For a century, evolutionary theory has predicted something that has been remarkably difficult to prove: that life doesn’t evolve at a smooth, steady pace. Instead, evolution moves through explosive bursts of rapid change, separated by long stretches of relative stability. University of Michigan researchers just confirmed this pattern with striking clarity — by turning artificial intelligence loose on more than 15,000 museum bird skeletons.
Published in Nature Ecology & Evolution, the study focused on Passeriformes, the enormous bird group that includes most of the world’s songbirds — and revealed that their evolutionary history unfolded in sudden sprints, many of which aligned precisely with major shifts in Earth’s climate.
A Century-Old Theory Finally Tested At Scale
“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,” explained Jake Berv, lead author of the study and postdoctoral fellow in the University of Michigan School for Environment and Sustainability.
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According to this long-standing theory, called adaptive radiation, sudden bursts of rapid evolutionary change can occur when a species encounters a new ecological opportunity — perhaps colonizing a new continent, or filling an ecological niche left empty after a mass extinction. Over time, as fewer new opportunities remain available, evolution naturally slows.
“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,” Berv said. “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.”
While fossil evidence had offered hints supporting this idea, directly testing it at large scale using modern specimens had remained extraordinarily difficult — until AI made it possible.
Teaching AI To Measure Bird Skeletons
To reconstruct the evolutionary history of passerines with genuine statistical power, senior author Brian Weeks and the U-M team needed an enormous dataset — one far beyond what any individual researcher could collect by hand.
The solution was Skelevision, an AI tool developed in Weeks’ laboratory in collaboration with David Fouhey’s laboratory at New York University, built over a seven-year collaboration.
Skelevision photographs bird skeletons positioned in front of a calibrated grid that establishes a consistent measurement scale. The AI model can then accurately measure 12 distinct bones across each skeleton — a task that would take a human researcher considerably longer to perform manually and consistently across thousands of specimens.
Using this system, researchers scanned and measured more than 15,000 museum specimens, most drawn from the collections of the University of Michigan Museum of Zoology. Each specimen took roughly 45 seconds to scan — allowing the team to digitize entire museum collections at a speed that would have been unimaginable using traditional manual measurement methods.
In total, the project assembled data on more than 2,000 species and over 170,000 individual skeletal measurements.
A New Statistical Method For Studying Whole Skeletons
Collecting the raw measurements was only part of the challenge. Berv also developed a new statistical method called bifrost, specifically designed to analyze each species’ complete skeleton as an integrated whole, rather than examining individual bones in isolation.
“The whole organism is an integrated, complex morphology, and each of the individual pieces is interrelated to every other part in the body,” Berv explained. “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?'”
Using this approach, the researchers reconstructed how passerine body shapes changed across roughly 45 million years of evolutionary history.
A Burst Of Evolution During Extreme Global Cooling
The results revealed a period of exceptionally rapid body-shape evolution occurring around 35 million years ago. This burst coincided precisely with the Eocene-Oligocene transition — a major geological period marked by intense global cooling, one of the most significant climate shifts in Earth’s history.
The team also identified a distinct cluster of evolutionary slowdowns occurring around 15 million years ago, coinciding with a separate 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.”
Modern Geography Tells The Same Story
To test whether this pattern held up beyond the fossil record, researchers also examined the current global distribution of the bird species in their dataset — and found a striking parallel.
Bird communities living at more extreme latitudes, where seasonal temperatures swing more dramatically, tend to include species that evolve faster than those living closer to the equator, where climate remains comparatively stable year-round.
Because this same pattern appeared both across millions of years of evolutionary history and across modern geographic regions today, the findings suggest that environmental variability may play a genuinely important and underappreciated role in shaping the pace of evolutionary change.
“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 Get A New Kind Of Scientific Value
This research also highlights something increasingly true across biology: the enormous, often underappreciated scientific value locked inside museum specimen collections — value that AI is now unlocking at a scale never previously possible.
“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,” Weeks 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.”
What This Means For Understanding Modern Climate Change
Beyond illuminating deep evolutionary history, the researchers believe their findings carry genuine relevance for understanding how species might respond to the rapid climate changes unfolding today.
“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.”
If past climate upheavals reliably triggered bursts of rapid evolutionary change in birds, understanding exactly how and why that happened could help scientists better anticipate how modern bird populations — and potentially other species — might respond to the accelerating environmental changes occurring right now.
Key Takeaways
- Using AI, researchers analyzed over 15,000 museum bird skeletons and 170,000 skeletal measurements to study songbird evolution across 45 million years
- The study confirmed a century-old evolutionary theory: change occurs in rapid bursts rather than at a steady pace
- A major burst of rapid evolution around 35 million years ago coincided with intense global cooling during the Eocene-Oligocene transition
- Birds living at more extreme latitudes today evolve faster than those near the equator, mirroring the historical pattern
- The findings suggest climate variability plays a significant role in driving evolutionary change, with possible relevance for understanding species response to modern climate change
Source: University of Michigan — July 29, 2026
Journal Reference: Jacob S. Berv, Charlotte M. Probst, Santiago Claramunt, J. Ryan Shipley, Matt Friedman, Stephen A. Smith, David F. Fouhey, Brian C. Weeks. Rates of passerine body plan evolution in time and space. Nature Ecology & Evolution, 2026.
DOI: 10.1038/s41559-026-03110-5
