Animal Behavior/Human Evolution

Human Evolution

"In the distant future ... psychology will be based on a new foundation, that of the necessary acquirement of each mental power and capacity by gradation. Light will be thrown on the origin of man and his history"—Charles Darwin, 1859

Biological evolution has led to the emergence of humans as a distinct species from other primates. The term "human", in the context of human evolution, refers to the genus Homo, but studies of human evolution usually also include other groups, such as the australopithecine hominids.

The distinct lineage of humans split off from among the other great apes around 4-7 million years ago, most likely in Eastern Africa. The earliest undisputed hominins emerged 4.4 million years ago, the earliest members of Homo arose 2.5 million years ago, while our species, Homo sapiens, is not older than 200,000 years. Although considerable questions remain, a consistent picture is slowly emerging. Ancestral hominins occupied mostly forested habitats, depended on a largely fruit-based diet, and lived in small, migratory, male dominated, social groups. Around 5 million years ago geological uplifting along the rift valley, along with increasingly drier and cooler climatic conditions of the pliocene, resulted in an expansion of savanna ecosystems across eastern Africa. Trees became more widely spaced, and allowed sufficient light to reach the ground for an unbroken herbaceous layer dominated by grasses. Australopithecines appear to have ventured out into these more open woodlands in search of the ungulate fauna thriving there. A variety of ecological demands contributed to rapid specializations in a host of morphological and behavioral traits. Footprints, preserved at Laetoli, Tanzania, when rain cemented powdery volcanic ash from a recent eruption into a layer of tuff, evidence that autralopithecines already lacked the mobile big toe of apes, had an arch more typical of modern humans, and habitually walked upright. With the advent of bipedal walk, the acquisition of a larger skull cavity helped balance the head during running. Australopithecines included meat into an increasingly omnivorous diet and coordinated group hunting efforts may have hastened the development of many critical skills ranging from communication, group cohesion, sophistication in hunting, to general problem solving. The emergence of fairly small, complex social units, with increasingly egalitarian roles of the sexes, encouraged the development of skills for allowing members to better navigate alliances, deception, warfare, cannibalism, as well as sharing, teaching, and compassion.

The divergence of the genus Homo from the australopithecines about 2.5 million years ago marks the beginning of the Palaeolithic epoch (i.e. the Old Stone Age). The discovery of a hominid fossil named Turkana Boy and, more recently, of 1.5M year old hominid tracks at Lake Turkana evidence that Homo erectus (including the sister taxa Homo ergaster and Homo heidelbergensis) combined into the characteristics of an efficient, endurance runner, including a highly efficient bipedal walk, a surprisingly modern foot, elongated limbs, decreased body hair with improved ability to cool down using sweat, and an increased brain case volume. The earliest known stone tools, dated to around 1.8 million years, are credited to this lineage. Behaviorally driven adaptations of food processing through tools and cooking appear to have dramatically enhanced feeding efficiency. This technologically advanced, now extinct, lineage exhibited a particular curiosity for exploring new terrains and it accounts for early expansions into Asia and Europe. With it fossils begin to emerge in the record in a number of new locals as diverse as Spain, Germany, Georgia, and Indonesia.

Denisova hominids were recently discovered as members of a distinct subspecies of Homo sapiens from a cave in the Altai Mountains in Siberia. They are viewed as direct descendants of Homo erectus' expansion into Europe and elsewhere. Subsequent study of the nuclear genome suggests that this group shares a common origin with Neanderthals, that they ranged from Siberia to Southeast Asia, and that they lived among and interbred with the ancestors of some present-day modern humans, with up to 6% of the DNA of Melanesians and Australian Aborigines deriving from Denisovans. A comparison with the genome of a Neanderthal from the same cave revealed significant local interbreeding, with local Neanderthal DNA representing 17% of the Denisovan genome, while evidence was also detected of interbreeding with an as yet unidentified ancient human lineage. The lineage Neanderthals (Homo neanderthalensis) may have also descended from the Homo erectus (also Homo heidelbergensis) lineage around 500-300k years ago. As hunter-gatherers they moved across Europe following the advancing and retreating glaciers of the ice age. Adapted to temperate, northern climates success largely hinged on warmer interglacial periods. Total population probably never exceeded 100,000. The presence of Neanderthal weapons in animal bone show a level of hunting sophistication that allowed them to harvest even such large game as elephants, rhinoceros, and horses. It is also now clear that Neanderthals must have cared for injured group members, conducted simple burials, and showed an appreciation for art including carvings, beads, and music. Discussion continues on whether Neanderthals were able to communicate using a spoken language, but their hyoid bones, involved in speech, were basically identical to those of modern humans.

40,000 years ago another major African expansion of hominins lead modern humans from the taxon Homo sapiens to Europe which eclipsed Neanderthals with a number of advanced and sophisticated technologies, The rise of the new arrivals (Cro-Magnons) parallels the decline and eventual vanishing of Neanderthal culture, although whether through extinction or by intermixing with modern humans remains unclear.

The mesolithic (i.e. the Middle Stone Age), running roughly from 10,000 to 6,000 years ago, marks the transition from a hunter-gatherer to a largely agricultural society. Different Mesolithic people began the initial stages of domestication. settled into villages of huts, developed basic tools into an industry of small implements, and invented ceramics.

The neolithic (i.e. the New Stone Age), marking the remaining period up to the beginning of the bronze age, witnessed the emergence of domestication of animals, including sheep and dogs, cultivation of plants, irrigation, writing, architecture, and government.