As we go through our lives -- driving cars, exploring the Internet, studying the world around us -- it is hard to imagine that we're related to Earth's other animals. It's even a stretch to see what connects us with the rest of the chordates, a group of about 50,000 species including the vertebrates like fish, amphibians, reptiles and birds, mammals and ourselves. But indeed, all chordates, from the worm-like amphioxus to Homo sapiens have three common features.

Each of us has a single hollow nerve bundle running up our backs that blossoms into a brain in the head of most chordates. Each also has a stiff rod, called a notochord, that contains fluid-filled cells sheathed in fibrous tissue. (In humans, as in most vertebrates, the notochord becomes part of the structure of the discs between the vertebrae of our spines.) The third trait shared by all chordates is the presence, at some stage of life, of gill slits in the throat. (Human gill slits close up while we're still embryos.)

Chordates, and particularly back-boned vertebrates, have achieved incredible success on Earth. Scientists attribute much of that success to evolutionary modifications in their genetic make-up. Vertebrates have many more genes than our invertebrate relatives. The additional set of genes have allowed us to develop complex body parts -- the backbones, skulls, jaws, teeth and sophisticated brains -- that have enabled vertebrates to dominate land, sea and air.

Before we as humans gloat over our success, we must remember we are not the sole survivors of this great journey of evolution. We are part of an epic story much larger than ourselves.

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