Evidence for Evolution: Paleontology

Paleontology is the study of past life based on fossil records and their associations to different geologic time periods.

When organisms die, they usually do not leave any traces of their existence as most of them are decomposed or consumed. However, some organisms are preserved as fossils, which are the remains of organisms from a past geologic age embedded in rocks by natural processes. These fossils are play an important role in understanding the evolutionary history of life on Earth, as they provide direct evidence of evolution and detailed information on the ancestry of organisms.

Fossil records

By organising fossils records in a chronological sequence, we are able to find out how a particular group of organisms, for example animals and plants, evolved.

Fossil evidence shows that organisms tend to gradually become more complex. The reason for this is that the organisms are constantly adapting to the ever-changing conditions in the environments, due to the varying geographical regions and climatic conditions. By studying the number and complexity of different fossils at different stratigraphic levels, it has been shown that older fossilized organisms have simpler structures, whereas younger fossils often have more complex structures.

This supports Darwin’s theory that organisms will develop certain characteristics to aid in their survival due to the competition for the limited resources. These characteristics would be passed down from generations to generations. Hence, in light of this evidence presented by fossil records, scientists readily accept the theory of evolution.

Example: Evolution of the horse

An almost-complete horse fossil record was found in North American sedimentary deposits from the early Eocene to the present, which provides one of the best examples of evolutionary history (phylogeny).

The course of development of horses from Hyracotheium to Equus (the modern horse) involved at least 12 genera and several hundred species. The major changes seen in the horse’s adaptation to changing environmental conditions may be summarized as follows:

1.Increase in size (from 0.4 m to 1.5 m);
2.Lengthening of limbs and feet;
3.Reduction of lateral digits (refer to picture);
4.Increase in length and thickness of the third digit (refer to picture);
5.Increase in width of incisors;
6.Replacement of premolars by molars; and
7.Increases in tooth length, crown height of molars.

The marshy, wooded country in which the Hyracotherium used to live in had become dryer, and thus the Hyracotherium adapted to the drier environment by means of an elevated head for gaining a good view of the surrounding countryside; and a larger hoofed foot to increases its speed so as to escape from predators. The original splayed-out foot would not be able to support the Hyracotherium on the drier, harder ground. The probable explanation for the changes in the teeth may be that the animal’s diet changed from soft vegetation to grass, and thus would need stronger teeth.

Example: Human Evolution

Human fossils show that the oldest human ancestor was an apelike creature that lived about 40 million years ago in Egypt. However, a gap in the fossil record means it is difficult to follow human evolution over the next 20 million years. Humanlike fossils have been found 4- to 5-million-year-old sediments in Ethopia and East Africa. It has been said that everyone can trace their family trees back to a common ancestor – one woman who lived in Africa. She has been called Eve, after the first woman in the Bible. The idea about ‘Eve’ has led to arguments among scientists.

Additional clues from fossils

Each new fossil contains additional clues that allow us to better understand our life's history and help us to learn about the evolutionary story of different organisms. Examples include:


Indication of interactions

This ammonite fossil (see right) shows punctures in an ammonite fossil have been interpreted by scientists as the bite mark of a mosasaur, which corresponds to the shapes and capabilities of mosasaur teeth and jaws.

Clues at the cellular level

Fossils contain clues about growth patterns in ancient animals. The cross-section through a sub-adult thigh bone of the duckbill dinosaur Maiasaura shows that it was a fast-growing bone as there were lots of blood vessels running through it, which can be seen from the white spaces.

Limitations

The fossil record is an important source for scientists when tracing the evolutionary history of organisms. However, there is a lack of continuous fossils in record and there are large gaps between major evolutionary ancestries (“missing links”). One of the reasons for this is that in general, the probability that an organism becomes fossilized after death is very low, and thus is a major limitation in tracing the descent of biological groups.

Source:
S Gater & V Wood-Robinson, "G.C.S.E Science Double Award-Biology", 1996, p. 154-155,
10th April 2007

Marshall Cavendish, "Exploring Life Science 4", 2000, p. 310, 10th April 2007
http://en.wikipedia.org/wiki/Evidence_of_evolution, 10th April 2007
http://evolution.berkeley.edu/evolibrary/article/0_0_0/lines_02, 10th April 2007

Ilisa Ishak [Science Investigator]