I don't think you need that much context if you start with an organ system. For example, I got interested in biology by learning about how the kidneys work. You can give a pretty handwavy explanation to get someone oriented and then go into more and more detail (for example about the structure of the nephron, the hormones involved, how filtration works at a molecular level, etc.)

The other organ systems are similar. Once you know about the organ systems development and genetics are well motivated and easier to pick up. And from there you have a pretty good scaffold from which to fill in the rest of human biology.