WEIT 2012 Chap2
WRITTEN IN THE ROCKS
What is a fossil?
A fossil is the petrified remains of an animal or a plant which has been preserved by time into sediments. Buried deeply beneath the earth's surface, most fossils are inaccessible. They also have been known since ancient times.
Fossils are really useful. Scientists consider them as a record, in the way that fossils give to them a lot of indications about the ancestors of species for example; how they looked like and they are the tangible historical evidence of 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, the remains of the animal or plant 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 and is naturally occurring material that is broken down by processes of weathering and erosion),for exemple if you take water from a river a you let it a while you will see the sediment at the bottom of the bottle ( find a definition of sediment + clear exemple). Once they are buried in the sediments, the hard parts of fossils become infiltrated or replaced by minerals which are dissolved. The only remains are a cast that is compressed into rocks by the pressure of the sediments that are piling up on top.
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 due to their soft parts. The first 80 percent of the history of life were soft-bodied animals or plants because of that we almost know nothing about them so of course we don't know anything about the origin of life. 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 been discovered that means we only have 0.1 to 1 percent of all the species.
Where do scientists find fossils?
Scientists must brave a lot of dangers to hope finding fossils.
really?--Pierre.brawand 22 mars 2012 à 06:29 (CET)Finding them demands
a lot of time, it's expensive and risky. 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.
The paleontologists can only work on the sediments when they are exposed or raised by the erosion of wind or rain. Furthermore, once the scientists have discovered the fossils, 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 and DNA sequences, 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.
How to determine the age of a rock?
The principle of superposition has been introduced by the Danish Nicolaus Steno in the seventeenth century. It consists in saying that there are different layers of rock and that the younger it is the more on top it will be. But this helps to know the relative ages of rocks,not their actual ages. For this, in 1945 the principle of using radioactivity has been introduced in order to measure the age of the rocks.Because there are radioisotopes (radioactive elements)in the rock, which decay at a certain speed so we can measure how much of them are still in the rock to know their age.John Wells proved that the use of radioactivity to age things is always true using fossil corals.First he used radioactivity,which gave him a certain date and after he used the fact that living corals produce daily and annual growth rings.
The Facts
How can we constitute evidence for evolution in the fossil ? First the big evolutionary picture through which we can see that earlier life is really simple and after time species became more complex so the youngest fossils are really similar to living species.To fond evidence of evolution the splitting species must be found. The new species must have traits that link them to the ancestors and so make them look like their descendants.The evolution is also observed as more we go on the top rocks more the fossils look like the modern species. So in adjacent layers the fossils are more similar than to further layers, it implies a gradual and continuous process of divergence.The youngest fossils ressemble the species living in that area and not the species living in other parts of the world.For exemple, fossil marsupials were found only in Australia where the most of the modern marsupials live so it proves that the modern species descend from the fossil ones.
Good start. Some minor information seems useless but the main message is there. Try to generate some Appendix documents in order to illustrate your comments. Keep going!--Pierre.brawand 22 mars 2012 à 06:32 (CET)
Absolutely no change since March 22.... :-(--Pierre.brawand 28 mars 2012 à 13:35 (CEST)
References
- retour à Evolution_3BIbDF10-11_2011/12
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