[deadbeeves]

>They're each extremely complex

Yes. Now. Not when they first emerged.

>but useless without the other two

Unsubstantiated assertion.

>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.

[Sprocklem]

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.

[zakki]

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?

[1] https://youtu.be/z2_-h3I_WXQ

[2] https://youtu.be/0TgKW-dj-wo

[deadbeeves]

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.

[ceejayoz]

> If the life formed by randomness, according to this old Discovery video[1], it defies law of probability.

Evolution is not random. The mutations that make it possible are, but natural selection makes it a non-random process.

If you make a random number generator, but keep more of the odd numbers, you'll get a non-random result set.