Well, it really should be “They said in the past, but now they say…”
Many of the science passages are structured around the idea that in the past scientists believed one thing, but new evidence is challenging that belief. “They say/ I say” comes from an excellent writing manual that is used in hundreds of universities to teach students how to write academic papers. If you want to get ahead of your peers, I suggest you buy a copy and start working your way through it. I have had dozens of students email me during first year because they have been assigned the book and they have already gone through the lessons with me. I own a copy of all the editions, but I like the second edition with readings the best because it contains 44 essays written that students can use to practice SAT level reading.
Understanding They Say/I Say structure will allow you to identify the main point of the article more easily. I have found that students become confused because they assume the main idea will always be at the beginning of the passage. This is not always the case. Sometimes the first paragraph (or two or three) will focus on what scientists believed in the past or the type of research questions focused on in the past. Then, there will be a shift into what the latest research indicates, or what scientists are focusing on now.
I think the key to unraveling these passages is being able to identify the Key Question. Scientific Method involves developing a question and designing experiments to test various hypotheses in order to, perhaps, answer the question. If you understand what the question is, then you can better understand what the scientists are doing to develop an answer to the question.
Also, don’t forget to read the passage’s title and the introductory information. These can provide you with the keys to unlocking what the passage is about.