Seriously, you need a gene pool of about 1,000 members if one is talking about reproducing a population of large mammals.
Here? Why would it make a difference? I think no matter where you post this question your responses will be in a similar vein. Might be better to rephrase the question, or give us more info. Do this couple share our biology for example? Is there anything specific about the world that might enable to get us on board with the idea, and help us to suspend our disbelief? ETA: This is venturing into sci-fi territory rather than fantasy. Most sci-fi enthusiasts don't mind a little bending or manipulation of facts but not blatant disregard for them.
Just name your characters Adam and Eve. That should do the trick. In all seriousness, something like this would be better suited for a fantasy novel IMO (or a sci-fi novel with fantasy elements). That way you can use some sort of fantastical device to explain why this is possible.
If that's the case then just choose the fantasy average number of kids and do the math. Other than population numbers, I'm not sure what you mean by "look like".
A couple of things. I know I said I was disregarding the facts. I said that because it's not what I want people to worry about when they answer the question. If I do end up putting this scenario, I'll be the one doing the fact checking. For now I just want to know how far a civilization progresses in 12 generations, and I don't care at the moment about their gene pool, because that isn't the question I'm asking right now. Also, how is it venturing into Sci Fi territory? Just because it's an alien planet means it's Sci fi? I don't think so.
As I see it, this isn't a question about how far a civilization progresses in twelve generations, because you don't have a civilization. A civilization allows for division of labor, different people pursuing different goals. You have two people. What do these two people know when they're stranded? The home/village created by a poet and a politician will be rather different from the one created by a mechanical engineer and a chemist. How hard is the environment? If they can just walk out of a comfortable cave and catch the fruit falling from the trees they'll have lots of time to care for and teach their children; if they spend most of their time protecting themselves from weather and predators and searching for food, they won't. What are the local diseases? It's not that unlikely that both people would die of some respiratory ailment in the first year or two, and quite likely that if they do survive to have children who have children, infectious diseases will come along to thin the population.
Granted, Fantasy can indeed take place in space. It was the biological bent to the original question brought what I said to mind. When you talk of a man and woman on an Alien world I think it's a fairly obvious assumption to make. I immediately thought: human beings in space, sci-fi... my mistake.
Two is too few. Making a living even in a fairly benign environment is hard work. Even with knowledge, there is a certain threshold population required before there is enough leisure to do research, mineral exploration, mining, and all the rest that will lead to civilisation, and that is not taking in account infant mortality, deaths during childbirth, deaths from hunting, accidents, etc, and this is assuming there are no diseases. Plus without paper and the time to write everything down, technology will be lost as time goes by and with every generation. By the third generation the survivors would likely be hunter gatherers with a small agricultural capacity, and they are likely to be stuck at that level for a long time.
Everything said by @ChickenFreak, plus, 12 generations is about 300 years if you use the standard 25 years per generation (though it's really closer to 20). Regardless, where you start, knowledge-wise, is very important. 300 years after starting with two people who understand that polymers, computers, plastics, metallurgy are at least possible, even if they themselves don't know exactly know how to make those things happen, is very different than 300 years after two people who start with no knowledge, or worse, are hampered by belief dynamics that restrict certain behaviors, experimentation or living arrangements that might prove beneficial but are taboo/sinful/you get the picture. Two people stuck on a planet, alone, are going to have nearly every minute of their waking life spent on survival-only tasks. So will many generations that come after them. You're looking at hunter-gatherers at a max. ETA: Also, the presence or absence of farmable plant species and animal species on your hypothetical world is going to make a huge difference in the size of given populations. Until the advent of farming, humans and our ancestors lived in groups of 20 to 25. That's the standard size of a pack of humans (we're naturally pack animals) living off of the land. We're largish creatures in the grand scheme of things and we individually need, at a hunter/gather level, about 10 square miles per person, of land resource to survive. If you don't have good farming, you don't have large populations. Know that AMS, anatomically modern sapiens, you and me, lived in those small groups of 20 to 25 people for between 100,000 and 200,000 years. Before that you're getting into the territory of no longer homo sapiens.
Silly me. You know what I left out? Experiencing pregnancy and childbirth and raising a baby out of infanthood with no medical infrastructure whatsoever. Edited to add: If I were that one woman and I were on the planet with the cave and the fruit and nice tropical breezes and a few medical supplies or a little medical knowledge, my top priority would be birth control. Both to give me a shot at personal survival, and because there's no way I'd want to subject children to a wilderness.
Welcome to the forum, seriously. We do like to answer questions. And we are mostly nice people. But don't get mad at us because your question was this vague. So how about backing up. To summarize a few things that have been brought up: 1) Which aspects of "look like" are you asking about? 2) What does the world look like to start with, what are their resources and hazards? 3) What kind of knowledge/technology do these people have to start with?
It's fantasy you can blow facts out of the water as long as you convince the reader it's possible. And if not, all that will happen is the nerds - - will argue your 'flubs' to death thereby talking about your story and making it more popular - you can't really go wrong. Plus, you could spin the idea off into new realms maybe they turn into a hybrid to adapt to the planet. Really, it's your story and you can do anything you want - plausibility isn't as important as the believability of the characters and their problems.
