Fossil Evidence of Evolution

What is a fossil?

…physical evidence of an organism that lived long ago.Examples: skeletons, shells, leaves, seeds, imprints, tracks, and even fossilized feces and vomit.The vast majority of fossils are the remains of the hardparts of extinct organisms.

How do fossils form?

Fossils form when body parts or impressions are buried in rock before they decompose.The evidence is preserved in the rock through geochemical processes. Fossils are not the actual bodily remains.Fossilization is an extremely rare event.Most ancient species are not represented in the fossil record.

What is the fossil record?

…the collection of fossils that represents the preserved history of living things on earth.The fossil record provides the dimension of time to the study of life.It shows that Earth’s organisms have changed significantly over extremely long periods of time.

General Patterns in the Fossil Record

Deeper rock layers were laid down before the layers above them. Thus, fossils in lower layers are older than those in upper ones.Fossils occur in a definite sequential order, from species that appear “primitive” to modern-appearing ones.The species representing different lines of descent become more similar to each other as they approach their common ancestors.

Example

Comparison of the early members of four families of odd-toed ungulates.

(a) Hyracotherium (Horses) (b) Hyrachyus (Rhinos)(c) Heptodon (Tapirs)(d) Eotitanops (Brontotheres)

Fossils Form in Sedimentary Rock

Geologic Time Scale

The boundaries between the major periods of geologic time are defined by major changes in the types of fossils found in the rocks deposited during these eras.

“Alternative” Explanation

Flood Geology: a single world-wide flood churned up all of the earth's inhabitants, and deposited them in a sequence of buried fossils that today we "misinterpret" as the geologic ages that document evolution.As it turns out, the distribution of many fossils in the fossil record is the exact opposite of that predicted by flood geology.

Dating the Fossil Record

The discovery of radioactivity enabled scientists to accurately determine the ages of fossils, rocks, and events in Earth’s past.Determining the age of a rock involves using minerals that contain naturally-occurring radioactive elements and measuring the amount of decay in those elements to calculate approximately how long ago the rock formed.

Age Determination Using Radioactive Isotopes

•Radioactive isotopes are useful in dating geological materials because they convert or decay at a constant, and therefore measurable, rate.•Age determinations using radioactive isotopes have reached the point where they are subject to very small errors of measurement, now usually less than 1%.

Intermediate Forms

Despite the claims of anti-evolutionists, so many “transitional” fossils have been found that it is often hard to tell when the transition actually occurred.Actually, nearly all fossils can be regarded as intermediates because they are connections between their ancestors and their descendants.

This drawing shows some of the fossils documenting the evolution of the modern camel from ancestral forms that existed in much earlier geologic ages. The ability to trace such lineages, time and time again, backwards in time shows that an evolutionary description of earth’s history fits the facts of the geologic record.

Direct Ancestor or Close Relative?

Ancestor-descendant relationships can only be inferred, not directly observed.No matter how long we watch, no two fossils will ever breed—we must look for other ways to determine relatedness.Because genetically similar organisms typically produce similar physical features, we can use fossils to help us recognize related species in the history of life.

Archaeopteryx: An Intermediate Form

Archaeopteryx:An Intermediate Form

A mosaic of 24 distinct anatomical features– 3 bird-like (13%)– 17 reptile-like (70%)– 4 intermediate (17%)

Sinornithosaurus: another feathered dinosaur?

Reptile to Mammal Transition

In mammals, each half of the lower jaw is a single bone called the dentary; whereas in reptiles, each half of the lower jaw is made up of three bones.Evolution of this jaw articulation can be traced from primitive synapsids(pelycosaurs), to advanced synapsids(therapsids), to cynodonts, to mammals.

In the evolution of mammals from reptiles, two of the extra lower jaw bones of synapsid reptiles (the quadrate and articularbones) became two of the middle-ear bones, theincus (anvil) and malleus(hammer). Thus, mammals acquired a hearing function as part of the small chain of bones that transmit air vibrations from the ear drum to the inner ear.

Evolution of Whales

The evolution of whales and dolphins is one of the best-documented transitions in the fossil record.The existence of whale fossils with hind legs provides evidence that whales evolved from four-legged land-dwelling mammals.

The fossil record shows that whales and dolphins probably evolved from a primitive group of hoofed mammals called Mesonychids. Some of these mammals crushed and ate turtles, as evidenced by the shape of their teeth.

Mesonychid

Later, a species existed that had front forelimbs and powerful hind legs with large feet that were adapted for paddling. This animal, known as Ambulocetus, could have moved between sea and land. Its fossilized vertebrae also show that this animal could move its back in a strong up and down motion, which is the method modern cetaceans use to swim and dive. Ambulocetus

A later fossil in the series from Pakistan shows an animal with smaller functional hind limbs and even greater back flexibility. This species, Rodhocetus, probably did not venture onto land very often, if at all.

Rodhocetus

Finally, Basilosaurus fossils from Egypt and the United States present a recognizable whale, with front flippers for steering and a completely flexible backbone. But this animal still had hind limbs (thought to have been nonfunctional), which have become further reduced in modern whales.

Basilosaurus

Conclusion

Many critical gaps in our knowledge remain.These gaps may or may not be filled by new evidence in the future.However, it is certain that important discoveries will continue to be made that will likely intrigue us, possibly surprise us, and definitely enrich our understanding of the evolutionary history of life.