The largest birds that ever lived—the now-extinct elephant birds—looked a lot like super-sized ostriches. They were fast-running and flightless, just like ostriches are. And their island home of Madagascar was just a short distance from mainland Africa, where ostriches live.
If you had to put money on the identity of the elephant birds’ closest living relative, the ostrich would be a safe bet. It would also be a spectacularly wrong one.
Elephant birds have been extinct for centuries, but many of their skeletons reside in museums around the world. By extracting DNA from these specimens and comparing it to DNA from living birds, Kieren Mitchell from the Australian Centre for Ancient DNA has discovered that the elephant birds’ closest living relative is… the kiwi.
“That blew us away completely,” says Alan Cooper, who led the study. “You have to try pretty hard to get a more disparate pair.”
He’s not exaggerating. Elephant birds probably plucked fruit from trees, while kiwis rummage through leaf litter for grubs and worms. Elephant birds lived in Madagascar, around 7,000 miles away from the kiwi’s home in New Zealand. The biggest elephant birds great up to 3 metres tall and weighed up to 275 kilograms; kiwis would bump against your shins and smaller species could fit inside an elephant bird’s gargantuan egg.
“Geographically, it didn’t make any sense. Morphologically, it didn’t make sense. Ecologically, it didn’t make any sense,” says Cooper. “We tested it pretty exhaustively because we were so surprised, but there’s no doubt in the genetic data.”
All the scientists I contacted agreed that the result is surprising. But it actually fits with a new narrative about the origin of ostriches, kiwis and their kin, which has been gaining support over the last few decades.
The elephant bird and kiwi belong to a group of birds called the ratites. These include the ostrich from Africa, the rhea from South America, the emu and cassowary from Australia, and the extinct moas of New Zealand.
Kiwis aside, these species are all big and flightless. Many scientists (quite reasonably) assumed that evolved from a common ancestor that was itself already big and flightless. This ancestral ratite probably lived at a time when all the southern continents were fused into a single land mass called Gondwana, and diverged into separate forms when the super-continent broke apart.
This ‘rafting’ story seems intuitive but it has crumbled in the face of genetic evidence.
As scientists compared ratite DNA, they found that geographical neighbours aren’t necessarily evolutionary neighbours. The moas and kiwis, for example, both hail from New Zealand. But when Cooper sequenced moa DNA in 1992, he found that kiwis are closer to the Australian emus and cassowaries than to their island neighbours. These birds arose after Australia and New Zealand had split so if they all evolved from an already flightless ancestor, the kiwis must have somehow rafted over a huge stretch of Pacific.
The ratites aren’t all flightless either. Genetic studies revealed that a group of flying, South American birds called tinamous are part of the ratite group. Stranger still, the closest relatives of the small, partridge-like tinamous are the huge, towering moas—a fact that Allan Baker from the Royal Ontario Museum confirmed earlier this month.
Cooper’s discovery mirrors the moa-tinamou relationship. Two groups of giant birds (moas and elephant birds) are more closely related to small, chicken-sized ones (tinamous and kiwis) from the other side of the world, than to similarly large neighbours (ostrich and rhea). Time and again, physique and geography have proved to be poor guides to ratite evolution.
There’s only one plausible explanation: the ratites evolved from small, flying birds that flapped their way between continents and independently lost the ability to fly on at least six separate occasions.
The rafting hypothesis is dead, and the kiwi-elephant bird is the “final nail in the coffin”, says Michael Bunce from Curtin University, who studies ancient DNA. “A number of text-books need to be re-written.”
Indeed, in his 2004 book The Ancestor’s Tale, Richard Dawkins writes, “I take delight in the power of natural selection, and it would have given me satisfaction to report that the ratites evolved their flightlessness separately in different parts of the world… Alas, this is not so.” Cheer up, Richard. It is so.
The ratites are an incredible example of convergent evolution—the process where living things turn up to life’s party wearing the same clothes. “They all started as these small, flighted, partridge-like things and most of them became these large, giant forms that were so close that everyone assumed they must have started off like that,” says Cooper.
Cooper thinks that the rise of the ratites took place shortly after the extinction event that wiped out most of the dinosaurs. Their absence created an ecological vacuum—there were lots of plants around and no big animals to eat them. The ratites filled those niches. Time and again, they evolved into big plant-eaters, losing the ability to fly in the process.
After around 10 million years, the mammals started doing the same thing, and their success stopped other birds from following in the ratites’ footsteps. “The window of opportunity had gone,” says Cooper. “No bird after that point could try and become big and flightless again, or they’d get eaten. The ratites survived by running like hell.”
This idea also explains why the kiwis and tinamous stayed small. Cooper thinks that they diversified in places that already had large flightless ratites—the moas and rhea. “These guys turned up after someone else had taken up the big and flightless niche, forcing them to do something alternative,” he says. “The kiwis became small, nocturnal insectivore. The tinamous kept flying.”
The discovery raises other puzzles about kiwi evolution. Some scientists believed that kiwis lay disproportionately huge eggs because they evolved from a much larger ancestor. “This new paper proposes that kiwis have always been small, suggesting that egg size independently became large in kiwis even when body size did not,” says Rebecca Kimball from the University of Florida. “That may stimulate some new and interesting research.”
If there’s a crack in this story, it’s that Cooper’s result is based on mitochondrial genomes—small, secondary sets of DNA within our cells. In the past, scientists have had to revise conclusions based on mitochondrial DNA after getting their hands on the main nuclear genome. “The mitochondria only give us part of the picture,” says Bunce. “The next challenge is to peek into the nuclear genome.”
“We’re working on that,” says Cooper. It’s not easy. Elephant birds tend to die in hot, humid, swampy conditions, which are awful at preserving DNA. Cooper’s team struggled for years to get enough material to sequence, before finally recovering enough from specimens stored in a New Zealand museum. It’ll be even harder to sequence the nuclear genome of these titans. Cooper adds, “The nuclear DNA from the other ratites, including the moa, confirms what we see from the mitochondria, so we’re not expecting too many surprises.”
Reference: Mitchell, Llamas, Soubrier, Rawlence, Worthy, Wood, Lee & Cooper. 2014. Ancient DNA reveals elephant birds and kiwi are sister taxa and clarifies ratite bird evolution. Science http://dx.doi.org/10.1126/science.1251981