Therians: They may sound as if they come from a far-off planet, but these are no alien creatures. Found in nearly every corner of the Earth, they count a surprising range of species among their ranks: the next-door neighbor's pet pooch, alpacas in the Andes, aardvarks in Africa, and even you and me.

Yes, we are all therians, a subclass of mammals that is the dominant mammalian form on the planet today. Successful in a big way with an almost completely global empire, the therian club has members underground (moles), in freshwater lakes and rivers (otters), on open grasslands (zebras), in the trees (squirrels) and in the open ocean (seals, whales and dolphins) -- in fact, in just about every habitat on Earth that you can think of.

We are vertebrates that nourish our young with milk our females produce from special mammary glands, and we all sport body hair at some point in our lives (even if only before birth, as is the case with some whale species).

Although we are also all warmblooded, that is not quite the distinction some of us were brought up believing in, since various other creatures have turned out to share that characteristic.

The largest living creature on Earth, the 150-ton blue whale, is a therian, as is the smallest shrew or bat, weighing in at just a few grams. We therians also share a few other more esoteric similarities that reflect our common origins from a single, simple starter kit.

Why else would our lower jaws, along with those of other primates, polar bears, Przewalski's horses and palm civets, be hinged directly to our skulls rather than via the quadrate bone like all other vertebrates?

In the same way, we all have trios of tiny sound-transmitting bones in our heads (we call them our ears), and a strange muscular band that stops our heart and lungs from coming into contact with our intestines (perhaps that makes hiccups a uniquely therian problem).

And the limited blueprints extend to the microscopic level too, for we therians all have red blood cells without nuclei, while all other vertebrates have nucleated red blood cells. From our furry exteriors to our variously shaped jaws, from our middle ears to our innermost blood cells, we share similarities that tell of a commonality of origins.

Our particular success comes from our versatility: The reason we are able to exploit the Earth so fully is largely because of our ability to regulate our body temperature and thus adapt to extremes of heat and severe cold.

While all therians are mammals, not all mammals are therians. Some ancient groups of mammals didn't make it through the selective sieve of time, but one astonishing group that did is the aptly named prototherians; they are an offshoot of the very earliest mammals.

The short-beaked echidna found on Kangaroo Island

I met my first prototherian on a small island south of Adelaide. There, pushing its way through the woodland leaf litter, was a spiny creature for all the world like a large hedgehog. It was my first echidna, and I could hardly contain my excitement. The prototherian was unimpressed by me, a modern therian, and ignored my presence while smells of food enticed it away on its rambling path.

We therians all give birth to live young, for we are either placentals or marsupials, but here on Kangaroo Island I was in the presence of a creature that does it differently: a monotreme. We take our own forms, our own patterns of life for granted, but here was a living example of a different blueprint from a group of relatives that had gone off down a different path. It was rather like being descended from a long line of teachers and then finding that there is another (rarely spoken of) branch of one's family that all became industrial chemists. Meeting up again raises a lot of questions.

Monotremes differ from us in some astonishing ways. The echidna nurtures its young on milk and has body hair, but also has a number of apparently reptilian features, such as a bare snout.

Even more bizarrely, the echidna does not give birth to live young. Instead of bearing baby echidnas, this creature actually lays eggs. Mention eggs and birds or reptiles instantly spring to mind, but monotremes are in all relevant ways mammals -- just rather different sorts of mammals.

Echidna mothers, for example, lay tiny 1 yen-coin-size eggs. A single egg is deposited directly in a fur-lined fold of skin on the female's belly. The egg is carried around in this fur-lined nest for about 10 days until the tiny "puggle" breaks free with another birdlike feature -- an egg tooth. The diminutive puggle then remains wedged in the skin flap, gorging itself on milk and growing at a phenomenal rate.

But here, too, monotremes are different. Echidna mothers do not have mammary glands with nipples as other mammals do; instead they have a patch of skin that "weeps" milk.

We may look askance at this ancient offshoot of early mammalian stock, but we must always remember that the creatures we share the Earth with today are descended from a very long line of survivors, and the ancestors of the short-beaked and long-beaked echidnas of New Guinea and Australia are certainly survivors -- they have been around for many millions of years.