>These explanations are always full of "must have" and "must have been".
Any explanation that starts with "must have" requires that there is a single theoretical framework that could possibly apply to the world as we see it today. Since you reject this necessity, you must think there's an alternative framework that accounts for all observed phenomena (or that accounts at least as well as the theory of evolution). What is that framework?
It IS strange, using the evolutionary framework itself, that useless mutations will hang around for thousands or millions of years (countless generations) before dropping perfectly into an extremely complex system, that itself is but a small part of a larger complex system.
Evolution teaches that small, immediately useful mutations build upon one another, reinforcing the beneficial nature of the mutation. It's impossible to build large, complex life systems (with a large number of prerequisites at each step) this way. For example, the Heart, Blood Vessels and Blood. They're each extremely complex but useless without the other two.
How would evolution slowly evolve a heart with no blood or blood vessels?
How would evolution slowly evolve blood but no blood vessels or heart?
How would evolution slowly evolve a network of blood vessels but no blood to carry or heart to pump?
>How would evolution slowly build a heart with no blood or blood vessels? How would evolution slowly build blood but no blood vessels or heart? How would evolution slowly build a network of blood vessels but no blood to carry or heart to pump?
You start with a fluid-filled body cavity where oxygen is transported by dissolution from the outside to the inside. This fluid already contains oxygen-transporting cells. Then you section off part of the cavity to enable oxygen-transporting cells to move more efficiently. At this point the OTCs are completely separated, so the fluid inside the conduits can be called blood. The animal can pump blood by squeezing its body during its normal locomotion. Eventually muscle cells are added to the circulatory system to enable oxygen circulation independently of the animal's movement, as well as to circulate it even more effectively. Later on the muscle cells become centralized as it's more efficient, not to mention that a big single muscle can pump more strongly than a distributed system of tiny muscles. Each step of the way you have a functional organism and each form performs the function of transporting oxygen more effectively.
I know you're going to say that this is a "must have" explanation, but you merely asked for an explanation. The above is a plausible series of events that could have led to the circulatory system as we see it today, so if you want to argue that the circulatory system couldn't have evolved, you'll need to argue why this explanation isn't plausible.
EDIT: Also, I can't help but notice you ignored the question I asked in my last paragraph.
I'm not sure how exactly the circulatory system developed in our ancestors, but I will add that simpler circulatory systems do exist in animals today, including the open circulatory systems that are found in, e.g., arthropods. In these animals, blood is pumped across an body cavity by a heart, before flowing back through the fluid-filled body cavity itself, outside of any blood vessels. It is definitely possible to have "partial" circulatory system that functions fine while lacking some of the components present in humans.
Similarly, simpler versions of, e.g, eyes have been observed in nature with structures that are thought to be analogous to those of our early ancestors when eyes were first developing.
I guess I can see where the parent thinking you replied came from. If the life formed by randomness, according to this old Discovery video[1], it defies law of probability.
If each step from simple to complexity is using this randomness evolution, it will be more and more defies law of probability.
Wikipedia said earth had water 4.4B year ago. The first cell formed 3.8B years ago.
This PBS video[2] said multicellular life emerged 1B years ago. Other source said 1.7B years ago.
And then the first modern human appeared around 300 thousands years ago.
So the question is: is 700M-1400M years enough time for the probability to create human?
There are crappy combination padlocks that can be picked by turning the combination wheels and pulling on the shackle. When you've turned a wheel into its unlocking position the shackle will shift open slightly, signalling that you've cracked that digit. So a padlock with 3 digits could be opened in at most 30 moves, instead of 1000.
When you have a selection mechanism, the randomness of mutations becomes kind of moot. Each step of the way that produces a new adaptation only has a handful of optimal (or optimal-enough) solutions. Now, suppose that we're an underwater species that's in the process of developing light-sensitive cells, and from our present genome there are 2^64 possible genes that will produce a good protein for the transduction step (of converting light into some other form of energy). Do we need to mutate 2^64 times to find the "correct" gene? No, all of those 2^64 genes are "correct". Our descendants should not fall into the trap of thinking that because they got gene #5541741487894936799 that it was a special outcome. They could have just as easily gotten gene #5541741556614413535.
An appeal to incredulity is not an argument.
>These explanations are always full of "must have" and "must have been".
Any explanation that starts with "must have" requires that there is a single theoretical framework that could possibly apply to the world as we see it today. Since you reject this necessity, you must think there's an alternative framework that accounts for all observed phenomena (or that accounts at least as well as the theory of evolution). What is that framework?