WEIT 2012 Chap2
WRITTEN IN THE ROCKS
What is a fossil?
A fossil is the remains of an animal or a plant which has been preserved by time into sediments. They sometimes have the appearance of animals or plants molding or the appearance of traces on the rocks.
A fossil is really useful. Scientists consider them as a record, in the way that it gives to them a lot of indications about the ancestors of species and about evolution. But that hasn't been always like that. Until the nineteenth century, they were explained simply as products of supernatural forces or remains of still-living species inhabiting remote and uncharted parts of the globe.
How are fossils formed?
An animal or a plant can be fossilized only with very specific circumstances. First, they must find their way into the water, then sink to the bottom and get covered by sediments (sediments are depots of wind, ice or water). Once they are buried in the sediments, the hard parts of fossils become infiltrated or replaced by minerals which are dissolved. It only remains a cast of a living creature that is compressed into rocks by the pressure of the sediments that are piling up on top.
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Terrestrial species, fragile creatures like birds, worms, jellyfish or bacteria are much rarer to find compared to aquatic ones, because they aren't easily fossilized. Then, fossils have to survive the endless shifting, folding, heating and crushing of the earth's crust, which most fossils obliterate.
Fossils have to be discovered, because wind, water or weather can efface semiexposed fossils. Scientists estimate that at most 4 billion species that ever lived on earth and only 250'000 fossil species have discovered.
Where do scientists find fossils?
Scientists must brave a lot of dangers to hope finding fossils. Finding them demands
a lot of time, it's expensive and risked. Indeed, scientists must face diseases, political troubles and inhospitable places, where the fossils are usually found, such as the Sahara Desert. They need also a lot of luck to find the place where a fossil is.
Furthermore, once the scientists have discovered them, they must then extract, prepare and describe them.
Fossils are found in the Sahara Desert, because in the time of the prehistory, there were a lot of lakes and swamps in this place, what facilitated the creation of fossils. To form a fossil, an organism needed some water, this is why a lot of fossils are found near oceans, lakes or rivers.
Why are fossils so important for Evolution?
"The story of life on earth is written in the rocks."
Thanks to fossils, scientists can understand the evolution of species and know how a common ancestor of a specific fossil looks like.
Without them, we will only know relationships between species through similarities in form, development ans DNA sequence, but not when a species appeared on earth or which is its common ancestor
Advances in sciences have provided and will provide precious insights from the past.
References
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