The one thing that is certain is that these organisms originated in waters, the seas which were swarming with a variety chemicals and organisms. A rich, viscose solution chock full of goodies. A delight for all sorts of organisms.
From the seas we have sprung and so great is our connection to the seas of the Earth that our blood is like an ocean-to-go in our bodies. A lasting, powerful connection to our origin on this planet. A tie that binds all animals to our common primordial origins.
As a constant reminder from whence we have sprung, our blood is a telling reminder of the powerful forces of our environment which shaped us through evolution. In this a case, a clear example of how the physics and chemistry of our planet Earth not only shaped evolution but also have been embedded into our very being. We are one with oceans and seas of the Earth and it's ancient rhythms flow through us like the tides that rock our planet.
The effects of the physical world upon evolution are everywhere and quite literally shaped the way life moved from a single cell to many celled organisms.
A single cell has a fixed, limited surface area to water ratio. It is tiny and the water surrounding it is mighty. Huge, large, almost infinitely larger in the case of a planetary ocean.
The microbes can travel only so far. Ocean currents push and pull even those with onboard locomotion great distances and in any direction.
One way to resist the the ebb and flow is to evade it by burrowing into sand, rocks, other organisms and/or through the water itself. But how does one burrow in the first place? An eukaryote's outer cell membrane is a little flimsy allowing for limited burrowing, while a prokaryote's have a more solid cell wall is but hampered by the lack of powerful enough locomotion. Indeed it looks like eukaryotes evolved into better eukaryotes by being able to control their amorphous-nature as evidenced in amoebas, thus making them in some way maybe super-amorphous but still limited in their movements through a substrate.
As the physics of Earth placed constraints on organisms, the only way to break free of these limitations was to evolve a better organism. One that that did not look too far off from our single cell ancestors but yet could do extraordinary things in comparison.
An organism that would, like single cells, get their living needs through their outer layer, be very simple in form but better able to either 1. sense the environment or 2. manipulate the environment.
One way to provide a sturdier organism is to form a structure that better resists the push and pull of currents, tides and other forces. One way for an organism to pass more easily through it's environment is to increase its tensile strength.
We can all see for ourselves that triangles are much more stable and incredibly strong structures that can resist outside, environmental stresses. One of the most powerful of all triangles are equilateral triangles. A key feature of equilateralism is the concept of bilateral symmetry. An entity that is the sum of two mirror images merged into one perfectly separable entity.
Symmetry and especially complete mirror or bilateral symmetry does not start with evolved organisms but the thing organisms are made of, molecules.
There are a few different types of symmetry in molecular structures but in particular enantiomers and stereoisomers paved the way long before organisms developed their own bilateral symmetry and before organisms even evolved.
When you look in the mirror you see a complete organism but one that can be thought of as the combination of two mirror image sides, the left and the right if we split ourselves down the middle.
We know single cell creatures use DNA to duplicate themselves into other single cell organisms. All that one particular single creature has to do is to duplicate itself once and at the same time, duplicate another mirror image of itself, and in that process bind to each other to create a multi-cellular organism. Given the way DNA and RNA behave it is not much of a stretch to suggest that some sort of RNA/DNA interaction could create a mirror image, as it were, of a single cell creature and since it came about as a result of one cell dividing, one creature is produced instead of two, the two fused together in the process.
Symmetry can be seen from both molecular and physical attributes to provide superior advantages over non-symmetry, even from before life begun on this planet.
Fortunately for us, we have an organism that displays the properties one might expect in going from the one to the many, flatworms or platyhelminthes. They are bilaterally symmetrical, allowing for much better movement through the environment while also displaying single-cell phenomena like getting essential respiration or metabolic energy in and out via diffusion and having no body cavity (coelem).
While blood may be like an ocean to go, the Coelom ('sea lum') is the case that allows us to take it with us.