The largest birds that ever lived – the now extinct elephant birds – looked like oversized ostriches. They were fast moving and unable to fly, just like ostriches. And their home island of Madagascar was not far from mainland Africa, where ostriches live.
If you had to invest money in the identity of the elephant bird’s closest living relative, the ostrich would be a sure bet. It would also be spectacularly wrong.
Elephant birds have been extinct for centuries, but many of their skeletons can be found in museums around the world. By extracting DNA from these specimens and comparing it to DNA from living birds, Kieren Mitchell of the Australian Center for Ancient DNA discovered that the elephant bird’s closest living relative … is the kiwi.
“We were completely blown away,” said Alan Cooper, who led the study. “You have to work hard to get a more different pair.”
He’s not exaggerating. Elephant birdsprobably picked fruit from trees while kiwis root in leaf litter for maggots and worms. Elephant birdslived in Madagascar, about 7,000 miles from the home of the kiwi in New Zealand. The largest elephant birdsgreat up to 3 meters tall and up to 275 kilograms; Kiwis would bump against your shins, and smaller species could fit inside the giant egg of an elephant bird.
“Geographically, it didn’t make sense. Morphologically it made no sense. That made no ecological sense, ”says Cooper. “We tested it pretty thoroughly because we were so surprised, but there is no doubt about the genetic data.”
All the scientists I contacted agreed that the result is surprising. But it actually fits in with a new story about the origins of ostriches, kiwis and their relatives, which has become more and more popular in recent decades.
The elephant bird and the kiwi belong to a group of birds called ratites. These include the ostrich from Africa, the rhea from South America, the emu and cassowary from Australia, and the extinct moas from New Zealand.
Aside from kiwi fruit, these species are all large and flightless. Many scientists (reasonably) assumed that this arose from a common ancestor who was already large and flightless itself. This ancestral ratite probably lived at a time when all of the southern continents were merging into a single landmass called Gondwana and disintegrating into separate forms when the supercontinent broke up.
This “rafting” story seems intuitive, but it has crumbled in the face of genetic evidence.
When scientists compared the DNA of ratites, they found that geographic neighbors are not necessarily evolutionary neighbors. The moas and kiwis, for example, both come from New Zealand. But when Cooper sequenced moa’s DNA in 1992, he found that kiwis are closer to Australian emus and casuanas than to their neighboring islands. These birds emerged after Australia and New Zealand separated. So if they all evolved from an already flightless ancestor, the kiwis must have flown over a vast Pacific somehow.
The ratites are also not all flightless. Genetic studies showed that a group of flying South American birds called Tinamous belong to the group of ratites. Even stranger, the tiny partridge-like Tinamous’ closest relatives are the giant, towering moas – a fact confirmed by Allan Baker of the Royal Ontario Museum earlier this month.
Cooper’s discovery reflects the Moa-Tinamou relationship. Two groups of giant birds (moas and elephant birds) are more related to small, chicken-sized (tinamus and kiwis) from the other end of the world than to similarly sized neighbors (ostrich and rhea). Physique and geography have proven time and again to be poor guides to the evolution of ratites.
There is only one plausible explanation: ratites evolved from small, flying birds that lived between continents and independent lost flight ability on at least six different occasions.
The rafting hypothesis is dead, and the kiwi elephant bird is the “last nail in the coffin,” says Michael Bunce of Curtin University, who studies ancient DNA. “A number of textbooks need to be rewritten.”
In his 2004 book The story of the ancestors, writes Richard Dawkins, “I am delighted with the power of natural selection, and I would have had satisfaction to report that ratites have developed their incapacities separately in different parts of the world … Unfortunately, it has not.” Cheer up , Richard. It is so.
The ratites are an incredible example of convergent evolution – the process by which living things appear in the same clothes at the party of life. “They all started out as those little, winged, partridge-like things, and most of them went into those big, giant shapes that were so close together that everyone thought they must have started that way,” says Cooper.
Cooper believes that the rise of the ratites came soon after the dinosaurs became extinct. Their absence created an ecological vacuum – there were many plants and no large animals to eat. The ratites filled these niches. Again and again they developed into large herbivores and lost their ability to fly.
After about 10 million years, mammals began to do the same, and their success prevented other birds from following in the footsteps of ratites. “The time window was over,” says Cooper. “After this point in time, no bird could try to become large and unable to fly again, or it would be eaten. The ratites survived by running like hell. “
This idea also explains why the Kiwis and Tinamous have stayed small. Cooper thinks they’ve diversified in places that already had large flightless ratites – the moas and rheas. “These guys showed up after someone else took the large and flightless niche and forced them to do something alternate,” he says. “The kiwis became small, nocturnal insectivores. The Tinamous flew on. “
The discovery raises other puzzles about the evolution of the kiwi. Some scientists believed that kiwifruit lay disproportionately large eggs because they evolved from a much larger ancestor. “This new paper suggests that kiwifruit have always been small, suggesting that kiwifruit egg size grew independently large even when body size didn’t,” says Rebecca Kimball of the University of Florida. “That could stimulate new and interesting research.”
If there’s a rift in this story, it’s because Cooper’s finding is based on mitochondrial genomes – tiny, secondary sets of DNA in our cells. In the past, scientists had to revise conclusions based on mitochondrial DNA after getting their hands on the main genome of the nucleus. “The mitochondria only give us part of the picture,” says Bunce. “The next challenge is to look into the nuclear genome.”
“We’re working on it,” says Cooper. It is not easy. Elephant birds tend to die in hot, humid, swampy conditions, which are horrific for DNA preservation. Cooper’s team struggled for years to get enough material for sequencing before it finally recovered enough of the specimens kept in a New Zealand museum. Sequencing the nuclear genome of these titans will be even more difficult. Cooper adds, “The core DNA of the other ratites, including the moa, confirms what we see of the mitochondria, so we don’t expect too many surprises.”
Relation: Mitchell, Lamas, Soubrier, Rawlence, Worthy, Wood, Lee & Cooper. 2014. Old DNA shows that elephant birds and kiwi are sister taxa and clarifies the evolution of ratites. Science http://dx.doi.org/10.1126/science.1251981