#1 Fossils
Fossils
Fossils are the remains or traces of organisms that once lived. Scientists who study fossils are
called paleontologists. The study of fossils in the earth provides evidence to support the idea that
life changed over time from simple to complex. Fossils distribution shows that life began in the
sea and the moved to land. Fossils help scientist understand how climates and land surfaces have
changed. It is through fossil evidence that we know that organisms existed over three billion years
ago. By the process of absolute dating scientists can determine the age of the earth’s rocks and
its fossils almost exactly based on the physical or chemical make-up. Scientists also use relative
dating (easier to use!) by observing the layers of sedimentary rocks.
Sedimentary Rock
Many fossils are found in sedimentary rock. The oldest rock layers lie under the younger layers.
Sedimentary rock is formed from layers of slowly deposited sediments. Sediments, such as
particles, silt, and mud, are usually deposited by water. After a long period of time and great
amounts of heat and pressure, sediments harden into rock forming visible layers. Skeletons,
imprints, shells, bones and other animal and plant remains become trapped and then become
fossils. Fossils in lower rock layers are assumed to be older than fossils found in upper
layers. Generally, fossils found at the bottom look the same as the fossils found at the top, except
they will be more complex at the top.
Types of Fossilization
Organisms can become fossils (fossilization) in a number of different ways. Whole organisms
can be preserved in amber (sap). Soft parts of organisms usually decay, but the hard parts may
form molds or casts. A mold is an indentation in rock. A cast is formed when the decayed
organism forms a mold and the mold is filled with some substance. Imprints, such as dinosaur
footprints, occur when a print is made in soft sediment, such as mud, that later turns to rock.
#2 Comparative Anatomy
Comparative anatomy is the science that studies the structures or parts (anatomy) of plants and
animals. When scientists compare skeletal structure of different vertebrates, they see a similar
basic structure. This observation shoes that organisms with similar bone structures may have
evolved from a common ancestor population. Organs or structural parts that seem to have a
common evolutionary origin (beginning) are referred to as homologous structures.
For example, the wing of a bat, the flipper of a whale, and a human arm are homologous structures
(look at the picture). Although homologous structures are similar in structure, they do not always
have the same function.
#3 Vestigial Structures
Vestigial structures are parts of an animal’s body that are NOT USED. These structures look like
structures that are fully developed and used by other animals. The human appendix is an example
and, as evolution continued, humans stopped using this organ. Other vestigial structures are
human ear muscles, and the leg bones of the python and porpoise. These structures provide
further evidence of changing structure and function. Another example is toes in horses; they once
had 5 toes many years ago!
#4 Cell Structure
Cells and cell organelles are basically alike from one group of organisms to the next. For example,
all eukaryotic cells have a nucleus, cell membrane, cytoplasm, ribosomes, mitochondria,
chromosomes and others. In terms of evolution, this is evidence that different kinds of living things
may share a common origin.
#5 Biochemistry
Similarities in the biochemistry (body chemicals) of living things, such as DNA, hormones and
enzymes, show a close relationship between various forms of life. Organisms that are closely
related, like the cat and the lion, have a greater similarity in their protein structure. Greater
differences in cell biochemistry are thought to show a lesser evolutionary relationship.
#6 Examples of Evolution
Scientists have found several complete series of fossil records that show gradual changes in
animals through the ages. Two of the best examples of vertebrate evolution are those of the horse
and the elephant.
Evolution of the Horse
The ancient ancestor of the horse, Eohippus, was about the size of a fox. It had four toes on its
front feet and three toes on its hind feet. The horse gradually got bigger and the length of its feet
increased. As time passed, some of the toes disappeared, until today the modern horse, Equus, is
one-toed. The middle toe is the one that remains, but the horse still has tiny splints of the two
other toes!
Evolution of the Elephant
The ancestor of the present day elephant was about the size of a pig and no tusks. Over time, the
size of the elephant’s body and head increased tremendously. The early trunk was much shorter
than the trunk of today’s elephant.