Tracing the origin of ratites helps unlock the secret of plate tectonics in the distributions of modern birds and mammals.
Fossil records are a useful tool to help decipher evolution. Unfortunately, the oldest kiwi fossil we have (a leg bone found on the coast near Bulls) is only one million years old. That is a mere blip in time compared with the 50- or 60-million year-old fossils we need to find.
Recent research into the kiwi’s DNA suggests the species are linked to the Australian emu and cassowary. This means kiwi developed outside New Zealand, after it split off from Gondwanaland 80 million years ago, and migrated here later. It is thought they arrived about 30 million years ago.
But just how that journey happened remains a mystery.
Three Possibilities
Three explanations have been put forward to explain the kiwi’s presence in New Zealand.
An Ancient Ancestor
This explanation suggests the kiwi ancestor was already around 60 million years ago when New Zealand broke away from Antarctica and Australia. If true, it removes the question of whether or not the kiwi could ever fly. It would also mean our national icon originated around the same time as the dinosaurs.
Walking to New Zealand
Islands rise up and submerge as tectonic plates move. A string of islands have come and gone between New Caledonia and Northland in New Zealand during the past 50 million years. And at some point since separating from Gondwanaland, New Zealand was connected to Australia. It is possible that the kiwi and other species moved from one island to the next as they rose and fell – using them like stepping-stones to reach New Zealand, or that today’s ancestors simply walked across when New Zealand and Australia were joined.
Least Likely Explanation
A flying kiwi is the least likely explanation. Of all today’s ratites, only the South American tinamous can fly - and it can't fly very well. If the original ratites in Gondwanaland could fly, it is too much of a coincidence that all but one has since lost the ability. It is more logical to assume that all ratites had a flightless ancestor – which makes the tinamous the exception, not the rule.
Another reason why kiwi probably never flew is the theory that the kiwi ancestor was much bigger than today's bird. The kiwi egg is so huge it should theoretically be laid by a bird two or three times bigger – closer to a cassowary in size. And that would have been much too big to fly across the Tasman Sea, even millions of years ago when the ditch was much narrower. |