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Australopithecus and Ardipithecus
Since Darwin, anthropologists have been looking for "the missing link" -- the last common ancestor of humans and apes. Some very old human ancestors like Australopithecus afarensis (represented by "Lucy"), which is 3.2 million years old, or Ardipithecus ramidus (represented by "Ardi"), which is 4.4 million years old, may be the missing link. Most anthropologists think the last common ancestor is still older -- 6 to 7 million years old. Whatever this ancestor is, a thread of ancestors stretches back from this one to the origin of life, billions of years earlier.
Mammals Among the Dinosaurs
Before 65 million years ago, in the age of dinosaurs, the ancestors of apes and men were small shrew-like animals. Mammals actually first appeared at about the same time as the dinosaurs, but dinosaurs came to rule the world while mammals remained small nocturnal animals trying to keep out of their way. NASA reports on the theory that everything changed abruptly one day 65 million years ago, when a large asteroid hit Earth in the Caribbean near the Yucatan Peninsula. All of the dinosaurs -- except the one line that led to birds -- were killed off. Sixty million years later, mammalian evolution led to the common ancestor of apes and man.
The First Land Animals
The first land animal came out of the sea and onto land about 365 million years ago. It was a bony fish that had learned to breath air -- similar to African lungfish today. It is thought these earlier explorers were fleeing a sea of fierce predators. The fossil evidence from the era indicates that the ancient ocean was a nightmare full of predators. The fish that first came on land probably stayed only for short periods of time. Eventually one of the air bladders fish use to stabilize themselves in the sea was modified to create the first lungs. These animals were the ancestors of all mammals, including apes and men.
Single Cells
Three billion years ago, a complex of inorganic materials started replicating itself and life began -- starting the long chain that ultimately led to both apes and men. For billions of years, only single-cell life existed -- until about 500 million years ago when an event known as the Cambrian explosion occurred. This was when cells found a way to come together to build more complex structures. All the basic body plans -- worms, insects and fish for example -- were developed. One of the fish eventually crawled up onto land. These fish evolved into amphibians, then reptiles, and then into mammals, which later split to become apes and men.