The first thing any embryo must do before it can develop into an organism is establish early polarity—that is, it must set up a way to distinguish its top from its bottom and its back from its front.
Topic
How embryos across different species establish polarity (body orientation), and how this phase differs from later development
Framework
Old / New (There is also a strong "Highlight Noteworthy" element as the passage spotlights a paradox, but the main structure is an Old/New contrast between early vs. later mechanisms.)
Main Point
The earliest step in embryonic development—establishing polarity (front/back, top/bottom)—uses very different mechanisms across species, but for later developmental stages, like forming limbs and organs, animals rely on remarkably similar genetic processes. (The Most Valuable Sentence is in the last paragraph: )
P1: Introducing Embryonic Polarity and Its Diversity
All embryos must first establish their basic body orientation (polarity), but the ways in which different organisms create this early spatial framework vary a lot—the signals flies use, for example, are very different from those used by nematodes or mammals.
P2: How Different Species Establish Polarity
The paragraph details how specific animals get polarity started: fruit flies prepare the "instructions" in the egg before fertilization; nematodes use the site of sperm entry as a guide, moving certain proteins to help create body orientation; some simple vertebrates use a sperm-driven cue, but mammals don't seem to. In humans and other mammals, polarity happens much later and the process remains mysterious.
P3: Paradox—Similarity in Later Development
Once body orientation is set up, all animals rely on a surprisingly similar set of genes to develop complex structures like eyes and limbs. This creates a paradox where embryos use very different strategies early on, but later use almost identical ones to elaborate their body plans.