WASHINGTON, Dec. 11 -- An international team led by Chinese researchers has sequenced the genomes of 48 species of birds to create the most reliable avian tree of life to date.

This massive project, which took more than four years to complete and involved hundreds of researchers from 20 different countries around the world, analyzed at least one genome from every major bird lineage, including the woodpecker, owl, penguin, hummingbird and flamingo, and produced dozens of reports, eight of which are published Thursday in Science.

The results "have enabled us to answer numerous fundamental questions to an unprecedented scale," said co-lead author Guojie Zhang of the National Genebank at BGI in China and the University of Copenhagen.

"This is the largest whole genomic study across a single vertebrate class to date. The success of this project can only be achieved with the excellent collaboration of all the consortium members," Zhang said.

The findings supported a "big bang" theory for the evolutionary expansion of birds during the 10 to 15 million years that followed a mass extinction event about 66 million years ago that killed off all dinosaurs except some birds.

This contradicted the idea that birds blossomed 10 to 80 million years earlier before the mass extinction event, as some recent studies suggested.

"Birds are dinosaurs," said co-author Ed Braun, associate professor of biology at the University of Florida. "They're the one lineage of dinosaurs that made it through the mass extinction at the end of the so-called dinosaurs."

Based on this new genomic data, a few bird lineages that survived the mass extinction gave rise to more than 10,000 species that comprise 95 percent of all bird species living with us today, the researchers said.

The researchers also found that birds lost thousands of genes in their early evolution after birds split from other reptiles, many of which have essential functions in humans, such as in reproduction, skeleton formation and lung systems.

"This is an exciting finding, because it is quite different from what people normally think, which is that innovation is normally created by new genetic material, not the loss of it," Zhang said.

This new tree supported three independent origins of waterbirds. It also indicated that the common ancestor of core landbirds, which include songbirds, parrots, woodpeckers, owls, eagles and falcons, was an apex predator, which also gave rise to the giant terror birds that once roamed the Americas.

Birdsong was found to evolve independently at least twice. Parrots and songbirds gained the ability to learn and mimic sounds independently of hummingbirds, despite sharing many of the same genes. This is surprising because animals with similar characteristics usually share a common ancestor, the researchers said.

In general, brain circuits for musical and vocal learning in birds and humans are similar but have been arrived at via different evolutionary paths.

The researchers also found five genes related to teeth formation were switched off some 116 million years ago, in an ancestor of today's birds, causing birds to lose their teeth since then.

Their research also showed the chicken has the most similar genome to an avian ancestor, which was thought to be a feathered dinosaur.

Crocodiles were be birds' closest living relatives, with a common ancestor that lived around 240 million years ago, said the research, which also showed that crocodiles have one of the slowest-evolving genomes, whereas the pace of genetic change has been much faster in birds.

Colorful feathers are thought to be evolutionarily advantageous, giving a male bird in a given species an edge over his competitors when it comes to mating. This research found that genes involved in feather coloration evolved more quickly than other genes in eight of 46 bird lineages.

They also estimated that the today's penguins first appeared around 60 million years ago.

Overall, the genomic structure of birds has stayed remarkably the same among species for more than 100 million years and the evolution rate across all bird species is slower compared to mammals.