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ScienceWise - Autumn 2010

The Mystery of the Ratites

Article Illustration
Dr Matt Phillips
Article Illustration
An emu

How Did Flightless Birds Cross the Oceans?

200 million years ago many of the great Southern Hemisphere landmasses such as Australia, New Zealand, South America, Africa Antarctica, Madagascar and India were merged into a supercontinent known as Gondwana. Over the course of time, movement of the tectonic plates created the separate geography we are familiar with today. Of course when separation happened any plants and animals living on these landmasses would have been split into isolated populations, each of which would have then taken their own separate evolutionary course.

However, there is a puzzle surrounding one group of animals, known as the Ratites. Ratites are a family of large flightless birds that includes ostriches, emus and New Zealand’s extinct giant moa. Large flightless birds would have been easy prey for many species of dinosaur so scientists believe the two never co-existed. The fossil record supports this idea, indicating that the Ratites evolved about 65 million years ago – about the same time the dinosaurs died out.

The problem is that by 65 million years ago there were vast tracts of ocean between places like Australia and Madagascar So if Ratites evolved that recently, how did their ancestors  get from one isolated landmass to another if they couldn’t fly?

The surprising answer to this puzzle has recently been discovered by a team, led by Dr Matthew Phillips, an ARC Postdoctoral Fellow at the ANU Research School of Biology. The team found that rather than having a common flightless ancestor as was originally thought, the Ratites’s closest relatives are in fact the small flying tinamous of South America. This implies that the evolutionary traits of being large and flightless evolved separately in each location driven by environmental factors such as plentiful food and a lack of large predators.

“Our study suggests that the flighted ancestors of Ratites appear to have been ground-feeding birds that ran well. So the extinction of the dinosaurs likely lifted predation pressures that had previously selected for flight and its necessary constraint, small size. Lifting of this pressure and more abundant foraging opportunities would then have selected for larger size and consequent loss of flight.” Dr Phillips explains.

“Not only have we shown that the separate ratite lineages evolved too recently to have been on Gondwana before its continents drifted apart, but from our analyses we infer that at least ostriches, and possibly ratites as a whole, have their origins in the northern continents.”

But how do you go about establishing the lineage of a group of birds some of which have been extinct for thousands of years? DNA is not a particularly durable molecule, so tissue fragments from animals that are long dead don’t always provide quality data. However in addition to the main DNA concentration in the nucleus of each cell, there is also a small amount of what’s known as mtDNA in the mitochondria.

Mitochondria are small organelles within each cell that facilitate the conversion of food into energy. It’s possible that in the very distant past they even existed as separate micro-organisms that became incorporated into host cells, hence retaining a little of their own specific DNA.

Mitochondrial DNA is often a better choice for scientists interested in extinct species because there are generally many mitochondria in each cell, meaning many more copies of the mitochondrial DNA exist. The relative simplicity of the mtDNA code is also an advantage in establishing family lineage.

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