Large Flightless Birds on Isolated Continents and Islands
are Nevertheless Related.
Why? Here is a Different Answer.

Richard Bruce PhC Economics University of California
former instructor St. John's University

Scientists have discovered that many species of flightless birds of the infraclass Palaeogathae lost their ability to fly independently of one another. The ancestors of the ostrich evolved into large flightless birds in Africa, the ancestors of the emu and cassowary did the same in Australia, and so forth.

These flightless birds are traditionally known as ratites. The ratites are simply the flightless members of the infraclass Palaeogathae. Palaeognathae includes the ratites: the African ostriches, the South American rheas, the Australian emu and cassowaries, the New Zealand kiwis and extinct moas, and the extinct elephant birds of Madagascar. Also in Palaeogathae are the various South American species of tinamous that do fly, but usually only for very short distances.

Previously it had been believed that all these birds but the various species of tinamous had evolved from a flightless bird on the southern continent of Gondwana. When Gondwana broke up forming Africa, South America, Australia, Madagascar, New Zealand, and other land masses the different flightless birds evolved into their present species and the recently extinct moas and elephant birds. Both the moas and elephant birds were driven into extinction by humans in the past few thousand years.

New Research

Recent research published in the Journal Science by Kieren J. Mitchell, Bastien Llamas, Julien Soubrier, Nicolas J. Rawlence1, Trevor H. Worthy, Jamie Wood, Michael S. Y. Lee1, and Alan Cooper argues that the ancestors of these flightless birds flew into their separate land masses after Gondwana broke up and independently evolved into large flightless forms.

Not everyone will be able to read more than the abstract on the Science site. I believe unpaid access is limited to members of the AAAS, American Association for the Advancement of Science. For the intelligent layman you can read an article posted by NPR, National Public Radio here.

Why did this happen? A Just So Story

As mentioned above, all these flightless birds and the several species of tinamous are the surviving members of the infraclass Palaeognathae. These species have since the days of Darwin been recognized as a group that split off early from the other birds. They have features that are more reptile-like than the rest of the birds.

All other birds are in Neognaths. So all the other birds are more closely related to each other than they are to the species in the infraclass Palaeognathae. For example, the hummingbird, robin, chicken, hawk, parrot, and penguin are all more closely related to one another than they are to the ostrich.

Close Relatives Can Infect You

Why is this important? The more closely related two species are the more communicable diseases they tend to have in common. For example, one of our two closest relatives, the chimpanzee, shares every communicable disease with us, and the other great apes share many. The great apes other than humans are in danger of extinction because the huge human population is fertile ground for new diseases to evolve and those diseases usually infect the great apes also.

Similarly, the closest relatives of the white-tailed deer in North America, the larger deer, for example, the elk and moose, find it difficult or impossible to survive in the white-tailed deer's range because it carries diseases that kill those larger deer, but not the white-tailed deer. (Note to Europeans, what Americans call a moose, you call an elk, and what Americans call an elk is closely related to the red deer.)

Relatives Particularly Dangerous When Numerous

So having close relatives in your environment can be a bad thing, especially when they are numerous. A large population will tend to have more diseases than a small population because it provides more food for the pathogens to live on. For example, humans are a food for the bacteria that causes strep throat.

But the large population has more individuals, any one of which might through mutation gain an immunity that allowed the animal to live with the pathogen. This animal will have more descendants and eventually the whole population will enjoy the immunity. The closely related species with a smaller population suffers from an increasing variety of pathogens but does not have the advantage of more individuals to evolve an immunity. This species will frequently be driven into extinction within the range of their more numerous relative. As mentioned above elk and moose have difficulty surviving in the range of the smaller and more numerous white-tailed deer.

The Danger of Immune Carriers

Survival of the larger species is particularly difficult when the smaller species carries the disease without being seriously harmed by it. In this case, the pathogen can become a symbiote. The animal that is immune but carries the disease gains the advantage that the disease kills competitors for its food supply, and perhaps even predators. The immune carrier can become essentially poisonous to predators.

Opportunity For the More Distantly Related

This creates opportunities for groups that are not so closely related to the immune carrier and therefore less likely to share the same diseases. The members of Palaeoganathae, ostriches, emus, etc. are by definition not susceptible to diseases that are specific to Neognaths. As this is the case they have an advantage in filling those niches that will support relatively few individuals, for example, large birds. If the birds are large enough they will not be able to fly, so this means that the primitive and therefore rare Palaeoganathae have an advantage over Neognaths in filling the large, flightless bird niche.

The Advantage of Losing Flight

This advantage became even greater when the declining Palaeoganathae group no longer included long distance flying birds that could reach the isolated land masses: Africa, South America, Australia, Madagascar, and New Zealand. When this happened the birds of the Palaeoganathae group on a small land mass were cut off from any Palaeoganathae specific diseases circulating among Palaeoganathae populations in the larger world. If a disease evolved among the African Ostriches that was specific to the Palaeoganathae, it had no way of reaching the Australian emus and cassowaries, the Madagascar elephant birds, the South American rheas and titamous, or the New Zealand moas and kiwis. The various species of titamous do fly, but only for short distances, so it can not carry Palaeoganathae specific diseases out of South America.

So the Palaeoganathae independently took over and monopolized the large flightless bird niche in Africa, South America, Australia, Madagascar, and New Zealand, because their competition the Neognaths were constantly flying in with new plagues that had evolved around the world. Some of those plagues infected and killed both Palaeoganathae and Neognaths, giving little advantage to either group, but others only infected and killed the Neognaths. This was because the birds that carried the diseases were Neognaths. No species of Palaeoganathae was capable of flying into the isolated land masses.

Large Species, Few Relatives

Isolation from relatives is particularly important for physically large species. Small species frequently have far larger populations, and shorter life cycles, so they can develop immunities to disease more quickly. Many physically large species are living fossils, the last of their kind, at least within their range. There are many examples of this phenomena.

For example, the largest trees Sequoias and coastal redwoods until recently had no relatives in their natural range with a common ancestor more recent than the Mesozoic. Like the ratites, they are part of an ancient lineage that had largely died out. In the ocean, the largest fish are the sharks, which are living fossils. In South America, the largest exclusively freshwater fish is another living fossil, the arapaima. In North America, the largest exclusively freshwater fish is still another living fossil, the alligator gar. Jumping from fish to spiders, the tarantulas, the largest spiders, are the second earliest branch off the spider family tree. This is but a small sampling of the examples that can be drawn from our present flora and fauna and natural history. Similarly, the largest birds the ratites have relatively few close relatives in their environment compared to the Neogaths.

A Similar Story, Marsupials and Placentals in Australia

The history of the placentals and marsupials in Australia appears to be similar to the story of Neognath and Palaeoganathae in Africa, Madagascar, Australia, New Zealand, and Australia.

The story of the how the marsupials took Australia from the placentals is counter-intuitive if you use traditional reasoning. Marsupials are part of a larger group, metatherians, that may have evolved in Asia. From there it is believed that the spread to North America where the first Marsupials evolved. The marsupials and other metatherians went extinct in Asia and North America, but they spread to South America where they survived. From South America, they made it through Antarctica to Australia.

It appears that a single species of marsupial rafted into Australia after it had already split with Antarctica. This means that one animal, a pregnant female, or several animals rode in on floating plant material. When this tiny marsupial reached Australia it had to compete with the placental mammals that were already there. As it was thought that placentals were out competing marsupials and driving them into extinction on other continents it was a real surprise to realize that in Australia the marsupials did not just survive, they drove the placentals into extinction. This looks like a real David and Goliath story, a triumph of the underdog. The above is mostly a summary of an article on the evolution of marsupials in Wikipedia.

Using the same reasoning that was used above we can make sense of this strange history.

Bats are placentals. Bats might have flown between Antarctica and Australia. It was the Eocene, which was much warmer than today, so Antartica might have had bats. The marsupials never evolved a marsupial bat. So marsupial bats could not fly in marsupial-specific diseases.

So the Australian marsupials would only to deal with any marsupial-specific diseases that came with the original rafter or rafters. They probably did not have any serious ones or they would not have successfully made the trip. The marsupials would also have to deal with any marsupial diseases that evolved in Australia. Finally, they would have to handle diseases that were common to both placentals and marsupials, but those diseases would afflict Australian placentals as well.

The Australian placentals, on the other hand, would have to deal with the placental-specific diseases that evolved on all the other continents. Today, the world as a whole has twenty times the land area of Australia. If the ratio between the world and Australia in the Eocene was roughly the same as it is today then placental-specific diseases would have twenty times the land area in which to evolve as the Australian placentals.

So the Australian placentals would face both the challenge of marsupial competition and the placental-specific diseases that evolved throughout the world and were transported to Australia by bats. The combination may have been too much for them which explains why the marsupial David drove the placental Goliath into extinction in Australia.

Superior and Inferior Clades

Now I know I have violated one of the rules of biology by assuming that there was a superior group, the placentals, and an inferior group the marsupials. They must both have been equally fit, as they both survived.

Part of the point I am making is that this rule is wrong. Successful groups suffer a disadvantage as a result of their success. As was explained above, as a group becomes numerous it is likely to have more communicable diseases. This makes it difficult for species within the clade to hold on to niches that involve relatively few individuals. Frequently an older, inferior, less fit clade will hold on to those niches. This is a major reason why there are so many "living fossils," and so many of the "living fossils" are large and/or predatory.

Biologists were confused about the evolution of ratites in part because they rarely consider communicable diseases as a factor. Often this is because they do not have empirical evidence on diseases. As an armchair theorist, I am used to doing without evidence. I simply use the well-known and well-established fact that more closely related two species are together the more diseases they tend to have in common. There are mountains of empirical evidence for this. As mentioned above we do not have to go farther than our closest relatives, the chimpanzees, to demonstrate the point. Using this point many of the mysteries of biology, especially those related to large organisms can be easily solved. By using this analysis based on communicable diseases we can understand the evolution of ratites and gain an important tool for understanding many other organisms.

Beyond this when one clad has species that fly, while a competing clad has no flying species, the clad without flying species may have a huge advantage on islands and isolated land masses. I have many other web pages that deal with the more general issue, this is the first that deals with the issue of flying. You can find the web pages on my biology index page.

Biology Index Page