What does "through our shared evolutionary origins" mean?
"... it is more reasonable to assume that other creatures, who share so much in common with us through our shared evolutionary origins, do, in fact, experience pleasure." I recall reading this recently. "The Emotional Lives of Animals" by Marc Bekoff?
Other species share a closer or more distant evolutionary relationship with us. All life on Earth descends from a common ancestor, as described by Charles Darwin. Our evolutionary relationship with another species of animal is the distance in time and degree of genetic variation from a common genetic ancestor.
Occam's Razor is the principle of logic that says that in explaining a thing no more assumptions should be made than are necessary. Unless you have evidence to the contrary, you should assume that the simplest explanation that fits the facts is correct. So one should prefer a simple explanation over a more complicated one.
So given that humans experience pleasure, and we are genetically closely related to other animals that also exhibit signs of pleasure, it is reasonable to assume that those other animals experience pleasure also.