Lynn Margulis is a famous evolutionary biologist. The Creationist book In The Beginning quotes her as saying:
"I have seen no evidence whatsoever that these [evolutionary] changes can occur through the accumulation of gradual mutations."
Science Vol. 252, 19 April 1991, p. 379
The obvious implication is that Dr. Margulis has doubts about evolution. Since she has been awarded membership in the National Academy of Sciences, and since she is the ex-wife of Carl Sagan, it would be high-profile if she did indeed see some big problem with evolution.
But she has said elsewhere that she doesn't doubt evolution. Her next two sentences clear up this puzzle:
"There's no doubt, of course, that they exist, but the major source of evolutionary novelty is the acquisition of symbionts - the whole thing then edited by natural selection. It is never just the accumulation of mutations."
So, she wasn't talking about the history of life in general. The "changes" were very specific details of how early bacteria evolved into single celled eukaryotes.
It is known that bacteria sometimes invade large cells, begin living inside the host cell, and eventually become necessary to their host. Dr. Margulis was saying that such symbiosis events explain various strange structures inside eukaryotes. She invented this once-controversial idea, and it is now the accepted explanation for chloroplasts and mitochondria. (Those structures actually use their own internal DNA to reproduce themselves, independently from the rest of the cell they are in.) According to her web page, she is still trying to prove that cilia arose this way.
Summary: what she actually said is that single-celled creatures sometimes evolved by symbiosis.