The ancestors of ostriches and emus were long-distance fliers – here’s how we worked this out

Aside from being a delight to watch, flight in birds is regarded by many cultures as a symbol of freedom, and a source of inspiration for humans to build our own flying machines. This makes those birds that have given up flight for a land-based way of life seem all the more intriguing.

In our new study of a 56 million-year-old fossil bird, my colleagues and I show that the distant ancestors of ostriches and other large flightless birds once flew great distances.

Many flightless birds belong to Palaeognathae, a taxonomic group containing ostriches, rheas, emus, cassowaries and kiwi, as well as the tinamous of Central and South America.

Unlike their large flightless relatives, tinamous can fly – but not very far. Spending most of their lives on the ground, they tend to fly only if startled by a predator. If you have ever been on a walk and startled a grouse or pheasant, this type of flight, known scientifically as burst flight, will be familiar to you.

Because they are flightless (or can’t fly far), the fact that palaeognaths are found on many different continents – South America, Africa, Australia and New Zealand – has been difficult for scientists to explain.

When the theory of plate tectonics became widely accepted in the 1960s, an answer seemed within reach. All of the continents were once united as the supercontinent Pangea, which slowly broke apart during the time of the dinosaurs, starting to split around 200 million years ago. Scientists wondered whether different populations of flightless palaeognaths could have just drifted apart from each other along with the continents they lived on.

However, this once-popular theory has since been discredited for two reasons. One is that the flying tinamous are genetically closer to some flightless palaeognaths than they are to others. This means that ostriches, rheas, emus, cassowaries and kiwi did not share a flightless common ancestor. Instead, in a remarkable case of parallel evolution, they all became flightless separately from each other.

The second reason is that genetic research shows palaeognath lineages started to separate many millions of years after Pangea broke up – far too late for the continental drift theory to be true.

This means palaeognaths had to have made it to South America, Africa,
Australia and New Zealand under their own power. Only able to fly in short bursts, a tinamou doesn’t stand a chance of flying across an ocean – but what about palaeognaths from the distant past? Could the ancestors of today’s palaeognaths have made these long journeys?

The collections of the Smithsonian National Museum of Natural History in Washington DC include an almost perfectly preserved sternum, or breastbone, belonging to an ancient palaeognath called Lithornis promiscuus that lived 56 million years ago. It was a fairly large bird, about the size of a grey heron.

Other researchers had determined that the sternum is a key piece of the skeleton for determining the flight style of a bird, so this fossil was our best chance to determine what this ancient bird was capable of.

Using a technique called geometric morphometrics, we compared the shape of the Lithornis sternum to those of over 150 living bird species. Our results show that Lithornis was not a burst flier like today’s tinamous. Instead, its sternum is most similar in shape to birds that fly huge distances, such as egrets and herons. This means means that, unlike their living relatives, Lithornis and other ancient palaeognaths would have been capable world travellers, able to establish new populations on different continents.

Why did these birds become flightless over and over again?

No matter how beautiful or inspiring we think flying is, it is also hard. If a bird species finds itself in a situation where it can get all of its food on the ground and doesn’t need to fly to escape predators, it will probably evolve towards being flightless.

Nowadays, these conditions are only met on islands, with the dodo being perhaps the most famous example. The dodo was a flightless bird that roamed Mauritius until it became extinct in the 1600s.

Dodos had no natural predators until humans arrived in the late 1500s (bringing with them other animals including rats). This meant dodos had not evolved a fear response, and there are records of them happily approaching humans.

Back when Lithornis and its relatives were alive, the world was very different. Just a few million years before, the dinosaurs had gone extinct. With no major predators around, birds were safe on the ground on continents as well as islands. And with a specialised bill tip organ as well as a keen sense of smell, Lithornis was well suited for probing for food in the soil, so it had no need to fly up into the trees to feed.

Therefore, ancient palaeognaths were set on a course towards flightlessness or low flight capacity wherever they went around the world. New mammalian predators evolved slowly, over millions of years, giving these flightless birds plenty of time to evolve new ways to escape and defend themselves.

After these long-distance flying ancestors went extinct, we were left with a puzzling distribution of these birds that could only be explained by the fossil record.