Originally posted by JesusIsAlive
The underlying principle of the theory of evolution states that all life (specifically all human and animal organisms) descended from a previous life form, all of which have a common ancestor. These life forms originally began as a primitive, single-celled amoeba and then evolved over billions of years into human beings (the higher, complex life form) due to natural selection.
yes and no.
There is a lot of interpretation FROM the principals of evolutionary theory in there, but way too much emphasis on human origins and the origin of life in general.
A better definition would be: Evolution is the change in any unit of replication over time, provided that the units have both properties of heredity and variance, based on the ability of these units to replicate in a given environment.
So, as many people point out constantly, the theory of evolution say nothing about the origins of life. so these points:
Originally posted by JesusIsAlive
all life (specifically all human and animal organisms) descended from a previous life form
Originally posted by JesusIsAlive
These life forms originally began as a primitive, single-celled amoeba
Are factually inaccurate.
I will deal with the latter first. Evolution does not say what the original life form on the planet was. The only scientific fact remotely similar to this is as follows:
All life on earth, that we have look at the DNA of, shares a certain very small percentage of identical genes. This indicates that at some point in time, there was one thing that was alive that propagated all life on earth. This does not mean there were no other living things at the time, just that whatever property this unit of replication had made it that much more likely to continue replication.
To the first there are 2 comments. Obviously, to say that all life comes from prior life is a framing tactic that puts me at an instant disadvantage. At some point, life must have began. Scientists have some theories as to how, but the conditions of the earth 5 billion years ago is pretty hard to reproduce in a lab. That has nothing to do with evolution. Evolution only deals with the units of replication ONCE THEY BEGIN TO REPLICATE, and not the origins.
Secondly, all life did not come from another life form. That insinuates that one day a fish gives birth to a frog. Evolution does not create such distinct divides between species, except over long periods of time.
For instance, say there is a population of birds. Now, there are small flying birds that are eventually going to evolve into an ostrich. (this isn't fact by the way, I don't know the evolution of ostriches, it is to illustrate a point). The important thing to remember here is that for speciation of this kind to occur, there must be a reason for 2 populations to stop mating with each other. Once they become so genetically different from each other that they can no longer mate and produce offspring, they are considered to be different species.
So, we have this small bird. Now, the bird has some chicks. Some of them are smaller and can fly around and get all the food they need, but some are a little bit larger and it is better for them to only fly short distances and forage in shrubs or whatever for food. Now, the two species can still interbreed and probably still will for the next few generations. However, as time passes, the birds that are too large to get enough food to fly around constantly have to become more aggressive and therefore larger, or else they will starve. That is to say, the birds that are born with a greater disposition to be aggressive for food will replicate their genes (replicator units) more often than those who do not.
At this point there may be an interesting phenomena. The small flying birds may still be able to mate with these large shrub birds, but because of behavioural differences, they wont. This allows multiple generations to pass where the genes do not get shared between the two populations, and they become distinct from one another.
So, now we have this distinct species of large foraging bird with limited flight. Some of them reproduce better as they get larger and bigger/ longer claws. Those with a longer neck are able to reach more food and thusly reproduce more children. Those with smaller wings require less food and are able to survive longer in drought.
The whole thing of microevolution and macroevolution is bunk. Everything is microevolution, however, at massive timescales, it looks like macroevolution is occurring. At any given time, all members of a single species should be able to reproduce with each other, but over in some cases thousands of generations, they lose that ability, so we call them different species. Basically, all life is a continuum.
Your focus on human evolution is a little misguided too, at seems to illustrate a bias in your thinking. Human evolution is one of the least understood and reliable fields in evolutionary research. That doesn't mean we don't have evidence of human evolution, it is because of the statistical population of relatives we have the ability to compare our evolution to, that being none.
When studying the evolution of pigs, it is really simple because there are many ancestors that can be compared between, thus being able to tease our certain causes for certain features. For the human fossil record, this is much more difficult. There have been VERY few species of human, and only ONE homo sapiens. Not that this in any way discredits evolution, just that it points out your definition is fixated too squarely on your bias toward the subject.
Besides, there is much more convincing DNA evidence for evolution than fossil.
EDIT: This kinda flys in the face of my don't debate with creationists thing, and I guess I only replied because I got a PM from mr JIA...
Probably not going to say any more than this, kinda embarassed I got suckered in anyhow...