Bird Genomes Reveal a Clearer Bird Family Tree

Published in Nature today, the largest study yet of bird genomes offers an unusually sharp view of the avian family tree. Our research indicates that most modern bird groups arose during the 5 million years following the dinosaurs’ extinction.

Birds are an important presence in our everyday lives, providing a reminder of nature even in urban areas. Although they are popular with the public and extensively researched by scientists, fitting them all into a single family tree has proved exceptionally challenging.

Our analysis of the genomes of more than 360 bird species has uncovered the basic relationships between the principal groups of living birds.

While the new family tree challenges several earlier assumptions about bird relationships, it also identifies previously unrecognised groupings.

Untangling complex bird relationships

Earlier research established that the bird family tree has three main branches. The first includes tinamous and ratites, encompassing flightless birds such as emus, kiwis and ostriches.

The second branch contains landfowl and waterfowl, including chickens, ducks and related birds. Every other bird belongs to the third branch, called the Neoaves, which accounts for 95% of bird species.

Within the Neoaves are ten bird groups. Biologists refer to most of them as the "Magnificent Seven": landbirds, waterbirds, tropicbirds, cuckoos, nightjars, doves and flamingos. The remaining three, called the "orphans", comprise shorebirds, cranes and the hoatzin, a South American species.

Establishing the relationships between these ten groups, and particularly among the orphans, has been extraordinarily difficult. Our genome research suggests that a solution is now attainable.

Introducing the 'Elementaves'

The genome analysis identified a new bird grouping, which we have called "Elementaves". Its name draws on the four ancient elements - earth, air, water and fire - and the group contains birds adapted to thrive on land, in the air and in water.

Several of these birds have names connected with the sun, representing fire. Elementaves includes hummingbirds, shorebirds, cranes, penguins and pelicans.

Our research also verifies a close connection between two of Australia’s best-known bird groups: passerines, comprising songbirds and their relatives, and parrots. These much-loved birds feature prominently in Australian Bird of the Year polls.

Songbirds represent nearly 50% of all bird species, including magpies, finches, honeyeaters and fairy-wrens. Originating modestly in Australia around 50 million years ago, they later spread worldwide and became the most successful bird group.

When did modern birds emerge?

Another aim of the study was to add a timescale to the bird family tree. We achieved this by modelling genome evolution with a technique called the "molecular clock". Information from almost 200 fossils allowed us to limit the possible ages of certain branches on the tree.

The research indicates that every living bird shares an ancestor that lived slightly more than 90 million years ago. Most modern bird groups, however, appeared roughly 25 million years later, in a narrow interval of only a few million years after the Cretaceous period ended about 66 million years ago.

This timing matches the mass extinction of dinosaurs and other organisms after an asteroid struck Earth. Birds therefore seem to have taken advantage of the opportunities created when these previously dominant forms of life disappeared.

One unanswered question remains

The genome study is the result of almost ten years of work undertaken through the Bird 10,000 Genomes Project. Its eventual aim is to sequence the genomes of all 10,000 living bird species.

This stage of the project sought to include species representing every major bird group, or family. Examining these 363 genomes involved a genuinely international collaboration, led by researchers from the University of Copenhagen, the University of California San Diego and Zhejiang University in China.

Despite this vast quantity of genome information, one section of the bird family tree is still unresolved. Our analysis was unable to establish with confidence the relationships of the hoatzin, one of the orphans. Native to South America, the hoatzin is a very distinctive bird and the only surviving member of its lineage.

The findings show that resolving some relationships in the tree of life requires very large amounts of genome data. They also highlight the value of combining genome and fossil evidence to understand the evolutionary history of life on Earth.

Jacqueline Nguyen, Scientific Officer in Ornithology, Australian Museum, and ARC DECRA Fellow, Flinders University and Simon Ho, Professor of Molecular Evolution, School of Life and Environmental Sciences, University of Sydney

This article is republished from The Conversation under a Creative Commons licence. Read the original article.