I grew up reading many books on how to argue creationism, to defend the faith from the evolutionists. When I got to college, I saw that arguing creationism wasn't working at all. This was very strange, because I knew all the arguments. What I didn't realize for a very long time is that science interprets itself. Science is a communal endeavor, and it grows through discussions, not arguments. The standard of truth in science is now basically postmodern.
Science is very different now than it was a hundred years ago; it is staggering under its own weight. Science has become a community of specialists. About a half a dozen people in the world understand what it is that I'm doing for my research right now, and why it's important. If anyone outside of that very small group wanted to double-check my work, it would probably take him weeks of reading old journal articles, doodling on marker boards, asking questions, and messing about with computer simulations written in Fortran.
When articles that I write go up for peer review, the validity of my work is considered by some of the few people who can actually understand it. The confidence that scientists outside that circle can place on my work rests on the trustworthiness of my reviewers. In short, scientific truth is determined by the community. This is a postmodern standard of truth.
We scientists would like it if everything we knew in science could be reasoned from first principles, or tested quickly and easily in a lab setting. We don't want to have to use a postmodern standard of truth, but we have little choice. It's not quite so bad as it sounds; because science is based on experimentation, observation, and reason, it doesn't become unhinged from reality nearly as quickly as disciplines which are based more on opinion and ideology. And science works; we've sent astronauts to the moon, our life expectancies are much improved over what they were a hundred years ago, and I can buy a pizza for a dollar and cook it in four minutes.
If I, as an individual, were to argue against the consensus of the entire scientific community, I would be presumptuous. Thomas had the luxury of being able to touch the holes in Jesus' hands and feet, and the wound in his side; then he could believe. The scientific community is composed of people who are as skeptical as Thomas, but, when it comes to theory of origin, the evidence is not as plain to us as it was to Thomas. No one person can understand biochemistry in sufficient detail to be able to consider whether it is probable that even a single cell came about by chance. When it comes to problems that require such a high degree of specialization, it is unreasonable to expect a scientist to take my word over that of the community.