New research has shattered the idea that New Zealand’s iconic kiwi bird is a close relative of Australia’s emu. Instead, the kiwi has a closer connection with a giant flightless bird that was the stuff of legends.
It’s more than 20 years since one of us (Alan Cooper) published research that has long caused deep national shame for New Zealand. It showed that the kiwi – the national bird of New Zealand – was merely an Australian import.
But two decades on, DNA extracted from bones of a mysterious extinct giant from the other side of the world has finally set the record straight – and solved an evolutionary mystery that has lasted for 150 years. The results are published today in Science Magazine.
The ratite birds
The ratite birds are an instantly recognisable group – comprising the kiwi, emu, cassowary, ostrich and rhea. They are all flightless and comparative giants (apart from the kiwi) of the bird world.
Two ratite groups that were recently hunted to extinction were even more gigantic: the New Zealand moa and the Madagascan elephant birds. Elephant birds in particular are among the largest birds ever, growing up to 3m tall and at least 275kg – one estimate was even a whopping 438kg.
Their enormous size is believed to be the inspiration for the Arabic mythological rukh of Sinbad fame: a giant eagle-like bird capable of carrying off an elephant in its talons (hence the name).
Ratite birds have not only captured the popular imagination, but have mystified evolutionary scientists since they were first encountered, as in many ways they appeared to be more like mammals than traditional birds.
Ratites are distinguished from all other birds on the basis some primitive skull features, earning the name palaeognathous or “old jawed”. Their primordial form, and presence only on the southern continents, quickly led to the idea that they were an ancient form of bird that survived only in the specialised role of giant herbivore.
Migration with the continents
When the concept of continental drift gained widespread recognition in the late 1960s, the flightless ratite birds quickly became a poster-child for the theory that animals and plants isolated by this movement would have “speciated vicariously”.
The idea was that ancient ratites were present over all of Gondwana, the Cretaceous supercontinent of the southern hemisphere. As this landmass slowly broke apart between 130-50 million years ago, various groups of ratites became isolated and evolved in different directions as the continents crept to their current positions.
The evolutionary tree of ratites initially seemed to support this idea. The ostrich was identified as the most divergent species, consistent with the very early separation of Africa from the other Gondwanan continents around 130 million years ago.
The first DNA from the extinct ratites
The first study of the extinct ratites (performed as part of Cooper’s PhD) was on the giant moa.
The moa DNA held a big surprise. It was not closely related to the kiwi, as would be expected if both had evolved from some ancestral ratite isolated on New Zealand when it separated from what remained of Gondwana around 80-60 million years ago.
Instead, the kiwi was closely related to the Australian emu and cassowary suggesting – shock, horror! – it had originated across the Tasman and dispersed to NZ.
Sensing that a career overseas might be a wise political move, Cooper spent the following two decades studying extinct species around the world.
But he was always delving into museum collections when the opportunity arose to search for bones of the remaining extinct ratite – the Madagascan elephant bird – in the hope that this would one day solve the puzzle.
The long search for elephant bird DNA
An expedition to Madagascar in 1996 failed to find suitable samples. The team arrived in the capital Antananarivo six months after the Queen’s Palace, the largest wooden structure in the southern hemisphere, burned to the ground. This was also the place where the museum collections were stored.
But recent advances in genomic technologies have made it worthwhile to revisit DNA extracts generated in the early 1990s from the Museum of New Zealand as part of Cooper’s PhD research.
When we analysed the DNA from the bones of two different elephant bird species we were surprised to find that the closest genetic relative was not the neighbouring ostrich from Africa, but instead the kiwi – from the other side of the world.
The odd couple
It’s hard to imagine a more unlikely candidate, from either a geographical or morphological perspective!
Elephant birds were herbivorous, diurnal (awake in the daytime) and enormous, while kiwis are bizarrely specialised omnivores, nocturnal (awake at night) and only about the size of a big chicken.
If anything, elephant birds more closely resemble the other New Zealand ratite, the extinct moa, but the results of our analyses are unequivocal. Kiwi and elephant birds are rather close cousins (separating around 50 to 60 million years ago).
By this stage Gondwana was well and truly fragmented and there is no way elephant birds and kiwis could have been connected via a land route. Instead, it is clear their ancestors must have flown, either between Madagascar and New Zealand, or more likely from somewhere else to both.
That somewhere else was likely Antarctica, the landmass at the centre of the southern continents, and a place with temperate rainforests until around 20 million years ago when its isolation via the circumpolar current caused it to start cooling into frozen tundra.
The elephant bird DNA showed us with great clarity that flighted dispersal, not continental movement, was behind the current distribution of the ratites.
The key role of dinosaurs in ratite evolution
The elephant bird DNA showed us with great clarity that flighted dispersal, not continental movement, was behind the current distribution of the ratites.
We were also able to use molecular clock studies of the elephant bird DNA to estimate when the ratites had started to disperse around the world. This was the big clue – the date was immediately after the extinction of the dinosaurs 65 million years ago.
The mass-extinction at that point created major ecological opportunities for large herbivores, as mammals didn’t really become large and diurnal for around 10 million years after the dinosaurs had left the stage.
The small flying ratites were perfectly positioned to exploit this opportunity, and indeed numerous enigmatic palaeognathous fossils appear in the fossil record around this period in Europe, North and South America. They give us a great idea about what the ancestral flying ratite looked like.
But once mammals became larger and started taking over the earth, the window of opportunity was gone. No bird could try to become big and flightless again without being eaten.
The exceptions of course, were on mammal-free islands, as shown by the dodo on Mauritius. This hypothesis neatly explains how flightlessness and gigantism arose at least five times convergently within ratites: as the flighted ancestors arrived and adapted to the new environments, similar evolutionary pressures resulted in near-identical body-plans.
This is one of the reasons the ratites have confounded evolutionary researchers for so long.