According to the Big Bang model, the universe expanded from an extremely dense and hot state and continues to expand today. A common analogy explains that space itself is expanding, carrying galaxies of stars with it, like raisins in a rising loaf of bread. The graphic scheme above is an artist's concept illustrating the expansion of a portion of a flat universe.
The ancient scripture called Genesis opens with "In the beginning was the Word". This is probably true of modern human history, but this book is about all of life on earth. The best explanation that science has today is still quite controversial and liable to change as we discover more things (especially fossils) and learn more about what probably happened millions of years ago.
If we look back in time, we can imagine a moment when everything was nothing! Describing how everything in the universe is made is very complicated so we will use a simple model. In this simple model, everything is made of atoms. Atoms are made up of two or three things - they all contain protons and electrons while most contain neutrons as well. Electricity is made up of electrons flowing along wires. Splitting apart the middle of an atom - the protons and neutrons - releases huge amounts of energy which is what happens in nuclear reactors and nuclear bombs.
When anything is heated up the atoms move apart, and the material gets bigger. Cool it down and it shrinks. Someone has plotted the amount of shrinkage that occurs in different materials, and it seems that there is an 'absolute zero' temperature (around −273.15°C) when all movement stops. Nobody has ever made things this cold though - even outer space is warmer than absolute zero.
At the beginning of our universe, which was about 13.7 billion years ago, all the material and energy we see now was compressed into a minute ball too small to see. Then something happened, nobody knows what, that made the ball suddenly expand and explode outwards creating the beginning of the universe. All the galaxies in the universe are still moving apart due to this original explosion.
One lump of this material cooled down and about 4.5 billion years ago became the planet Earth, on which we live. It was a hot and hostile chemical soup that eventually cooled down and became solid. There was no life on this planet until about 4 billion years ago, when the ancestors of modern bacteria were single-celled micro-organisms that were the first forms of life to develop on Earth. Exactly how or why inorganic chemicals became rearranged into living cells is a hot topic in science - some think it was lightning or volcanoes that sparked life, but really we do not yet have a reasonable explanation that fits the evidence.
That is science. Perhaps if you enjoy and study these topics, you will become a scientist and help us humans evolve a good story that fits the facts. If you do, then like Charles Darwin, you will probably run into a lot of argument. The scientific method is a technique used to try and prove if an idea is really the truth. This means making a prediction about how something might have happened then working out what would happen if the idea was true. Then the scientist sets out to see if the facts fit the predictions. Scientists all over the world will continue to test the idea to see if it is still true in the light of new discoveries. This is why ideas evolve. A famous scientist, Issac Newton, predicted how many things worked. For a long time all the evidence made it seem like he was right until many years later another famous scientist - Albert Einstein - showed he was wrong. It wasn't long until Einstein was shown to be wrong, and so it goes on. One day we may know the truth, or maybe not.
Now you have probably heard about bacteria and viruses in medicine, because some of these tiny things can cause a lot of serious illness. Actually, most of them are harmless. A virus is not actually a living cell, but a fragment of life that can invade and change the nature of a cell. A bacteria is a simple living cell. When viruses invade bacteria they tend to either kill it or change its internal structure. Thus about 2 billion years ago complex living cells called eukaryotes emerged.
Following these initial breakthroughs, the time-line of the evolution of life on earth seems to looks like this:
1Radar topography reveals the 180 kilometer(112 mile) diameter ring of the crater; clustered around the crater's trough are numerous sinkholes, suggesting a prehistoric oceanic basin caused by the impact (Image courtesy NASA/JPL-Caltech). billion years of multicellular life,a bit like seaweed
600 million years of simple animals,which became sea cucumbers, which seem to link plant and animal life
570 million years of arthropods (ancestors of insects, arachnids and crustaceans),
550 million years of complex (but still microscopic) animals, the first truly independent and mobile living creatures able to search for food
500 million years of fish and proto-amphibians, now recognizable animals whose fossilized remains started this story
475 million years of land plants such as ferns which regenerate rather than produce seeds
400 million years of insects and plants which produced seeds which includes some initial protein for the infant plant to sprout
360 million years of amphibians, animals who emerged from the sea and began to live on dry land as well as in the water
300 million years of reptiles, - so called 'cold blooded' animals including the Great Lizards we call dinosaurs that need warm sun to function
200 million years of mammals, which are like us, able generate heat within their bodies and so function at night
150 million years of birds, which are the modern survivors of the age of dinosaurs
130 million years of flowers, which attract animals and insects in order to spread their pollen and reproduce by fertilizing distant plants
65 million years since the non-avian dinosaurs died out, probably after a huge meteorite struck earth at Chicxulub in Mexico, a moment called the K-T boundary
2.5 million years since the appearance of the genus Homo, of which large family we modern Homo Sapiens Sapiens belong, as well as several other similar animals
200,000 years since Modern humans started looking like they do today, and actually became wise (sapiens means wise, so we are the wisest of the wise animals)
25,000 years since Neanderthals died out. These were fairly wise cousins of modern man, who knew about using tools, but did not manage to survive through the ice ages.
One current view of the rise and fall of different human-like populations. Other interpretations differ mainly in the description and geographical distribution of various hominid (human like) species.
What seems to have differentiated us, the wisest of the wise from the other human type creatures was we alone were smart enough to survive the ice ages! Evolution is all about adapting to survive the threat of extinction from natural causes such as bad weather, disease or being eaten by predators. Evolution is all about sex and violence. Nature is said to be 'red in tooth and claw'
Probably you have noticed that quite a lot of creatures including many plants and animals come in male and female forms. The male provides pollen or semen with half the genetic code of its species, and fertilizes a seed or egg which has the other half of the code, so that it is almost impossible for any two creatures of the same species to be exactly alike, but they will all be very similar. We call these differences "diversity". Since we inherit half of our genetic code from our mother and the other half from our father, we will inherit some of the characteristics of each. You will also have noticed that we all have different skills: some like sports, others dancing, and some exploring for example. When something nasty happens, some of us cope better than others. In times of famine, when there is not much food, smaller people, who use less energy moving about, have an advantage over bigger people who have to eat more. If some of the big people die of hunger, future generations will be made up more of the children of the small people who survived. Small people tend to have smaller children, so the trait for people to be small begins to dominate. Evolution means that the best adapted individuals survive, and the worst do not.
