After traveling more than 1,000 miles, five frozen bison embryos arrived in Minnesota by airplane last week.

They started out in Yellowstone National Park, made their way to Colorado State University in Fort Collins and finally the Minnesota Zoo in Apple Valley. Hours after arriving, they were implanted in the zoo’s four bison cows.

If all goes according to plan, there will be bison calves at the zoo next spring.

“If this project is successful and a Yellowstone bull calf is born at the [zoo], he will be widely used in the future within the Minnesota Bison Conservation Herd to spread his valuable genetic material,” said Tony Fisher, director of animal collections, in an e-mail.

North American bison once numbered in the tens of millions, but hunting drove the number down to fewer than 1,000 by the late 1800s. The zoo partnered with the Minnesota Department of Natural Resources (DNR) in 2012 to build a herd of 500 genetically pure bison — part of a state-mandated effort to restore the animals to Minnesota prairies.

It’s one of many efforts across North America to restore bison. It’s also one part of the zoo’s work to conserve species, from prairie butterflies to Asian wild horses.

Though the number of North American bison has recovered somewhat, most are not genetically pure. Mixed with cattle, they’ve lost some of the traits that help them survive on the prairie. According to the zoo, it’s estimated that less than 1 percent of American plains bison in the world have tested free of cattle genes.

“We think that bison that are best adapted to survive over the long-term will be those animals that have the genetic makeup of a wild population,” said Ed Quinn, natural resource program supervisor at the DNR.

To demonstrate the significance of genetics, Quinn points to an example in the plant world: Queen Anne’s lace. The delicate white flower is genetically similar to the common carrot. It’s an extreme case, but it shows why genetic purity matters, he said.

With bison, the hope is to preserve those that are as similar as possible to how the animals occurred historically.

The DNR has been working on bison restoration since the 1960s, starting with a small herd at Blue Mounds State Park. Since partnering with the zoo, that herd has grown to more than 100 animals, some of which have been moved to Minneopa State Park in Mankato.

Incorporating bison with a pure Yellowstone genetic heritage into the existing herd has been a goal for years, Quinn said. Of all the genes that show up in the herd, he said, Yellowstone genes are the least well-represented.

“Obtaining a live bull from Yellowstone National Park has always remained impossible due to a transfer moratorium by government authorities,” Fisher said. “Recent embryo transfer research has now opened up the possibility of finally being able to acquire a Yellowstone bull.”