How did they arise? How did they diverge from one another? How were each of the three related to the two others? Which came first? Why did just one of the three lineages lead onward to all visible, multicellular organisms—all animals, all plants, all fungi, ourselves—while the other two remained unicellular and microscopic, though still vastly abundant, diverse, and consequential? And what kind of creature, or process, or circumstance preceded them all? Where was the tree of life rooted?
