Chinese and US scientists have teamed up to achieve breakthrough findings that dispel myths and explain how today's avian species came to be, Yang Feiyue reports.

Earth is home to about 11,000 bird species, accounting for the greatest diversity among terrestrial vertebrates. Yet question marks have long loomed over their origins and evolution. How closely are they related to dinosaurs? How did climate change influence their evolution? And how did they diversify biologically over time? Significant recent breakthroughs are helping answer these questions, thanks to genomic research on terrestrial vertebrates led by Chinese scientists, who collaborated with their US counterparts to make strides in unraveling the mysteries surrounding the origins of modern birds.

"Using new approaches to mine genomic information among 124 species, covering most modern birds' diversity, we found that the main lineages first divided into two groups — one mostly land-based and the other containing waterbird species," says Wu Shaoyuan, a professor from Jiangsu Normal University in Xuzhou, who led the research team.

The waterbirds were found during the study to be of a new evolutionary lineage, named Aquaterraves, which include seabirds and their close relatives.

Aquaterraves, along with the previously identified land birds (Telluraves), constitute the two major lineages of Neoaves, a clade to which almost 95 percent of the roughly 10,000 known species of extant birds belong.

After four years of efforts, the team has also shown that modern birds date back further than previously assumed — much earlier than the dinosaurian extinction event, which seems to have had a limited impact on their evolution.

Instead, a global warming event around 55 million years ago appears to have triggered the diversification that produced today's seabirds.

The researchers extracted DNA-sequence data from 25,640 genetic loci, referred to fossil records from different geological periods, and reconstructed the evolutionary history and trajectory of modern birds through phylogenetic tree construction, molecular clock estimation and species diversity differentiation rate analyses.

The scientists estimated when the branches split into new lineages by comparing the mutations that accumulated along the branches. The older the split between two branches, the more mutations each lineage built up.

"Remarkably, these two major bird lineages diverged during the Late Cretaceous Period — long before the famous dinosaur extinction event," Wu says.

"Our study indicates that the radiation (divergence out from a central point) of modern birds was in remarkable lockstep with that of flowering plants and other organisms."

Traditional theories have long linked the evolutionary history of modern birds to the mass extinction event around 66 million years ago, suggesting that they evolved rapidly after the dinosaurs vanished.

However, the research indicates that the mass extinction event did not have a significant impact on the evolution of modern birds. Instead, the global warming event that occurred around 55 million years ago, known as the Paleocene-Eocene Thermal Maximum, led to the turnover of modern marine bird evolution, a process by which species in a particular ecological niche or geographic area are replaced by new species.

"It has thus challenged our traditional understanding that modern bird origins can actually be traced back to the dinosaur era," Wu says.

The findings were published on Feb 12 in the paper Genomes, Fossils, and the Concurrent Rise of Modern Birds and Flowering Plants in the Late Cretaceous, in the Proceedings of the National Academy of Sciences of the United States, a peer reviewed journal of the National Academy of Sciences.

"I imagine this will ruffle a few feathers," The New York Times quoted Scott Edwards, an evolutionary biologist at Harvard and one of the study's authors, as saying.

Scientists in various fields in both countries took part in the project out of their shared interest in birds, according to Wu.

The team included prominent researchers like Zhou Zhonghe from the Institute of Vertebrate Paleontology and Paleoanthropology affiliated with the Chinese Academy of Sciences, Edwards from Harvard and Liu Liang from the University of Georgia.

Wu had previously examined genomic data with US scientists to understand species' origins and relationships when he was a college student.

Similar studies have been conducted on mammals and angiosperm (flowering plants), both of which Wu says account for a big part of biodiversity in the Cenozoic, Earth's current geological era.

"Everyone played a crucial role in the project," Wu says. "The scientists from abroad came to China to engage in academic exchanges with us, and discussions were direct and sparkling."

Chinese paleontologists played a big role in fine-tuning the genetic estimates by examining the ages of 19 fossils.

"They have helped to convert the mutation rates seen in the genomes into specific times through fossil analyses," Wu explains.

If a branch appeared to be newer than a fossil that belonged to it, they would adjust the computer model that estimated the evolutionary pace.

Ultimately, biological data from the past was acquired and compared with data from modern times to trace the origin of birds, which Wu says will offer valuable information for humanity.

"The Earth has experienced major climate and environmental changes, which have affected biodiversity, and the data will offer us an insight and thus better prepare us for the future," Wu says. "As we are experiencing global warming, we need international cooperation to address the issue."

The study also revealed that living birds share a common ancestor that lived 130 million years ago. New branches of its family tree split off throughout the Cretaceous and afterward at a fairly steady pace, which Wu says was a result of the growing diversity of flowering plants and insects during the period.

These new discoveries have prompted scientists to reexamine bird evolution and pose a bold question about Earth's biological history: How exactly did birds embark on their evolutionary journey during the dinosaur era?

Experts believe the discoveries support Charles Darwin's classic theory of gradual species evolution and provide new evidence for the impact of global climate change on the unfolding of biodiversity.

"This research has led us to a new understanding of the timeline of bird evolution," says Zhou from the Institute of Vertebrate Paleontology and Paleoanthropology.

In contrast to the previously widely accepted model of rapid evolution, the evolution of modern birds appears to lean toward a slower and more continuous process, gradually changing through natural selection and showing synchronous trends with the evolutionary radiations of angiosperms, mammals, fish and insects, Zhou explains.

"This major discovery has inspired us to contemplate the evolution of life and Earth's history," he says.

"It not only deepens our understanding of the evolutionary history of birds and life on Earth but also enlightens us as to new considerations regarding the relationship between global climate change and the evolution of biodiversity," Zhou adds.

Wu says the new discoveries mark a first step in future studies.

"We will continue to collect and compare biological, climate and environmental data to develop a bigger picture and thus provide guidance and insight into the impact on biodiversity, and how to address challenges."

Contact the writer at yangfeiyue@chinadaily.com.cn