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 always been 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 are naturally occurring material that is broken down by processes of weathering and erosion), for exemple if you take water from a river and you let it a while you will see the sediment at the bottom of the bottle. 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?
Finding fossils 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, which 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 rocks, 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.
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.
How happened Fossilized Evolution and Speciation?
The evolution is gradual but it can be regular or not at all because the changes were driven by environmental factors such as fluctuation in climate or salinity. And as environments change regularly sometimes and not regularly at all some other times, the strength of natural selection will be increasing and decreasing. The evolution of species can be seen from the beginning to the end, but we can also notice that there are some intermediate species
What is a Missing Link?
What is really interesting is the transitional form between two species. Those fossils that show the transition between two very different kinds of living organisms. The transitional form is the evidence of evolution. For every two species, who aren't the same, there is once a single species that is the ancestor of both, which is called a missing link. The chance of finding that single ancestral species in fossil record is very small, it is why when scientists find some other species in the fossil record , close cousins to the missing link, they say that they are common ancestry equally. For example, if they have anatomical similarities, as in the structure of the body of the species. See the Appendix 1
What is the transition between Fish and Amphibians?
In 2004, the scientists discovered a fossil of a transitional form between fish and amphibians, the Tiktaalik. 390 millions years ago, the only vertebrates were fish but about 30 millions years later they found creatures recognized as tetrapods which walked on the land. Tetrapods looked like modern amphibians and showed strong links with earlier fishes.
How did early Fish evolve to survive on land ?
The Tiktaalik was a fish but also has amphibianslike features. Its head is flattened and has the eyes and nostrils on top rather than on the side of the skull. The fins had become stronger to aloud the fish to pull itself out of the water to survey its surroundings. This fish had a neck and normaly fish don't, it also had a set of sturdy ribs, it could breath both with its lungs and from its gills, and had fewer and sturdier bones in the limbs, which were in fact part fin, part leg, the two of them are new traits. Obviously Tiktaalik was well adapted to live in shallow waters, peek above the surface and breath air. Its descendants, who were bold enough, venture out of the water, leading to the evolution of every land-dwelling creature with a backbone. The Tiktaalik was not ready to go out of the water because it had not a limb, which would allow it to walk and it had still internal gills for breathing underwater. It is the most tangible evidence that evolution is true. See the Appendix 2
How did Species evolve?
How to passe from a reptile to a bird?
The first step of the evolution of flight is gliding. The Colugo (a "flying lemur") has a membrane from head to tail . The next step is the flapping of colugolike limbs to produce true flying, like bats. The Archaeopteryx lithographica was discovered in Germany . It has just the combination of traits of a transitional form, the reptilian features including separate fingers on the wing, which in modern birds are fused, and a neck attached to its skulll from behind instead of from below as modern birds and it has large feathers and an opposable big toe as birdlike traits. Its asymmetrical feathers suggest that it could fly. A parade of feathered theropod dinosaurs has been found in China in the mid-1990s as the Sinornithosaurus millenii whose entire body was covered with long, thin feathers, it still have some reptilian features or as the Microraptor gui, which has fully feathered arms and legs. Feathered dinosaures probably continued to exist and we should be able to find older feathered dinosaurs, but the feathers are preserved only in special sediments like in lake beds or lagoons and these conditions are very rare. As the fossils get younger , the reptilian tail shrunk, the teeth disappear, the claws fusing together and the appearance of a large breastbone to anchor the flight muscles.
Why did it evolve like that?
The use of the feathers was probably for insulation because theropods were partially warm-blooded so the feathers would have helped maintain body temperature. Early carnivorous dinosaurs evolved longer forelimbs and hands, this would favor the evolution of muscles that would quickly extend the front legs and pull them inward, exactly the motion used for the downward stroke in true flight. There are two ways flight could then have evolved. The first is the "trees down" senario and the second is the "ground up" therory and is more probable to be the one which has happened. The next step in evolving flight is very short airbone hops. Natural selection is also of course involved in the evolution of the flight. The other innovations that would come is hollow bones for lightness and a large breastbone. There is a progession from theropods having thin, filamentous body to ones with distinct feathers. The evolution af bird is the refashioning of old features into new ones. See the Appendix 3
Whales
Whales happen to have an excellent fossil record, their bones are easily fossilized. This is one of our best examples of an evolutionary transition since we have a chronologically ordered series of fossils, perhaps a lineage of ancestors and descendants, showing their movement from land to water. It has been recognized that whales are mammals : they are warm-blooded, produce live young whom they feed with milk and have hair around their blowholes.
Evidence from whale DNA, as well as vestigial traits like their rudimentary pelvis and hind legs, show that their ancestors lived on land. We can make a link with the next chapter about vestiges. Biologists believe now that the closest living relative of whales is the hippopotamus, which is a very good example of a transitional form. Hippopotamus are related to terrestrial mammals and are as aquatic as a land mammal can get, they live most of their time in the water and they have their eyes, nose and ears atop their head, which can be tightly close underwater. Although, whales have their own features that set them apart from their terrestrial relatives such that the absence of rear legs, a short neck, special features of the ear that allow them to hear underwater or also robust projections on top of the vertebrae to anchor the strong swimming muscles of the tail.
Sixty millions years ago, there were a lot of fossils mammals but no whales. The creatures that resemble modern whales appeared 30 millions later, indeed the fossils involved in this transition are from a period between 52 and 40 millions years ago.
After the Indohyus, almost certainly the cousin of the whale's ancestor, follows a rapid series of fossils that become more and more aquatic with time.
Between the Indohyus and the modern baleen whale, four species appeared, the Pakicetus, the Ambulocetus, which has as elongated skull and reduced but still robust limbs that still ended in hooves, the Rodhocetus, which has its nostrils backward and a more elongated skull, because of its tail muscules it would have been handicapped on land, and the Basilosaurus and Dorudon, which were totally aquatic mammals with short neck and blowhole atop the skull. The consequences of this evolution happened in the elongating of the skull, fastly (10 millions years) its limbs were reduced until becoming a reduced pelvis and reduced hindlimbs, which were unconnected to the rest of the skeleton.
Scientists don't know really why a reverse migration happened but there are several ideas. One possibility is that the disappearance of the fierce marine cousins of the dinosaurs, who were in competition with aquatic mammals for food, let the sea empty of predators and loaded with food. That's why all the ancestors of the whales came into the water.
See the Appendix 4
What do the fossils say?
What are the last important things to know about fossils?
The Sphecomyrma freyi is the "transitinal" ant, it has almost exactly the combination of antlike and wasplike features. The earliest known snake has a small pelvic girdle and reduced hind legs so it shows that snakes are an evolution of lizardlike reptiles. The Haikouella lanceolata, a small eel with a frilly dorsal fin, has been found in China. It had a head, a brain, a heart and a cartilaginous bar along the back, the notochord. It may be the roots of all the vertebrates including ourselves.
What are the three things fossils learn us?
First, they speak of evolution. They confirm several prediciton of evolutionary theory: gradual change within lineages, splitting of lineages and the existance of transitional form. Second, They occur in the fossil record precisely when they should. They occur in an order that makes evolutionary sense; the earliest birds appear after dinosaurs, but before modern birds. Finally, evolutionary change nearly always involves remodeling the old into the new; the legs of land animals are variations on the limbs of ancestral fish, the wings of birds were fashioned from legs of dinosaurs. Natural selection can act only by changing what already exists. Darwinism predicts, that new species will be modified versions of older ones and the fossil record confirms this prediction. See the Appendix 5
References
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