University of Michigan scientists have used artificial intelligence to uncover a striking pattern in the evolution of Passeriformes, the vast bird group that includes most songbirds. Their analysis suggests these birds did not evolve at a steady pace. Instead, major changes often arrived in rapid bursts that lined up with shifts in Earth’s climate.
Evolutionary theory has long proposed that life diversifies through periods of fast change followed by slower phases. Fossils have offered clues that this pattern exists, but the new study identified it by analyzing skeletal measurements from modern bird specimens. The researchers also found that several of the most important evolutionary bursts occurred around periods of major climate change.
Evolution in Sudden Bursts
“This is really important for evolutionary theory because there’s a long history, going back 100 years, that predicts the emergence of new groups, called evolutionary radiations, is often associated with an explosive burst of diversification. Evolutionary theory predicts that adaptive radiations may account for a large portion of the diversity of life on Earth,” said Jake Berv, lead author of the study and postdoctoral fellow in the U-M School for Environment and Sustainability.
“This could be because of a new ecological opportunity, or it could be because a group dispersed to a new continent, resulting in dramatic accelerations in their rate of evolution. The idea is that, over time, there’s less opportunity as evolution proceeds, and so it slows down, and that this occurs in pulses across time. That’s what theory predicts, and that seems to be what we see in the data as well.”
The results were produced with artificial intelligence and a large statistical model. The findings are published in Nature Ecology & Evolution and were primarily supported by Schmidt Sciences and the David and Lucile Packard Foundation.
AI Measures Thousands of Bird Skeletons
To reconstruct the evolutionary history of passerines, the University of Michigan team, including senior author Brian Weeks, studied more than 2,000 species and assembled over 170,000 individual skeletal measurements.
Collecting data at that scale was possible because of Skelevision, an AI tool developed by Weeks’ laboratory in collaboration with David Fouhey’s laboratory at New York University.
Skelevision photographs specimens, which in this case were bird skeletons, in front of a grid that provides a consistent measurement scale. During a seven-year collaboration, Weeks and Fouhey created an AI model capable of accurately measuring 12 bones across a bird’s skeleton.
The researchers used the system to scan and measure more than 15,000 museum specimens. Most came from the collections of the U-M Museum of Zoology. Each specimen can be scanned in about 45 seconds, allowing researchers to digitize entire museum collections far more efficiently than would otherwise be possible.
Reconstructing 45 Million Years of Change
Berv also created a new statistical method called bifrost, which allowed the team to analyze each species’ complete skeleton rather than examining individual bones in isolation. Using this approach, the researchers estimated how passerine body shapes changed over roughly 45 million years of evolution.
“The whole organism is an integrated, complex morphology, and each of the individual pieces is interrelated to every other part in the body,” Berv said. “The question from the model’s perspective is, ‘What is the sequence of evolutionary changes that needs to happen to explain the variation we can see today?'”
The analysis revealed a period of especially rapid body-shape evolution around 35 million years ago. This burst coincided with the Eocene-Oligocene transition, a period marked by intense global cooling.
The statistical results also identified a cluster of evolutionary slowdowns around 15 million years ago, which coincided with another major geological event.
“Our findings have definitely shifted my thinking about how the world works,” said Weeks, associate professor of ecosystem science and management at U-M’s School for Environment and Sustainability. “This pattern we found with rare, big increases in the rates of evolution and lots of small decreases in the rate of evolution is really consistent with a pattern where lineages are exploring new ecological space and changing rapidly to take advantage of that opportunity.”
Climate and Geography Shape Evolution
The researchers then tested the pattern by analyzing the global distribution of the birds in their dataset. They found that geography also helps predict the average rate of morphological evolution.
Bird communities at more extreme latitudes, where seasonal temperatures vary more sharply, tend to include species that evolve faster than those living closer to the equator. Because similar patterns appeared both across millions of years and across modern geographic regions, the findings suggest environmental variability may play an important role in driving changes in body shape.
“It looks like there’s a connection between latitudinal gradients and rates of morphological evolution that has been underappreciated,” Weeks said. “I hope our findings will inspire a new integration of rates of morphological change into other big areas of interest, things like the very well-known latitudinal gradients in biodiversity.”
Museum Collections Gain New Value Through AI
The work also highlights the scientific importance of museum collections. According to Weeks, artificial intelligence is making it possible to extract information from preserved specimens at a scale that would have been difficult to imagine in the past.
“It’s especially clear how important it is to invest in museums when you think about the scale of an analysis like this; it’s so far beyond the scope of what can be done using specimens contributed by an individual collector,” he said. “It’s also fun to imagine what early collectors would make of how we’re using the specimens they collected — I imagine it would blow their minds to learn that a computer has analyzed a photograph of these specimens. It’s just another example of how impossible it is to foresee the full future value of a specimen.”
Lessons for Modern Climate Change
The researchers say the findings may also help scientists think about how species could respond to the rapid climate changes now unfolding around the world.
“Right now, we’re in this moment in human history where there’s dramatic global climate change, and we don’t know what’s going to happen over even a 10-year period, let alone over a 10-million-year period,” Berv said. “To have a chance of understanding the long-term impact of human activity on Earth, we have to study the relationship between events in Earth’s history and evolutionary transitions.”
The research was also supported by the Michigan Institute for Data & AI in Society, the Natural Sciences and Engineering Research Council of Canada, and the National Science Foundation.