Author: Halliday Elizabeth
Institution: Marine Biology
Date: July 2005
The urge to discover humanity's common past, who we are and where we came from, is perhaps as ancient as the human species. First explained by myth, the story of Homo sapiens is continually being pieced together by scientists of different fields. The excavation of our ancestors utilizes traditional methods just as much as new technology and represents a unique cooperation of anthropology, genetics, and ecology.
One discovery stands out as particularly influencing common knowledge of our ancestors. On Nov. 24, 1974, Donald Johanson and his student Tom Gray were headed back to their Land Rover after a long morning of surveying for fossils. They were in northern Ethiopia, and it was about noon.The temperature was approaching 110, and they hadn't found anything significant since their start that morning. Hiking back to his Land Rover, a fossil caught Johanson's eye. Sticking out of the barren, dusty landscape was part of a hominid elbow. Digging turned up another bone, and then another until a partial hominid skeleton emerged from the ancient Hadar sediments.
That night, as the scientists examined their find, they listened to Beatles music. The petite, upright-standing female was formally assigned the name Australopithicus afarensis, but will forever be known as Lucy, after "Lucy in the Sky with Diamonds."
Figure 1. Lucy, pictured above, is famous both because of her skeletal completeness and because her species, Australopithicus afarensis, occupies a pivotal place in the evolution of hominids. Image Courtesy of The Institute of Human Origins, Arizona State University.
Lucy is just less then 3.18 million years old. She belongs to the family Hominidae, commonly called hominids, which includes all species that evolved after the ancestral split and divergence that would yield on one side modern humans, and the other African apes. As Homo sapiens, the only surviving species of the family Hominidae, our story is continually unfolding as scientists from fields such as paleoecology, paleoanthropology and molecular anthropology combine their knowledge to explain our shared ancestry.
The site of Hadar, where Lucy was found, is now a dusty desert but was once a forested oasis supporting a huge diversity and variety of life. Plants flourished and animals, including early humans, roamed the land. The only evidence today of that past comes from animal fossils that date back over 3 million years. Upon their death animals were incorporated in the deposits of Hadar. Today, erosion exposes their fossilized bones. The fossils of many browsing animals in the area suggest that there were many plants to sustain them, as well as plants to sustain early hominids. This reconstruction of the environment of our ancestors is called paleoecology.
Paleoanthropologists, like Johanson, use human fossils to learn from the anatomy of our ancestors. Although others have discovered older specimens since then, Lucy remains for several reasons the benchmark by which all human fossils are judged.
"From the public point of view, Lucy is by virtue of her name so widely known," Johanson said. Whenever an anthropologist unearths a new hominid, the media compares it to Lucy is it older, younger, closely or distantly related to Lucy?
Figure 2. Sunrise at Hadar, Ethiopia. Now a desert, Hadar once was an oasis supporting huge biological diversity including ancestral hominids. Image Courtesy of The Institute of Human Origins, Arizona State University.
"Even 30 or 35 years later, her completeness is one of the things that is most useful," explains Johanson. "We can look at the details of what a bipedal pelvis looks like." Complete skeletons are rarely found. The angle of her bones and the curvature of her spine show that Lucy used bipedal locomotion, meaning she walked upright, an attribute of hominids.
Johanson and others have found over 380 hominid bones in Hadar, but Lucy is still a special find. A large sampling of an ancient species is unique and extremely valuable to those piecing together the history of a species. Just as modern humans vary in height and weight, no two members of the ancient afarensis species were exactly alike. Thus the afarensis findings, including Lucy, are, according to Johanson, "very important as a reference collection not only of the entire anatomy but also of a species spanning a half million years."
"Evolutionarily speaking, afarensis is a species which sits at the kind of pivotal place between primitive apelike hominids and more derived, robust hominids," says Johanson. With this kind of a well-represented reference collection, scientists can measure the stability of the species and its change over time.
Researchers also harness molecular biology and genetics in their search for our common ancestor. Until about twenty years ago, anthropologists estimated a common ancestor occurred about 20 million years ago. Molecular data from modern DNA has moved that date up to between seven and nine million years ago. Johanson and other paleoanthropologists appreciate molecular data, because it "gives us a good idea of relationships and in a broad way an understanding of the timing of those events."
Recently molecular data has been used for more than just dating purposes. Sarah Tishkoff, a molecular anthropologist at the University of Maryland utilizes the fossil record of the human genome mutations, passed from one generation to another to discover the origin of mankind.
This requires intensive fieldwork. Tishkoff traveled through the African bush to collect blood samples from as many tribal groups as possible. At night, she hooked a centrifuge to her Land Rover's battery and processed the samples to measure the frequencies of mutations in the DNA of different populations. Tishkoff assembled a genetic database that reinforced the fossil record and theories of migration, in addition to yielding some new discoveries as well. She found that several populations of modern humans, who apparently originated in northeastern Africa, are ancient, dating back 90,000 years. She concluded that the African gene pool is older than any other, and that at some relatively recent time in the past, a small group of people migrated out of East Africa and populated the rest of the world.
Hopefully, the combination of scientific fields and cooperation across disciplines will continue to yield information about humankind's shared history. Johanson believes that as a species, Homo sapiens have a profound interest in answering fundamental questions about the natural world. Who are we, and where do we come? Yet there is even more to the inquiry into our past then a satiation of our natural curiosity there are lessons to be learned. Our evolution emphasizes how closely linked we are to the natural world. "We got here through the same processes as other creatures," says Johanson, and "the future of the planet and our fellow travelers in this journey will to a large extent count on the decisions we make as a species. In that sense, destroying the natural world could be likened to destroying your creator."
Interested in learning more? Check out:
A broadband documentary at www.becominghuman.org