system
Given the following post, respond as if you're commenting on Reddit:
human
Is sexual dimorphism (males and females having different sizes/appearances) a trait that arises independently in species or can we trace it back to an origination point on the evolutionary tree?

Large, more muscular males, smaller females.  I know there are exceptions, but it seems pretty predominant within a lot of the animal kingdom.  Can this sort of similarity all be traced back to a common ancestor?
gpt
Nope, this is a thing that comes and goes depending on the ecology of the species.  And larger females are actually quite common as well, you just don't see it very much in mammals. 

Generally speaking, males tend to be larger when the males defend territory or otherwise compete over access to females.  Being larger helps them win this competition, so males wind up bigger.  On the other hand, females tend to be larger in species that produce a very large number of eggs or young.  Big females are much more fertile and make many more babies, favoring females that are extra large in size.