Life On Earth

Life on earth
The first signs of life appeared on earth about three billion years ago in the form of algae and protozoa . these organisms are the smallest living things .they are unicellular forms of live . their biological functions are very elementary and their organization is very simple .
Since then living things have undergone slow butpermanent change . some of these protozoa grouped together in a single organism like a colony as a result different members of the colony specialized in different functionsfor example some cells specialized in obtaining food some others in food digestión the sponge is an example of this form of life.
The changes continued some organisms developedautonomous movement andbecame animals . others developed the ability to produce food and they becama plants.
The first animals were fish. Some fish climbed out of the wáter anddeveloped amphibians and reptiles new changes produced birds and mammals from the reptiles all these changes had one important cause and one important effect the causewas the struggle for survival the effect was the complexity ofbiological organizationmodern plants and animals constitute a diversified collection of beings with highlyspecialized functions.
Adolescence
Adolescence is a period of biological transition . however it affects the person in his social behavior and in his psychologicaldevelopmentit usually covers the period between the ages of twelve and eighteen years .
Adolescence is marked by the maturation of the reproductive función the development of the sex organsproduces special chemicals or hormones the hormones determine changes in the appearance of the body they prepare the body for the important function of reproduction ....