Fossil Dating

Uranium and its compounds emit radiation at a constant rate independent of environmental conditions. Consequently, uranium is converted to lead in a highly predictable fashion. When the earth was formed, uranium was doubtless present as a component of all radioactive substances. By determining the relative quantities of lead and radioactive minerals in rocks, it was established that the greater the age of any given rock sample, the greater was the proportion of lead to radioactive materials. Calculation of the duration of the various geological eras has accordingly been based primarily on the half-life for conversion of uranium to lead. For example; it has been estimated that the Paleozoic (or primary) era lasted for about 300 million years. This method of dating is referred to as radio chronology.

Life in the Cambrian oceans:
The Cambrian period, this began some 600 million years ago is the first period in the Paleozoic era. Despite its antiquity, numerous traces of it have remained in the fossil record. We know that there were many aquatic organisms during this period, including the unicellular forms, algae and multicellular animals, such as sponges, coelenterates, worms, shellfish and crustaceans. When there is only single cell, it must perform all the vital activities like feeding, respiration and movement. However, several cells are grouped together in a single organisms, division of labour can take place, permitting greater efficiency. By developing this principle to an ever-increasing degree, animals have progressively achieved higher levels of perfection.
One dominant characteristic of many of the living organisms of the Cambrian period is bilateral symmetry, similar to the found in human beings: the body can be divided along the mid-line into two halves which are virtually images.
The Cambrian was also a very important period for the evolution of plants.