In the header image, you see a very simplified diagram of the human heart that I drew in my first year of college. If you will, study it closely. What you see before you is a very complex and highly functional design, created with the intention of keeping its owner alive.
The heart is a complex organ made up of numerous components, each of which serves an important role. If even one of those parts was absent or defective, the entire organ would cease to function normally, the circulatory system would fail, and the owner of the heart would very likely die. Take, for example, the valves. To understand their purpose, think back to your childhood. Perhaps you remember "Frubes". Frubes (depicted right) are long, tubular yoghurts that were eaten by tearing the top off, and forcing the yoghurt upwards by squeezing. Imagine if the hole through which you would drink the yoghurt had a twin hole on the other end. What would happen if you squeezed the frube? Obviously, the yoghurt would come out of both ends.
This is exactly what would happen to your blood every time your heart beats, which for obvious reasons is not a good thing. However, the valves within your heart are a one way passage. This means rather than blood going in both directions, your heart pumps blood in a single direction. The blood enters the heart through the Vena Cava (and no, contrary to popular belief, it is not blue when it does so). It then travels through the first valve into the right ventricle, where it is pumped through a second valve, up through the pulmonary artery. It will travel through this artery to the lungs, where it will collect oxygen, before being sent back to the heart through the pulmonary vein. It then travels through a third valve into the left ventricle, before finally being pumped through a fourth valve and being sent through the aorta at more than half a mile per hour. Every minute, around 5 litres of blood travels through your heart at resting rate.
With that explanation alone, we have only begun to explain the complexity of the circulatory system. Not depicted in my diagram are various nodes, which obviously need regulating, that cause the heart to beat. The various blood vessels are designed in different ways according to their respective purposes. Even the very blood cells are shaped in a very specific way to allow it to perform the function it does.
Each of these features of the circulatory system are vital to its purpose. If any of it was any different to the way it is, the system would fail, or at the very least struggle. This irreducible complexity in the circulatory system alone makes Evolution of the heart completely impossible. The heart had to be formed instantly, with each and every part of the system pre-planned and designed as necessary. Any intermediate species between a creature that did not need a heart and a creature that did would have no chance what so ever of surviving, and if you don't survive, you don't evolve.
And all of that is just an extremely simplified explanation of the circulatory system with special emphasis on the heart. Your body is an incredible machine. That machine is controlled by a supercomputer, which is wired to all the individual systems in your body, all of which are made up of many different components, which are themselves made of millions and billions of even tinier machines which are all working together in great harmony. These machines are made following the most complex language system in all the universe: DNA. With all this in mind, it is more likely that the device on which you are reading this text was created by throwing a stick of dynamite into the kitchen than for a single cell in your body to have formed without intelligent intervention. Irreducible complexity is one of the strongest arguments that can be made against Evolution. Through it, we see that Evolution is 100% impossible.