A pair of Carnegie Museum scientists are part of an international team going backwards in time to recreate the complicated evolutionary journey of life.
The furry little creature in question has a long tail, lots of whiskers, and tiny teeth. To us humans, it looks more like something you might chase out of the house with a broom than a member of the family. But researchers at Carnegie Museum of Natural History conclude this inauspicious, shrew-like animal is in fact the elusive “hypothetical common placental mammal ancestor.” In other words, its descendants evolved into rats, cats, whales, monkeys, and all other placental mammals—including humans.
The mother of all placentals may not look robust, but findings of the recent Assembling the Tree of Life (ATOL) study, supported by the National Science Foundation, suggest it could be a hearty survivor of the same climatic changes that devastated the mighty dinosaurs.
Illustration: Stacy Innerst
For the massive project, which has brought cross-disciplinary teams of scientists together in pursuit of the ultimate evolutionary map of all major lineages of life, two Carnegie Museum of Natural History scientists joined forces with 21 fellow researchers from North America, South America, and Europe. Over six years, they created the most comprehensive mammalian Tree of Life to date. Results of the collaboration were recently published in the journal Science.
Since Darwin, the path of evolutionary change, often depicted as a branching tree, has intrigued humans. “The Tree of Life is fundamental,” says John Wible, curator of mammals at Carnegie Museum of Natural History and co-author of the Science paper. “It’s the avenue that lets you approach all the other questions that are interesting. There is a certain curiosity about our roots. ‘Where did I come from?’”
The current popularity of at-home DNA testing underscores our human yearning to understand our origins, Wible adds. “This Tree of Life study is like a home DNA kit, but one on steroids.”
DNA and Anatomy
While scientists have been mapping evolutionary change for decades, ATOL represents the largest study of placental mammalian ancestry undertaken to date. (The overwhelming majority of mammals are placentals, meaning they bear live young, nourish offspring with milk, and have hair or fur.) Results, which were drawn from a variety of data sources, have shed light on a controversy between two approaches to evolutionary research: molecular data (DNA) and morphological data (anatomical features).
Wible and his 22 ATOL teammates are morphologists and paleontologists. They hypothesize evolutionary relationships between both living and extinct mammals based on common appearance and anatomy. By tracing certain traits backwards, morphologists found that placental mammals appeared 65 million years ago around the Cretaceous-Tertiary boundary (more commonly known as the K-T boundary). Around that time, a meteorite is believed to have struck Earth, triggering a catastrophic drop in global temperatures that wiped out three-fourths of all life, including non-avian dinosaurs.
This morphologists’ version of events was widely accepted by scientists across the disciplines. “If placental diversification happened shortly after the dinosaurs went extinct, then large-scale extinction brought us modern mammals,” says Michelle Spaulding, the Rea Post-doctoral Fellow in mammals at Carnegie Museum of Natural History and Wible’s ATOL teammate. “That is the story paleontologists have been telling for decades.”
But researchers using DNA evidence have put forth a different scenario: that placental mammals first appeared 80 to 130 million years ago during the Early Cretaceous Period, when dinosaurs still ruled the land. Under this theory, placental mammal diversification is unrelated to catastrophic climate change or the mass extinction that followed.
As part of the groundbreaking study, John Wible and Michelle Spaulding looked at all aspects of mammalian anatomy—from skulls and skeletons to internal organs, muscles, and fur patterns. Photo: Joshua Franzos
Molecular scientists study evolution by examining genetic similarities among living species. By assuming that similar DNA sequences derive from a common ancestor, they have their own way of tracing the thread of evolution backward in time. They also make estimations of dates of origins for various groups based on DNA differences. Wible and his fellow morphologists remain skeptical of the molecular predictions of origin dates for placentals, because no placental fossils have surfaced in Early Cretaceous rock samples. “Our molecular colleagues say that rodents were living 80 million years ago,” he says. “We’ve been asking, ‘Where are they?’”
“ The Tree of Life is fundamental. It’s the avenue that lets you approach all the other questions that are interesting. There is a certain curiosity about our roots. ‘Where did I come from?’”
- John Wible, Curator of Mammals, Carnegie Museum of Natural History
In the end, the ATOL team combined molecules and morphology to maximize the amount of information. The results of their study support the morphologists’ theory—for now.
Though the molecular-morphological debate can get heated, Spaulding said it’s a friendly banter, not a trashtalking rivalry. “We’re all trying to answer the same question,” she says. “This is the first time there has been collaboration on this scale.”
In many ways, the team’s own diversity was an asset. Mapping the Tree of Life is a thorny scientific puzzle.
“Solving evolutionary problems is like solving a murder mystery,” says Guillermo Rougier, professor of anatomical sciences and neurobiology at the University of Louisville and a researcher on the project. “But instead of finding a dead body, you are looking for clues in lots of dead bodies in the form of fossils. You are using the accounts of unreliable witnesses in the form of living mammals.”
To sleuth for clues about our mammalian ancestors, researchers compiled a survey of 4,500 characters or distinct anatomical traits, compared to just 400 in previous studies. Sometimes they would pore over a single mammal for three weeks, painstakingly recording everything from paw pad shape to fur patterns to tail length. Then they would download photographs to a central database to preserve the results and standardize parameters such as size. Through this tedious process emerged evolutionary pathways and patterns, which were compiled in a shared web application called Morphobank.
By building a tree of relationships and then tracing features back though evolutionary time to the common ancestor, researchers were able to identify a tremendous amount of reconstructed characteristics of the earliest placental mammal from which all others evolved. Working from their description, natural history illustrator Carl Buell breathed life into a shrew-like creature.
Interest in the original mammalian ancestor was so intense that study colleagues at the American Museum of Natural History in New York teamed up with RadioLab, creator of a popular science-based podcast, to sponsor a “Naming of the Shrew” contest. The winning name was “Shrewdinger,” but Wible and Spaulding call the critter by one of the finalists’ names—Ralph. They think it’s less pretentious and more accurate because, well, it’s not technically a shrew. And let’s face it—Ralph rolls off the tongue much easier than “hypothetical common placental mammal ancestor.”
What exactly Ralph is hypothesized to look like could change as scientists dig up more fossils.
Illustration: Carl Buell
Though the morphology and DNA data provide clues, no one really knows what our earliest placental ancestor looked like. “Ralph is a figment of our imagination,” says Wible. “But a scientifically accurate figment of our imagination,” Spaulding interjects with a laugh.
Just as a human family tree might include branches of redheads and brunettes, a mammalian Tree of Life can reveal the point at which certain branches of mammals developed wings or lost their hair or tails. The project unites researchers who specialize in specific species, inviting comparisons and contrasts among the 5,100 living varieties of placental mammals.
“You have specialists on elephants, specialists on rodents, specialists on whales,” notes Wible. “We need to bridge all those disciplines. We need to make comparative statements that this tooth in an elephant is equal to this tooth in a primate.”
Morphobank is key to the project because it democratizes public access to information. Unlike most journal articles, it can be accessed for free anywhere in the world. “Mongolia is a treasure trove of fossils,” Spaulding says. “Someone from Mongolia—even an institution—might not be able to afford to look at the results of the fossils of their homeland in a magazine. But our matrix [on Morphobank] is open access.”
The site also ensures that the data will stay posted indefinitely, and preserving the online database will make it easier for scientists to add new fossils to the Tree of Life without starting the tedious process from scratch. Though Ralph emerged as the star of this completed project, what exactly Ralph is hypothesized to look like could change as scientists dig up more fossils to add to the existing record. The Tree of Life is a work in progress, a never-ending quest for our past.
As Wible puts it: “We’re never going to know the answer. We can’t go back in a time machine and reveal the history of life on Earth.”
That hasn’t stopped him from searching. Wible has long wanted to create such a project. In fact, when he interviewed for his job at the museum in 1998, he was asked where he saw himself in five years. “I want to have a family tree for mammals,” he said. Turns out it took him 10 years longer and 22 teammates to pull it off.
Scientists have been sketching Trees of Life since the 1800s. While the early attempts were often insightful, they were rarely based on rigorous classification schemes, says Wible. But that changed in the 1960s when scientists began using a rigorous methodology called cladistics. This led to a more standardized approach that allowed scientists to expand upon one another’s work.
Clues from the distant past may enable researchers to tackle modern problems related to mammalian conservation. “If you preserve two animals that are far apart on the chain, you are optimizing conservation efforts,” says University of Louisville researcher Rougier.
Looking back at how various animals adapted or failed to adapt to climate change might also help scientists predict the future. “If there is massive-scale catastrophe or extinction, you can expect there will be massive changes on Earth,” adds Zhe-Xi Luo, a paleontologist at the University of Chicago and former associate director of science at Carnegie Museum of Natural History who was also part of the ATOL project. “It’s always the little guy who manages to survive. The larger organisms tend to have bigger problems. The size of the trouble goes with the size of the beast.”
Ralph is a good example. Though it may look trifling, it weathered catastrophic climate change to spawn a new branch of the evolutionary tree. “If you look at our lineage, it’s not glorified by a tremendous amount of beauty or handsomeness,” Luo said of the creature. “It is just a little shrew, but it is a very important shrew.”