The evolution of human intelligence is closely tied to the evolution of the human brain and to the origin of language. The timeline of human evolution spans approximately seven million years, from the separation of the genus Pan until the emergence of behavioral modernity by 50,000 years ago. The first three million years of this timeline concern Sahelanthropus, the following two million concern Australopithecus and the final two million span the history of the genus Homo in the Paleolithic era. Many traits of human intelligence, such as empathy, theory of mind, mourning, ritual, and the use of symbols and tools, are somewhat apparent in great apes, although they are in much less sophisticated forms than what is found in humans like the great ape language. History Hominidae The great apes (hominidae) show some cognitive and empathic abilities. Chimpanzees can make tools and use them to acquire foods and for social displays; they have mildly complex hunting strategies requiring cooperation, influence and rank; they are status conscious, manipulative and capable of deception; they can learn to use symbols and understand aspects of human language including some relational syntax, concepts of number and numerical sequence. One common characteristic that is present in species of "high degree intelligence" (i.e. dolphins, great apes, and humans - Homo sapiens) is a brain of enlarged size. Along with this, there is a more developed neocortex, a folding of the cerebral cortex, and von Economo neurons. Said neurons are linked to social intelligence and the ability to gauge what another is thinking or feeling and are also present in bottlenose dolphins. Homininae Around 10 million years ago, the Earth's climate entered a cooler and drier phase, which led eventually to the Quaternary glaciation beginning some 2.6 million years ago. One consequence of this was that the north African tropical forest began to retreat, being replaced first by open grasslands and eventually by desert (the modern Sahara). As their environment changed from continuous forest to patches of forest separated by expanses of grassland, some primates adapted to a partly or fully ground-dwelling life. Here they were exposed to predators, such as the big cats, from whom they had previously been safe. These environmental pressures caused selection to favor bipedalism: walking on hind legs. This gave the Homininae's eyes greater elevation, the ability to see approaching danger further off, and a more efficient means of locomotion. It also freed