Colder climates brought the devil into the world. The Tasmanian devil, that is, which took advantage of the ecological changes brought about by a cooling world to overtake the Tasmanian tiger, or thylacine, and its relatives.
Fifteen million years ago multiple thylacine species roamed the thick forests of eastern Australia. The species ranged widely in size and were able to fill many niches in what was then a warm, wet climate. However, over a period of 2 million years, the world cooled 7ºC (13ºF) and most of these closed forests were replaced by open woodland. A few thylacine species survived, with the Tasmanian version lasting until the 20th century, but most couldn't cope with the new conditions.
Shimona Kealy, a PhD student at the Australian National University, reports in BMC Evolutionary Biology that into this gap sprang the devil and its better-tempered relatives the quolls, collectively known as dasyurids.
Kealy told IFLScience the cooler, drier environment sparked great diversification among dasyurids, such that for a time there were several species of devils roaming up and down Australia's east coast.
The fact that multiple thylacine species died out about the same time, in each case to have their ecological niche filled by dasyurids, suggests a common weakness independent of size.
“We think the structure of tigers’ feet and ankles might have made them better suited to closed forests with uneven surfaces, such as roots and logs, and less well suited to open woodlands,” Kealy said in a statement. She added to IFLScience the way the thylacine's muscles joined their joints was inefficient for moving over flatter ground. “Dasyurids, on the other hand, have ear-bone structures that appear to be better adapted to open woodlands, allowing them to hear over greater distances than thylacines,” Kealy continued.
The fossil record for this period lacks the detail to tell us whether dasyurids drove thylacines to extinction, or if the thylacines died out first and the dasyurids took advantage of the newly empty niches.
Unfortunately, Kealy told IFLScience, fossil records for the devil species on the Australian mainland are in short supply, so we know little about what happened after they diversified. Most devil species were gone by the time humans arrived on the continent, but the timing and cause of their disappearance remain a mystery.
By the time the Bass Strait filled, isolating Tasmania, the last surviving devils and tigers were both in a bad way, lacking genetic diversity, and barely clinging to life on the mainland. That lack of diversity meant the Tasmanian tiger was in severe trouble even before the mad hunting by early European settlers, and has made the far more numerous devil vulnerable to the transmissible cancer pushing it to the edge.