Since each of these layers seems so specialized it is easy to conclude that one type of creature gave rise to the next type of creature over the course of whatever time it took to form the various layers between them.
Radiometric dating and many other techniques are used to support the idea that this transformation process took tens and hundreds of millions of years.
Like extant organisms, fossils vary in size from microscopic, even single bacterial cells one micrometer in diameter, to gigantic, such as dinosaurs and trees many meters long and weighing many tons.
A fossil normally preserves only a portion of the deceased organism, usually that portion that was partially mineralized during life, such as the bones and teeth of vertebrates, or the chitinous or calcareous exoskeletons of invertebrates.
Since the geologic column represents millions of years of Earth's history, then obviously the fossils in each of the layers must be the same age as the layer in which they are found.
are the preserved remains or traces of animals, plants, and other organisms from the remote past.
The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous (fossil-containing) rock formations and sedimentary layers (strata) is known as the fossil record.
Although the hominid fossil record is far from complete, and the evidence is often fragmentary, there is enough to give a good outline of the evolutionary history of humans.
The time of the split between humans and living apes used to be thought to have occurred 15 to 20 million years ago, or even up to 30 or 40 million years ago.