Is the Carolina Wren our cousin?

ACarolina wren is nesting in the potted dwarf Alberta spruce tree on our front sidewalk The tiny nest, about 2 feet off the ground, has four bright-blue eggs with some speckling.

This busy bird begs several questions. Why hasn't the house cat noticed this tasty morsel? Why have the black rat snakes, now transiting the yard in search of mates, missed the nutritious eggs?

And the big question: Why are the wren and I so similar -- and so different?

Many similarities are obvious. The wren and I both have heads. We have a pair of eyes. We both have an opening to take in food. We both have a pair of legs. We have beating hearts and blood flow.

But our differences are overwhelming. The wren has a beak to crunch food. I have teeth. The wren has a feather skin covering. I have hair, not feathers. And in the nest are four eggs.

Humans are born live. Birds are born encased in a hard shell, and mature further before cracking out of it.

And the wren flies. Not me.

How could we be so similar and so different at the same time? The answer lies in our evolutionary history.

Scientists discover relationships -- such as my kinship to a wren -- using fossils and DNA. The questions boil down to this: Where on the family tree of life is there a common ancestor of humans and birds, when did this ancestor live, and what was it like?

The answer lies in a group of land-dwelling animals living 310 million years ago, the amniotes. Their name comes from the amniotic egg sac they developed. This common ancestor explains the things the wren and I share.

Over time, some of these amniotes developed into mammals and some became reptiles.

In the 310 million years of evolution of the amniota, humans evolved along one path, and wrens along another. It is this 310 million years of separate development that explains our differences.

For us, mammals divided into marsupials (kangaroos and opossums), platypuses and mammals with a placenta (this group includes humans).

Further subdivision, about 65 million years ago, led to primates -- a group that includes us, as well as chimpanzees, gorillas and orangutans.

Birds evolved along a different branch of the family tree. From amniotes, there evolved a group called archosaurs with crocodiles, dinosaurs and birds. After the appearance of dinosaurs, one group of bipedal predatory dinosaurs gave rise to birds. Birds are most closely related to tyrannosaurs and velociraptors.

Birds have further evolved into modern species, with songbirds closely related to each other, and ducks and ostriches being more distant relatives.

So the answer to the question, how closely related are the Carolina wren and his I, is, "We shared a common ancestor 310 million years ago -- a great grandfather millions of times over, who was very little like either of us."

Virginia's science Standards of Learning cover the relatedness of species in detail in Biology and Life Sciences, but their fundamentals begin in kindergarten (K.6).