland Australia and disappeared completely about 3,000 years ago, leaving thelast remaining group in Tasmania. In 1888, the Tasmanian government put a bounty on the thylacine’s head because it was believed to attack sheep, threatening farmers’ livelihoods. The thylacines were killed indiscriminately, with the last known animal in the wild murdered in 1930.
Its genetically closest living relative is debated. Some studies place the thylacine closer to the Tasmanian devil, a black furry four-legged animal about two feet long. Others say it’s the numbat, a.k.a. the banded anteater.
However, thylacines looked more like dogs and wolves than the devil or the numbat. “What is so striking about the thylacine and dogs is just how closely they resemble each other [but] just how distantly related they are,” says Andrew Pask, a reproductive biologist at the University of Melbourne in Australia and one of the authors of the study.
Although they diverged from a common ancestor about 160 million years ago, thylacines and dogs evolved to look alike to adapt to similar environments, a process called convergent evolution.Thus sequencing both their DNA provides a unique opportunity to test if their genomes are similar and to understand what genes drove their similar development.
Pask and colleagues extracted DNA from the soft tissue of a 108-year-old specimen of a young thylacine preserved in alcohol at the Museums Victoria in Melbourne, Australia. They sequenced the DNA and then compared it to that of the Tasmanian devil and two other marsupials.
They parsed this data using a method called the pairwise sequentially Markovian coalescent (PSMC) analysis, and they were able to determine how the thylacine population varied over time. Using a single genome and mathematical tools, this method helps trace when two different lineages of a population coalesced to a common ancestor. The population sizes are related to the distribution of coalescence times, which can be derived from mathematical analysis.
It showed that the genetic diversity of the thylacine underwent a drastic decline 70,000-120,000 years ago, during an ice age. So the thylacine populations had already started declining before the arrival of humans in Australia, believed to be about 65,000 years ago, and much before the land bridges from Australia to Tasmania were swallowed by the sea 10,000-14,000 years ago. This made the animals highly susceptible to fatal diseases like facial tumour disease (found among Tasmanian devils). While this conclusion has been reported before, the previous study suggested the decline in genetic diversity was more recent – after Tasmania became an island. The new study takes this back by several thousand years, showing that loss in genetic diversity began much earlier.
However, this doesn’t mean that the thylacine would have died out anyway because of diseases if humans hadn’t hunted them. “The thylacine is definitely extinct because we hunted it,” says Pask. “But what it does mean is that if we hadn’t hunted them into extinction, the population would be in very poor genetic health, like the devil.”
Thomas Gilbert, a geneticist at the University of Copenhagen, Denmark, agrees. “The key point is that the population was in decline before humans entered the range, suggesting while we may have knocked it over the edge, it was in trouble before.” But he cautioned that the PSMC method can sometimes lead to erroneous results, although he stressed that that may not be the case with the present study. “One just needs to realise that these things are not always 100% clear.”
PSMC sometimes yields false results because of the assumptions made. For example, a recent study on passenger pigeons suggested its numbers were fluctuating drastically before the birds went extinct – a conclusion that disappeared when the data used in the study was reanalysed. It seems passenger pigeons had such immensely large populations that they lost the ability to live in small flocks as they were being hunted to extinction.
Another question Pask and colleagues sought to answer was what genes could have motivated the similarities between thylacines and dogs even though they belonged to very different animal families. “We know that organisms evolve to look so similar when they occupy the same niche,” says Pask – just like fish and dolphins have adapted to look similar as they live in water. “But what we don’t know is which genes in the genome drive these developmental changes. That is, which are the genes on which evolution acts.”
The researchers compared the thylacine genome with that of a reconstructed ancestor of the dog family and found the genes that drive the similarity are not in the genome that codes proteins – but in the DNA that controls when and where a gene is expressed. The portion of the genome that coded for proteins was still different between dogs and thylacines.
According to one study, natural selection leaves a signature on the protein-coding portions of an animal’s genome, leading to the development of genes that allowed, for example, land animals to adapt to life in water. This is how modern-day whales, manatees and walruses came to be. But the signature were left only on a handful of genes, and did not mean that something else may also have happened. This ‘something else’ is what differentiated thylacines and dogs.
Although the thylacine went extinct before we could grasp the ideas of DNA and genes, the tools available today allow us to extract an enormous amount of information from tiny fragments of their remains. “The interesting thing about extinct animals from the conservation biology perspective is why did they die out in the first place. There is much debate over such questions, including was it the humans or environment” or something else, according to Gilbert. “The more extinct genomes we have, the more we can say about why things went extinct.”
If or how it will help conserve highly endangered animals remains to be seen but such studies will help us understand population changes and genetic diversity – as well as the fact that some animals like the Tasmanian devil may be in trouble even without human interference. “The only way to find out is to increase the amount of data available to study.”
Lakshmi Supriya is a freelance science writer based in Bengaluru.