A large-scale genome analysis is changing scientists’ understanding of koala evolution. According to research, koalas (Phascolarctus Greius) A severe population decline occurred about 100,000 years ago, long before humans arrived in Australia.
All koalas alive today are thought to have descended from a single ancestral population that endured dramatic environmental changes, including severe ice ages.
However, modern koalas now face a different set of dangers. These include hunting, widespread land clearing, wildfires, and disease.
The study, led by researchers from the University of Sydney and Texas A&M University, challenges previous research that found koala numbers declined only after humans arrived in Australia. The survey results are molecular biology and evolutionpublished by Oxford University Press.
“This study rewrites the timeline of the genetic history of koalas in Australia,” said PhD student Toby Kovacs, who led the study.
“By calculating the mutation rate of modern koala populations, we can extrapolate and construct a genetic chronology going back 100,000 years, providing a glimpse into the genetic diversity and size of ancient koala populations.”
Koala DNA reveals hidden population history
Mr Kovacs said fossil evidence was too limited to say exactly how many koalas lived in Australia 100,000 years ago. Genomic data can help fill these gaps by preserving clues about changes in initial population size and genetic diversity.
“Genome analysis shows that koalas have experienced significant population declines in the past due to climate change and habitat loss. As environmental conditions improved, koala populations rebounded and expanded across much of eastern Australia.”
“It is important to be clear that many of the threats facing modern koalas, such as habitat loss and hunting, are caused by humans.”
By studying how koalas responded to previous population declines and recoveries, scientists may be able to develop better conservation strategies to protect the species today.
How researchers reconstructed koala evolution
To reconstruct the koala’s genetic history, scientists focused on the species’ mutation rate within its genome (an organism’s genetic information).
Every time an organism reproduces, new mutations (genetic changes) naturally appear in its genome. Mutation rate describes how many of these changes occur in each generation. The rate varies by species, with some animals accumulating genetic changes faster than others.
The research team analyzed the genome sequences of the four parent-child trios and counted newly emerged mutations. This allowed them to calculate the exact mutation rate in koalas, which was about half the mutation rate seen in humans.
The researchers then applied the newly measured rates to the genomes of 457 koalas. This gives us a more accurate way to track how koala populations have expanded, contracted and separated over thousands of years.
This study is the first to directly estimate mutation rates in koalas or members of the marsupial order Diprotodontida, which also includes wombats, kangaroos and possums.
Previous research had concluded that koala populations have declined since modern humans arrived in Australia about 65,000 years ago. However, these estimates were based on mutation rates obtained from distantly related mammals such as humans and mice.
Koalas went into decline before humans arrived
New analysis suggests that the large-scale decline in koala populations began about 100,000 years ago. Then, about 60,000 years ago, their numbers reached a critical genetic bottleneck.
This collapse coincided with a period of large-scale environmental destruction during the most recent ice age in the Late Pleistocene. In terms of timing, decline occurs long before contact with humans.
The landmass that would eventually become Australia was primarily covered by moist forests during the Paleogene period (23 to 66 million years ago). During the Miocene (5 to 23 million years ago), the situation changed dramatically as the Australian tectonic plate moved northward.
During the Pleistocene (2.5 million to 11,700 years ago), Australia repeatedly transitioned between ice ages, characterized by cold, dry conditions, and interglacial periods, characterized by warmer, wetter conditions. These cycles steadily reshaped the continent, making its landscape drier and increasingly vulnerable to fire.
About 70,000 years ago, the expanding Nullarbor Plain created a vast semi-arid bushland. This reduced suitable habitat for koalas and separated koala populations in eastern and western Australia.
The western koala population eventually disappeared. A small population in the east managed to survive the most difficult glacial conditions.
a small group re-established the species
During the current interglacial period, conditions improved, allowing the surviving population in the east to expand. Between 16,500 and 6,000 years ago, they split into five genetically distinct populations.
These groups eventually produced the koala population that is now found along Australia’s east coast.
“Given these results, we want to know whether other Australian species, including relatives of extinct megafauna, also experienced population declines before the arrival of humans,” Kovacs said.
Newly calculated mutation rates reveal more than just koalas’ ancient past. Scientists can also use this to study recent changes in population size and improve modern conservation plans.
Estimates for recent koala generations show that koala populations in Queensland and New South Wales continue to decline. In contrast, Victoria’s population appears to be recovering.
Koalas are officially listed as endangered in Queensland, New South Wales and the Australian Capital Territory from 2022.
Genomic research could lead to conservation
Kovacs said: “Understanding whether koala populations are in decline can help conservationists act early, before koala populations lose genetic diversity and face increased risks from inbreeding.”
“Koalas have experienced large-scale population declines in the past due to climate change and the disappearance of suitable habitat. The surviving koalas are once again experiencing similar population declines, but this time due to human land clearing, bushfires, hunting and disease.”
“Our team is generating a vast genomic resource for koalas, but to fully understand what these datasets can tell us, we also need to know how quickly new genetic changes occur in this species,” Kovacs says.
“Estimating mutation rates will improve our ability to reconstruct the history of koala populations, understand their adaptive capacity and make more informed conservation decisions for the future.”
