All organisms were born from a
common ancestor and differentiated at some point to evolve separately. Organisms are believed to have originated 3.8 billion years ago, suggesting
that our common ancestor was born then. Most existing organisms are thought to
have differentiated in the Cambrian period (approximately 500 million years
ago) at the beginning of the Phanerozoic era, which is because the Precambrian
(Paleozoic) geological records (i.e., fossils) have not shown any multicellular
organisms that appear to be our ancestors. In addition, the “Phanerozoic era”
is an era in which the existence of organisms can be clearly confirmed. The
periodization of the Phanerozoic era, including the present time, is divided at
the points when the biota (type and number) changed significantly, based on
information from fossils. A major change in the biota indicates that a great
extinction event of organisms occurred immediately prior, followed
by the rapid evolution of the surviving organisms. A great extinction event
occurred immediately before the Cambrian period (although there is no
evidence), and biological evolution is thought to have occurred rapidly to fill
the void. Since then, many great extinction events have repeated to the present
day.
