Humans are primates, mammals in the order Primates, along with great apes, monkeys, and simians such as lemurs and lorises. Human evolution began, as did the evolution of all existing species, with the first single-celled organism, the first living thing on Earth. For simplification, most evolutionary biologists focus on the evolution of humans beginning with bipedalism, or the ability to walk upright on two feet. This trait arose between four and six million years ago. The next major step in human evolution was encephalization, the development of a large brain. About four million years ago, a group of hominids in a genus known as Australopithecus emerged. They walked upright and had large brains, about a third the size of the brains of modern humans (Homo sapiens). The skeleton of a member of Australopithecus afarensis called Lucy shows these telltale signifiers of the human evolutionary tree.
A little less than three million years ago, the genus Homo emerged, and this is the genus to which modern humans belong. This genus is distinct from its Australopithecus ancestors by virtue of an even larger brain, smaller facial features, and changes in the feet that led to a greater ability to walk upright but a decreased dexterity with the phalanges. The first species in the genus was Homo habilis (three million years ago to one million years ago), a species whose members used tools made of stone. These individuals for a time lived alongside Homo erectus (1.9 million years ago to 140,000 years ago), a direct ancestor of H. sapiens. H. erectus had a larger brain than H. habilis and is thought to have been the first species to use fire as well as stone tools. There is evidence that H. erectus not only cooked food but used fire to make clay tools. H. erectus individuals were also travelers: some spread from Africa into modern-day Europe and as far east as western Asia. They eventually gave rise to several other species, notably H. sapiens (which emerged between 100,000 and 200,000 years ago) and Homo neanderthalensis (about 130,000 years ago to 40,000 years ago). The exact dates at which each of these species emerged are still topics of heated debate.For most of their existence, H. sapiens and H. neanderthalensis lived alongside each other. Like their ancestors, they used fire and tools. They lived in tribal groups and ate a diet consisting of meat they hunted and plants they gathered. It seems likely that tribes of H. neanderthalensis were smaller than tribes of H. sapiens. Anatomically, the two are very similar, with the largest brains of any hominids (H. neanderthalensis had a slightly larger brain). H. neanderthalensis was likely comparable in size as an adult to H. sapiens, standing 150 to 168 cm (60 to 66 in.) tall and weighing 66 to 78 kg (146 to 171 lbs). However, H. neanderthalensis was stronger, with shorter limbs and a larger torso. There is strong evidence to suggest that these two species interacted, and that interbreeding between them occurred. Also under investigation is the cause of the extinction (loss of all members of a group) of H. neanderthalensis about 40,000 years ago. Hypotheses are varied and include disease, climate change, or mere absorption into H. sapiens through interbreeding. Whatever the reason, the extinction of H. neanderthalensis left only H. sapiens in the Homo genus.