The echidna is among the first modern mammals to take the "extremely rare" path out of water for life on the land, surprising Australian researchers.
The discovery came during analysis of a fossilised bone of an ancient echidna found 30 years ago in Victoria.
When studying the 108-million-year-old upper arm bone, Professor Suzanne Hand noticed it had attributes similar to a semiaquatic mammal.
More than 30 modern mammal species have transitioned from land to water, such as whales, otters and dolphins.
But the professor said it is "extremely rare" to see the opposite occur.
"We've had mammals going from water to land and land to water," Prof Hand told AAP on Wednesday.
"But in terms of modern mammals (those living after the dinosaur age), we've mostly seen mammals go from land to water."
The modern-day prickly creature also has muscles that help it conserve oxygen when burrowing, similar to the tissues platypuses use to dive.
Scientists found similar muscles in the fossil, helping them conclude that echidnas evolved from a water species.
Echidnas and platypuses, both belonging to the family of egg-laying mammals, are considered distant relatives as they have backwards feet and similar beak receptors to detect prey.
"The microstructure of the fossil's humerus (arm bone) is more like the internal bone structure seen in platypuses," Prof Hand said.
"The (muscles) are high in the platypus, but are higher than expected in echidnas, even as burrowers."
But a burning question arises: when did the echidna move to land?
Scientists predict it happened between 23 million and 66 million years ago.
They hope to find fossil proof in northwest NSW that can confirm their suspicions.
"We're increasing our efforts to help investigate the opalised fossils of Lightning Ridge which are about the same age as the Victorian fossil deposits," Prof Archer said.
"We're hoping we'll discover other ancestral monotremes (egg-laying mammals) that will help unravel the early history of this most fascinating group of mammals."
Researchers will also examine the Kryoryctes cadburyi fossilised bone through imaging and scans to understand it better.
