With respect to the ZPA, what kind of digit develops depends on how close the digit is to the source of Sonic hedgehog.

We would never have scales, feathers, or breasts if we didn't have teeth in the first place.

When the finely tuned balance among the different parts of bodies breaks down, the individual creature can die. A cancerous tumor, for example, is born when one batch of cells no longer cooperates with others. By dividing endlessly, or by failing to die properly, these cells can destroy the necessary balance that makes a living individual person. Cancers break the rules that allow cells to cooperate with one another. Like bullies who break cooperative societies, cancers behave in their own best interest until they kill their larger community, the human body.

Each galaxy, star, or person is the temporary owner of particles that have passed through the births and deaths of entities across vast reaches of time and space. The particles that make us have traveled billions of years across the universe; long after we and our planet are gone, they will be a part of other worlds.

We were staring at the origin of a piece of our own bodies inside this 375-million-year-old fish. We had a fish with a wrist.

Take the entire 4.5-billion-year history of the earth and scale it down to a single year, with January 1 being the origin of the earth and midnight on December 31 being the present. Until June, the only organisms were single-celled microbes, such as algae, bacteria, and amoebae. The first animal with a head did not appear until October. The first human appears on December 31. We, like all the animals and plants that have ever lived, are recent crashers at the party of life on earth.

With respect to the ZPA, what kind of digit develops depends on how close the digit is to the source of Sonic hedgehog.