mos to mean "everything there is". We will see that the 
About 300 t housand years after the Big Bang, the Universe had cooled enough for electrons to be captured by protons and alpha particles to form atoms. 
nd of billions of stars, separated from other galaxies by a vast ocean of almost empt y space. In this story we look at one particular galaxy (the Milky Way), since that is the one we know best, the one where we live. But we should not forget that, scattered far and wide across the Universe, there are billions of other galaxies, probably very similar to ours.
e seen that a small red giant, up to 1.5 times the size of the Sun, turns into a white dwarf when it dies. Larger red giants, however, die in a m ore spectacular way. 
play a major part in this story. The first molecules were formed in space. Some atoms which came out of a supernova were too light, and so moved too fast, to glue together with ionic bonds. They t raveled out of the star until they were cool enough to attach to a dust grain. There these light atoms met and join together with a new type of bond called a covalent bond. Molecules are the building blocks of life, made mostly of the atoms hydroge n, oxygen, nitrogen and carbon. The shell model of the atom explains what molecules can be formed by which atoms.
Planets are lumps of gas and rock held close to a star by the force of gravity. We live on planet Earth going round star Sun, along with eight other planets. Together these are called the solar system.
In this web site we will focus down on one tiny planet: the Earth. Notice from our diagram and model of the solar system how small it is compared to the Sun and the giant planets Jupiter and Saturn. If we didn't live here we probably wouldn't even notice it! Yet there is a very good reason why we should look at this planet and no other. The Earth is the only planet on which water forms a liquid, which is essential for life. The reason has to do with its distance from the Sun. A planet further from the Sun, like Mars, is so cold that water freezes into ice. Closer to the Sun, like Venus, water boils and all the molecules fly apart. Only on the Earth can water form that marvelous substance, liquid water. The Earth, like most of the other planets in the Solar System, has an almost perfectly circular orbit. This is unusual. In most of the other planetary systems studied the planets have oval (elliptical) orbits. If the Earth had an oval orbit, travelling sometimes near to the Sun and sometimes far from it, life could not have evolved on the planet. At times the oceans would have boiled and at times they would have frozen, and life as we know it would have been difficult if not impossible. 
ow when because the rocks have been s quashed and changed so much since then, but it is important because it is still happening today. Some people think that it is one cause of ice ages. 
r covering. This armor plating around their front ends was made of bone. Perhaps it was a defense against sea scorpions. They first appeared around 500 million years ago, during the Ordovician period as small fish-like animals called ostracoderms.
skates and rays evolved from jawless fish about 390 million years ago. Although their ancestors had bone, the shark group at some stage lost the ability to make bone, and their skeletons are made of cartilage. Cartilage is what we call "gristle" and in humans it makes the ears, nose and much of the skeletons of babies.
ade of cartilage. Cartilage is what we call "gristle" and in humans it makes the ears, nose and much of the skeletons of babies.
rom jawless fish about 390 million years ago. They lived in fresh waters subject to seasonal droughts. Their swim bladder evolved into a sac able to breathe air, similar to the lungfish of today. They had fleshy bases to their fins which were strong and flexible, letting them leave drying pools to find those ponds with water. They evolved into the amphibians. Most are now extinct, except for coelacanths and the lungfishes. They were the ancestors of the amphibians and all higher types of vertebrates, including man.
Some fish lived in ponds which dried up in summer. Their swim bladders evolved into lungs which they used to breathe air. They used their fins to crawl from one pond to another and these evolved into legs, two at the front and two at the back.
n years ago ferns the size of trees were common. They had solved most of the problems of living on land but were still tied to moist ground for their reproduction. Many ferns grew in swamps. They grew from a small underground growth called a prothallus. The sperm swam from one prothallus to fertilize the egg on another. Without water or wet ground they could not reproduce.
a group of vertebrates called reptiles solved the problem of how to reproduce without water, and they did it in exactly the same way, as the insects. Fertilization occurred before the eggs were laid, by the male injecting sperm into the female's body.
the egg. Also it took over a year for the plant to store enough food in the seed to make sure the baby plant could grow.
Improvements in reproduction were happening in the vertebrate world too. Instead of laying their eggs, female mammals kept them inside their bodies while they developed. In this way they protected their young, feeding them and giving them oxygen. The young could develop larger brains and more advanced bodies than any reptile.
