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According to the theory of evolution, the development of genus Homo spans over 2 million years although the fossil record is far from complete; as paleoanthropologists discover new evidence, models of human evolution change.

Anthropologists generally recognize three species of hominenes:

• Homo habilis,
• Homo erectus
• Homo sapiens

All human beings today belong to the same subspecies, Homo sapiens sapiens.

There is ongoing debate over whether "Neanderthal Man" was a separate species, Homo neanderthalensis, or a subspecies of H. sapiens.

H. habilis, the first species of genus Homo, evolved in South and East Africa in the late Pliocene or early Pleistocene (2 - 2.5 million years before present) when it diverged from the Australopithecines ( Australopithecines and Hominenes are collectively referred to as Hominids). Both genera were bipedal. H. habilis had smaller molars and larger brains than the Australopithecines, and tools made from stone and perhaps animal bones.

In the Early Pleistocene, from 1.5 to 1 million years ago, hominines in Africa, Asia, and Europe, evolved larger brains and made more elaborate stone tools; these differences and others are sufficient for anthropologists to classify them as a new species, Homo erectus.

Human babies have far more fat reserves than other primates. It has been hypothesized that this is necessary to ensure brain development during times of food shortages (the brain consumes 60% of a babies energy intake).

The conventional view of human evolution states that humans evolved in inland savanna environments. However, these areas would not have consistently provided a steady and varied enough diet to allow for proper brain development for many infants of early non-hunting hominids, says Stephen Cunnane of the University of Toronto. Shore based environments, however, would have been able to provide enough calories, iodine and omega fatty acids essential to brain growth. Such observations have lead, in part, to the aquatic ape theory. Unfortunately, proving this hypothesis will be difficult because of the fact that many near-shore environments available to our early ancestors are now submerged due to a global rise in sea level.

Between 400,000 years ago and the second interglacial period in the Middle Pleistocene, around 250,000 years ago, the trend in cranial expansion and the elaboration of stone tool technologies developed, providing evidence for a transition from H. erectus to H. sapiens. The direct evidence suggests that there was a migration out of Africa of H. erectus, then a further speciation of H. sapiens from H. erectus in Africa and a subsequent migration out of Africa which replaced the dispersed H. erectus. There is little evidence that this speciation occurred elsewhere, even though some fossil evidence for H. erectus has been found in China. However, the current evidence doesn't preclude multiregional speciation, either. This is a hotly debated area in paleoanthropology.

Important fossils:

• Petralona, Greece, about 300k years old. Contained many features of H. erectus.
• Arago, France, about 300k years old. Oldest skull clearly of H. sapiens origin.

The origins of humanity is a subject of great political and religious controversy in the United States and certain other countries. See: creationism.

See also: Homo neanderthalensis.

Location in the evolutionary tree:

• Kingdom Animalia
• Phylum Chordata
• Class Mammalia
• Order Primates
• Suborder Anthropoidea
• Infraorder Catarrhina
• Superfamily Hominoidea
• Family Hominidae

Note: the superfamily Hominoidea includes two other families, Pongidae, which includes the great apes, and Hylobatidae (gibbons and siamangs). Many taxonomists put Pongidae and Hominidae together as one family; see that article.