HOW DID HUMANS EVOLVE?

Don Johanson's "First Family," a group of 13 individuals of different ages and sexes found in Hadar, Ethiopia in 1975.

Howdid it move?

The broad heels of this creature, could withstand the pressure of walking upright. Like human heels, they are filled with shock-absorbing "spongy" bone, rather than the more solid bone found in the heels of apes.

Toe bones found among the First Family are long compared to those of humans, but they don’t curve forward toward the heel as they do in modern tree-climbing primates.

The very first fossils of this species -­ found by paleoanthropologist Donald Johanson at Hadar in 1973 -­ were the parts of a knee joint. Like a human knee, it could "lock," allowing its owner to stand straight-legged. The knees of chimps and other living primates, by contrast, always stay slightly bent.

As an isolated piece of evidence, this knee couldn't tell much about its owner. But it helped entice Johanson back to Hadar the following year, when he discovered Lucy.

What did they

look like alive?

Are they all the same species?

• The remains of Lucy, the first Australopithecus afarensis find, discovered in Hadar, Ethiopia in 1974 by a team of fossil hunters including Don Johanson.

How did it move?

Of all primates living today, only we humans walk fully upright. But Lucy and other fossil finds reveal that more than 3 million years ago, a relatively small-brained, ape-faced human ancestor walked steadily on two feet.

With dangling, powerful arms, Lucy was likely a good climber, and she may have spent some time in trees. Her upper arm bones are long and sturdy relative to those of modern humans.

With dangling arms and long, slightly curved hands and feet, Lucy and other members of her species may have also been good climbers. Scrambling up trees could protect them from predators and help them reach choice foods. Some scientists point to these "tree-dwelling" traits to argue that A. afarensis was not fully bipedal. The debate among anthropologists may continue, but the reputation of A. afarensis as "the ape that walked upright" makes it a celebrity species in the story of human evolution.

Lucy’s pelvis hints that she walked upright on two legs. When her crushed remains were carefully reconstructed by anthropologist C. Owen Lovejoy, her pelvis looked much like a modern woman’s. Broad, fan-shaped bones on either side, called the iliac blades, curve to form a pelvic basin. This basin helped support her internal organs and entire upper body as she moved.

What did they look like alive?

• The Laetoli Footprints discovered by Mary Leakey in Laetoli, Tanzania in 1978;

How did it move?

Like wet sand on a beach, the volcanic ash of Laetoli captured the traces of two individuals strolling side by side. One set of prints is clearly larger than the other. Perhaps these individuals were a parent and child, or a large male and his diminutive mate. The details of the prints are so fine that scientists can even tell that they walked at a leisurely pace.

As they stepped, their feet fell close together. Like humans today, they maintained good balance because their center of gravity moved forward in a straight line. By contrast, when a chimpanzee tries to walk upright, its feet stay wide apart, its center of gravity shifts side to side, and it awkwardly teeters for only a short time.

Why did they look like alive?

This print is remarkably like one a barefoot human would leave in wet sand. The foot that made this print had a big toe in line with the other toes, rather than set apart, as in a tree-dwelling chimp. It also had an arch between the ball and heel that acted as a shock absorber while walking or running.

Fossils for males and females often differ, particularly in a species like Lucy’s that is sexually dimorphic: Males are much larger than their mates, and their fossils are marked by deeper muscle scars. The pelvic bones of males and females may also be distinctive; females often have a wider "pelvic inlet" (the bony part of the birth canal) that is adapted for giving birth.

Are they all the same species?

When did they live?

First family: 3,2 million years ago

Laetoli: 3.6 million years ago

Lucy: 3.2 million years ago