Scientists have taken a deep dive into the wild world of mammalian evolution.
An international team of more than 100 researchers has analyzed genetic information from 240 mammalian species. Using the data, scientists can begin to answer questions about human disease, the brain, the arc of mammalian life on Earth and more.
Called the Zoonomia Project, the research involved collecting DNA from species across mammals’ family tree, including an aardvark, human, fat-tailed dwarf lemur and a tapir, the team reports in 11 studies in the April 28 Science.
“It’s a really nice survey of the mammals that are out there,” says Irene Gallego Romero, a human evolutionary geneticist at the University of Melbourne in Australia who was not a part of the work.
For every one of those mammals, scientists read out the slew of DNA “letters” that make up an animal’s genetic instruction book, or its genome, work first described in 2020. Then, the team lined up the text of each mammal’s book and looked for differences. Some text stayed nearly identical across all species examined, the researchers found.
Those spots, which have remained mostly unchanged across some 100 million years of evolution, may be parts of the genomes that are “doing something important,” geneticist Elinor Karlsson of the Broad Institute of MIT and Harvard and the University of Massachusetts Chan Medical School in Worcester said in an April 25 news briefing. Sections with lots of changes are interesting too, she says, and can offer clues about how a particular species may be adapting to its environment.
Karlsson recognizes that the 240 studied species, all placental mammals, represent just a tiny slice of living mammals — roughly 4 percent. And one animal not included “will annoy me to no end,” she said with a laugh. “For some reason, we couldn’t get a raccoon.”
But the team now has enough species to zero in on potentially essential bits of the mammalian genome, and to start to piece together new stories about mammals’ evolution, including our own. Here are five findings that we thought were especially cool.
1. A legendary sled dog’s genetic changes may have helped it survive in an extreme environment.
In 1925, a sled dog named Balto helped deliver diphtheria medicine to children in Nome, Alaska, a feat that made him famous. He was part of a dog population that had adapted to the region’s harsh conditions, scientists now propose. An analysis of Balto’s DNA, extracted from his remains, hints at what made these sled dogs so tough.
Balto most likely had an enhanced ability to digest starch and carried relatively few potentially damaging mutations, the researchers report. He was also less inbred than modern dogs — a sign that his population was genetically healthy.
Scientists also uncovered gene variants linked to bone and skin development. They can’t say for sure what advantages those variants offered, but it’s possible they gave Balto tough foot pads that helped him endure the cold and ice, says study coauthor Heather Jay Huson, an animal geneticist at Cornell University. “He was likely a very hardy dog.”
2. Mammalian evolution ramped up well before the dinosaurs died out.
The origin of placental mammals may trace back to 102 million years ago, one Zoonomia analysis suggests.
Scientists have long debated the timing of when mammalian evolution got underway. One traditional view is that the number of new mammalian species began to grow after an asteroid (or two) hit Earth some 66 million years ago and wiped out the dinosaurs (SN: 8/22/22).
But the new study supports a much longer timeline for different groups of mammals to arise. Instead, mammalian evolution may have kicked off around the time that the continents were breaking apart, and then continued over time, while Spinosaurus and Oviraptor still roamed Earth. Later, after the mass extinction event that marked the Cretaceous Period’s end, even more new mammalian species evolved.
While people may think mammals didn’t branch out until after the dinosaurs went extinct, “the data really suggest that they started to diverge earlier on,” Kerstin Lindblad-Toh, a geneticist at the Broad Institute and Uppsala University in Sweden, said in the news conference.
3. Mammalian genomes could help identify genes that drive human cancer.
In people with cancer, scientists can find mutations speckled through the genome. As cancers grow, these genetic changes tend to pile up. But it can be hard to know which mutations matter — which ones trigger or nudge along the disease. Data from the Zoonomia Project could help pinpoint which ones are important.
One of the new studies examined mutations that occur in medulloblastoma, a malignant brain tumor that mainly affects kids. The researchers looked for mutations in spots in the mammalian genome that don’t typically change. Because that DNA is shared across hundreds of mammal species, it’s likely essential for health, Gallego Romero says. Any disruptions could have big consequences — like cancer.
So a cancer mutation in these areas “is probably one of the ones to focus on,” says Gallego Romero, who wrote a commentary on the studies in the same issue of Science. It could be one of the key genetic changes responsible for setting off the disease, she says. Homing in on these potentially problematic mutations could help scientists identify suspicious genes and figure out treatments.
4. Some “jumping genes” may be more likely in meat eaters.
Scientists have known for decades that some stretches of organisms’ genomes can hop from one location to another. Now, one Zoonomia study estimates just how much of the genome is mobile among mammals, and how an animal’s diet may come into play.
Among the hundreds of mammals analyzed, aardvark genomes were the most stuffed with “jumping genes,” researchers found. These mobile elements make up nearly 75 percent of the animal’s genome.
Meat-eating mammals also stood out. Compared with their noncarnivorous counterparts, meat eaters tended to accumulate more of one jumping gene type. “That was a little bit shocking to me,” says Nathan Upham, an evolutionary biologist at Arizona State University in Tempe who was not involved with the work, but also cowrote an accompanying commentary.
It’s possible that predators somehow receive DNA from the prey they’re eating. That suggests that an animal’s ecology — in particular, what it eats — shapes the evolution of its genome, Upham says.
5. Comparing the genomes of humans and other mammals might reveal secrets about the brain.
Researchers have long sought what in our DNA sets us apart from other animals.
Lining up mammals’ genomes and looking for regions in human DNA that have undergone lots of changes unearthed some clues. Those regions represent areas that are evolving faster in humans than in other mammals — and could hint at what makes us unique, researchers report in one Zoonomia study.
The scientists suggest that the human genome folds up in such a way that brings fast-changing regions physically close to genes involved in brain development, like two shoelace ends brought closer together when tying a knot. That proximity may ultimately dial up gene activity — perhaps changing how the human brain works.
For now, scientists don’t know exactly what such genomic alterations might do. But, Lindblad-Toh says, “it’s important to remember that what makes us human is not one change, but many, many changes.”