2.4 million years ago to 1.5 million years ago
Homo habilis, which actually means "handy man," may have been the first species to make and use primitive stone tools. About five-feet-tall and weighing 100 pounds, H. habilis had a brain that was larger than the largest Australopithecus brain, but smaller than the Homo erectus brain.
1.8 mya to 300,000 years ago
The first example of Homo erectus, known as "Java Man," was discovered in Indonesia in 1893. Fossil remains of H. erectus have since been found throughout Africa and Asia, making it the first known wide-ranging hominid. Despite the primitive appearance of its skull, the H. erectus skeleton is very similar to that of modern humans, although more robust (thicker and heavier). H. erectus was probably the first hominid to use fire.
800,000 to 200,000 years ago
Sometimes classified as Homo sapiens archaic, this species contains a range of specimens that share features with both H. erectus and modern humans. In general, its brain was larger and more rounded than H. erectus, but smaller than that of a modern human. Fossil remains of H. heidelbergensis have been found in Africa and Europe.
230,000 to 30,000 years ago
Neanderthals are classified by some as Homo sapiens neanderthalensis -- a subspecies of H. sapiens. Averaging five-and-a-half feet in height and possessing short limbs, Neanderthals were well-adapted to living in a cold climate. Attached to their thick, heavy bones were powerful muscles. The Neanderthal brain cavity was larger than that of today's humans, but that may be related to the Neanderthals' greater bulk in general. Neanderthals were mostly found in Europe, and their skeletons show they lived brutal lives. Earlier theories suggested that modern humans are descended from Neanderthals, but most paleontologists have ruled out that idea. The fossil record suggests the two groups co-existed in some areas. Some speculate that Neanderthals interbred with modern humans, but genetic studies suggest the two groups didn't mate.
Homo sapiens idaltu
In the June 12, 2003 issue of Nature, a team led by Tim White reports finding fossils worthy of a new subspecies of Homo sapiens: Homo sapiens idaltu. The skull of an adult male found in Middle Awash, Ethiopia, is slightly larger than the upper limits seen in contemporary humans, but it shares more characteristics -- in particular, less prominent brow ridges -- with modern humans than any other fossils found to date.
Homo sapiens sapiens (modern)
120,000 years ago to present
Modern Homo sapiens, also known as Homo sapiens sapiens, have been around for at least the past 120,000 years. Homo sapiens living about 40,000 years ago made elaborate tools out of bone, antler, ivory, stone and wood, and produced artwork in the form of carvings and cave paintings. In the last 100,000 years, the fossil record shows that even among this species, there is a trend toward smaller tooth sizes and lighter body frames.
Sources: Smithsonian Institute National Museum of Natural History, PBS