For at least 300 years, scientists have been gathering the evidence for evolutionary
change. Much of this vast database is observational, and the evidence came to
light with the study of fossils (paleontology) and the rock record (geology).
This essay focuses on the evidence about evolution from the fossil record.
Documentation of ancestor-descendant relationships among organisms also comes
from the fields of biogeography, taxonomy, anatomy, embryology and, most recently,
genetics particularly DNA analysis. Information from these fields can
be found in the materials listed in the Suggested Readings.
The fossil record remains first and foremost among the databases that document
changes in past life on Earth. Fossils provide the dimension of time to the
study of life. Some of the most basic observations about fossils and the rock
record were made long before Darwin formulated his theory of descent with
modification. The fossil record clearly shows changes in life through
almost any sequence of sedimentary rock layers. Successive rock layers contain
different groups or assemblages of fossil species.
Sedimentary rocks are, by far, the most common rocks at Earths surface.
They are formed mostly from particles of older rocks that have been broken apart
by water, ice, and wind. The particles of gravel, sand, and mud, which are collectively
called sediment, settle in layers at the bottoms of rivers, lakes, and oceans.
Shells and other limy materials may accumulate in the oceans. As the sediments
accumulate they bury shells, bones, leaves, pollen, and other bits and pieces
of living things. With the passing of time, the layers of sediments are compacted
by the weight of overlying sediments and cemented together to become the sedimentary
rocks called limestone, shale, sandstone, and conglomerate. The buried plant
and animal remains become fossils within the sedimentary layers.