Dodo25
07-11-2010, 10:06 AM
The claim 'evolution is wrong' has come up (again) in the religious texts section, and I would like to address and refute it here. There is another thread about evolution already, yet I prefer to open up a new one, because the old one (http://www.online-literature.com/forums/showthread.php?t=44359&highlight=creationism), altough interesting, is dead for quite some time, and rather long, which usually dilutes the actual topic.
The intent of this first post is to establish that evolution is a scientific fact as well as a standart theory. The intent of this thread is to debate or extend the arguments I will put forward here. Discussions about specific aspects or examples from nature are of course welcome too. Anyone who can challenge or improve the arguments I'll put forwards is welcome to join in.
By the way, I'm not a scientist, in fact I haven't even started college yet (but I'm pretty sure I'll study evolutionary (or socio-) biology). My arguments should not be believed because of anyone's qualifications, they should be convincing because of their factual value. People are welcome to google the issues I raise or ask real biologists about it, especially if you mistrust my intellectual honesty or my ability to filter out wrong propaganda from accurate, or at least accredited information. I've done a vast amount of research on the subject (creationism and ID too), so I'm confident I can give an accurate representation of the science behind evolution. I know this post is very long, but I try to condense the relevant information, and make it a worthy read (also for non creationists).
First of all, we need to clear up the terminology. Then I'll briefly describe the current knowledge about how life is related, and then I'll come to the evidence for first an old earth and then for evolution.
Terminology
'Evolution' has several meanings, it's most common one is 'biological (/chemical) evolution', and unless stated otherwise, whenever I'll use the word 'evolution', it means biological evolution, which states:
Living organisms have descended through modification from earlier species (the technical definition is 'changes of allele frequencies over time in the gene pool'). Evolution happens to the greatest extent due to natural selection, genes (alleles) that are better at copying themselves than others (meaning, make the organism survive and reproduce better) will for these very reasons be favored by natural selection, the mechanism responsible for the apparent 'design' we find in nature.
Evolution makes NO claims about how life began in the first place. The scientific hypothesis (not a theory) that deals with this is called 'abiogenesis'. Evolution needs a replicating entity to get started. Presumably the first 'life' was a self-replicating macromolecule (or molecule cycle), and once there is replication with variation, the famous mechanism, natural selection, jumps in and we get, over a very very long amount of time, the complexity and 'design' we now find in nature.
Creationists like to point out 'evolution is JUST a theory'. They don't (or don't want to) understand the scientific terminology. Admittedly, it is a bit confusing. Stephen J. Gould has put it down succinctly in his essay 'Evolution as Fact and Theory':
"In the American vernacular, "theory" often means "imperfect fact"--part of a hierarchy of confidence running downhill from fact to theory to hypothesis to guess. Thus the power of the creationist argument: evolution is "only" a theory and intense debate now rages about many aspects of the theory. If evolution is worse than a fact, and scientists can't even make up their minds about the theory, then what confidence can we have in it? […] Well, evolution is a theory. It is also a fact. And facts and theories are different things, not rungs in a hierarchy of increasing certainty. Facts are the world's data. Theories are structures of ideas that explain and interpret facts. Facts don't go away when scientists debate rival theories to explain them. Einstein's theory of gravitation replaced Newton's in this century, but apples didn't suspend themselves in midair, pending the outcome. And humans evolved from ape-like ancestors whether they did so by Darwin's proposed mechanism or by some other yet to be discovered."
The scientific method goes from 'Observation' to 'Hypothesis' to 'Experiments' based on the hypotheses to 'Theory', which is a hypothesis that has succesfully predicted the outcome of numerous experiments and has never been falsified.
Scientists (or at least an overwhelming consensus, I've several times seen figures as high as 99.98%, definitely more historians deny the holocaust than biologists deny evolution) do regard it as a fact that organisms evolved. How they did it is explained by the theory of evolution. A theory can never become a fact, yet this doesn’t mean that natural selection is at all likely not to be the main mechanism behind it. Any current scientific theory, such as the theory of relativity or the germ theory of disease, has successfully predicted the outcomes of hundreds of experiments and has never been falsified by evidence. The modern synthesis ('Neo-Darwinism') is among the most rigorously tested theories; the likelihood that it has got the main mechanisms wrong is astronomically low.
Another prevalent misunderstanding is that humans somehow represent the 'top of an evolutionary ladder'. This is wrong, humans didn't evolve from apes (or monkeys, monkeys have tails, apes don't), they share a common ancestor. Evidence (universality of DNA) suggests that all life evolved from one single 'origin of life'. One can imagine it like a huge tree of life, the bottom of the tree represents the past, with the origin of life around 3.5 billion years ago, and at the top, all the millions of tiny twigs represent living species. The 'trunks' at the bottom also represent (in retrospect!) whole phyla, meaning i.e. animalia, fungi, bacteria (which encompass many different phyla actually). Then the tree branches further, some branches on the 'trunk' of the phylum animalia would i.e. be 'reptiles', and from the reptiles branch branched off the branches 'mammals' and 'birds', while the reptile branch continued to evolve, upwards, in time. Then at the most detailed level, the 'twigs', there are single species, i.e. among the family of the great apes, we have orang-utans, gorillas, chimpanzees (, bonobos) and humans.
Note however that at the trunk of the tree of life, what we now, with hindsight, call the lineage of a whole phylum, was only one 'normal' species that happened to be ancestral to a whole phylum.
The three also has many 'dead ends', for roughly 99% of all the species that have ever lived have gone extinct.
One more term that needs to be defined is species.
Species are not fixed Platonic ideas, the whole idea of evolution is that they change. The creationist term 'missing link' is thus misleading, because every species is changing, and there is no distant goal, a species exists because it is good at making a living. The biological distinction is as follows:
If two types of organisms are incapable of producing fertile offspring, then they belong into different species. The definition fades into fuzzyness when we talk about single celled organisms that reproduce asexually, in these cases, important characteristics are used, yet of course the distinction becomes fuzzy.
Now that we have the definitions, I will sketch some convincing (I hope so) lines of evidence.
EVIDENCE
The Age of the Earth
People who accept that the earth is very old can skip the next few paragraphs.
The earth is around 4.55 billion years old, this has been verified by methods of radiometric dating, several different isotopes (which decay in different ways even, beta or alpha particles and other types still) with different half-life times INDEPENDENTLY produced the same results over and over. The most famous dating technique is the C-14 (radiocarbon) method. Yet because this particular decay has a relatively short half-life time (around 6000 years), and because it is based on carbon, it is only used for relatively new, organic things (i.e. wood tools, coal paintings on rock walls etc.). Yet even this method establishes that the earth is older than Young Earth Creationists believe.
"120 million of us place the big bang 2,500 years after the Babylonians and Sumerians learned to brew beer."
- Sam Harris
It's a tragedy! And by 'us' he means the United States I think.
Dating rocks works similarly, you need volcanic rocks (sediments can only be dated relatively, if they're 'sandwiched' in between layers of volcanic rock), these rocks contain small bubbles of gas that was formed by radioactive decay. Then one can measure the ratio of 'parent' isotope and 'daugther gas', and because we know the half-life of these isotopes, we can calculate when the stone was formed (because this sets the daugther isotope at zero (lava)). For instance, in 'potassium-argon' dating, a specific isotope of potassium decays to argon at a half-life rate of 1.3 billion years. This method is used for the oldest rocks, because of its long half-life time. By comparing the ratio of the potassium isotope to argon (gas) in the rock, the age can be calculated. And again, the beauty of it is that many times the half-life of these atomic 'clocks' overlap, and different methods reach the SAME CONCLUSIONS!
Evolution
Now some lines of evidence for evolution. There are many, I'll just focus on my few favorites:
Many creationists think fossils are the only evidence for evolution, and for some reason, they also think that there are almost no crucial fossils. In fact, there are lots of fossils documenting all major evolutionary changes (amphibians - reptiles, reptiles - birds, reptiles - mammals, ape-like ancestor - humans and so on), creationists are lying when they say 'there are no missing links'. Yet even without a single fossil, we would know that evolution is true, because of evidence from other fields of biology (and geography, chemistry etc).
I'll just pick two examples for the fossil evidence, reptiles (more precise: dinosaur) - bird transition, and of course, the ape-like ancestor - human transition.
Reptiles - Birds
The more predictions a theory can make, the greater is it's scientific value. Evolution has made hundreds of predictions. One would be that birds must share ancestors with other lineages, i.e. they could be closely related to the mammals, or to reptiles. Before it was known that birds evolved from dinosaurs, it was already predicted that they must fit in somewhere. So we should be able to find fossils sharing similarities of both modern birds and dinosaurs, or modern birds and 'young' mammals (yet not both, they could only have come from one branch of the tree of life, two branches evolving to the same place independently would be a very uncanny coincidence!).
The most popular fossil for the dinosaur - bird transition is archaeopterix. Up to now, about 11 specimen have been found. There was a short period where some scientists thought archaeopterix has been severely misinterpreted, and this meme (of course) lives on in creationist circles, yet this was due to a lie by the not-even-biologist (but astronomer and creationist) Fred Hoyle. Anyway, archaeopterix had teeth, like dinosaurs, but also a primitive beak. It's ribs (chest bones) were partially waxen together, as it is in modern birds. It also had feathers and 'wings', the hind legs were a bit fatter than in most birds, yet they are bird-like. By the way, of course not all dinosaurs evolved into birds (imagine i.e. brontosaurus changing into a bird), only one species did (a small predator, like velociraptor but a bit smaller presumably). There are more such 'transition' fossil species than archaeopterix, but I won't go into details.
Early Hominid fossils:
The popular creationist response to such fossils is either 'these are just extinct apes' and 'these are humans, just a bit different (they then refer to some Bible passages that talk about some strange tribe or so, I can't recall the name, it's not important though because it's nonsense, as I shall show)'. creationists also claim that models of such hominids are misleading, because artists paint 'humanlike' facial features and add hair / no hair, even though one cannot reconstruct a face accurately from just skull bones. This claim is true to some extent, yet it really isn't about faces, it is about comparative anatomy based on bones!
When we compare modern humans to modern chimpanzees, the most striking differences are the front (chimps have none), the pelvis (human one is big and broad, chimp one long and slim), the teeth (chimps have canine teeths) and their arrangement (chimps have them U-formed, humans parabolic), cranial capacity and of course the backbone (and how it is connected to the skull, if it is right under the brain this indicates uprightness for walking).
The oldest known fossil (I'm using the singular here, but there are always several specimen that have been examined, in the case of this one, it's only about 2-3, most of the time it is many more though) on the human side of the chimpanzee-human split (after the common ancestor of BOTH split into two separate populations which then underwent evolution in different directions) is 'Ardipithecus ramidus'. It's about 4.5 million years old, could walk upright (yet not perfectly, probably only used it in certain situations), and it's skull, teeth and pelvis were all ape-like.
Then a newer species in between would be Australopithecus afarensis (i.e. 'Lucy'), which lived around 3.5 million years ago. It was a bit taller than Ardipithecus (yet still small compared to us), walked upright, yet it too had mostly ape-like features.
Then we reach the genus homo, with Homo habilis (around 2 million years ago). This is where the cranial capacity became significantly bigger (yet no front), no canine teeth anymore, yet still arranged in an apelike way. Homo habilis was the first animal to use tools (some modern chimps have learned this too actually).
Then comes Homo erectus (around 1 million years ago), it looked already very humanlike with cranial capacity even bigger than Homo habilis. The teeth were arranged humanlike, yet in certain specimen, there is a gap between side teeth and front teeth, this is an indicator that it descended from animals with canine teeth, which have this gap too (so it doesn't bite itself with the long teeth when it closes its mouth).
Another popular Hominid fossils is Homo neanderthalensis (neanderthal man). Recent genetic analysis (yeah, they could even isolate its DNA!) has however shown, that it was not our ancestor, more of a close relative. There is now an interesting debate going on about whether Homo sapiens has interbred with Homo neanderthalensis (which died out 40'000 years ago, most likely because 'we' killed it, as we have most likely killed all 'monster sized' birds and mammals except elephants, yet that's another subject). What I find fascinating is that there is evidence (bodies buried with flowers and other 'valuable' stuff) that Neanderthal man had religious beliefs. This fact has interesting theological implications if one holds any theological views at all.
Fossils of our species, Homo sapiens, appear from around 150'00 years onwards. We seem to have not changed significantly in anatomy. Yet around 40'000 years ago something special happened to the content in our brains, because then the first cave paintings appeared. Some anthropologists say it was the origin of language, others place this back further, even to homo erectus. Either way, it represented the 'Great Leap Forward', the discovery of culture. Maybe it was a new feature of language, such as the 'conjunctiv' for imagination..
Genome Comparison
One of the most convincing lines of evidence comes from molecular biology: There are certain viruses called 'endogeneous retroviruses (ERV)'. ERVs insert themselves into the genomes of infected organisms. If this happens in the germ line (sperms or egg cells), these viral DNA sequences (viral RNA changed into DNA) can get passed on to the offspring of the infected individual.
In the human genome, 8% of all genes are actually the remains of such viral invasions. We know this because they have very characteristic DNA 'markers' that normally belong to viruses, and we can even determine the 'species' of ERV (there are many different ones) these remains are from. Now how is this relevant in regard to evolution? Well, viral remains are found in all animals. And the great apes, especially our closest living relatives, the chimpanzees, share the VERY SAME types of ERV at the VERY SAME PLACES in the chromosomes! How did this huge coincidence happen if animals were created separately? Did through a cosmic accident the same virus type happen to insert itself at the same place, independently, in humans, chimps and gorillas?
The only explanation that makes sense is that humans and chimpanzees (and gorillas, and everything else once you keep comparing) share common ancestors (that already had the viral remains and then passed them on to the descendants).
The same tree of ancestry can be arrived at by comparison of DNA sequences. One just takes sequences that don't get expressed in the phenotype (because these sequences are under selection pressure and thus don't mutate at a constant rate) and compares them from species to species. For all kinds of different genes, the same tree of ancestry (a specific, nested hierarchy) is the result.
Traces of Evolutionary History
Because evolution works on 'preexisting' structures, it can't go back to an empty drawing board and 'intelligently design' things from scratch. This means that we should find 'imperfect' features in organisms that have been exapted to fit a new purpose (or remains that serve none at all). We do so indeed, some examples:
Whales (mammals who evolved from land mammals, actually, the whale's closest living relative is the hippo) have tiny bones where their back legs would be. In some cases, when a normally repressed gene, due to a mutation, expresses itself in the whale, the whale actually grows 'real' back legs with three distinct bones, exactly as i.e. hippos do (it doesn't have toes anymore, but the leg bones are still there, yet normally they're repressed into tiny bones).
Chickens sometimes grow teeth, a hint to their reptile past. Because of mutations, the teeth producing genes are split, broken or repressed, but sometimes it comes back in individuals.
Human embryos have a yolk sack. Why would they need it? It is a remain from when we were fish and amphibian-like. It develops backward in later embryo stages, but it can be found early. This led to the prediction that there should be broken genes for 'yolk' producing in the human genome. And indeed, three such (broken) genes have been identified!
In fact, the whole field of embryology is full of evidence for evolution, because embryo development in mammals goes through fish and amphibian stages first. Early on, mammal embryos actually have inchoate gills, and in some unfortuate individuals the genes aren't turned off and gill like structures (altough non-functioning of course, because so much is missing) do develop (and must be removed surgically, if the baby survives).
Another funny fact is that babies grow hair on their shoulders in the late stages of pregnancy. The hair goes away a couple of months after birth, it too is a remainder to our primate past.
All these things show how evolution can't go back to the drawing board. It needs to twiddle with preexisting structures, random mutations which sometimes happen to have a beneficial effect are favored by selection. This is the opposite from intelligent design, yet it works, and the 'designs' it has come up with are often brilliant and stupefying!
Mistakes in Design
Other times the designs work, yet they are very idiotic. These 'mistakes in design' are also evidence for evolution, similar to the remains of evolutionary history. Examples include the laryngeal nerve, which connects the larynx to the brain, yet not straightforwadly, because it first goes down the whole neck and around the blood vessels surrounding the heart (making a U-turn) and then going back up the neck to the brain. This becomes particularly funny when the animal we're talking about is a giraffe. It is like this by the way because fishes don't have necks, and in fishes, the nerve is just perfect like it is. Because a mutation that would unwrap the nerve from the heart blood vessels is so unlikely to happen and would most likely mess up so much structure that it would kill the organism, when the neck develop, the nerve just had to get along with it (literally).
Another example, ironic because it affects the 'crown of creation', is the mammal eye (which includes the human eye). The photons must first pass through a layer of nerve cells before they hit the active spots on the retina were they're transformed into signals leading to the brain. The nerve tissue is on the 'wrong' side of the retina, so it absorbs photons to some parts and thus impoverishes vision. This is also the reason why we have a 'blind spot' on the retina, where all the nerve 'wires' need to leave the eye. It could be otherwise, as is neatly illustrated by the eyes of squids (which evolved independently).
Geographical Distribution
...of Fossils
The older a fossil, the less complex it is. If animals were all created at the same instant, this would not be the case. A single rabbit fossil in the Cambrium would refute evolution. Yet of course, mammals only appear around 300-200 million years ago (first they look reptile-mammal like, the platypus actually still has some interesting features from that era). This distribution itself is strong evidence for evolution. Young Earth creationists have found some very disturbing arguments 'against' this, it involves density of animals, different layers of sediments that are formed at the same time, intelligence and run-away capacity of animals, and the 40 day Biblical flood..
...of Organisms
The classical example here are islands. The fauna and flora on islands is always closely related to the nearest mainland, even in cases where there are significant differences in climate. On many islands, whole groups of animals are simply 'missing', because no ancestor of such a group ever made its way to such an island. For instance, until human settlers brought them there, there were absolutely no mammals in Australia, and the marsupials filled the 'niches' mammals would normally fill in other parts of the world.
Furthermore, the geographical distribution of animals and fossils, and their relatedness, matches up perfectly with the theory of continental drift. If for instance we found different groups, families of organisms with similiarities, yet they naturally live on continents which are now far apart from each other, continental drift LATER (another prediction coming true) explained that these land masses were ones joined together.
The intent of this first post is to establish that evolution is a scientific fact as well as a standart theory. The intent of this thread is to debate or extend the arguments I will put forward here. Discussions about specific aspects or examples from nature are of course welcome too. Anyone who can challenge or improve the arguments I'll put forwards is welcome to join in.
By the way, I'm not a scientist, in fact I haven't even started college yet (but I'm pretty sure I'll study evolutionary (or socio-) biology). My arguments should not be believed because of anyone's qualifications, they should be convincing because of their factual value. People are welcome to google the issues I raise or ask real biologists about it, especially if you mistrust my intellectual honesty or my ability to filter out wrong propaganda from accurate, or at least accredited information. I've done a vast amount of research on the subject (creationism and ID too), so I'm confident I can give an accurate representation of the science behind evolution. I know this post is very long, but I try to condense the relevant information, and make it a worthy read (also for non creationists).
First of all, we need to clear up the terminology. Then I'll briefly describe the current knowledge about how life is related, and then I'll come to the evidence for first an old earth and then for evolution.
Terminology
'Evolution' has several meanings, it's most common one is 'biological (/chemical) evolution', and unless stated otherwise, whenever I'll use the word 'evolution', it means biological evolution, which states:
Living organisms have descended through modification from earlier species (the technical definition is 'changes of allele frequencies over time in the gene pool'). Evolution happens to the greatest extent due to natural selection, genes (alleles) that are better at copying themselves than others (meaning, make the organism survive and reproduce better) will for these very reasons be favored by natural selection, the mechanism responsible for the apparent 'design' we find in nature.
Evolution makes NO claims about how life began in the first place. The scientific hypothesis (not a theory) that deals with this is called 'abiogenesis'. Evolution needs a replicating entity to get started. Presumably the first 'life' was a self-replicating macromolecule (or molecule cycle), and once there is replication with variation, the famous mechanism, natural selection, jumps in and we get, over a very very long amount of time, the complexity and 'design' we now find in nature.
Creationists like to point out 'evolution is JUST a theory'. They don't (or don't want to) understand the scientific terminology. Admittedly, it is a bit confusing. Stephen J. Gould has put it down succinctly in his essay 'Evolution as Fact and Theory':
"In the American vernacular, "theory" often means "imperfect fact"--part of a hierarchy of confidence running downhill from fact to theory to hypothesis to guess. Thus the power of the creationist argument: evolution is "only" a theory and intense debate now rages about many aspects of the theory. If evolution is worse than a fact, and scientists can't even make up their minds about the theory, then what confidence can we have in it? […] Well, evolution is a theory. It is also a fact. And facts and theories are different things, not rungs in a hierarchy of increasing certainty. Facts are the world's data. Theories are structures of ideas that explain and interpret facts. Facts don't go away when scientists debate rival theories to explain them. Einstein's theory of gravitation replaced Newton's in this century, but apples didn't suspend themselves in midair, pending the outcome. And humans evolved from ape-like ancestors whether they did so by Darwin's proposed mechanism or by some other yet to be discovered."
The scientific method goes from 'Observation' to 'Hypothesis' to 'Experiments' based on the hypotheses to 'Theory', which is a hypothesis that has succesfully predicted the outcome of numerous experiments and has never been falsified.
Scientists (or at least an overwhelming consensus, I've several times seen figures as high as 99.98%, definitely more historians deny the holocaust than biologists deny evolution) do regard it as a fact that organisms evolved. How they did it is explained by the theory of evolution. A theory can never become a fact, yet this doesn’t mean that natural selection is at all likely not to be the main mechanism behind it. Any current scientific theory, such as the theory of relativity or the germ theory of disease, has successfully predicted the outcomes of hundreds of experiments and has never been falsified by evidence. The modern synthesis ('Neo-Darwinism') is among the most rigorously tested theories; the likelihood that it has got the main mechanisms wrong is astronomically low.
Another prevalent misunderstanding is that humans somehow represent the 'top of an evolutionary ladder'. This is wrong, humans didn't evolve from apes (or monkeys, monkeys have tails, apes don't), they share a common ancestor. Evidence (universality of DNA) suggests that all life evolved from one single 'origin of life'. One can imagine it like a huge tree of life, the bottom of the tree represents the past, with the origin of life around 3.5 billion years ago, and at the top, all the millions of tiny twigs represent living species. The 'trunks' at the bottom also represent (in retrospect!) whole phyla, meaning i.e. animalia, fungi, bacteria (which encompass many different phyla actually). Then the tree branches further, some branches on the 'trunk' of the phylum animalia would i.e. be 'reptiles', and from the reptiles branch branched off the branches 'mammals' and 'birds', while the reptile branch continued to evolve, upwards, in time. Then at the most detailed level, the 'twigs', there are single species, i.e. among the family of the great apes, we have orang-utans, gorillas, chimpanzees (, bonobos) and humans.
Note however that at the trunk of the tree of life, what we now, with hindsight, call the lineage of a whole phylum, was only one 'normal' species that happened to be ancestral to a whole phylum.
The three also has many 'dead ends', for roughly 99% of all the species that have ever lived have gone extinct.
One more term that needs to be defined is species.
Species are not fixed Platonic ideas, the whole idea of evolution is that they change. The creationist term 'missing link' is thus misleading, because every species is changing, and there is no distant goal, a species exists because it is good at making a living. The biological distinction is as follows:
If two types of organisms are incapable of producing fertile offspring, then they belong into different species. The definition fades into fuzzyness when we talk about single celled organisms that reproduce asexually, in these cases, important characteristics are used, yet of course the distinction becomes fuzzy.
Now that we have the definitions, I will sketch some convincing (I hope so) lines of evidence.
EVIDENCE
The Age of the Earth
People who accept that the earth is very old can skip the next few paragraphs.
The earth is around 4.55 billion years old, this has been verified by methods of radiometric dating, several different isotopes (which decay in different ways even, beta or alpha particles and other types still) with different half-life times INDEPENDENTLY produced the same results over and over. The most famous dating technique is the C-14 (radiocarbon) method. Yet because this particular decay has a relatively short half-life time (around 6000 years), and because it is based on carbon, it is only used for relatively new, organic things (i.e. wood tools, coal paintings on rock walls etc.). Yet even this method establishes that the earth is older than Young Earth Creationists believe.
"120 million of us place the big bang 2,500 years after the Babylonians and Sumerians learned to brew beer."
- Sam Harris
It's a tragedy! And by 'us' he means the United States I think.
Dating rocks works similarly, you need volcanic rocks (sediments can only be dated relatively, if they're 'sandwiched' in between layers of volcanic rock), these rocks contain small bubbles of gas that was formed by radioactive decay. Then one can measure the ratio of 'parent' isotope and 'daugther gas', and because we know the half-life of these isotopes, we can calculate when the stone was formed (because this sets the daugther isotope at zero (lava)). For instance, in 'potassium-argon' dating, a specific isotope of potassium decays to argon at a half-life rate of 1.3 billion years. This method is used for the oldest rocks, because of its long half-life time. By comparing the ratio of the potassium isotope to argon (gas) in the rock, the age can be calculated. And again, the beauty of it is that many times the half-life of these atomic 'clocks' overlap, and different methods reach the SAME CONCLUSIONS!
Evolution
Now some lines of evidence for evolution. There are many, I'll just focus on my few favorites:
Many creationists think fossils are the only evidence for evolution, and for some reason, they also think that there are almost no crucial fossils. In fact, there are lots of fossils documenting all major evolutionary changes (amphibians - reptiles, reptiles - birds, reptiles - mammals, ape-like ancestor - humans and so on), creationists are lying when they say 'there are no missing links'. Yet even without a single fossil, we would know that evolution is true, because of evidence from other fields of biology (and geography, chemistry etc).
I'll just pick two examples for the fossil evidence, reptiles (more precise: dinosaur) - bird transition, and of course, the ape-like ancestor - human transition.
Reptiles - Birds
The more predictions a theory can make, the greater is it's scientific value. Evolution has made hundreds of predictions. One would be that birds must share ancestors with other lineages, i.e. they could be closely related to the mammals, or to reptiles. Before it was known that birds evolved from dinosaurs, it was already predicted that they must fit in somewhere. So we should be able to find fossils sharing similarities of both modern birds and dinosaurs, or modern birds and 'young' mammals (yet not both, they could only have come from one branch of the tree of life, two branches evolving to the same place independently would be a very uncanny coincidence!).
The most popular fossil for the dinosaur - bird transition is archaeopterix. Up to now, about 11 specimen have been found. There was a short period where some scientists thought archaeopterix has been severely misinterpreted, and this meme (of course) lives on in creationist circles, yet this was due to a lie by the not-even-biologist (but astronomer and creationist) Fred Hoyle. Anyway, archaeopterix had teeth, like dinosaurs, but also a primitive beak. It's ribs (chest bones) were partially waxen together, as it is in modern birds. It also had feathers and 'wings', the hind legs were a bit fatter than in most birds, yet they are bird-like. By the way, of course not all dinosaurs evolved into birds (imagine i.e. brontosaurus changing into a bird), only one species did (a small predator, like velociraptor but a bit smaller presumably). There are more such 'transition' fossil species than archaeopterix, but I won't go into details.
Early Hominid fossils:
The popular creationist response to such fossils is either 'these are just extinct apes' and 'these are humans, just a bit different (they then refer to some Bible passages that talk about some strange tribe or so, I can't recall the name, it's not important though because it's nonsense, as I shall show)'. creationists also claim that models of such hominids are misleading, because artists paint 'humanlike' facial features and add hair / no hair, even though one cannot reconstruct a face accurately from just skull bones. This claim is true to some extent, yet it really isn't about faces, it is about comparative anatomy based on bones!
When we compare modern humans to modern chimpanzees, the most striking differences are the front (chimps have none), the pelvis (human one is big and broad, chimp one long and slim), the teeth (chimps have canine teeths) and their arrangement (chimps have them U-formed, humans parabolic), cranial capacity and of course the backbone (and how it is connected to the skull, if it is right under the brain this indicates uprightness for walking).
The oldest known fossil (I'm using the singular here, but there are always several specimen that have been examined, in the case of this one, it's only about 2-3, most of the time it is many more though) on the human side of the chimpanzee-human split (after the common ancestor of BOTH split into two separate populations which then underwent evolution in different directions) is 'Ardipithecus ramidus'. It's about 4.5 million years old, could walk upright (yet not perfectly, probably only used it in certain situations), and it's skull, teeth and pelvis were all ape-like.
Then a newer species in between would be Australopithecus afarensis (i.e. 'Lucy'), which lived around 3.5 million years ago. It was a bit taller than Ardipithecus (yet still small compared to us), walked upright, yet it too had mostly ape-like features.
Then we reach the genus homo, with Homo habilis (around 2 million years ago). This is where the cranial capacity became significantly bigger (yet no front), no canine teeth anymore, yet still arranged in an apelike way. Homo habilis was the first animal to use tools (some modern chimps have learned this too actually).
Then comes Homo erectus (around 1 million years ago), it looked already very humanlike with cranial capacity even bigger than Homo habilis. The teeth were arranged humanlike, yet in certain specimen, there is a gap between side teeth and front teeth, this is an indicator that it descended from animals with canine teeth, which have this gap too (so it doesn't bite itself with the long teeth when it closes its mouth).
Another popular Hominid fossils is Homo neanderthalensis (neanderthal man). Recent genetic analysis (yeah, they could even isolate its DNA!) has however shown, that it was not our ancestor, more of a close relative. There is now an interesting debate going on about whether Homo sapiens has interbred with Homo neanderthalensis (which died out 40'000 years ago, most likely because 'we' killed it, as we have most likely killed all 'monster sized' birds and mammals except elephants, yet that's another subject). What I find fascinating is that there is evidence (bodies buried with flowers and other 'valuable' stuff) that Neanderthal man had religious beliefs. This fact has interesting theological implications if one holds any theological views at all.
Fossils of our species, Homo sapiens, appear from around 150'00 years onwards. We seem to have not changed significantly in anatomy. Yet around 40'000 years ago something special happened to the content in our brains, because then the first cave paintings appeared. Some anthropologists say it was the origin of language, others place this back further, even to homo erectus. Either way, it represented the 'Great Leap Forward', the discovery of culture. Maybe it was a new feature of language, such as the 'conjunctiv' for imagination..
Genome Comparison
One of the most convincing lines of evidence comes from molecular biology: There are certain viruses called 'endogeneous retroviruses (ERV)'. ERVs insert themselves into the genomes of infected organisms. If this happens in the germ line (sperms or egg cells), these viral DNA sequences (viral RNA changed into DNA) can get passed on to the offspring of the infected individual.
In the human genome, 8% of all genes are actually the remains of such viral invasions. We know this because they have very characteristic DNA 'markers' that normally belong to viruses, and we can even determine the 'species' of ERV (there are many different ones) these remains are from. Now how is this relevant in regard to evolution? Well, viral remains are found in all animals. And the great apes, especially our closest living relatives, the chimpanzees, share the VERY SAME types of ERV at the VERY SAME PLACES in the chromosomes! How did this huge coincidence happen if animals were created separately? Did through a cosmic accident the same virus type happen to insert itself at the same place, independently, in humans, chimps and gorillas?
The only explanation that makes sense is that humans and chimpanzees (and gorillas, and everything else once you keep comparing) share common ancestors (that already had the viral remains and then passed them on to the descendants).
The same tree of ancestry can be arrived at by comparison of DNA sequences. One just takes sequences that don't get expressed in the phenotype (because these sequences are under selection pressure and thus don't mutate at a constant rate) and compares them from species to species. For all kinds of different genes, the same tree of ancestry (a specific, nested hierarchy) is the result.
Traces of Evolutionary History
Because evolution works on 'preexisting' structures, it can't go back to an empty drawing board and 'intelligently design' things from scratch. This means that we should find 'imperfect' features in organisms that have been exapted to fit a new purpose (or remains that serve none at all). We do so indeed, some examples:
Whales (mammals who evolved from land mammals, actually, the whale's closest living relative is the hippo) have tiny bones where their back legs would be. In some cases, when a normally repressed gene, due to a mutation, expresses itself in the whale, the whale actually grows 'real' back legs with three distinct bones, exactly as i.e. hippos do (it doesn't have toes anymore, but the leg bones are still there, yet normally they're repressed into tiny bones).
Chickens sometimes grow teeth, a hint to their reptile past. Because of mutations, the teeth producing genes are split, broken or repressed, but sometimes it comes back in individuals.
Human embryos have a yolk sack. Why would they need it? It is a remain from when we were fish and amphibian-like. It develops backward in later embryo stages, but it can be found early. This led to the prediction that there should be broken genes for 'yolk' producing in the human genome. And indeed, three such (broken) genes have been identified!
In fact, the whole field of embryology is full of evidence for evolution, because embryo development in mammals goes through fish and amphibian stages first. Early on, mammal embryos actually have inchoate gills, and in some unfortuate individuals the genes aren't turned off and gill like structures (altough non-functioning of course, because so much is missing) do develop (and must be removed surgically, if the baby survives).
Another funny fact is that babies grow hair on their shoulders in the late stages of pregnancy. The hair goes away a couple of months after birth, it too is a remainder to our primate past.
All these things show how evolution can't go back to the drawing board. It needs to twiddle with preexisting structures, random mutations which sometimes happen to have a beneficial effect are favored by selection. This is the opposite from intelligent design, yet it works, and the 'designs' it has come up with are often brilliant and stupefying!
Mistakes in Design
Other times the designs work, yet they are very idiotic. These 'mistakes in design' are also evidence for evolution, similar to the remains of evolutionary history. Examples include the laryngeal nerve, which connects the larynx to the brain, yet not straightforwadly, because it first goes down the whole neck and around the blood vessels surrounding the heart (making a U-turn) and then going back up the neck to the brain. This becomes particularly funny when the animal we're talking about is a giraffe. It is like this by the way because fishes don't have necks, and in fishes, the nerve is just perfect like it is. Because a mutation that would unwrap the nerve from the heart blood vessels is so unlikely to happen and would most likely mess up so much structure that it would kill the organism, when the neck develop, the nerve just had to get along with it (literally).
Another example, ironic because it affects the 'crown of creation', is the mammal eye (which includes the human eye). The photons must first pass through a layer of nerve cells before they hit the active spots on the retina were they're transformed into signals leading to the brain. The nerve tissue is on the 'wrong' side of the retina, so it absorbs photons to some parts and thus impoverishes vision. This is also the reason why we have a 'blind spot' on the retina, where all the nerve 'wires' need to leave the eye. It could be otherwise, as is neatly illustrated by the eyes of squids (which evolved independently).
Geographical Distribution
...of Fossils
The older a fossil, the less complex it is. If animals were all created at the same instant, this would not be the case. A single rabbit fossil in the Cambrium would refute evolution. Yet of course, mammals only appear around 300-200 million years ago (first they look reptile-mammal like, the platypus actually still has some interesting features from that era). This distribution itself is strong evidence for evolution. Young Earth creationists have found some very disturbing arguments 'against' this, it involves density of animals, different layers of sediments that are formed at the same time, intelligence and run-away capacity of animals, and the 40 day Biblical flood..
...of Organisms
The classical example here are islands. The fauna and flora on islands is always closely related to the nearest mainland, even in cases where there are significant differences in climate. On many islands, whole groups of animals are simply 'missing', because no ancestor of such a group ever made its way to such an island. For instance, until human settlers brought them there, there were absolutely no mammals in Australia, and the marsupials filled the 'niches' mammals would normally fill in other parts of the world.
Furthermore, the geographical distribution of animals and fossils, and their relatedness, matches up perfectly with the theory of continental drift. If for instance we found different groups, families of organisms with similiarities, yet they naturally live on continents which are now far apart from each other, continental drift LATER (another prediction coming true) explained that these land masses were ones joined together.