Biology/Evolution/The Fossil record

The Fossil Record

Some of the most important clues to Earth’s past are fossils. A fossil is

the preserved remains or traces of an organism that lived in the past.

Most fossils form when organisms that die become buried in

sediments. Sediments are particles of soil and rock. Layers of sediments

build up and cover the dead organism. Over millions of years, the layers

harden to become sedimentary rock. Some remains that become buried

in sediments are actually changed to rock. These fossils are called

petrified fossils. Sometimes shells or other hard parts buried by

sediments are gradually dissolved. A hollow space in sediment in the

shape of an organism or part of an organism is called a mold. Sometimes

a mold becomes filled in with hardened minerals, forming a cast.

Organisms can also be preserved in ice, tar, or amber.

Scientists can determine a fossil’s age in two ways: relative dating and

absolute dating. Scientists use relative dating to determine which of two

fossils is older. In a sequence of rock layers, the layers at the top are younger

than the lower layers. Therefore, fossils found in top layers are younger than

fossils found in bottom layers. Another technique, called absolute dating,

allows scientists to determine the actual age of fossils. The rocks that fossils

are found near contain radioactive elements, unstable elements that decay,

or break down, into different elements. The half-life of a radioactive element

is the time it takes for half of the atoms in a sample to decay. Scientists can

compare the amount of a radioactive element in a sample to the amount of

the element into which it breaks down to calculate the age of the rock.

The millions of fossils that scientists have collected are called the fossil

record. Despite gaps in the fossil record, it has given scientists a lot of

important information about past life on Earth. Almost all of the species

preserved as fossils are now extinct. A species is extinct if no members of

that species are still alive. Scientists have calculated the ages of many

different fossils and rocks. From this information, they have created a

“calendar” of Earth’s history called the Geologic Time Scale that spans more

than 4.6 billion years. The largest length of time in the scale is Precambrian

Time. After the Precambrian, the scale is divided into three major blocks

called the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era.

According to one theory, called gradualism, evolution occurs slowly

but steadily. Tiny changes in a species gradually add up to major changes

over very long periods of time. According to another theory, called

punctuated equilibria, species evolve during short periods of rapid

change. Species evolve quickly when groups become isolated and adapt

to new environments. Most scientists think that evolution can occur gradually at some times and fairly rapidly at others.