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Fossils occur in layers which can be sequenced and dated. Generally speaking, the older layers are lower down. These layers correspond to historical periods or eras. Each era contains specific types of fossils. As of 2013 over 250,000 fossil species have been recovered.
The fossil record shows many creatures that no longer exist as they have become extinct. The most famous extinct animals are the Dinosaurs. It is estimated that 99% of all species that have ever lived are extinct - which implies not so intelligent design.
Another point to note is that the youngest fossils (the ones in the uppermost layers) are of creatures most similar to ones living today. As we go deeper (further back in time) we find stranger and more unusual animals. The oldest fossils are of Photosynthetic Bacteria which are 3,500 million years old. Multi-cellular life (Worms, Jellyfish and Sponges) dates from around 600 million years ago. Around 543 million years ago there was suddenly a profusion of life, an event called The Cambrian Explosion.
In the diagram below, the first appearance of a class of living animal or an event (like the beginning of photosynthesis) is shown in the appropriate place in the rock strata. In this diagram the youngest layers are at the top, the oldest at the bottom.
Even without adding dates to this diagram, some interesting observations can be made.
Human and hominid fossils are only ever found in the more recent Tertiary layers. They are never found in any older layer.
No human fossils have ever been found in the Cretaceous layers (and certainly none have ever been found associated with dinosaur fossils which, themselves, are only ever found in the Cretaceous era layers). Such finds would be evidence against Evolution.
Primates are another group of fossils only ever found in the Tertiary layers. But they are found in lower (older) rocks than human or hominid fossils.
Mammals and Birds are found in several eras: recent ones like the Tertiary and older ones like the Cretaceous, Jurassic and Triassic. The earliest bird and mammal fossils are found in the Triassic.
Reptiles are found in Tertiary, Cretaceous, Jurassic, Triassic and all the way back to the beginning of the Permian.
No mammal or bird fossil has ever been found in layers older than ones containing the earliest reptiles (in other words, the Permian). Again, such a find would be evidence against Evolution.
Amphibians and their fossils go back one era further than reptiles - back to the beginning of the Carboniferous. No reptile fossil has been found in layers older than the Carboniferous. A reptile found in layers older than the oldest amphibian would again be evidence against Evolution.
Jawed fish make their first appearance in the fossil record in the Silurian layers. Jaw-less fish appear in the Ordovician. Going back even further, Vertebrates appear in the Pre-Cambrian layers along with other multi-cellular animals.
The fossil record (even without dating the layers) implies the appearance of humans at a later time than primates which in turn appeared at a later time to other mammals. Mammals themselves appear later in the fossil record than reptiles which themselves appear at a later time than amphibians. Even earlier we see the appearance of Jawed Fish, Jawless Fish and Vertebrates in that sequence of layers getting older.
The fossil record is consistent with the hierarchy derived from form and features. It is also consistent with studies of DNA genetics. In addition, it corresponds to the evolutionary sequence indicated in the development of embryos.
All of these observations taken together reinforce each other's conclusions and are consistent with the Theory of Evolution.
Most supporters of Creation are often heard to say that the fossil record is incomplete. This is true but not relevant. The Theory of Evolution makes predictions about what types of fossils should be found in different layers. It also allows itself to be disproved by predicting where certain fossils should not be found.
Some Creationists say that the fossil record is the bones of the animals killed in a world wide flood in the last few thousand years. If this were true, we would expect fossils to be jumbled up with all types of animals mixed together. But as I mentioned earlier, dinosaur fossils are never found with human fossils. Fossils occur in strict layers that make evolutionary sense. Also there is no geological evidence for a world-wide flood in the last few thousand years. In addition, the dates of the layers are much older than a few thousand years.
Many former so-called "missing links" have been found in recent years. A better name for these is Transitional Forms.
An example is Archaeopteryx, a dinosaur with feathers, a beak and an opposable toe (all features of birds) but with teeth and claws like a reptile. This fossil is essentially a cross between a reptile and a bird. It dates from 145 million years ago which is before the appearance of true birds. The first was discovered in the 1860s - twelve specimens are now known (2013).
In the 1990s a series of feathered Therapod Dinosaur fossils were found in China. One of these (Mei Long) shows bird-like behaviour as it is sleeping with its head tucked under its wing, just like many modern birds.
As the series of fossils gets younger, the reptilian tail gets shorter, the teeth disappear and claws fuse together like a modern bird's wing skeleton. Finally the breast bone appears onto which flight muscles can be anchored.
Features and DNA indicate a relationship between Birds and the Therapods. The finds of feathered therapods is at the expected time in the fossil record.
For the transition between fish and amphibians, Eustenpteron was discovered as far back as 1881. This is a fish but its fins have digit-like bones. Its flat skull and its teeth are similar to an amphibian's.
In 2004 a fossil called Tiktaalik was found. This is a large creature with a crocodile-size head on a salamander-like body but with a fish-like rear. It has a neck, something that no fish has. This creature had both gills and lungs as well as sturdy ribs to help in the pumping of air.
The Puijila Darwini is a seal / sea lion that is 20 million years old (before the two diverged). Unlike modern seals and sea lions which have flippers, this creature had webbed feet and its teeth resembled that of an otter.
In 2006 a 90 million year old snake was discovered in Patagonia. Its fossil shows a small pelvis and reduced hind legs. This is expected as snakes evolved from lizards.
A 220 million year old fossil (called Odontochelys semitestaccea) is a toothed turtle with half a shell and a longer tail than modern turtles. Supporters of Creation often ask questions like "what good is half a shell?" and deny that such a structure is viable.
Finally, a 530 million year old fossil called Haikouella lanceolata was found in China in the late 1990s. This creature is only 40mm long. It resembles a small eel and has a frilly dorsal fin, a head, brain, heart and teeth (but no jaw). The creature has no bones but what makes it interesting is the presence of a cartilaginous bar called a Notocord. This appears to be a very early example of a Chordate, animals that would evolve a backbone and become Vertebrates. As such, it is a possible ancestor to all fish, reptiles, mammals and, of course, humans.
Series of fossils taken together and placed in age sequence show patterns of evolutionary change. The fossil record has many gaps but there are enough to show many examples of the slow evolution of species from ancestor versions to modern versions. The fossil record also shows changes from simpler organisms to more complex ones. We have already discussed the feathered therapods. These are more examples.
One example is a series of fossils showing the evolution of the modern Horse from a smaller ancestor.
The sequence begins with a small animal called Hyracotherium which was 0.4m tall and lived 54 million years ago. Several intermediates fossils have been found including Mesohippus (0.6m), Merychippus (1.0m), Piohippus (1.25m) leading to the modern horse (Equus) which is 1.6m high.
Another sequence shows the evolution of Whales back into the oceans from hippo-like ancestors. This sequence shows clearly the slow loss of hind limbs, the flattening of the tail and the change from front limbs to fins for swimming.
The earliest ancestor to the whale is Indohyus, a racoon-like animal that was partially aquatic and lived 48 million years ago. Through a series of intermediaries we arrive at Dorudon (40 million years) with its small hind limbs unconnected to the rest of its skeleton (like a modern whale) and a blow hole at the top of its head. The entire sequence from racoon-like Indohyus to something recognisable as a modern whale required just 10 million years, rather a fast time scale in evolutionary terms.
Recently a fossil called Ambulocetus was found - like its name implies, it is a walking whale.
Whales have descended from mammalian ancestors that lived on land. Like their ancestors, whales still breathe air through lungs. They are warm blooded and give birth to live young with a placenta. Dolphins are similar and are in fact smaller whales.
The question as to why a sea creature should have to come to the surface to breath air while other vertebrates breathe underwater through gills can only be answered by the Theory of Evolution. This is not Creation or Intelligent Design. The whale only makes sense as a creature descended from land-living mammalian ancestors. This is clearly shown by the fossil record and by the DNA evidence discussed earlier that shows a close relationship between whales and hippos.
Dolphins further show their mammalian ancestry when they swim by moving their spines up and down, the same way a horse gallops. Proper fish swim by moving their spines side to side, a feature replicated in their lizard descendants.
The Theory of Evolution makes predictions about the fossil record that can be tested.
When Marsupials (pouched mammals) first appeared, the continents of South America, Antarctica and Australia were linked together. There are still a few marsupial species living in South America and there are fossils of their ancestors in the rocks. The oldest of these date from 40 million years ago.
Australia, of course, is famous for its marsupials. There are over 200 living species of marsupials in Australia and hundreds of fossils of their ancestors, many of which are now extinct. The earliest Australian marsupial fossils date from 30 million years ago.
A prediction made by the Theory of Evolution was that fossils of marsupials would be found in Antarctica since this continent was once linked to both South America and to Australia. These fossils would be dated between 30 to 40 million years ago.
Marsupial fossils were eventually found in Antarctica in 1982. A dozen species and they dated to the correct period, between 35 and 40 million years ago. Marsupials originated in North America 80 million years ago and migrated to South America, Antarctica and Australia. They thrived in Australia because of the lack of Placental Mammals on that continent to compete with them.
When it was realised that humans are closely related to apes, it was predicted (by Charles Darwin) that pre-human humanoid fossils would be found mainly in Africa. This is because Africa is the continent that most apes are indigenous to. An exception is the Orangutan in East Asia.
This prediction made by the Theory of Evolution has been verified many times by the discovery of many pre-human hominid fossils.
Some have been found in Europe, mainly a close relative to humans called Neanderthal Man. A few have been found in Asia, mainly our closest pre-human ancestors (Homo Habilis). But, as predicted, the vast majority of pre-human hominid fossils, and the older more ape-like ones, are found in East Africa.
Using the fossil record along with DNA evidence, we can work out that our species Homo Sapiens migrated from Africa about 100,000 years ago and spread into Asia and Europe. In the last 20,000 years, modern humans have crossed the Pacific Ocean to occupy its many islands and crossed a land bridge to the Americas.
No pre-human hominid fossils have ever been found in the Americas. That corresponds to the idea that the Americas were the last continents to have been colonised by humans. This happened so recently that, by that time, all other pre-hominid humans had become extinct.
We can see from the discussion above that fossils are not the only evidence for Evolution. The gaps in the fossil record are not detrimental to the Theory. Rather, the fossil record confirms and amplifies what is known from the studies of animal classification, the DNA record and studies of embryology.
The fossil record shows gradual changes within a single lineage. It shows species splitting into two or more species. It shows the development of new traits by the modification of older ones (fins become limbs, fore-arms become wings or paddles, etc).
And it shows transitional forms precisely where they should occur in the fossil record.
One group of transitional fossils show the change from the reptilian three boned lower jaw to the single boned lower jaw of mammals. During the transition, two of the jaw bones shrink and move to the mammalian ear where they form an early version of the three ear bones that give mammals good hearing.
If evolution were not happening fossils would not occur in the order that makes evolutionary sense. Biologist J B S Haldene was once asked what would disprove evolution. He responded "Fossil rabbits in the Pre-Cambrian".
When an atom decays, the nucleus fires out particles and changes into another type of atom, called a Daughter Atom. The rate of decay is constant. If a hundred radioactive atoms are taken it is not possible to say which individual ones will decay or change. However we can measure something called the Half Life.
Going back to our 100 atoms, after a single half life has passed, half of the 100 atoms will have decayed leaving 50. After another half life has passed half of the remainder will have decayed (leaving 25). This type of decay is called Exponential Decay. After about six half lives have passed the number of radioactive atoms has become too low to measure accurately.
All radioactive dating has an error of about 1%.
Different atoms have different half lives.
Carbon-14 is replenished in the Earth's atmosphere by radiation from space reacting with the Earth's Nitrogen. In living organisms the ratio of Carbon-14 to Carbon-12 ("normal" carbon) is known and constant. The living organism exchanges carbon with the environment so the ratio between the two is the same as for the non-living environment. Once the organism dies, the carbon becomes frozen in place as no more is exchanged. As the Carbon-14 decays, the ratio between Carbon-14 and Carbon-12 falls. After a single half life has passed (5,730 years) the ratio will have fallen by a half.
Carbon-14 is only useful for dating items that were once alive like wood, cloth, paper, mummified skin. Carbon-14 has a relatively short half life so it is only useful for items that are less than about 25,000 years old.
Uranium-238 was created in an exploding star before the Sun and Earth were formed. There is still Uranium-238 on the Earth because its half life is very long. This atom is present in most rocks, especially volcanic rocks. When a rock solidifies that freezes its atoms into place. Most of these rocks will also contain lead. Lead atoms come in many varieties (Lead-206, Lead-207, Lead-208) and these are stable (non-radioactive).
As the rock ages its Uranium-238 decays forming Lead-206 via a series of intermediaries. Again by measuring the Uranium and Lead ratio, the age of the rock (since it solidified) can be calculated. This method also has a 1% error associated with it.
Sedimentary rock (where the majority of fossils are found) cannot be dated like this but these rocks are often found sandwiched between volcanic (or igneous) rock. This can be dated. In addition, other radioactive atoms can be used (Uranium-235, this is the form used in nuclear power stations and Thorium-232). These decay at different rates so can be used to check results against each other.
A few fossils contain growth rings which mark out the cycle of the day and the year. The Earth's day is changing; it is getting longer by one second every 100,000 years because of the gravitational effect of the Moon.
One group of ancient fossil corals were found in the Devonian layers and were dated at 380 million years old using radioactive clocks. The length of day and year when these corals were alive was measured by studying their rings. They indicated a year consisting of about 400 days. Working backwards from a 24 hour day, we can calculate how the length of the day and year have varied in the Earth's past using the Theory of Gravity (incidentally, a theory that few people ever say "it's only a theory" to). The Earth's day is mainly affected by the Moon so the calculation took into account the changes in the Moon's orbit and how it affected the Earth's rotation. For the period 380 million years ago the day was just 22 hours long and there were 396 days in the year. This is in excellent agreement with the number of days in the coral rings. These types of measurements have always agreed with the dating of the fossil by the radioactive clocks.
Some people have argued that we cannot know if the rate of radioactive decay has always been constant. There is at least one piece of evidence that it has. When stars explode (a supernova), the exploding star brightens quickly and fades slowly. The fading is with a half life of just over 60 days. This is identical to the half life of an element called Californium-254 which is created in the explosion and can be detected in the stellar debris; its decay matches the fading of the star.
This half life of supernovas and their associated Californium-254 decay can be observed within our galaxy, in nearby galaxies and also in galaxies at the very edge of the Universe. When we look at very distant galaxies we are looking into the past because of the finite value of the speed of light.
Dating relies on many properties in physics ranging from the very small (radioactivity) to the very large (the effects of gravity and exploding stars).
Creationists who believe in a Universe only a few thousand years old can only do so if they ignore much of modern science. If the Universe really is only a few thousand years old and Evolution does not occur, then science must abandon much of its current knowledge of Biology, Microbiology, Genetics, Quantum Mechanics (radioactivity), Relativity (the overall properties of the Universe), Cosmology, Newton's Law of Gravity, Geology, Plate Tectonics and large areas of Medicine (e.g the use of antibiotics).
Wikipedia page listing the evidence of evolution.
Time Line of Life
A historical time line of life on Earth.
Dating using Radioactivity