Using DNA extracted from 2000-year-old bones, scientists have discovered that an extinct giant eagle from New Zealand is most closely related to an Australian bird one-tenth its size. The new family tree suggests the bird's progenitor underwent an exceptionally large and rapid size increase once it reached its new island home.
The extinct Haast's eagle (Harpagornis moorei) was 30-40% heavier than the largest living eagle. With a wingspan of 2.5 to 3 meters and weighing in at 10 to 14 kilograms, it must have been a fearsome sight for New Zealand's first human settlers, some 700 years ago. Whether they attacked people is unknown, but evidence suggests the eagles made short work of 200 kilogram moas--extinct, emu-like herbivorous birds--dispatching them with a single strike to the neck or head.
Based on physical characteristics, New Zealand paleobiologist Richard Holdaway had previously suggested that Haast's eagle was most closely related to the Australian wedge-tailed eagle, a large predator at 4.5 kilograms. So when Holdaway, molecular biologist Mike Bunce--then at Oxford University in the United Kingdom--and colleagues reanalyzed the eagle group using mitochondrial DNA, they were surprised to find that Haast's eagle is more closely related to some of the world's smallest eagles: the little eagles of Australia and Papua New Guinea and the booted eagle of Eurasia, each of which weigh around a kilo or less. Their last common ancestor lived somewhere between 0.7 and 1.8 million years ago, the team concludes in the 4 January issue of the Public Library of Science Biology.
There are other examples of large and rapid size changes in animal evolution--some Galapagos marine iguanas living on different islands, for example, have a 10-fold size difference, notes Eric Palkovacs, an evolutionary biologist at Yale University in New Haven. But for birds, which cannot simply get larger by living longer, as lizards do, the transformation is remarkable, he adds.
Bunce speculates that Haast's eagle was able to become so big thanks to the absence of predatory mammals in prehistoric New Zealand. Unlike birds of prey in Africa, for example, which have to compete with hyenas and lions, Haast's eagle would have been allowed to dine in peace rather than have to fly off with its prey, he says.