Is there a scientific reasen for the fact that the left arm and leg is connected to right hemisphere?

This all comes down to the way that vertebrates evolved, and it is a classic example of evolution producing a workable though complicated design that no intelligent designer would ever produce unless she were on drugs.

To explain let’s go back to the very earliest chordates. We are going back here to a time before our ancestors were fish, before they even had backbones. These things existed as essentially free-swimming worms a few inches long. No fins, no jaws, not even a real mouth, no limbs of any sort, just a body and a head. Every body segment aside form the head was identical and had just 4 muscle blocks.

These things lived by swimming around in the water filtering plankton from the water through a hole in their front end. That hole wasn’t a mouth, it was just a hole lined with beating hairs that drew in water.

With such a simple body plan these creatures didn’t require much nervous control by the ‘brain’ to the body. Each body segment was controlled by a simple reflex arc from those 4 muscle blocks to the spinal cord. The brain could make them more or less responsive to stimulation and so cause the animal to turn left or right, but direct control from the brain to the body was extremely limited and there were correspondingly few nerves running from the brain to the body.

What I haven’t mentioned is that this creature had EYES, and therein lies the reason for the crossing over. The early chordate brain was excessively devoted to operating some rather complex eyes that allowed it to see both food and predators. So why are the eyes so important? Because when light enters they eyes it… crosses over. It inverts. You can follow the link below to get a better idea of this, with diagrams. But for now simply remember that the eye has a lense, and a lense inverts an image left to right as well as up and down. So with a brain devoted to processing inverted images collected through a lense it makes sense for the left brain to control the right body. After all the information that comes from the left eye will in large part be telling the brain about conditions on the right side of the body.

And so that is exactly what happened. These earliest chordates had (and still have) a brain with massive visual centres and limited control of the body itself but, and this is vital, the left visual centres are largely located in the right brain. To move towards food on the left side of the body for example the signal form the left eye is processed by the right brain, and the right brain then sends the appropriate signals to the left body. To move towards visually acquired stimuli the signal has to cross over from the side of the brain that processed the data to the OPPOSITE side of the body.

As chordates became larger and more complex one of the things they did was to grow paired fins. They ceased being wormlike and became fishlike. But paired limbs require far more fine and centralised control than can be provided by a reflex arc to the spinal cord. So the brain assumed control.

The same brain that had evolved a complex crossover so that visual data corresponded to the correct side of the body was now being asked to assume direct control of paired limbs. Of course evolution works with what it has, not with what is best. The brain had a complex crossover for controlling the muscles of the body, and it couldn’t re-wire that, so it simply built on it. Limb control also crossed over form the left to the right brain even though it was no longer under visual control.

And as we became more and more complex we simply kept the same convoluted design. The reason the left brain control the right arm today is simply because hundreds of years ago information form the left eye was processed by the right brain, which was a very sensible solution at the time.

Honestly, I don't know, but I would say that is a great science question.

If we looked at it from an evolutionary process, it must have given us some advantage somehow. I guess the answer would lie in researching the biological advantages to having alternating hemispheric control of the body, versus same hemisphere control (Do other animals have similary physiology as we do, or are we unique would be another good follow up question.)

Ask a doctor, biologist, or anthropologist. You might want to consider studying such a thing in college or advanced biology in high school. That insightful question could lead you into a good line of work.

Man shares 99.997% of his genetic code with chimpanzees. The cimp itself has evolved from a series of creatures that ultimately bring it to a creature that crawled upon its belly. At that stage, the creature did not need to look forward into the ground but rather up toward the sky and away from the ground. So, in a sense it head was on pointing in the 180 degrees from today’s man. As a result its head was facing its back. Thus, right hemisphere controlled right side, and left to left. Evolution in all her glory saw no reason to change as we became bipedal and developed heads to look forward.

The left half of the body is contrlled by the right half of the brain and vice versa. Since the Nerves which are controlling the body decussate in th midbrain.

this is because the nerves inside you brain star out at the correct side but when they get to the medulla they cross over and continue on the wrong side all the way down through the body