EVIDENCE FOR EVOLUTION
There are many lines of evidence that support the idea that organic evolution actually
For example, the fact that some organisms are well-adapted to the particular
environment in which they live is consistent with natural selection.
livings, like similar cell, embryonic, and skeletal structures, would also be expected if life
forms share a common ancestor.
and bones that serve no function can
be explained by evolution from an ancestor that had the same, but functioning, organ or
Finally, systematic changes in fossils are sometimes preserved, providing a record
of directional change in species.
FOSSILIZATION AND EARLY METAZOANS
Fossils are any evidence of past life.
are the actual remains of hard or soft
tissues of the organism itself.
are evidence of the activity of ancient life
such as footprints, burrows, coprolites, and gastroliths.
Fossils can be made by:
of the organism (i.e., an organism trapped
in ice or amber), 2)
of organic materials by minerals (i.e., petrified wood),
(a carbonized film), 5) molds, and 6) casts.
two types of molds.
An external mold
forms when a shell that is surrounded by
sediment dissolves and leaves behind a void space that preserves the external features of
forms when a shell that is filled-in with sediment dissolves
and leaves behind a three-dimensional object that preserves the internal features of the
forms when an external mold is later filled in with sediment, creating a
three-dimensional object that preserves the external features of the shell.
Fossil collections may be
(may not accurately reflect the types and abundances of
plants and animals living in the past) by:
1) the lack of preservation of some organisms
in the ancient ecosystem, 2) the collection of only certain types of fossils, and 3) the
improper identification of fossils.
Two fossil deposits, the Ediacara deposit and the
Burgess Shale (discussed below) are particularly unbiased with respect to preservation
and reveal an amazing diversity of life.
are many-celled animals.
Some of the oldest fossil metazoans are found in
deposit in southwestern Australia.
These animals lived in a shallow marine
environment at the end of the Precambrian time period.
They are extremely well-
preserved, which suggests that the organisms were buried rapidly.
The fossils in the
Ediacara deposit include soft-bodied organisms like jelly fish and worms that were
passive filter feeders.
The Ediacara fossil deposit represents a fairly primitive ecosystem.
Some of the Ediacara animals had bizarre body plans, unlike anything living today.