Roughly 45 seconds. That’s how long a computer needs to measure the skeleton of a single dead songbird, and that one number carries more weight in this study than almost any other detail — it’s what converted a century-old argument among researchers into a question you can finally answer with data.
Researchers at the University of Michigan turned to artificial intelligence to extract a pattern from the evolutionary history of Passeriformes, the vast bird group that includes most songbirds. Evolution here didn’t tick along at a constant rate. The major changes arrived in rapid bursts, and several of those bursts align with shifts in Earth’s climate.
The concept itself isn’t new. The source of the evidence is.
Fossils suggested it. Present-day skeletons confirmed it.
For decades, evolutionary theory has held that life diversifies through stretches of rapid change punctuated by slower phases. Fossils offered hints. This study picked up the very same signal by measuring bones from modern bird specimens stored in museum drawers.
“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,” Berv said.
Getting there took AI paired with a large statistical model. The work appears in Nature Ecology & Evolution, with primary support coming from Schmidt Sciences and the David and Lucile Packard Foundation.
Seven years of training a model to measure 12 bones
Working with senior author Brian Weeks, the Michigan team covered more than 2,000 species and assembled over 170,000 individual skeletal measurements. That isn’t something anyone does by hand.
The tool behind it is Skelevision, an AI system developed in Weeks’ lab together with David Fouhey’s lab at New York University. It photographs specimens — bird skeletons, in this case — against a grid that holds the measurement scale steady. Across a seven-year collaboration, Weeks and Fouhey trained a model capable of accurately measuring 12 bones on a single bird’s skeleton.
In total, more than 15,000 museum specimens passed through the system, the majority drawn from the U-M Museum of Zoology’s collections. At about 45 seconds per specimen, digitizing a whole museum collection is no longer a project that eats an entire career.
A global chill written into the bones
Berv also developed a new statistical method, bifrost, that allowed the team to analyze each species’ skeleton as a whole rather than dissecting individual bones one by one. Using it, they reconstructed how passerine body shapes changed over roughly 45 million years.
“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?'”
About 35 million years ago, body-shape evolution accelerated sharply. That spike lands on the Eocene-Oligocene transition, a period of severe global cooling. The statistics also picked out a cluster of slowdowns roughly 15 million years ago, coinciding 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.”
The farther from the equator, the faster the change
The team then ran a second test, this time examining where the birds in the dataset live today. Geography, it turned out, also predicts the average rate of morphological evolution.
Bird communities at more extreme latitudes — places where seasonal temperatures swing more violently — contain species evolving faster than their counterparts nearer the equator. What makes that compelling is seeing the same pattern emerge both across millions of years and across the map as it looks now. Together they point to environmental variability as a driver of 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.”
A case for museum funding, tucked inside a bird study
A second argument runs underneath this paper, and Weeks states it plainly: AI is now extracting information from preserved specimens on a scale that simply wasn’t feasible before.
“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.”
The uncomfortable part
According to the researchers, the work could help scientists reason about how species respond to the climate change unfolding right now. Berv is cautious about the mismatch in timescales — as he should be.
“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.”
Further support came from the Michigan Institute for Data & AI in Society, the Natural Sciences and Engineering Research Council of Canada, and the National Science Foundation.
If there’s a single takeaway worth keeping, it’s this: the birds that changed fastest were those living where the weather never settled. A 45-million-year record is telling us instability is the engine. And we’re currently running the fastest version of that experiment ever attempted, with nobody yet holding the results.