What is especially interesting is that the fossils do appear to show a progression from the most "simple" of organisms, such as single celled creatures like bacteria, to the most "complex" organisms, such as vertebrates, mammals, and of course humans.
This evolutionary progression seems to be clearly demonstrated in that certain kinds of creatures in the upper layers are rarely if ever seen in lower layers.
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For most scientists all of this seems so obvious that it is difficult to question.
It goes against human nature to challenge long held ideas of truth.
Fossils occur on every continent and on the ocean floor.
Through paleontology (the scientific study of fossils), it is possible to reconstruct ancient communities of living organisms and to trace the evolution of species.
The term also is used to describe the fossil fuels (oil, coal, petroleum, and natural gas) that have been formed from the remains of ancient plants and animals.
Other subdivisions reflect the evolution of life; the Archean and Proterozoic are both eons, the Palaeozoic, Mesozoic and Cenozoic are eras of the Phanerozoic eon.
The three million year Quaternary period, the time of recognizable humans, is too small to be visible at this scale.
Fossils of single-celled organisms have been recovered from rocks as old as 3.5 billion years.
Animal fossils first appear in rocks dating back about 1 billion years.