Figure 7.17: About 40 to 60 million years ago this mosquito and fly were trapped in the
gooey stuff, called resin that comes from trees. The fossils in the movieJurassic Park, were
trapped in resin. ( 14 )
happened.
Geologist use a method called radiometric dating to determine the age of rocks and fossils in
each layer of rock. This technique measures the decay rate of radioactive materials in each
rock layer (Figure7.18).
Radiometric dating has been used to determine that the oldest known rocks on Earth are
between 4-5 billion years old. The oldest fossils are between 3-4 billion years old.
Vestigial Structures
Millions of species of animals, plants and microorganisms are alive today. Even though
two different species may not look similar, they may have similar internal structures, and
chemical processes that indicate they can have a common ancestor.
Some of the most interesting kinds of evidence for evolution are body parts that have lost
their use through evolution (Figure7.19). Most birds need their wings to fly. But the wings
on an ostrich have lost their original use. These are calledvestigial structures. Penguins
do not use their wings to fly in the air; however they do use them to ”fly” in the water. A
whale’s pelvic bones-which were once attached to legs- are also vestigial structures (Figure
7.20).
If you look at an x-ray of the bones in your back (called vertebrae), you will see several