Hi Tornapart,
"I know anyone who believes in evolution is going to say that there is no 'if' about it. However..."
Evolution is of course a scientific theory that explains the origin of species. Like all scientific theories it should be believed based on its ability to explain and predict things we find in the natural world. Nonetheless there are still many specific things about the arrival of a given species such as us (homo sapiens) that still being discovered.
"If man evolved over hundreds of thousands of years ago, why were there only about 200-300 million people alive 2,000 years ago? Surely there'd have been many billions by then?"
I think it would be helpful to break your question up here a bit. Please bear with me.
"If man evolved" seems to imply a single event in time that happened then remained static. I am the father of four children, each of my girls is 1 evolutionary generation from me. My first born has now given birth to my first grandchild, who is 2 evolutionary generations from me. I have found it best to think of evolution as a continuum, wherein 1 generation to the next always continues to be of the same species, but there is no specific hard barriers that would prevent a given line of descent from accumulating enough DNA difference to qualify as a genuinely new species.
"over hundres of thousands of years ago" is based on the initial study of DNA that is passed through females only. The mitochondrial DNA you have should be nearly identical to your mother and her mother, etc. Only mutations can effect this DNA. Based on estimates of the rate of mutational changes to this DNA it was worked out that homo sapiens are in the range of 100,000 to 250,000 years old. Since then a sequence of DNA unique to the Y chromosome has been used to cross-check this work. The Y-chromosome DNA of course passes through the male line only and is unaffected by DNA from the mother. The two studies build confidence in the 100K to max 250K numbers.
"why were there only about 200-300 million people alive 2,000 years ago?" This is a question about population dynamics. One part of the answer is the power of exponential growth of a population left unchecked by other forces. Thus humans were able to move their population number from 1 billion to 7 billion in little over 100 years! If we imagine human population was very small 100K years ago, perhaps as little as a few thousand it takes time and some good fortune to take that number up to the range of 100s of millions. Some have already suggested forces that would keep a population in check, such as disease, disasters and competition for resources, etc. However, I think it is kinda interesting to look at the flip side of this problem. If the human population was being kept in check with very little upward movement what might have released the flood gates of growth?
A fun documentary to watch on this subject is "How Beer Made the World." It is a bit tongue and cheek, but it does touch upon what must have certainly been a few of the major factors. The ending of the last ice age created a favorable conditions for humans to learn to farm. Small but significant mutations may also be very recent such as the FOXP2 sequence combined with increased brain power unleashing our ability to communicate and thus drive our numbers up. Even the act of cooking food, which is connected fire, has been seen by some as a key to allowing human brain power to increase.
What you're doing though is good. In considering any scientific theory we should look for ways to kill it. In fact for a theory to qualify as scientific there must be some method to show it is false. If DNA mutation and recombination were impossible, for example, it would deal a severe blow to the theory of evolution. Time and reproduction rates are also important, if enough time were not available or reproduction rates too low, evolution would be in trouble as a theory. Your question is there too much time, such that we should expect higher population numbers, is a good question and without a proper answer might get our understanding of life on earth in trouble.
Cheers,
-Randy
