Like the platypus, the echidna has a spur on its hind foot, which doesn't get the attention it deserves, maybe because unlike the platypus, the spur is nonvenomous. "What's the fun of having a spur if it's nonvenomous?" is what amateur naturalists want to know. Nevertheless, the echidna has many unique characteristics in its own right; for example, the echidna penis has four heads; I bet you didn't know that. Well, the female echidna knows it.
Echidnas might strike people as more interesting if they didn't look so much like other, more common mammals. Waddling around with their spiny hides they look like porcupines or spiny anteaters, which they are no more closely related to than they are to Bruce Willis, another creature whom they superficially resemble.
Placental mammals are apt to view monotremes with contempt. "Why don't you get with the program?" they ask. "If you're going to have mammary glands, then have a live birth, why don't you?" Placental mammals think just because they're in the majority, anything that's not placental is backward or abnormal. As far as they're concerned marsupials are better than monotremes, but not much. The way they see it, giving birth to a live, relatively fully developed individual is the only way to go for a fully-evolved mammal.
The issue of which reproductive method is superior is one an echidna might put to a human woman in the throes of labor. "How's it going, dearie?" the echidna would ask. "You're pushing a nine-pound infant through your vaginal tract? Oh, my. Isn't that a little like trying to get a bowling ball down a tube sock? I just lay a soft leathery egg which I store in my pouch until it hatches. Then I'll nurse it for about a month before putting it in a little burrow and returning once every five days until it's weaned. How often will that little cub of yours need to be nursed? Oh, my. As often as that? But when will you sleep? Well, well, well. It certainly must be a challenge being all advanced and fully evolved like you. Oh, by the way, did I mention my mate has a penis with four heads?"
Unlike many placental mammals I could name, the monotremes are doing quite nicely, thank you, and only one of them - Attenborough's Echidna is considered Critically Endangered. The truth is, there's no such thing as being "more evolved" or "less evolved." We're all evolved; echidnas just found what worked for them and stuck with it before the rest of us.
