What are fossils?
Fossils are the preserved remains or traces of any organisms from the remote past. Fossils may be: - Direct fossils such as bones, teeth, shells, and leaves, OR - Indirect fossils such as footprints, tooth marks, tracks, or burrows. Fossilisation requires an unusual combination of specific circumstances to occur, specifically: - Rapid burial, resulting in high pressure - Lack of oxygen, preventing decomposition by bacteria - Preservation of remains (i.e. not consumed or removed by scavengers) |
Figure 6.1 A pyritised Echioceras ammonite
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The totality of fossils (both discovered and undiscovered) is known as the fossil record, and it provides arguably the most compelling evidence for evolution to date.
How do we date fossils?
There are several techniques for dating fossils, generally categorised into relative and absolute dating techniques.
Relative dating uses geological evidence to assign comparative ages of fossils, using the following process:
- The earth is arranged into sedimentary layers (stratification), with younger stratum sitting on top of the older stratum, as stated by the law of superposition.
- The relative and evolutionary history of fossils can be traced by mapping the relative ages of the strata in which different fossils are found.
- Index fossils (remains that represent short lived species that only appear in a specific period of time on the geological time scale) are used to date the rock strata, and the age of the fossil is hence determined.
Absolute dating uses radiometric data analysis to determine more exact ages of fossils. It involves comparing the ratio of radioactive isotopes in fossilised samples to those found in the atmosphere. As radioactive isotopes decay at a constant rate, they can be used to determine the age of fossils, as the amount of radioactive isotope remaining in the fossil or surrounding rock is proportional to the age of the fossil. The carbon-14 isotope can be analysed for fossils up to ~50,000 old, while Uranium-238 and Potassium-40 are useful for older specimens.
There are several techniques for dating fossils, generally categorised into relative and absolute dating techniques.
Relative dating uses geological evidence to assign comparative ages of fossils, using the following process:
- The earth is arranged into sedimentary layers (stratification), with younger stratum sitting on top of the older stratum, as stated by the law of superposition.
- The relative and evolutionary history of fossils can be traced by mapping the relative ages of the strata in which different fossils are found.
- Index fossils (remains that represent short lived species that only appear in a specific period of time on the geological time scale) are used to date the rock strata, and the age of the fossil is hence determined.
Absolute dating uses radiometric data analysis to determine more exact ages of fossils. It involves comparing the ratio of radioactive isotopes in fossilised samples to those found in the atmosphere. As radioactive isotopes decay at a constant rate, they can be used to determine the age of fossils, as the amount of radioactive isotope remaining in the fossil or surrounding rock is proportional to the age of the fossil. The carbon-14 isotope can be analysed for fossils up to ~50,000 old, while Uranium-238 and Potassium-40 are useful for older specimens.
Figure 6.2 A fossilised leaf
Figure 6.3 A cast of the archaeopteryx fossil
Modern uses of fossils - a world of possibility
Fossils today can be used for many other purposes than simply comparing and contrasting shapes and sizes of different structures, and predicting relationships. The development of DNA-DNA hybridisation techniques has allowed for the extraction of ancient DNA to be put to a whole new use - comparing the number of gene sequence differences between two species and using this to derive the number of years since those organisms diverged. DNA sequencing has also enabled us to learn more about interbreeding of species, such as H. sapiens and H. neanderhalensis. Thanks to these techniques, we have discovered that humans today share anwhere between 1-4% of their DNA with Neanderthals! These techniques can help answer questions which have puzzled scientists for years, but generate a lot of food for thought as well... |
Limitations of fossil evidence
When studying hominid evolution, we are looking at evidence that is over 7 million years old. That should be a big hint to us all that obviously, some problems are going to be encountered when studying and interpreting this evidence, which make it difficult to gain a comprehensive understanding of our ancestry. So what issues arise when studying fossil evidence? Common problems include:
Figure 6.4 The extraction of DNA-containing bone marrow from a fossil
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