Students of early man sometimes refer to the most modern incarnation of homo sapiens as a separate sub-species known as homo sapiens sapiens, meaning, roughly, "wise thinking humans". They use this nomenclature to designate a new kind of homo sapiens that emerged sometime around 100,000 to 70,000 years ago. This new sub-species of homo sapiens did not so much have a larger brain (ounce for ounce, the brains of homo neanderthalis, to take another sub-species of humans, were bigger than ours) but one that seems to have been "wired" differently, allowing for more imaginitive thought. Among the advantages of this kind of brain was the ability to create larger, cooperative groupings of humans and to coordinate activities among them. With such groupings, homo sapiens sapiens, it is believed, were able to either defeat or out-compete other sub-species of human beings. And indeed, as homo sapiens sapiens spread across the world, other sub-species of the genus homo disappeared. Yet other casualties of the spread of homo sapiens sapiens, say some paleontologists, were various forms of megafauna. Below is a chart of large mammals that existed in North America, for example, around the time of the first arrival of homo sapiens sapiens. But this theory of human-caused mass extinction is hotly contested. Changing climate, with end of the ice age, may have played a bigger role. Moreover, there is evidence that homo sapiens sapiens arrived in North America tens of thousands of years earlier than is indicated on the above map, long before some of the species below disappeared.