| >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. |
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.