I thought obvious that the joke refers to the misconception some people host that 'homo sapiens' is a less evolved species than modern people, something we out-evolved.
Although, debatably, homo neanderthalensis is not a subspecies of homo sapiens, thus negating the need for a homo sapiens neandterthalensis and homo sapiens sapiens classification differentiation to exist, making homo sapiens always the modern man (who yes, is also an ugly caveman) and homo neanderthalensis the most recent full time ugly caveman.