One of the deep problems that both supporters and detractors of the theory of evolution wrestle with is, effectively, the problem with emergent design. How did things get to be the way they are? More accurately, given the enormously complex and intricately interdependent interplay of fully formed, specialist agents in today's world – how did life get from there to here? If life started as unicellular organisms (or even further back as amino acids), how can something as complex and apparently well-designed as a human being, with all its well-designed component bits and pieces, emerge? To cite a commonly used example, how could something like the eye emerge?
Whatever mechanisms are at play, we can be sure that there was never a time in the long history of life on earth when eyes, or hearts and lungs and livers for that matter, were self sustaining organisms, able to look after their own survival and evolving in their own right up to the point where they were useful to some other organism and were co-opted. That means that somehow, on the long road from eukaryotes (prokaryotes in fact) to human being - hearts, lungs and livers, etc. evolved along with the organism; that is to say, we started as a minimum viable product (self-metabolising unicellular organism) and iterated from there to here.
Easily posited, but, in the case of the cardiovascular system for instance, errors will be selected on mercilessly, intermediate forms of the lungs, or heart, are even harder to imagine than intermediate forms of the eye. If we examine our most recent ancestors in the tree of life or glance sideways at our cousins (chimps, bonobos etc.) and other close relatives we can see this confirmed – there are no substantive differences between cardiovascular systems current in mammals, including seafaring mammals like dolphins and whales.
But there are land animals whose respiratory & circulatory systems differ substantively from ours; insects have a so-called open circulatory system, they have no lungs and do not breathe as we do, and their hearts are very different to ours too. Insects have small holes in the sides of their exoskeletons (spiracles) that allow air to enter a system of tubes known as trachea. The trachea extend to all parts of the body, terminating as the thinnest of capillaries in the tissues where the oxygen will be delivered. The tissues are flooded with the rest of the fuel mix ('blood'; food, water and other essentials) as the heart only 'pumps' (by peristalsis) so far. This all means that the biochemical magic of metabolism in insects resembles so many tiny generators firing away on-site.
Mammals and insects are indeed extremely distant relatives, having diverged on the tree of life when vertebrates and invertebrates diverged, but I ask you to bear with me because an open circulatory system as such can act as a credible model for an intermediate form on the long journey from there (unicellular organisms) to here (human beings with closed circulatory systems). Nature, self-organising between limits and tipping points, acting recursively though selection, effects any transition through inexorable cumulative change over many many generations. We of course can be a little more direct, even if our method takes its cue from evolution. Thinking in terms of building software:
Expanding the generator(s) metaphor, we could define our architectural metaphor as replacing all those on-site generators with a single big generator working with a couple of heavy-duty pumps, whereby the fuel pipes have been exapted as power lines to deliver directly consumable energy to the cells.
Then we would start small: by de-coupling a single cell from its own power source for instance. To achieve that for that one cell, we would already need to have modified a small section of fuel pipe so that it could act as a power line, and have moved 'power generation' to outside the cell. We would also have had to protect the rest of the system from possible problems (given that they would probably be fatal!) by surrounding our experiment with tests, in short, our first delivery would have to represent a walking skeleton of our solution, a minimum viable feature.
Thereafter, iteration by iteration, through continuous inspect and adapt and constant re-factoring, our design is refined simultaneously both piecemeal and as a whole, emerging along the way in perfect tandem with the (details of the) goal that we are discovering.