Extreme cousins of ours who are no more: From massive Gigantopithecus to diminutive Flores “hobbits.”

Our closest relatives are other primates, both living and ancestral. While all species have evolved features that separate them from other groups, there are some who, when viewed through the lens of time, stand apart as being “extreme.” Some have effectuated this by evolving to gargantuan or diminutive size, or by accruing unique responses to biological systems such as breathing. Among these, two stand apart for size: the largest primate, Gigantopithecus , and the smallest hominin taxon, Homo floresiensis . Gigantopithecus was a 10‐foot tall orangutan look‐alike first described in 1935 known largely from jaws and teeth. They inhabited SE Asia 9 mya to 100 kya when they succumbed to climate change affecting the plants and fruits these vegetarians ate. Why so big? We don't know. Clearly, however, their size gave them protection from predators and thus freedom to roam and feed as often as needed (which was often, given their size.) On the other extreme is “Flores Man,” discovered in 2003 on the Indonesian Island of Flores. Nicknamed “hobbit” due to their standing only 3.5 feet (cranial and postcranial remains recovered) Flores is argued to exhibit sufficient uniquely derived characters to warrant species‐level distinction separating them from preceding H. erectus or contemporaneous H. sapiens and are placed in the taxon H. floresiensis. Their average brain size is in about 400 cm 3 , in the range of Plio‐Pleistocene australopiths and chimpanzees, leading some to claim the specimens are microcephalic H. sapiens . Adherents of H. floresiensis attribute their diminutive size to “insular dwarfism” which also affected other island species. Flores died out circa 50 kya, arguably from contact with arriving H. sapiens. While these groups represent size extremes, perhaps the best example of an extinct relative is our closest cousin, the Neanderthals. While Neanderthals are us in many ways, studies have shown that they exhibited extreme specializations in their upper respiratory tracts (e.g., large noses with unique features; large paranasal sinuses; larynges likely higher in the throat than in living humans). This anatomy may have made them ideally equipped to process cold and dry air and be among features that separate them on a species level – H. neanderthalensis ‐ from H. sapiens . Neanderthals may thus have evolved to be “Respiratory Extremists” to conquer their world. Unfortunately, our ancestors lived in that world too, and perhaps what led to their demise was transmission of our respiratory pathogens to them. Whether it was the largest ape, the smallest Homo or our closest relative, our primate extremists seem to all have met the same end: extinction.