How did the lineage of extinct human ancestors change over the course of millions of years? What changes made our predecessors more humanlike? When did those changes occur and why?
Answers lie in the extensive fossil record of hominids, the family of animals most closely related to humans. Additionally, the soil and rock deposits in which those specimens are found reveal the environmental conditions that existed when they lived.
Did they live in forests, on open savannahs or lake shores? Additionally, fossils from more recent times are accompanied by tool artifacts that reflect intellectual development of our predecessors.
From this data we have clues for how these creatures made their living and how they interacted with one another. Of paramount importance, over the course of several million years, hominids became increasingly bipedal. That is, they came to walk on two legs.
Fossilized remains of the earliest hominids show they could walk short distances but primarily took refuge in trees. This is demonstrated by their arm, leg, hand and foot bones. The proportions of these bones and their grasping toes show them to have been primarily tree-dwelling animals.
The foramen magnum is the hole at the base of the skull through which the spinal cord passes. In animals that stand upright, the brain is situated above the spine. Their foramen magnum is in a forward position relative to animals walking on all four legs.
This transition to a forward position is well documented in hominid fossils. At the same time one sees modifications to bones of the hands, pelvis, legs, and feet. Eventually, grasping toes were lost and hominids became more suited to walking greater distances across open expanses and less adapted to forests.
Hands became progressively less important for moving about in trees. At the same time they were easily adapted to manipulating and carrying objects. Artifacts found with fossils from later time periods show the use and manufacture of tools.
Throughout the transition of hominids to more upright, bipedal and eventually tool-making creatures, the volume of the brain cavity increased. Also, contours on the inside of fossil skulls show development of parts of the brain associated with new adaptations.
The number of hominid fossils is too great to treat each individually in this short account, but a condensed outline is illuminating.
Very near the beginnings of the first branch on the human family tree is a creature dating from 5.8 million years ago called Ardipithecus kadabba. Its fragmentary remains were discovered from several sites in Ethiopia between 1997 and 2003. Evidence from the excavation of five individuals at one site shows they lived in a wooded environment.
The ancestors of chimpanzees and bonobos had diverged from those of A. Kadabba some million years earlier. The last common ancestors of chimps, hominids and gorillas dates yet another 1 million years earlier, nearly 8 million years ago.
October 2009 brought an announcement of the discovery of a related species to A. Kadabba, Ardipethecus ramidus. Nicknamed Ardi, this specimen, dated from about 4.4 million years ago, created great excitement.
Its skeleton offers a glimpse of the transition from life in trees to life on the ground. Evidence also indicates this change was accompanied by a move toward more monogamous relationships.
A. ramidus became a dead-end branch on the hominid family tree. However, emerging at nearly the same time is another branch, a whole new genus whose members are called Australopithecines. Fossils from a variety of species of Australopithecines have been found from the extreme south of Africa to what are now Africa's northern deserts.
Some species are slender (gracile), and appear in the fossil record from about 4 to 2 million years ago. A stouter variety is thought to have arisen from gracile Australopithecines. They are classified by some paleoanthropologists in a genus called Paranthropus. The robust species existed from about 2.7 to 1.2 million years ago.
Gracile species of Australopithecines include A. anamensis, A. afarensis, A. garhi, and A. africanus. Three robust species are generally recognized: P. aetheipicus, P. robustus, and P. boisei.
Australopithecines stood about 4 to 4¬? feet tall. Their brains were 35 to 40 percent the size of modern humans. Their teeth and jaws were distinctly hominid, less like those of apes.
The most famous A. afarensis discovery is that of a skeleton found in the Afar region of Ethiopia in 1974. Nearly 40 percent of this skeleton, commonly referred to as Lucy, was recovered. It is dated from 3.2 million years ago and demonstrates clearly that Lucy walked upright.
Supporting the notion that A. afarensis was bipedal was the discovery in 1976 of footprints in Laetoli, Tanzania. The paths of two individuals walking upright next to each other are "cemented" in a layer of volcanic ash. These awesome impressions are dated at 3.5 million years ago
Evidence for A. afarensis making and using tools comes from cut marks on fossil bones of "cow-like" and "goat-like" animals. These date from about 3.4 million years ago, 800,000 years earlier than any other known use of tools.
Larger, more complex brains followed with the emergence of hominids that manufactured tools. Each adaptation was essential and would eventually lead to hominids that depended on complex social interactions and communication for survival.
Steve Luckstead is a medical physicist in the radiation oncology department at St. Mary Medical Center. He can be reached at firstname.lastname@example.org.