Skip to navigation | Skip to content

Debate over platypus and kangaroo link

Don Colgan

Don Colgan - debate not over yet (Pic: Australian Museum.

A new US study that debunks the relationship between the kangaroo and the platypus is unlikely to settle an age-old debate, says an Australian expert.

The study published in the July 1 issue of the journal Mammalian Genome strongly supports the "Theria" hypothesis that marsupials such as the kangaroo are more closely related to mammals such as humans, who bear live young, than to egg-laying mammals (monotremes) such as the platypus.

The competing "Marsupionta" hypothesis, which links marsupials and monotremes as the more closely related sister groups, has been supported by analysis of mitochondrial DNA - the same method used to link whales and hippos.

Mitochondrial DNA has traditionally been easier to access and has the advantage of not undergoing recombination as does nuclear DNA. A downside, however, is that it is a relatively small sample of the genome.

The authors of the new study, a team at Duke University, say their analysis of nuclear DNA is "more comprehensive" and shows that the use of mitochondrial DNA is "statistically unreliable when it comes to evolutionary genetics".

"This is the first molecular evolutionary study that seriously and powerfully says the paleontologists have been right all along in grouping mammals the way they did," says Duke researcher Keith Killian. "It turns out that common sense is correct"

"Using prominent genes like IGF2R to classify mammals has allowed us to close the Theria versus Marsupionta debate once and for all," he adds.

However Don Colgan, Principal Research Scientist at the Australian Museum's Evolutionary Biology Unit disagrees.

Colgan says that despite the statistically weak evidence from mitochondrial DNA analyses, the debate will continue because there's enough data to keep the competing hypothesis alive.

Of the seven data sets based on nuclear DNA, he says four support the Theria hypothesis, one is equivocal and two support the Marsupionta hypothesis.

The Duke team argue that mitochondrial DNA provides misleading results because it requires more human input to decide which information is fed into the computer, thereby raising the risk of human bias.

"In fact when the data were given to three different laboratories for anlaysis, they generated three different family trees," says Killian.

However Colgan says that both methods are subject to potential human bias and believes both have an important contribution to make in understanding evolution.

"Even in their own data set, Killian and team said 'regions of ambiguous homology' were 'excluded prior to phylogenetic analysis'. That's code for human input," he says.

"When a resolution for this debate comes it will be as a result of a combination of both methods."