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September 8th, 2011
12:48 PM ET

'Lucy' discoverer: Why I study human evolution

Donald C. Johanson is the director of the Institute of Human Origins at Arizona State University.   He discovered the 3.18 million year old skeleton popularly known as "Lucy."

My deep commitment to understand the origins of humankind was ignited when I read Thomas Henry Huxley’s 1863 book "Man’s Place in Nature." The core idea that gripped my teenage mind was the suggestion that humans and African apes shared a common ancestor that roamed Africa millions of years ago.

I was riveted by the 1959 discovery of a 1.8 million-year-old skull at Olduvai Gorge and I knew that I wanted to travel to Africa and join the search for our ancestors. The allure of conducting fieldwork in remote unexplored regions of Africa dominated my thoughts throughout my undergraduate and graduate studies.

In 1970, Professor F. Clark Howell at the University of Chicago invited me to join his expedition to the remote Omo region of southern Ethiopia.  From the moment I set foot on African soil I knew my life would be filled with adventure and hopefully discovery.  The thrill of field research surpassed my dreams. Every day I collected fossil remains of pigs, elephants, hippos, monkeys, gazelles, and other creatures.  These discoveries were revealing the past world in which we evolved.

Skeletons more than 1 million years younger than 'Lucy' hold more evolution clues

After three summers of not finding a scrap of human bone, my resolve was beginning to wane.  But in 1973 I signed on as co-director of an expedition to the remote and poorly explored Afar region of Ethiopia.  Here vast fossil fields offered unparalleled opportunity and I was convinced that this is where I would find hominid fossils.  On that very first expedition to a place locally known as Hadar I found a 3.4 Million Years Ago fossil hominid knee joint.  Extensive study of the bones revealed that in virtually every detail the knee was identical to ours.  This creature had walked upright and therefore deserved a place on the human family tree.

On my eager return to Hadar in the fall of 1974 the major goal was to find fossils complete enough to identify the species that had walked with that knee.  Almost immediately we found teeth and jaws of roughly the same antiquity as the knee.  The anatomy of these specimens was more ape-like than any that had been previously found in Africa.  Darwin and Huxley would have been elated had they been alive to see the finds.

On November 24, 1974, a hot Sunday morning I was completing a routine mapping exercise and I spotted a three-inch long bone fragment that would change my life.  As I kneeled down to more closely inspect the anatomy of the fragment I knew instantly it was part of a hominid elbow.

Looking around to see if more of this individual was there I saw a chunk of lower jaw, a shard of skull, a fragment of a vertebra and ribs.  Each specimen was unquestionably hominid and I knew it was a partial skeleton of a human ancestor that had lain in suspended animation for 3.2 million years and had been exposed by rain erosion.

This was childhood dream come true and I knew my life would dramatically change.  The bones were diminutive, probably a female and sometime during the evening celebration the skeleton was christened Lucy (after Beatles' song, "Lucy in the Sky with Diamonds").

Extensive laboratory examination revealed that Lucy belonged to a new species, Australopithecus afarensis, a conclusion that prompted a redrawing of the human family tree. Current thinking places afarensis at the pivotal point on our family tree where two evolutionary lineages diverged.  One led to several extinct side branches of Australopithecus while the other led to our own genus Homo that eventually evolved into modern humans, Homo sapiens.

Lucy is just one of the thousands of fossil hominid specimens, spanning the last 6 million years, that allows us to document the African origins of all humans alive today.  Some 6 to 8 million years ago a common ancestor (still undiscovered) to humans and the chimpanzee lived somewhere in Africa.  Our ancestors first stood up in Africa, and by 2.6 MYA they began to fashion stone tools.  Our brains first expanded around 2 MYA, and by 200,000 years ago Homo sapiens made an African appearance.

Modern humans are not the final resting place in our evolution.  If the past is any guide to us, we too may someday face extinction as a species.  We are united by our past, we are a single species of human on the planet today, and our future is intimately tied to how responsibly we care for the unique planet on which we live.

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Get a behind-the-scenes look at the latest stories from CNN Chief Medical Correspondent, Dr. Sanjay Gupta, Senior Medical Correspondent Elizabeth Cohen and the CNN Medical Unit producers. They'll share news and views on health and medical trends - info that will help you take better care of yourself and the people you love.