Evolution

Date: 5-10 million years ago

The last common ancestor of chimpanzees and humans remains a holy grail in science, but clues about this species have recently surfaced, particularly with the ground-breaking discovery of the world's oldest known probable human ancestor, Ardipithecus ramidus, also known as "Ardi." The sequencing of human and other primate genomes reveals that our genes mutate at certain rates over time. Based on such estimates, "We can conclude that humans and chimpanzees probably last shared a common ancestor between five and seven million years ago," said Blair Hedges, a professor of biology at Penn State.

Date: 4.4 million years ago

Ardipithecus ramidus represents the oldest and most complete skeleton ever found of a probable ancestor to modern humans. The skeleton, nicknamed "Ardi," belonged to an 110-pound female with a chimp-sized brain. Ardi could walk on the ground with two legs, but she had long arms and was also a skilled tree-climber in her Ethiopian, forest setting.

Date: 3 to 3.9 million years ago

Before the discovery of Ardi, "Lucy" (Australopithecus afarensis) was the world's oldest and most complete hominid skeleton. Also from Ethiopia, Lucy was 3.5 -feet tall and weighed only about 55 pounds. There's a good chance Lucy was distantly related to Ardi. Uc Berkeley Paleontologist Tim White added that, especially for her diminutive size, Lucy had big teeth, so dietary changes may have even led to the Ardipithecus/Australopithecus split. Lucy's species, "Australopithecus became a superior omnivore, able to eat tubers and roots with more fiber and grit, adapting it better to times of scarcity during periods of extended drought," he explained.

Date: 1.8 to 1 million years ago

Originating in Africa and possibly spreading to China and Java, Homo erectus ("upright man") got around. Paleontologist Tim White of UC Berkeley said, "When hominids had stone tools, they expanded out of Africa like crazy." One reason may be tied to food processing, according to Richard Wrangham of Harvard University.

Date: 130,000-30,000 years ago

Neanderthals (Homo neanderthalensis) were stocky, brainy hominids from Europe and parts of western and central Asia. They lived in family groups and fearlessly hunted big game, often stabbing large prey with spears at very close range. Their anatomy, along with archaeological finds, suggests they possessed language, art and music. Modern humans and Neanderthals even shared the same geographic region for a period.

Date: 200,000 years ago to present

The recent discovery of Ardipithecus ramidus indicates that social, rather than environmental, change