The heart, a hollow organ with four chambers, can be imagined as an unbelievably strong pump. It is about the same size as a fist. Electrical signals coming from the brain cause the heart muscle to contract about 70 times in a minute in an adult. The contractions are more frequent in children and while we are playing sports. These contractions pump the blood through the whole body through the circulatory system. This system consists of two types of vessels: arteries with blood rich in oxygen and veins with blood poor in oxygen. While the arterial blood supplies oxygen to every cell in the body, the venous blood is sent to the lungs where it is purified by removing the waste product carbon dioxide and absorbing new oxygen.
Our body consists of millions of cells, which must be supplied with water properly. This supply is the responsibility of the blood. The red blood cells transport vital oxygen around the body. The blood also absorbs nutrients from the digestive tract, distributes them to the organs, and in return, takes away the waste products from the organs. Blood contains platelets that seal the wounds and white blood cells that help us fight disease-causing germs.
The human body is made up of 80% water, which is present in blood and other cells. Since we lose a lot of fluid through sweat, urine, and breath, we need to replenish it constantly. Drinking less water can cause immediate problems. The body cells become dry, and the skin wrinkles. Blood becomes thicker, and the heart has to make more effort to pump it through the body. Since the blood is responsible for transporting oxygen around the body, oxygen is not supplied adequately to the organs. The heart and the brain react immediately to a lack of oxygen—they get damaged in just a few minutes. 
The Neanderthals were people who lived 130,000-30,000 years ago, but then died out due to unknown reasons. One theory is that other lines of humans were stronger and eliminated them. The Neanderthals were about 1.70 m tall, had a heavily built body and were very powerful. They needed this power and strength to hunt woolly mammoths and rhinoceros with their home-made spears and knives. They made clothes of animal skins, lived in caves, and used fire. Their name is derived from the place where their remains were found, the Neander valley near Dusseldorf. 
Today, one can think of the apes and humans as cousins. They have common ancestors, but have not descended directly from each other. About 5-7 million years ago, a line of man-like apes grew apart, and evolved independently of the other apes. The oldest, almost completely intact skeleton of an upright ancestor of human – the Australopithecus – was found in 1974 in Ethiopia (Africa). The scientists named it ‘Lucy’.
Biologists classify humans as animals and part of the animal kingdom. Like our nearest relatives, the apes, humans are also mammals. Humans have evolved from apes, but there are features that differentiate us from other animals—including apes. These include the upright walk, our opposable thumb, our special ability to communicate by a language, and our highly developed brain. Our brain allowed our ancestors, like the Neanderthals, to invent tools. [custom_search_and_filter]