A fossil is the remains of a once-living organism. Usually only the harder parts of organisms, like bones, teeth, shells, and horns preserve. Sometimes inorganic compounds like calcium carbonate or silica replace these remains, making it possible for these remains to preserve for millions and millions of years.
What is the Significance of Fossils?
Fossils have probably always been known to humans, but their significance has not always been understood. Old Chinese texts refer to “dragon bones”, many of which were probably actually the fossils of now extinct animals, perhaps dinosaurs, mammoths, and Homo erectus. In 1676, a large lower leg bone was discovered by Reverend Plot in England, who thought it might have belonged to a giant.
Handaxes, which are stone tools that were manufactured by Homo erectus 1.8 million years ago (Ma) - 200 thousand years ago (ka) were thought of as extraterrestrial “thunderbolts” in Medieval times. The first Neanderthal specimens ever recovered in the mid 1800s were regarded by many as pathological human remains.
Even with the lack of understanding surrounding fossils, fossil hunting and collecting was in vogue during the 18th and early 19th century. Mary Anning is one of the most famous fossil hunters during this time period and is credited with the discovery of Ichthyosaurus. She discovered this nearly 20 foot long fossil near Lyme Regis in southwest England. It was found to be a type of marine reptile that no longer existed and the discovery created a stir among those who were questioning the supposed antiquity of the earth at that time. Her discoveries, together with the work of many important figures including Gideon Mantell and William Buckland (who described the first dinosaurs) set the foundation for the field of investigation that would come to be known as palaeontology.
What Do Fossils Tell Us?
Fossils tell us many things. First, fossils reveal that plants and animals have been on the earth for a long time, being associated with rocks and sedimentary layers that have great antiquity.
Second, they show that many of these plants and animals have gone extinct. There are strange forms that differ from any known living organisms.
Third, and most importantly, fossils reveal that change has occurred over time. When fossils are lined up and compared with each other, very strong patterns are apparent. Some fossils can be grouped together based on their form (e.g. skull shape, tooth shape and size, body plan, etc.). When the oldest fossils are compared to the youngest ones, they may look very different from each other, but when the oldest fossil is compared to the second oldest fossil, there are only subtle differences between them. The third oldest fossil differs only slightly from the second oldest fossil, etc.
Each of the fossils along the sequence can be considered a transitional form or a “missing link”. The term “missing link” is now antiquated and avoided in scientific literature. In reality, many transitional forms in many evolutionary lineages, including our own, have been discovered and are no longer “missing”.
How Do We Know How Old Fossils Are?
There are two kinds of dating: relative and absolute.
Relative dating consists simply of knowing which fossils are older or younger.
Absolute dates (or chronometric dates) are based on solar years and give an actual age reported as “years before present". These dates are based on the measures of certain types of elements that change over time at a known rate. Radiocarbon dating is one example of an absolute dating technique. Other methods include pottasium-argon analysis, amino acid racemization, and thermoluminescence.
The methods used by scientists to date a fossil depend on the types of materials that are available for dating and the puported relative age of the fossils. Some methods only work for very young fossils, and others only work for very old fossils. Only when the results of various dating methods are redundant and confirm each other is a scientist confident in the absolute age of a fossil specimen or species.
Fossils Are the Hard Evidence for the Evolution of Life
Geology tells us that the earth is very old. It has taken 4.6 billion years to create the patterns of uplift, erosion, and sedimentation that characterize the earth’s surface today. Darwin’s theory of natural selection provides the mechanism for evolution. Trait frequencies change over time because of the differential success of certain traits, some of which confer reproduction advantages in certain environments. We know evolution can happen, because the earth is old enough for enough of these small changes in trait frequencies to accumulate and cause species to change. Fossils demonstrate that evolution has happened; the traits of organisms have changed over time.
Sources:
Haviland, W. A. and Crawford, G. W. 2009. Human Evolution and Prehistory. Nelson Education Ltd.
Evans, D. and Selina, H. 2005. Introducing Evolution. Icon Books: Thriplow.
