“You haven’t changed a bit!”

It’s nice when people lie about our Throwback Thursday pictures this way, but it’s also true that some of us stay better preserved than others.

David Gohman Luke wondered which animals have maintained their look the longest—but he's talking millions of years, not decades. Luke asked Saturday’s Weird Animal Question of the Week via Facebook: “What is the oldest surviving mammal that has gone unchanged by evolution?”

Don’t Go Changin'

“'Unchanged' is a tricky word,” Nizar Ibrahim, a paleontologist at the University of Chicago and 2014 National Geographic Emerging Explorer, says via email.

With only fossils to go by, scientists can examine an ancient animal's skeletal structure, but it's not the whole story. Physiology and DNA change somewhat over time, he says, both through the basic process of evolution as well as random genetic changes.

That said, two mammals that have undergone the fewest evolutionary shifts are the platypus and the opossum, says Samantha Hopkins, associate professor of geology at the University of Oregon.

Put That on My Bill

You could say the platypus is a survivor: It's one of the few living descendants of an ancestor that diverged from all the other mammals about 150 million years ago, Hopkins says.

The platypus has “a number of primitive features,” Ibrahim says, “both from what we know from fossils and from what we can see in their [modern-day] anatomy."

Those include their leathery eggs and a lack of nipples, both traits that don't exist in mammals that evolved after platypuses.

February 6, 2009—National Geographic researchers are trying to collect DNA samples from these odd duck-billed mammals to determine whether there are separate subspecies.





Eggs, you might be wondering? You heard right. Egg-laying mammals are called monotremes, and though once more diverse, today that group contains only the platypus and two species of echidna.

Skulls of platypus-like ancestors have been found dating back to the Cretaceous period (63 million to 138 million years ago), Hopkins says. Their duck-like bill and unique jaws help scientists identify such fossils. (Also see "Giant Platypus Found, Shakes Up Evolutionary Tree.")

Ibrahim adds that the oldest fossils that look like the modern platypus date to the Quaternary period, which is about 2.5 million years ago.

Playing Possum

They don’t go back as far as the monotremes, but opossums are also quite ancient, "with little change over the last few tens of millions of years,” Ibrahim says.

Today there are more than 60 species of possum, which live throughout the Americas, as well as Australia, New Zealand, Tasmania, and New Guinea.

An unwary marsupial suddenly finds itself face to face with its fifteen minutes of fame.





It Pays Not to Be Picky

It’s not fully understood why some species remain unchanged for such a long time, Ibrahim says, but stable environments and little competition might play a role.

For instance, there aren't many Australian mammals that have evolved to be aquatic—which means the water-loving platypus doesn't have much competition to deal with.

And sometimes, evolution just gets it right.

For instance, Hopkins points out that most species of opossum, like North America's Virginia opossum, are "dietary generalists." Their attitude toward food is “No bugs right now? I can eat trash. No trash? I’ll just forage on this rotting fruit," she says—an adaptability that lets them live just about anywhere. (Also see "The Living Dead: Animals That Pretend to Go Belly-Up.")

View Images There are more than 60 species of opossums (pictured, a Virginia opossum), most of which haven't changed much over the millennia. Photograph by Joel Sartore, National Geographic Photo Ark

On the other hand, extremely specialized animals like giant pandas, whose diet is 99 percent bamboo, are more vulnerable to extinction if their preferred food is no longer available.

See there? The more agreeable you are to change, the more you might just get to stay the same.