There is a relatively large and active community engaged in reconciling the current claims of mainstream biological science with what is known as "Young Earth Creationism." Young Earth Creationists believe that God's creation of the earth took seven literal twenty-four-hour days, and that it took place roughly 6,000 years ago.
It is common among that community to endorse the reality of microevolution, which is biological evolution that creates relatively minor changes over brief spans of time, while rejecting macroevolution, which is evolution capable of creating new species, and which requires spans of time longer than 6,000 years. This, of course, requires distinguishing between micro- and macroevolution as two separate things, one true, one false.
For many atheist biologists, their chief point of contact with the Christian community is with YEC scientists, so they understandably take YEC claims as central to Christian doctrine. However, there are many Christians who are not Young Earth Creationists.