I recently borrowed a book from my sister who has a Master's in biology, Zoology by Dorit, Walker and Barnes, about 900 pages, intended audience is college biology students I believe. I've been reading it over the summer, and I must say it's the most interesting book I've read in years. I can't recall the last time I've learned so much in such a short time span.
The book goes through what stuff cells are made of, how respiration and cell division works, how protein compounds are made, how it is thought various solutions to problems evolved, how various features of species stem from physical limits and constraints in the environment. For instance, tiny animals don't have lungs. They don't need them because no part of their internal body is ever far from air or water. The complicated blood and lung system of humans is really an extraneous necessity required by our huge size.
The book then covers the various animal groups one by one. As it turns out, most of the stuff I at least tended to associate with humans and higher animals is really nothing new. Sex was not invented with the monkeys. Also air is a pretty inhospitable environment, as the book puts it, every animal not living in water is constantly living at the risk of desiccation.
One of the things I've learned is that the world of cells is an extremely complicated world in its own. A big animal, like a human, is really a big blob of cooperating cells, each cell being a living being in its own right, although of course dependent on the others. The cells in your foot are dependent on the cells in your mouth to swallow food and the cells in your intestine for taking in nutrients from the soup that passes through. And vice versa, no feet means no catching food, at least in nature.
It's a purpose of life, feeding your cells, and a good one I believe. But each cell is actually a pretty complex thing, with small bacteria-like sub-components called organelles like the mithocondria that take in oxygen and various other stuff and output cell fuel, ATP, not to mention the complex chemical machinery that replicates and executes the DNA code. Cells have a life of their own. For instance some cells in the body feed on the bacteria and other stuff that enters the tissue by engulfing and digesting it; that's an important aspect of the immune system.
Likewise, the interaction between the cells is highly developed, not the least keeping in mind that the chemical cell machinery must at all stages be able to self-repair. It is also grown, unlike a human-made machine in the macro world assembled from ready-made pieces, all animals and all their parts must be made from tiny single cells that collect nutrients and out of them build bigger structures.
And everything is controlled by the laws of physics and long self-replicating double-helix strands of 4-character code that specifies what happens when.
It is simply amazing.