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“Might Lucy be our direct ancestor, a missing gap in the human family tree?”

The ‘Lucy’ fossil rewrote the story of humanity

Forty years ago in east Africa, a team of scientists found a fossil that changed our understanding of human evolutionBy Melissa Hogenboom27 November 2014

Forty years ago, on a Sunday morning in late November 1974, a team of scientists were digging in an isolated spot in the Afar region of Ethiopia.

Surveying the area, paleoanthropologist Donald Johanson spotted a small part of an elbow bone. He immediately recognised it as coming from a human ancestor. And there was plenty more. "As I looked up the slopes to my left I saw bits of the skull, a chunk of jaw, a couple of vertebrae," says Johanson.

It was immediately obvious that the skeleton was a

momentous find, because the sediments at the site were known to be 3.2 million years old. "I realised this was part of a skeleton that was older than three million years," says Johanson. It was the most ancient early human – or hominin – ever found. Later it became apparent that it was also the most complete: fully 40% of the skeleton had been preserved.

At the group's campsite that night, Johanson played a Beatles cassette that he had brought with him, and the song "Lucy in the Sky with Diamonds" came on. By this time Johanson thought the

Taung child

Lucy

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the skeleton was female, because it was small. So someone said to him: "why don't you call it

Lucy?" The name stuck immediately. "All of a sudden," says Johanson, "she became a person."

It would be another four years before Lucy was officially described. She belonged to a new species calledAustralopithecus afarensis, and it was clear that she was one of the most important fossils ever discovered.

But at the campsite the morning after the discovery, the discussion was dominated by questions. How old was Lucy when she died? Did she have children? What was she like? And might she be our direct ancestor, a missing gap in the human family tree? Forty years later, we are starting to have answers to some of these questions.

Though she was a new species, Lucy was not the firstAustralopithecus found. That was the Taung Child, the fossilised skull of a young child who lived about 2.8 million years ago in Taung, South Africa. The Taung Child was discovered in 1924 and was studied by anatomist Raymond Dart. He realised that it belonged to a new species, which he called Australopithecus africanus.

Dart wrote: "I knew at a glance that what lay in my hands was no ordinary anthropoidal brain. Here in lime-consolidated sand was the replica of a brain three times as large as that of a baboon and considerably bigger than that of an adult chimpanzee…" The Taung Child's teeth were more like a human child's than an ape's. Dart also concluded that it could walk upright, like humans, because the part of the skull where the spinal cord meets the brain was human-like.

The Taung Child was the first hint that humans originated from Africa. But when Dart published his analysis the following year, he came in for stiff criticism. At the time, Europe and Asia was thought to be the crucial hub for human evolution, and scientists did not accept that Africa was an important site. The Taung Child was denounced by

the prominent anatomist Sir Arthur Keith as just an ape and of no major importance.

Over the next 25 years, more evidence emerged and showed that Dart had been right all along. By the time Lucy came along, anthropologists accepted that australopithecines were early humans, not just apes. So upon her discovery, Lucy became the oldest potential ancestor for every known hominin species. The immediate question was: what was she like?

Lucy had an "incredible amalgam of more primitive and more derived features that had not been seen before," says Johanson. Her skull, jaws and teeth were more ape-like than those of other Australopithecus. Her braincase was also very small, no bigger than that of a chimp. She had a hefty jaw, a low forehead and long dangly arms.

For Johanson, in the field at Hadar, it was immediately apparent that Lucy walked upright, like the Taung Child. That's because the shape and positioning of her pelvis reflected a fully upright gait. Lucy's knee and ankle were also preserved and seem to reflect bipedal walking. Later studies of A. afarensis feet offer even more evidence.

When she was discovered, Lucy was hailed as the oldest direct ancestor of modern humans. "A. afarensis took us one small step closer to that common ancestor we share with chimpanzees," says Tim White of the University of California, Berkeley. "We knew we were genetically incredibly close to chimpanzees, with the last common ancestor we shared with them estimated to be around six million years ago. Lucy had closed a gap in our knowledge."

It now looks like Lucy did not take us as close to our common ancestor with chimps as everyone thought. The latest genetic studies suggest we actually split from chimpanzees much earlier, perhaps as much as 13 million years ago. If that is true, the 3-million-year-old Lucy arrived quite late in the story of human evolution. Older fossils, such as the 4.4-million-year-old Ardipithecus described by White and his colleagues, are closer to our ape ancestors.

A replica of Lucy’s skull