From the Arctic to the Amazon rainforest, from New York City to Tokyo, people may appear a little different, but those differences are superficial.
Under the skin, our bodies look the same and work in identical ways. What is remarkable, though, is how adaptable we are. Thanks to their initiative and intelligence, people have adapted to a variety of lifestyles in contrasting locations
ANCESTORS In African forests, around seven million years ago, our apelike ancestors started to walk on two legs. Being upright, their hands could perform tasks, and they could spot predators from afar.
Over millions of years, evolution equipped hominins (the human line) with bigger brains, the ability to harness fire, make tools, and develop culture.
HOMO HEIDELBERGENSIS Heidelberg man (Homo heidelbergensis) was taller and bigger-brained than Homo erectus, but still had big brow ridges and a flat forehead.
海德堡人海德堡人（HOMO HEIDELBERGENSIS Heidelberg man，海德堡人）比直立人更高，大脑更大，但仍然有大的眉脊和平坦的前额。
Possibly a direct ancestor of both Neanderthals and modern humans, they lived in Asia, Africa, and— a first for hominins—Europe between 800,000 and 250,000 years ago. They were not scavengers, but skilled hunters, who, after the kill, butchered deer, rhino, and other prey using stone tools.
CELLS Imagine you could take a tiny sample of body tissue and look at it under a microscope. You would see that it was made up of tiny, living building blocks, called cells.
In all, among the 100 trillion cells it takes to build a complete body, there are some 200 different types of cells, each with their own shape, size, and function.
MULTIPLICATION Each one of us started life as a fertilized egg. That single cell gave rise to the trillions of cells that make up the body by a process of multiplication called cell division, or mitosis.
The nucleus of every cell contains 23 pairs of threadlike chromosomes that hold the instructions for life.
During mitosis, as shown here, chromosomes are duplicated and separated into two new identical cells. As well as enabling us to grow and develop, mitosis produces billions of new cells every day that are used to repair and maintain the body.
SYSTEMS A total of 12 systems work together to make the human body work. Each system consists of a collection of organs that cooperate to carry out a particular function or functions.
For example, the organs making up the digestive system dismantle complex molecules in food to release usable substances, such as glucose or amino acids, that are utilized by body cells to supply energy or build structures.
LIVING IMAGES In the past the only way to look inside the living body was to cut it open. In 1895, X rays were discovered, providing a way of imaging the body’s insides from the outside.
Today, doctors have access to many imaging techniques that help them to diagnose disease so they can start treatment quickly. Many techniques use computers to produce clear, precise images of not just bones—as early X rays did—but of soft tissues and organs as well.
HAIR We may not have the luxuriant fur of our mammal relatives, but our skin is still covered with millions of hairs.
Most of those on the body are short, fine vellus hairs that, when tweaked by visiting insects, warn us that we might be bitten or stung. Thicker, longer terminal hairs are found on the head, eyebrows, and eyelashes.
Head hair is protective and also forms an important part of our appearance. Eyebrows and eyelashes both help to protect the eyes. In men, terminal hairs also grow on the face and ches
PASSENGERS Unknowingly we are carrying on our skin a variety of, mainly microscopic, passengers. Most are parasites that feed on our skin cells, secretions, or blood.
Everyone, without exception, has billions of bacteria on their skin. Many of us are home to tiny eyelash mites, distant cousins of spiders. Less common are other types of mites and their relatives, the ticks.
Children often become infected with small insects called head lice. Another blood-feeding insect, the human flea, is much rarer. Other hangers-on include fungi and leeches.
LIFE STORY Every person, provided they have a normal lifespan, follows the same sequence of mental and physical changes from infancy to old age. Our life story includes rapid development and learning as an infant and child.
Then, the great changes of adolescence during the teenage years, when we switch from being children to adults. As adults, we mature before starting to age and “slow down.
BONES These living organs consist of a matrix of collagen fibers, which produce strength and slight springiness, and mineral salts, particularly calcium phosphate, which give hardness.
This matrix is made and maintained by bone cells. Bones are both strong and light because denser, heavier bone tissue is found only on their outsides, with lighter, spongy bone inside. As well as supporting the body, bones also make blood cells and store calcium
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