What is a life cycle?
When people think about living things and their life cycles they often also begin to wonder about life itself. The question "Where did living things come from?" has been asked down the centuries but it is only in the twentieth century that scientists have been able to put different pieces of information together and make a suggestion. They used information from fossils and from their studies on how they thought the early Earth might have been.

Fossils are the preserved remains of animals and plants. They form if a living thing is buried so quickly after it dies that its body cannot rot away. As scientists studied fossils they found that the deeper they dug into the ground, the older the fossils they found. As they looked at fossils from different times they found that the most recent ones had shells and bones and the earliest ones had soft bodies. They found that the very earliest fossils were very tiny.

From studying the Earth's rocks, scientist have also been able to find out what the early Earth was like. They think that the Earth formed from rocks in space colliding together. They formed a huge molten ball of rock. Its surface cooled down and became solid. Thousands of volcanoes erupted across the Earth's surface and shot out steam and gases to make the first atmosphere. There were huge storms and long periods of rain that formed rivers and seas on the Earth's surface.

Rocks are made from chemicals called minerals. As the water flowed over the rocks it dissolved many of the minerals and carried them into the sea. In time, the sea became like a chemical soup.

The early atmosphere contained different gases from the ones that are present today. As lightning flashed in the storms, it changed some of the gases in the atmosphere into those chemicals that we find in living things today. Scientists think that these chemicals joined the others in the sea. At the edge of the sea, you often find rock pools. These were also present on the edges of ancient seas. It is thought that some of the water in these rock pools evaporated. This made the water more concentrated and brought the chemicals dissolved in it closer together. In some way that has yet to be worked out, the chemicals joined together to make sheets which rolled up and formed tiny balloons. Inside each balloon other chemicals gathered. A tiny balloon with its chemicals formed the first living thing and the first life cycle began.

What was the life cycle of the first living thing like?
It is thought that it fed by taking in more chemicals. As it did this, it grew. When its body was large enough, it reproduced by splitting in two. Each of the offspring then began their life cycle. They too would feed, grow and reproduce. In time the seas teemed with tiny living things.

Do any living things today have a life cycle like the early life forms?
Yes. They are very tiny and you need a microscope to see them. One of the best known is a living thing called an amoeba. It lives in ponds. An amoeba is like a tiny balloon of jelly. In the centre of its body is a dark egg-shaped structure. It is called the nucleus and contains the chemicals that keep the amoeba alive. The amoeba feeds by forming tiny arms that stick out from its jelly body and wrap around other small living things. It traps them in a bubble of water and digests them. When an amoeba is fully grown, its jelly and nucleus divide to produce two offspring.

How did more complicated living things form?
Scientists think that some of the tiny balloons stuck together and formed a ball. This led to some of the balloons on the outside of the ball developing paddles to move the ball along, while the balloons in the centre remained unchanged. Later, some of the balls of balloons stuck to rock and formed cylinders with an open top. The balloons on the outside formed a protective skin while the balloons on the inside specialised in digesting any food that entered the cylinder. They passed their food to the other balloons in the body.

Are living things made of these tiny balloons today?
Yes, they are. We call them cells. The human body is made from about sixty billion of them. There are many different types of cell but almost all of them have the same basic structure of the first life forms - a skin called a membrane, a body full of a watery jelly containing many chemicals and a nucleus.

Why is the nucleus so important?
It contains chemicals which carry instructions. The instructions tell the cell how to develop.

When does a life cycle begin?
Some scientists believe that the life cycle of every living thing begins when its first cell forms. This cell divides into millions of cells to make the body of the living thing. The early stage of this development may take place inside a seed if the living thing is a plant, or inside an egg if the living thing is a bird or an insect. We usually say that the life cycle begins when the seed sprouts or the egg hatches. The early stage of development of humans and other mammals takes place inside the mother. We usually say that the life cycle of a human or mammal begins when they are born.

Do scientist have any other ideas about how life on Earth began?
The chemicals that are needed for life have been found on rocks and dust which have landed on the Earth from space. Every day tonnes of dust enter the Earth's atmosphere from space. Some scientists believe that this dust could have helped to form the first living things on the planet. They also think that, as there are these chemicals in space, they could have landed on other planets in other places and helped life start there too.

What would the life cycles of living things on other planets be like?
They would be like the life cycles of living things here. They would start, there would be a period of taking in nourishment and growing, there would be a period of reproduction and, at some time after that, the life cycle would end as the living thing died.